ALBERT

Description: ABSTRACT Based on the line integral (LI) and maximum difference reduction (MDR) methods, an automated iterative forward modelling scheme (LI-MDR algorithm) is developed for the inversion of 2D bedrock topography from a gravity anomaly profile for heterogeneous sedimentary basins. The unknown basin topography can be smooth as for intracratonic basins or discontinuous as for rift and strike-slip basins. In case studies using synthetic data, the new algorithm can invert the sedimentary basins bedrock depth within a mean accuracy better than 5% when the gravity anomaly data have an accuracy of better than 0.5 mGal. The main characteristics of the inversion algorithm include: (1) the density contrast of sedimentary basins can be constant or vary horizontally and/or vertically in a very broad but a priori known manner; (2) three inputs are required: the measured gravity anomaly, accuracy level and the density contrast function, (3) the simplification that each gravity station has only one bedrock depth leads to an approach to perform rapid inversions using the forward modelling calculated by LI. The inversion process stops when the residual anomalies (the observed minus the calculated) falls within an ‘error envelope’ whose amplitude is the input accuracy level. The inversion algorithm offers in many cases the possibility of performing an agile 2D gravity inversion on basins with heterogeneous sediments. Both smooth and discontinuous bedrock topography with steep spatial gradients can be well recovered. Limitations include: (1) for each station position, there is only one corresponding point vertically down at the basement; and (2) the largest error in inverting bedrock topography occurs at the deepest points.

Description: As part of the Mercury Experiment to Assess Atmospheric Loading in Canada and the U.S. (METAALICUS), different stable Hg(II) isotope spikes were applied to the upland and wetland areas of a boreal catchment between 2001 and 2006 to examine retention of newly deposited Hg(II). In the present study, a Geographical Information Systems (GIS)-based approach was used to quantify canopy and ground vegetation pools of experimentally applied upland and wetland spike Hg within the METAALICUS watershed over the terrestrial loading phase of the experiment. A chemical kinetic model was also used to describe the changes in spike Hg concentrations of canopy and ground vegetation over time. An examination of the fate of spike Hg initially present on canopy vegetation using a mass balance approach indicated that the largest percentage flux from the canopy over one year post-spray was emission to the atmosphere (upland: 45%; wetland: 71%), followed by litterfall (upland: 14%; wetland: 10%) and throughfall fluxes (upland: 12%; wetland: 9%) and longer term retention of spike in the forest canopy (11% for both upland and wetland). Average half-lives (t1/2) of spike on deciduous (110 ± 30 days) and coniferous (180 ± 40 days) canopy and ground vegetation (890 ± 620 days) indicated that retention of new atmospheric Hg(II) on terrestrial (especially ground) vegetation delays downward transport of new atmospheric Hg(II) into the soil profile and runoff into lakes.

Description: We present a lake-wide investigation of Lake Superior carbon and organic matter biogeochemistry using radiocarbon, stable isotope, and carbon concentrations. Dissolved inorganic carbon (DIC) abundance in the lake was 121–122 Tg C, with offshore concentration and δ13C values being laterally homogenous and tightly coupled to the physical and thermal regime and biochemical processes. Offshore Δ14C of DIC (50–65‰) exhibited lateral homogeneity and was more 14C enriched than co-occurring atmospheric CO2 (∼38‰); nearshore Δ14C of DIC (36–38‰) was similar to atmospheric CO2. Dissolved organic carbon (DOC) abundance was 14.2–16.4 Tg C. DOC's concentration and δ13C were homogenous in June (mixed lake), but varied laterally during August (stratification) possibly due to spatial differences in lake productivity. Throughout sampling, DOC had modern radiocarbon values (14–58‰) indicating a semilabile nature with a turnover time of ≤60 years. Lake particulate organic carbon (POC, 0.9–1.3 Tg C) was consistently 13C depleted relative to DOC. The δ15N of epilimnetic particulate organic nitrogen shifted to more negative values during stratification possibly indicating greater use of nitrate (rather than ammonium) by phytoplankton in August. POC's radiocarbon was spatially heterogeneous (Δ14C range: 58‰ to −303‰), and generally 14C depleted relative to DOC and DIC. POC 14C depletion could not be accounted for by black carbon in the lake but, because of its spatial and temporal distribution, is attributed to sediment resuspension. The presence of old POC within the epilimnion of the open lake indicates possible benthic-pelagic coupling in the lake's organic carbon cycle; the ultimate fate of this old POC bears further investigation.

Description: ABSTRACT In 2008, a wide azimuth simultaneous source (SimSrc) 3D data set was acquired over an area coinciding with an existing conventional wide azimuth survey. Using two (nearly) simultaneous sources provided twice the number of shots, an increased shot density and twice the fold for essentially the same acquisition time. Although the improved sampling for the SimSrc data was predicted to yield improvements in processes such as 3D multiple attenuation and migration, cross-talk between the sources was a concern. Cross-talk can be suppressed by known techniques such as separation algorithms that estimate the individual shots and thereby separate SimSrc records into their constituent shots. However, separation algorithms available at the time suffered from aliasing and were not effective for severely shot-aliased data such as those encountered in the experimental data set, which had a 75-m in-line shot spacing, half that typical for wide azimuth geometries. In this paper we present a new separation technique called the Alternating Projection Method (APM), which is demonstrated to be robust in the presence of aliasing. Through comparisons with conventional wide azimuth data we show that 3D multiple removal and migration are both aided by the increase in shots. Moreover, the high quality of the APM separated data indicates that prestack products such as migration angle gathers will have better quality than was achieved a few years ago using a passive separation approach. We found that by carefully selecting and tailoring the method's projection operators, excellent separation results can be achieved efficiently.

Description: ABSTRACT Velocity dispersion is not usually a problem in surface seismic data processing, as the seismic bandwidth is relatively narrow and thus for most Q values, dispersive effects are not noticeable. However, for highly absorptive bodies, such as the overpressured free gas accumulations associated with some gas hydrates or high-porosity normally pressured gas sands, dispersive effects may be seen. In this work I analyse one such data set from the offshore north-east coast of India. I demonstrate that the effect is measurable and that compensating for it in either data processing or migration can improve the wavelet character, as well as delivering an estimate of the effective Q values in the associated geobody. I also raise the question as to whether velocities derived using low-frequency waveform inversion over such dispersive geobodies are wholly appropriate for migration of full seismic-bandwidth data.

Description: ABSTRACT The dynamic response of a semi-infinite fluid-filled borehole embedded in an elastic half-space under a concentrated normal surface load is analysed in the long-wavelength limit. The solution of the problem is obtained with integral transforms in the form of a double integral with respect to the slowness and frequency. The partial P- and SV - wave responses are further transformed to path integrals along Cagniard paths in the complex slowness plane. Unlike the traditional Cagniard-de Hoop technique based on the Laplace transform of time dependence, this paper is based on the Fourier transform. The tube-wave response is presented as a causal integral over a slowness range. The resultant representation in the time-domain is suitable for the numerical evaluation of the complete response in the fluid-filled borehole, especially at large distances. Asymptotic analysis of seismic phases arising in the borehole is performed on the basis of the obtained solution. The complete asymptotic wavefield consists in P - and SV - waves, the Rayleigh wave and the low-frequency Stoneley (tube) wave. Pressure synthetics obtained by the use of the asymptotic formulas are shown to be in good agreement with straightforward calculations.

Description: ABSTRACT We conducted a detailed experimental investigation of the effect of CO 2 injection on the electrical conductivity of water bearing porous media, needed for an improved geophysical monitoring of CO 2 storage reservoirs. Therefore, we developed an experimental set-up that allows to investigate electrical characteristics of the injection process as well as the impact of dissolved CO 2 on pore water conductivity. We found that a gaseous, fluid and supercritical pure CO 2 phase bears no relevant conductivity at pressures up to 13 MPa and temperatures up to 50° C. When CO 2 dissolves in pore water, pressure-dependent dissociation processes can double the pore water conductivity, that can be used in leakage detection. This is quantified by an adaptation of Archie’s law. The empirical adaptation and the experimental data are confirmed by combined geochemical-geoelectrical modelling. Furthermore, water-saturated sand samples were investigated while CO 2 displaced the pore water at pressures up to 13 MPa and temperatures up to 40° C. A decrease in electrical conductivity by a factor of up to 33 was measured, corresponding to a residual water saturation of 14–19%. Qualitatively, a decrease was also demonstrated under supercritical conditions. As an integrative interpretation, a conceptual model of electrical rock properties during CO 2 sequestration is presented.

Description: ABSTRACT We used a laboratory scale model to study the effects aligned fractures might have on seismic wave propagation at a larger scale in real Earth imaging. Our main objective was to investigate the effect of aligned fractures on seismic P-wave amplitude through the estimation of the induced attenuation. The physical model was constructed from a mixture of epoxy resin and silicon rubber, with inclusions designed to simulate two sets of inclined fractures at an angle of 29.2° with each other. Two-dimensional reflection data were acquired using the pulse and transmission method in three principal directions relative to the fracture strike azimuth with the model submerged in a water tank. We used the Quality Versus Offset (QVO) method, an extension of the classical spectral ratio method for determining attenuation to estimate the induced attenuation (inverse of the seismic quality factor) from the Common Mid Point (CMP) pre-processed gathers. The results of our analysis show that the induced P-wave attenuation is anisotropic, with elliptical (cos2θ) variations with respect to the survey azimuth angle θ. The minor axis of the Q ellipse corresponds to the fracture normal. In this direction, i.e. across the material grain, the attenuation is a maximum. The major axis corresponds to the fracture strike direction (parallel to the material grain) where minimum attenuation occurs. These attenuation results show consistency with the azimuthal anisotropy observed in the stacking velocities in the fractured-layer and are all consistent with the physical model, and thus provide a physical basis for using attenuation anisotropy to derive fracture properties from seismic data.

Description: ABSTRACT Young's modulus and Poisson's ratio are required for geomechanics applications such as hydraulic fracture design, analysis of wellbore stability and rock failure, determination of in situ stress and assessment of the response of reservoirs and surrounding rocks to changes in pore pressure and stress. Shales are usually anisotropic and models that neglect shale anisotropy may fail to describe geomechanical behaviour correctly. Anisotropy in shales results from a partial alignment of anisotropic clay particles, kerogen inclusions, microcracks, low-aspect ratio pores and layering. For shales, the Young's modulus measured parallel to bedding E 1 is usually greater than the Young's modulus measured perpendicular to bedding E 3 . However, the Poisson's ratio ν 31 corresponding to stress applied perpendicular to bedding and strain measured parallel to bedding can be greater than, equal to, or less than the Poisson's ratio ν 12 for stress applied parallel to bedding and strain measured parallel to bedding. For transverse isotropy, the elastic anisotropy resulting from a partial alignment of clay particles can be written in terms of the coefficients W 200 and W 400 , which describe the impact of clay particle orientation on the anisotropy of shales. Disorder in the orientation of clay particles acts to reduce W 400 faster than W 200 , since W 400 is a higher order moment of the clay particle orientation distribution function than W 200 . This is confirmed by analysis of measured anisotropy parameters for shales. A partial alignment of clay particles is consistent with the measured Young's moduli for shales and with values of Poisson's ratio ν 31 〉 ν 12 but not with values ν 31 〈 ν 12 . These values can be explained if there exist kerogen inclusions, microcracks, or low-aspect ratio pores aligned parallel to the bedding plane.

Description: ABSTRACT In this paper, we examine the influence of a velocity model and microseismic source frequency on microseismic waveforms and event locations. Finite-difference waveform synthetics are generated based on the Cotton Valley hydraulic fracture experiment, where we vary the vertical heterogeneity of the velocity models as well as the microseismic source frequencies. We find that differences between plausible velocity models lead to changes in arrival times of approximately 0.0035 seconds for P-waves and 0.0085 seconds for S-waves. Based on the average P- and S-wave velocities, the difference in the P- and S-wave traveltimes is equivalent to approximately 20 m in location difference. Significant increases in the waveform coda develop with increasing model heterogeneity and increasing source frequency. The presence of signal noise as well as other sources of error (e.g., uncertainty in geophone location) will likely lead to further increase in uncertainty in location error estimates. Thus we note that location error due to incorrect velocity models cannot be ignored.

Description: ABSTRACT Projection filtering has been used for many years in seismic processing as a tool to extract a signal out of noisy data. The effectiveness of projection filtering reaches a limit when seismic events are affected by static shifts. Such shifts degrade the lateral coherency of the data, which is the strongest assumption made by projection filtering. We propose an algorithm to estimate projection filters and static shifts simultaneously in order to perform noise attenuation in the presence of static shifts in the data. We then show results on synthetic and real data to demonstrate the denoising capabilities of our algorithm.

Description: Atmospheric deposition is one of the major sources of nutrients bringing trace metals to remote marine biota. In this study, total atmospheric deposition and crustal aerosol concentrations were monitored at Kerguelen Islands (49°18′S; 70°07′E) in the Southern Ocean during a short campaign in early 2005 and then continuously for about 2 years (2009–2010). Results show very low levels of atmospheric dust and trace metals concentrations but higher deposition fluxes than expected. The averaged total dust deposition flux as derived from Al deposition measurements is 659 μg m−2 d−1. Simultaneously measured Fe and Co deposition fluxes are respectively 29 μg m−2 d−1 (520 nmol m−2 d−1) and 0.014 μg m−2 d−1 (0.24 nmol m−2 d−1), giving typically crustal elemental ratios to Al of 0.54 and 2.6 10−4. Measured dust deposition is in relatively good agreement with those simulated by current atmospheric models, but suggest that previous indirect calculations from field experiments are too low by a factor of 20. Observations and model results show that dust is transported above the marine atmospheric boundary layer to Kerguelen Islands, and thus that surface concentrations are not representative of the total dust column. Indeed, using surface concentrations leads to very large computed wet scavenging ratios, and to the conclusion that it is not appropriate to derive deposition fluxes from surface concentrations at remote ocean sites.

Description: Northern high-latitude rivers transport large amounts of terrestrially derived dissolved organic matter (DOM) from boreal and arctic ecosystems to coastal areas and oceans. Current knowledge of the biodegradability of DOM in these rivers is limited, particularly for large rivers discharging to the Arctic Ocean. We conducted a seasonally comprehensive study of biodegradable dissolved organic carbon (BDOC) dynamics in the Yukon River and two of its tributaries in Alaska, USA. Distinct seasonal patterns of BDOC, consistent across a wide range of watershed size, indicate BDOC is transported year-round. Relative biodegradability (%BDOC) was greatest during winter, and decreased into spring and summer. Due to large seasonal differences in DOC concentration, the greatest concentrations of BDOC (mg C L−1) occurred during spring freshet, followed by winter and summer. While chemical composition of DOM was an important driver of BDOC, the overriding control of BDOC was mineral nutrient availability due to wide shifts in carbon (C) and nitrogen (N) stoichiometry across seasons. We calculated seasonal and annual loads of BDOC exported by the Yukon River by applying measured BDOC concentrations to daily water discharge values, and also by applying an empirical correlation between %BDOC and the ratio of DOC to dissolved inorganic N (DIN) to total DOC loads. The Yukon River exports ∼0.2 Tg C yr−1 as BDOC that is decomposable within 28 days. This corresponds to 12–18% of the total annual DOC export. Furthermore, we calculate that the six largest arctic rivers, including the Yukon River, collectively export ∼2.3 Tg C yr−1 as BDOC to the Arctic Ocean.

Description: Previous work estimating the N2 excess above background due to denitrification has suggested that nitrate deficit-type methods may be an underestimate of fixed nitrogen (N) loss in the major oxygen deficient zones of the ocean. The N2 excess approach has the advantage over nitrate deficit-type methods in that it does not depend on stoichiometric assumptions of fixed N to phosphate or oxygen utilization and avoids any uncertainly regarding the pathway of N loss. Measurements of N2:Ar from two stations within the eastern tropical North Pacific and from one station within the Arabian Sea oxygen deficient zones were used to determine the N2 excess due to denitrification. In both of the regions, the N2 excess was comparable in shape and magnitude to the concurrent fixed nitrogen deficit. In the eastern tropical North Pacific oxygen deficient zone, the N2 excess was near zero at the surface and rose to maxima of 13.7 ± 1.8 and 10.8 ± 1.9 μM N, compared to maximum fixed N deficits of 13.5 ± 1.5 and 12.3 ± 1.5 μM N, respectively. In the Arabian Sea oxygen deficient zone, the maximum N2 excess was 11.1 ± 1.5 μM N, compared to a maximum deficit of 12.5 ± 1.0 μM N. These results suggests that previous estimates of fixed N loss based on fixed N deficit calculations in these regions are likely reasonable, given the same considerations of volume and residence time of the water of the oxygen deficient zone.

Description: Empirical transfer functions are derived for predicting the total benthic nitrate loss (LNO3) and the net loss of dissolved inorganic nitrogen (LDIN) in marine sediments, equivalent to sedimentary denitrification. The functions are dynamic vertically integrated sediment models which require the rain rate of particulate organic carbon to the seafloor (RRPOC) and a proposed new variable (O2-NO3)bw (bottom water O2 concentration minus NO3− concentration) as the only input parameters. Applied globally to maps of RRPOC and (O2-NO3)bw on a 1° × 1° spatial resolution, the models predict a NO3− drawdown of 196 Tg yr−1 (LNO3) of which 153 – 155 Tg yr−1 is denitrified to N2 (LDIN). This is in good agreement with previous estimates using very different methods. Our approach implicitly accounts for fixed N loss via anammox, such that our findings do not support the idea that the relatively recent discovery of anammox in marine sediments might require current estimates of the global benthic marine N budget to be revised. The continental shelf (0 – 200 m) accounts for 〉50% of global LNO3 and LDIN, with slope (200 – 2000 m) and deep-sea (〉2000 m) sediments contributing ca. 30% and 20%, respectively. Denitrification in high-nitrate/low-oxygen regions such as oxygen minimum zones is significant (ca. 15 Tg N yr−1; 10% of global) despite covering only ∼1% of the seafloor. The data are used to estimate the net fluxes of nitrate (18 Tg N yr−1) and phosphate (27 Tg P yr−1) across the sediment-water interface. The benthic fluxes strongly deviate from Redfield composition, with globally averaged N:P, N:C and C:P values of 8.3, 0.067 and 122, respectively, indicating world-wide fixed N losses (by denitrification) relative to C and P. The transfer functions are designed to be coupled dynamically to general circulation models to better predict the feedback of sediments on pelagic nutrient cycling and dissolved O2 distributions.

Description: We compared net community production determined from an in situ O2/Ar mass balance (O2/Ar-NCP) with incubation measurements of new and primary production in the subarctic northeast Pacific. In situ O2/Ar-NCP was strongly correlated to new production from 24-h 15NO3− uptake integrated over the mixed layer (15N-NewP), if measurements were separated into high and low-productivity conditions. Under high-productivity conditions, O2/Ar-NCP estimates were similar to 15N-NewP, whereas under low productivity conditions O2/Ar-NCP was up to two times higher than 15N-NewP. The relationship between O2/Ar-NCP and 24-h 13C primary production (13C-PP) was more variable, but with a consistent mean O2/Ar-NCP: 13C-PP ratio of 0.52 ± 0.17 when only low-productivity, summer measurements were considered. This relationship with primary production is perturbed by high productivity events such as a late-summer, iron-stimulated bloom observed at the offshore stations. Finally, we show that diapycnal mixing usually dominates the O2/Ar mass balance in winter in the subarctic Pacific, preventing the determination of NCP by the O2/Ar method at that time, except for one unusual stratification event in February 2007.

Description: The soil system represents the dominant terrestrial reservoir of carbon in the biosphere. Deforestation, poor land management, and excessive cropping lead to a decrease in soil carbon stocks, but intensive cropping can reverse this trend. We discuss long-term soil organic carbon data from two major rice-growing areas: Java (Indonesia) and South Korea. Soil organic carbon content in the top 15 cm for both countries has increased in recent decades. In South Korea, the top 15 cm of soils store about 31 Tg (1012 g) of carbon (C) with a sequestration rate of 0.3 Tg C per year. In Java, the agricultural topsoils accumulated more than 1.7 Tg C per year over the period 1990–2010. We attribute the increase in measured SOC mainly to increases in above- and below- ground biomass due to fertilization. Good agronomic practices can maintain and increase soil carbon, which ensures soil security to produce food and fiber.

Description: The Mauritanian upwelling system is one of the most biologically productive regions of the world's oceans. Coastal upwelling transfers nutrients to the sun-lit surface ocean, thereby stimulating phytoplankton growth. Upwelling of deep waters also supplies dissolved inorganic carbon (DIC), high levels of which lead to low calcium carbonate saturation states in surface waters, with potentially adverse effects on marine calcifiers. In this study an upwelled filament off the coast of northwest Africa was followed using drifting buoys and sulphur hexafluoride to determine how the carbonate chemistry changed over time as a result of biological, physical and chemical processes. The initial pHtot in the mixed layer of the upwelled plume was 7.94 and the saturation states of calcite and aragonite were 3.4 and 2.2, respectively. As the plume moved offshore over a period of 9 days, biological uptake of DIC (37 μmol kg−1) reduced pCO2 concentrations from 540 to 410 μatm, thereby increasing pHtot to 8.05 and calcite and aragonite saturation states to 4.0 and 2.7 respectively. The increase (25 μmol kg−1) in total alkalinity over the 9 day study period can be accounted for solely by the combined effects of nitrate uptake and processes that alter salinity (i.e., evaporation and mixing with other water masses). We found no evidence of significant alkalinity accumulation as a result of exudation of organic bases by primary producers. The ongoing expansion of oxygen minimum zones through global warming will likely further reduce the CaCO3 saturation of upwelled waters, amplifying any adverse consequences of ocean acidification on the ecosystem of the Mauritanian upwelling system.

Description: Net primary productivity (NPP) is a key flux in the terrestrial ecosystem carbon balance, as it summarizes the autotrophic input into the system. Forest NPP varies predictably with stand age, and quantitative information on the NPP-age relationship for different regions and forest types is therefore fundamentally important for forest carbon cycle modeling. We used four terms to calculate NPP: annual accumulation of live biomass, annual mortality of aboveground and belowground biomass, foliage turnover to soil, and fine root turnover in soil. For U.S. forests the first two terms can be reliably estimated from the Forest Inventory and Analysis (FIA) data. Although the last two terms make up more than 50% of total NPP, direct estimates of these fluxes are highly uncertain due to limited availability of empirical relationships between aboveground biomass and foliage or fine root biomass. To resolve this problem, we developed a new approach using maps of leaf area index (LAI) and forest age at 1 km resolution to derive LAI-age relationships for 18 major forest type groups in the USA. These relationships were then used to derive foliage turnover estimates using species-specific trait data for leaf specific area and longevity. These turnover estimates were also used to derive the fine root turnover based on reliable relationships between fine root and foliage turnover. This combination of FIA data, remote sensing, and plant trait information allows for the first empirical and reliable NPP-age relationships for different forest types in the USA. The relationships show a general temporal pattern of rapid increase in NPP in the young ages of forest type groups, peak growth in the middle ages, and slow decline in the mature ages. The predicted patterns are influenced by climate conditions and can be affected by forest management. These relationships were further generalized to three major forest biomes for use by continental-scale carbon cycle models in conjunction with remotely sensed land cover types.

Description: Modern-day coral reefs have well defined environmental envelopes for light, sea surface temperature (SST) and seawater aragonite saturation state (Ωarag). We examine the changes in global coral reef habitat on multimillennial timescales with regard to SST and Ωarag using a climate model including a three-dimensional ocean general circulation model, a fully coupled carbon cycle, and six different parameterizations for continental weathering (the UVic Earth System Climate Model). The model is forced with emission scenarios ranging from 1,000 Pg C to 5,000 Pg C total emissions. We find that the long-term climate change response is independent of the rate at which CO2 is emitted over the next few centuries. On millennial timescales, the weathering feedback introduces a significant uncertainty even for low emission scenarios. Weathering parameterizations based on atmospheric CO2 only display a different transient response than weathering parameterizations that are dependent on temperature. Although environmental conditions for SST and Ωarag stay globally hostile for coral reefs for millennia for our high emission scenarios, some weathering parameterizations induce a near-complete recovery of coral reef habitat to current conditions after 10,000 years, while others result in a collapse of coral reef habitat throughout our simulations. We find that the multimillennial response in sea surface temperature (SST) substantially lags the aragonite saturation recovery in all configurations. This implies that if corals can naturally adapt over millennia by selecting thermally tolerant species to match warmer ocean temperatures, prospects for long-term recovery of coral reefs are better since Ωarag recovers more quickly than SST.

Description: We investigated the phenology of oceanic phytoplankton at large scales over two 5-year time periods: 1979–1983 and 1998–2002. Two ocean-color satellite data archives (Coastal Zone Color Scanner (CZCS) and Sea-viewing Wide Field-of-view Sensor (SeaWiFS)) were used to investigate changes in seasonal patterns of concentration-normalized chlorophyll. The geographic coverage was constrained by the CZCS data distribution. It was best for the Northern Hemisphere and also encompassed large areas of the Indian, South Pacific, and Equatorial Atlantic regions. For each 2° pixel, monthly climatologies were developed for satellite-derived chlorophyll, and the resulting seasonal cycles were statistically grouped using cluster analysis. Five distinct groups of mean seasonal cycles were identified for each half-decade period. Four types were common to both time periods and correspond to previously identified phytoplankton regimes: Bloom, Tropical, Subtropical North, and Subtropical South. Two other mean seasonal cycles, one in each of the two compared 5-year periods, were related to transitional or intermediate states (Transitional Tropical and Transitional Bloom). Five mean seasonal cycles (Bloom, Tropical, Subtropical North, and Subtropical South, Transitional Bloom) were further confirmed when the whole SeaWiFS data set (1998–2010) was analyzed. For ∼35% of the pixels analyzed, characteristic seasonal cycles of the 1979–1983 years differed little from those of the 1998–2002 period. For ∼65% of the pixels, however, phytoplankton seasonality patterns changed markedly, especially in the Northern Hemisphere. Subtropical regions of the North Pacific and Atlantic experienced a widespread expansion of the Transitional Bloom regime, which appeared further enhanced in the climatology based on the full SeaWiFS record (1998–2010), and, as showed by a more detailed analysis, is associated to La Niña years. This spatial pattern of Transitional Bloom regime reflects a general smoothing of seasonality at macroscale, coming into an apparent greater temporal synchrony of the Northern Hemisphere. The Transitional Bloom regime is also the result of a higher variability, both in space and time. The observed change in phytoplankton dynamics may be related not only to biological interactions but also to large-scale changes in the coupled atmosphere–ocean system. Some connections are indeed found with climate indices. Changes were observed among years belonging to opposite phases of ENSO, though discernible from the change among the two periods and within the SeaWiFS era (1998–2010). These linkages are considered preliminary at present and are worthy of further investigation.

Description: Total (TOC) and dissolved (DOC) organic carbon vertical profiles were analyzed from 11 stations located in various regions of the Mediterranean Sea, together with the distribution of other physical, chemical and biological parameters. TOC showed the highest concentrations (68–83 μM) above the pycnocline, followed by a marked decrease to values of 45–48 μM at 100–200 m. Below 200 m, values of 40–45 μM were observed. The excess TOC and DOC occurring at each station was calculated by subtracting 48 μM from the observed concentrations. The stock of the excess TOC and DOC increased eastward; while surface DOC mineralization rates decreased from 1.5 μM d−1 to 0.26 μM d−1 eastward. The integrated average of the biological parameters in the above-pycnocline layer showed a bacterial production versus particulate primary production (BP/PPP) ratio ranging from 22% in the Ionian Sea (MIO station) to 31% in the Ligurian Sea (Dyfamed station), while bacterial carbon demand versus PPP was higher than 100%, considering a bacterial growth efficiency of both 15% and 30%. The data here reported indicate various scenarios of carbon dynamics. At the stations west of the Sardinian Channel, the microbial loop was very active, and a high flux of carbon to the microbial loop (large bacterial and protist abundance) may be hypothesized, which would result in a low DOC concentration. At the stations east of the Sardinian Channel, no significant longitudinal variation was found in DOC and BP. DOC accumulated at these stations, possibly due to bacteria P-limitation, to DOC chemical composition and/or to the occurrence of different prokaryotic populations with a different ability to consume the available DOC.

Description: ABSTRACT I introduce a new explicit form of vertical seismic profile (VSP) traveltime approximation for a 2D model with non-horizontal boundaries and anisotropic layers. The goal of the new approximation is to dramatically decrease the cost of time calculations by reducing the number of calculated rays in a complex multi-layered anisotropic model for VSP walkaway data with many sources. This traveltime approximation extends the generalized moveout approximation proposed by Fomel and Stovas. The new equation is designed for borehole seismic geometry where the receivers are placed in a well while the sources are on the surface. For this, the time-offset function is presented as a sum of odd and even functions. Coefficients in this approximation are determined by calculating the traveltime and its first- and second-order derivatives at five specific rays. Once these coefficients are determined, the traveltimes at other rays are calculated by this approximation. Testing this new approximation on a 2D anisotropic model with dipping boundaries shows its very high accuracy for offsets three times the reflector depths. The new approximation can be used for 2D anisotropic models with tilted symmetry axes for practical VSP geometry calculations. The new explicit approximation eliminates the need of massive ray tracing in a complicated velocity model for multi-source VSP surveys. This method is designed not for NMO correction but for replacing conventional ray tracing for time calculations.

Description: ABSTRACT Many natural phenomena, including geologic events and geophysical data, are fundamentally nonstationary - exhibiting statistical variation that changes in space and time. Time-frequency characterization is useful for analysing such data, seismic traces in particular. We present a novel time-frequency decomposition, which aims at depicting the nonstationary character of seismic data. The proposed decomposition uses a Fourier basis to match the target signal using regularized least-squares inversion. The decomposition is invertible, which makes it suitable for analysing nonstationary data. The proposed method can provide more flexible time-frequency representation than the classical S transform. Results of applying the method to both synthetic and field data examples demonstrate that the local time-frequency decomposition can characterize nonstationary variation of seismic data and be used in practical applications, such as seismic ground-roll noise attenuation and multicomponent data registration.

Description: ABSTRACT A new uncertainty estimation method, which we recently introduced in the literature, allows for the comprehensive search of model posterior space while maintaining a high degree of computational efficiency. The method starts with an optimal solution to an inverse problem, performs a parameter reduction step and then searches the resulting feasible model space using prior parameter bounds and sparse-grid polynomial interpolation methods. After misfit rejection, the resulting model ensemble represents the equivalent model space and can be used to estimate inverse solution uncertainty. While parameter reduction introduces a posterior bias, it also allows for scaling this method to higher dimensional problems. The use of Smolyak sparse-grid interpolation also dramatically increases sampling efficiency for large stochastic dimensions. Unlike Bayesian inference, which treats the posterior sampling problem as a random process, this geometric sampling method exploits the structure and smoothness in posterior distributions by solving a polynomial interpolation problem and then resampling from the resulting interpolant. The two questions we address in this paper are 1) whether our results are generally compatible with established Bayesian inference methods and 2) how does our method compare in terms of posterior sampling efficiency. We accomplish this by comparing our method for two electromagnetic problems from the literature with two commonly used Bayesian sampling schemes: Gibbs’ and Metropolis-Hastings. While both the sparse-grid and Bayesian samplers produce compatible results, in both examples, the sparse-grid approach has a much higher sampling efficiency, requiring an order of magnitude fewer samples, suggesting that sparse-grid methods can significantly improve the tractability of inference solutions for problems in high dimensions or with more costly forward physics.

Description: ABSTRACT Based on the line integral (LI) and maximum difference reduction (MDR) methods, an automated iterative forward modelling scheme (LI-MDR algorithm) is developed for the inversion of 2D bedrock topography from a gravity anomaly profile for heterogeneous sedimentary basins. The unknown basin topography can be smooth as for intracratonic basins or discontinuous as for rift and strike-slip basins. In case studies using synthetic data, the new algorithm can invert the sedimentary basins bedrock depth within a mean accuracy better than 5% when the gravity anomaly data have an accuracy of better than 0.5 mGal. The main characteristics of the inversion algorithm include: (1) the density contrast of sedimentary basins can be constant or vary horizontally and/or vertically in a very broad but a priori known manner; (2) three inputs are required: the measured gravity anomaly, accuracy level and the density contrast function, (3) the simplification that each gravity station has only one bedrock depth leads to an approach to perform rapid inversions using the forward modelling calculated by LI. The inversion process stops when the residual anomalies (the observed minus the calculated) falls within an ‘error envelope’ whose amplitude is the input accuracy level. The inversion algorithm offers in many cases the possibility of performing an agile 2D gravity inversion on basins with heterogeneous sediments. Both smooth and discontinuous bedrock topography with steep spatial gradients can be well recovered. Limitations include: (1) for each station position, there is only one corresponding point vertically down at the basement; and (2) the largest error in inverting bedrock topography occurs at the deepest points.

Description: ABSTRACT In geological materials, anisotropy may arise due to different mechanisms and can be found at different scales. Neglecting anisotropy in traveltime tomographic reconstruction leads to artefacts that can obscure important subsurface features. In this paper, a geostatistical tomography algorithm to invert cross-hole traveltime data in elliptically anisotropic media is presented. The advantages of geostatistical tomography are that the solution is regularized by the covariance of the model parameters, that known model parameters can be used as constraints and fitted exactly or within a prescribed variance and that stochastic simulations can be performed to appraise the variability of the solution space. The benefits of the algorithm to image anisotropic media are illustrated by two examples using synthetic georadar data and real seismic data. The first example confirms suspected electromagnetic anisotropy in the vadose zone caused by relatively rapid water content variations with respect to wavelength at georadar frequencies. The second presents how sonic log data can be used to constrain the inversion of cross-well seismic data and how geostatistical simulations can be used to infer parameter uncertainty. Results of both examples show that considering anisotropy yields a better fit to the data at high ray angles and reduces reconstruction artefacts.

Description: ABSTRACT Dense, wide-aperture and broad frequency band acquisition improves seismic imaging and potentially allows the use of full waveform inversion for velocity model building. The cost of dense acquisitions however limits its applicability. Blended or simultaneous shooting could lead to a good compromise between cost and dense acquisition, although the cross-talk between simultaneous sweeps may reduce imaging capabilities. Onshore, a compromise is achieved with distance separated simultaneous sweeping acquisition, because the shots are easily separated when the processing focuses on pre-critical reflected events. Full waveform inversion for velocity model building however relies on post-critical reflected, refracted and diving events. These events can interfere in a distance separated simultaneous sweeping acquisition. By using a single vibrator, single receiver data set recorded in Inner Mongolia, China, a distance separated simultaneous sweeping data set is created to study the robustness of full waveform inversion in this acquisition context. This data set is well suited for full waveform inversion since it contains frequencies down to 1.5 Hz and offsets up to 25 km. Full waveform inversion after a crude deblending of the distance separated simultaneous sweeping data set leads to a result very similar to the one obtained from the single vibrator, single receiver data set. The inversion of the blended data set gives a slightly less good result because of the cross-talk but it is still quite satisfactory.

Description: ABSTRACT We have developed a straightforward and ray based methodology to estimate both the maximum offset and reflection imaging radius for multi-layered velocity models, which can be used for a 2D/3D VSP survey design. Through numerical examples, we demonstrate that the presence of a high-velocity layer above a target zone significantly reduces the maximum offset and reflection imaging radius. Our numerical examples also show that including in a migration VSP data acquired beyond a recommended maximum offset, radically degrades the quality of the final VSP image. In addition, unlike the conventional straight-line based approximation that often produces an incorrect large reflection imaging radius, our methodology predicts the VSP imaging radius with more accuracy than does the conventional approximation.

Description: ABSTRACT Harmonic or subharmonic noise is often present in vibroseis data as reverberation-like, laterally coherent bands occurring parallel to and before or after, the main events. Such periodic noise is typically generated during the standard correlation process when the actual source signal travelling through the subsurface is, for whatever reason, different from the desired source signal, i.e., the pilot-sweep controlling the baseplate and used for correlation. A typical cause can be that harmonic or subharmonic frequency partials are generated in addition to the vibroseis sweep's desired fundamental frequencies. These harmonics produce strong ‘ghost events’ during correlation of the geophone trace with the pilot-sweep, originating from additional correlations between the fundamental and harmonic frequencies. Especially subharmonic ‘ghosts’ will overlap with ‘good’ fundamental signals, since for typically used up-sweeps they are folded to later traveltimes, where the signal/noise-ratio is already lower, thus aggravating or preventing a reliable interpretation of possible later reflections. Here, a method is introduced to remove these unwanted noise trains (with only negligible impact on the fundamental signal) by transforming the seismogram traces into a so-called ‘(sub)harmonic domain’. In this domain, the respective harmonic noise portions are focused and separated from the fundamental signals, enabling easier detection and appropriate suppression. After back-transformation to the x-T domain, the records are free from the corresponding harmonic contamination and can then be processed as usual. The method operates in a data-driven fashion, i.e., the traces are not uniformly processed but are processed depending upon their actual (sub)harmonic content. The decontamination procedure can be applied universally, i.e., to uncorrelated/correlated and/or vertically unstacked/stacked data either in a manual, semiautomated or fully automated manner. The method works perfectly for synthetic vibroseis traces with or without harmonic/subharmonic portions. The application to real, crustal-scale vibroseis records that were acquired in 2006 in the Dead Sea region, Israel and that were severely contaminated by subharmonic ground-roll ghosts covering reflectivity from the basement to the Moho, shows the robustness and success of the presented method.

Description: ABSTRACT We formulate the Kirchhoff-Helmholtz representation theory for the combination of seismic interferometry signals synthesized by cross-correlation and by cross-convolution in acoustic media. The approach estimates the phase of the virtual reflections from the boundary encompassing a volume of interest and subtracts these virtual reflections from the total seismic-interferometry wavefield. The reliability of the combination result, relevant for seismic exploration, depends on the stationary-phase and local completeness in partial coverage regions. The analysis shows the differences in the phase of the corresponding seismic interferometry (by cross-correlation) and virtual reflector (by cross-convolution) signals obtained by 2D and 3D formulations, with synthetic examples performed to remove water layer multiples in ocean bottom seismic (OBS) acoustic data.

Description: ABSTRACT Inversion of gravity and/or magnetic data attempts to recover the density and/or magnetic susceptibility distribution in a 3D earth model for subsequent geological interpretation. This is a challenging problem for a number of reasons. First, airborne gravity and magnetic surveys are characterized by very large data volumes. Second, the 3D modelling of data from large-scale surveys is a computationally challenging problem. Third, gravity and magnetic data are finite and noisy and their inversion is ill posed so regularization must be introduced for the recovery of the most geologically plausible solutions from an infinite number of mathematically equivalent solutions. These difficulties and how they can be addressed in terms of large-scale 3D potential field inversion are discussed in this paper. Since potential fields are linear, they lend themselves to full parallelization with near-linear scaling on modern parallel computers. Moreover, we exploit the fact that an instrument’s sensitivity (or footprint) is considerably smaller than the survey area. As multiple footprints superimpose themselves over the same 3D earth model, the sensitivity matrix for the entire earth model is constructed. We use the re-weighted regularized conjugate gradient method for minimizing the objective functional and incorporate a wide variety of regularization options. We demonstrate our approach with the 3D inversion of 1743 line km of FALCON gravity gradiometry and magnetic data acquired over the Timmins district in Ontario, Canada. Our results are shown to be in good agreement with independent interpretations of the same data.

Description: ABSTRACT In this paper, we derive analytical expressions for one-way and two-way kinematical parameters in elliptical tilted transverse isotropy media. We show that the homogeneous elliptical tilted transverse isotropy models result in hyperbolic moveout with a reflection point sideslip x 0 , which can be considered as an additional traveltime parameter for one-way wave propagation. For homogeneous elliptical tilted transverse isotropy models we show that the inversion of one-way traveltime parameters suffers from the ambiguity for large tilts. It is shown that the accuracy of the inversion is sensitive to the error in x 0 . We also derive and invert the traveltime parameters for a vertically heterogeneous elliptical tilted transverse isotropy model with a tilt gradually changing with depth. The a priori knowledge of parameter δ is very important for inversion. The wrong choise of this parameter results in significant errors in inverted model parameters.

Description: ABSTRACT Nowadays, geostatistics is commonly applied for numerous gridding or modelling tasks. However, it is still under used and unknown for classical geophysical applications. This paper highlights the main geostatistical methods relevant for geophysical issues, for instance to improve the quality of seismic data such as velocity cubes or interpreted horizons. These methods are then illustrated through four examples. The first example, based on a gravity survey presents how a geostatistical interpolation can be used to filter out a global trend, in order to better define real anomalies. In the second case study, dedicated to refraction surveying, geostatistical filtering is used to filter out acquisition artefacts and identify the main geological structures. The third one is an example of porosity being integrated geostatistically with a seismic acoustic impedance map. The last example deals with geostatistical time to depth conversion; the interest of performing geostatistical simulations is finally discussed.

Description: ABSTRACT Field estimates of the normal and shear compliance of water saturated fractures were obtained from a seismic experiment carried out on a wave-cut platform of Upper Caithness Flagstone on the North coast of Scotland. The rocks are cut by two orthogonal sets of vertical fractures. Vertical geophones were glued to the rock surface and seismic waves were generated by striking the surface with a sledge hammer. First arrival traveltimes were obtained for source-receiver distances up to 30 m. Differences of compressional velocity with direction across the platform were interpreted as resulting from the compliance of the fractures intersected by the transmitted waves. The average normal compliance for fractures at this site is 4.0 × 10 −12 ± 1.0 × 10 −12 m/Pa. There is much less certainty about the precise value of shear compliance. However, the normal/shear compliance ratio is determined to be ≤0.1.

Description: ABSTRACT Seismic imaging is a linearized inversion problem relying on the minimization of a least-squares misfit functional as a function of the medium perturbation. The success of this procedure hinges on our ability to handle large systems of equations – whose size grows exponentially with the demand for higher resolution images in more and more complicated areas – and our ability to invert these systems given a limited amount of computational resources. To overcome this ‘curse of dimensionality’ in problem size and computational complexity, we propose a combination of randomized dimensionality-reduction and divide-and-conquer techniques. This approach allows us to take advantage of sophisticated sparsity-promoting solvers that work on a series of smaller subproblems each involving a small randomized subset of data. These subsets correspond to artificial simultaneous-source experiments made of random superpositions of sequential-source experiments. By changing these subsets after each subproblem is solved, we are able to attain an inversion quality that is competitive while requiring fewer computational and possibly, fewer acquisition resources. Application of this concept to a controlled series of experiments shows the validity of our approach and the relationship between its efficiency – by reducing the number of sources and hence the number of wave-equation solves – and the image quality. Application of our dimensionality-reduction methodology with sparsity promotion to a complicated synthetic with a well-log constrained structure also yields excellent results underlining the importance of sparsity promotion.

Description: ABSTRACT Prestack reverse time migration (RTM) is a very useful tool for seismic imaging but has mainly three bottlenecks: highly intensive computation cost, low-frequency band imaging noise and massive memory demand. Traditionally, PC-clusters with thousands of computation nodes are used to perform RTM but it is too expensive for small companies and oilfields. In this article, we use Graphic Processing Unit (GPU) architecture, which is cheaper and faster to implement RTM and we obtain an order of magnitude higher speedup ratio to solve the problem of intensive computation cost. Aiming at the massive memory demand, we adopt the pseudo random boundary condition that sacrifices the computation cost but reduces the memory demand. For rugged topography RTM, it is difficult to deal with the rugged free boundary condition with the finite difference method. We employ a simplified boundary condition that avoids the abundant logical judgment to make the GPU implementation possible and does not induce any sacrifice on efficiency. Besides, we have also done some tests on multi-GPU implementation for wide azimuth geometries using the latest GPU cards and drivers. Finally, we discuss the challenges of anisotropy RTM and GPU solutions. All the jobs stated above are based on GPU and the synthetic data examples will show the efficiency of the algorithm and solutions.

Description: ABSTRACT Potential, potential field and potential-field gradient data are supplemental to each other for resolving sources of interest in both exploration and solid Earth studies. We propose flexible high-accuracy practical techniques to perform 3D and 2D integral transformations from potential field components to potential and from potential-field gradient components to potential field components in the space domain using cubic B-splines. The spline techniques are applicable to either uniform or non-uniform rectangular grids for the 3D case, and applicable to either regular or irregular grids for the 2D case. The spline-based indefinite integrations can be computed at any point in the computational domain. In our synthetic 3D gravity and magnetic transformation examples, we show that the spline techniques are substantially more accurate than the Fourier transform techniques, and demonstrate that harmonicity is confirmed substantially better for the spline method than the Fourier transform method and that spline-based integration and differentiation are invertible. The cost of the increase in accuracy is an increase in computing time. Our real data examples of 3D transformations show that the spline-based results agree substantially better or better with the observed data than do the Fourier-based results. The spline techniques would therefore be very useful for data quality control through comparisons of the computed and observed components. If certain desired components of the potential field or gradient data are not measured, they can be obtained using the spline-based transformations as alternatives to the Fourier transform techniques.

Description: ABSTRACT In this paper, a method is proposed to invert permeability from seismoelectric logs in fluid-saturated porous formations. From the analysis of both the amplitude and the phase of simulated seismoelectric logs, we find that the Stoneley wave amplitude of the ratio of the converted electric field to the pressure (REP) is sensitive to porosity rather than permeability while the tangent of the REP's phase is sensitive to permeability. The REP's phase reflects the phase discrepancy between the electric field and the pressure at the same location in the borehole. We theoretically derive the frequency-dependent expression of the REP of the low-frequency Stoneley wave and find that the tangent of the REP's argument is approximately in inverse proportion to permeability. We obtain an inversion formula and present the permeability inversion method by using the tangent of the REP's phase. To test this method, the permeabilities of different sandstones are inverted from the synthetic full-waveform data of seismoelectric logs. A modified inversion process is proposed based on the analysis of inversion errors, by which the relative errors are controlled below 25% and they are smaller than those of the permeability inversion from the Stoneley wave of acoustic logs.

Description: ABSTRACT New magnetotelluric (MT) data from two perpendicular profiles in the Kristineberg area, northern Sweden, were analysed and modelled. In the Skellefte Ore District, the Kristineberg volcanic hosted massive sulphide (VHMS) deposit mine is one of the largest and deepest (1250 m). Seventeen broadband magnetotelluric stations were installed along two existing seismic reflection lines. The profiles were 6 and 12 km long with 500 m and 1 km site spacing, respectively. The obtained MT transfer functions in the period range of 0.0015–200 s are of fairly good quality. Detailed strike and dimensionality analysis reveal consistent but period dependent, strike directions, indicating a change in the geoelectrical strike with depth. From the two-dimensional inversion of the determinant of the impedance tensor, two stable conductivity models with good data fit were obtained. The addition of seismic reflection information from the co-located survey, improved the data fit of one of them. Extensive sensitivity analyses helped to delineate the well resolved regions of the models and to determine the position of pronounced boundaries. The results are in good agreement with previous studies, especially regarding the presence of a deep conductor interpreted as a structural basement to the district. They also reveal with more detail the configuration of the main geological units of the Skellefte Ore District, especially of the ore bearing volcanic rocks and the embedded alteration zones.

Description: ABSTRACT The shifted hyperbola approximation is widely used in seismic applications. Mostly, this approximation is applied to reflection moveout in multilayered media. The traditional domain for this application is the t - x domain. In this paper, we discuss the use of this approximation in the τ - p and t - p domains. The accuracy of the shifted hyperbola approximation defined in different domains is illustrated by analytical and numerical examples.

Description: ABSTRACT Wave-equation based shot-record migration provides accurate images but is computationally expensive because every shot must be migrated separately. Shot-encoding migration, such as random shot-encoding or plane-wave migration, aims to reduce the computational cost of the imaging process by combining the original data into synthesized common-source gathers. Random shot-encoding migration and plane-wave migration have different and complementary features: the first recovers the full spatial bandwidth of the image but introduces strong artefacts, which are due to the interference between the different shot wavefields; the second provides an image with limited spatial detail but is free of crosstalk noise. We design a hybrid scheme that combines linear and random shot-encoding in order to limit the drawbacks and merge the advantages of these two techniques. We advocate mixed shot-encoding migration through dithering of plane waves. This approach reduces the crosstalk noise relative to random shot-encoding migration and increases the spatial bandwidth relative to conventional plane-wave migration when the take-off angle is limited to reduce the duration of the plane-wave gather. In turn, this decreases the migration cost. Migration with dithered plane waves operates as a hybrid encoding scheme in-between the end members represented by plane-wave migration and random shot-encoding. Migration with dithered plane waves has several advantages: every synthesized common-source gather images in a larger aperture, the crosstalk noise is limited and higher spatial resolution is achievable compared to shot-record migration, random shot-encoding and linear shot-encoding, respectively. Computational cost is also reduced relative to both random and linear shot-encoding migration since fewer synthesized common-source gathers are necessary to obtain a high signal-to-noise ratio and high spatial resolution in the final image.

Description: ABSTRACT From the late 1990s, many studies on local geoid construction have been made in South Korea. However, the precision of the previous geoid has remained about 15 cm due to distribution and quality problems of gravity and GPS/levelling data. Since 2007, new land gravity data and GPS/levelling data have been obtained through many projects such as the Korean Land Spatilaization, Unified Control Point and Gravity survey on the Benchmark. The newly obtained data are regularly distributed to a certain degree and show much better improvement in their quality. In addition, an airborne gravity survey was conducted in 2008 to cover the Korean peninsula (South Korea only). Therefore, it is expected that the precision of the geoid could be improved. In this study, the new South Korean gravimetric geoid and hybrid geoid are presented based on land, airborne, ship-borne, altimeter gravity data, geopotential model and topographic data. As for the methodology, the general remove-restore approach was applied with the best chosen parameters in order to produce a precise local geoid. The global geopotential model EGM08 was used to remove the low-frequency components using degree and order up to 360 and the short wavelength part of the gravity signal was dealt with by using the Shuttle Radar Topography Mission data. The parameters determined empirically in this study include for Stokes’ integral 0.5° and for Wong-Gore kernel 110–120°, respectively and 10 km for both the Bjerhammar sphere depth and attenuation factor. The final gravimetric geoid in South Korea ranges from 20–31 m with a precision of 5.45 cm overall compared to 1096 GPS/levelling data. In addition, the South Korean hybrid geoid produces 3.46 cm and 3.92 cm for degrees of fitness and precision, respectively and a better statistic of 2.37 cm for plain and urban areas was achieved. The gravimetric and hybrid geoids are expected to improve further when the refined land gravity data are included in the near future.

Description: Gross photosynthetic O2 production (GOP) rates in the subpolar North Atlantic Ocean were estimated using the measured isotopic composition of dissolved oxygen in the surface layer on samples collected on nine transits of a container ship between Great Britain and Canada during March 2007 to June 2008. The mean basin-wide GOP rate of 226 ± 48 mmol O2 m−2 d−1 during summer was double the winter rate of 107 ± 41 mmol O2 m−2 d−1. Converting these GOP rates to equivalent 14C-based PP (14C-PPeqv) yielded rates of 1005 ± 216 and 476 ± 183 mg C m−2 d−1 in summer and winter, respectively, that generally agreed well with previous 14C-based PP estimates in the region. The 14C-PPeqv estimates were 1–1.6× concurrent satellite-based PP estimates along the cruise track. A net community production rate (NCP) of 87 ± 12 mmol O2 m−2 d−1 (62 ± 9 mmol C m−2 d−1) and NCP/GOP of 0.35 ± 0.06 in the mixed layer was estimated from O2/Ar and 17Δ measurements (61°N 26°W) during spring bloom conditions in May 2008. Contrastingly, a much lower long-term annual mean NCP or organic carbon export rate of 2.8 ± 2.7 mol C m−2 yr−1 (8 ± 7 mmol C m−2 d−1) and NCP/GOP of 0.07 ± 0.06 at the winter mixed layer depth was estimated from 15 years of surface O2 data in the subpolar N. Atlantic collected during the CARINA program.

Description: Previous case studies have illustrated the strong local influence of tropical cyclones (TCs) on CO2 air-sea flux (FCO2), suggesting that they can significantly contribute to the global FCO2. In this study, we use a state-of-the art global ocean biochemical model driven by TCs wind forcing derived from a historical TCs database, allowing to sample the FCO2 response under 1663 TCs. Our results evidence a very weak contribution of TCs to global FCO2, one or two order of magnitude smaller than previous estimates extrapolated from case studies. This result arises from several competing effects involved in the FCO2 response to TCs, not accounted for in previous studies. While previous estimates have hypothesized the ocean to be systematically oversaturated in CO2 under TCs, our results reveal that a similar proportion of TCs occur over oversaturated regions (i.e. the North Atlantic, Northeast Pacific and the Arabian Sea) and undersaturated regions (i.e. Westernmost North Pacific, South Indian and Pacific Ocean). Consequently, by increasing the gas exchange coefficient, TCs can generate either instantaneous CO2 flux directed from the ocean to the atmosphere (efflux) or the opposite (influx), depending on the CO2 conditions at the time of the TC passage. A large portion of TCs also occurs over regions where the ocean and the atmosphere are in near equilibrium, resulting in very weak instantaneous fluxes. Previous estimates also did not account for any asynchronous effect of TCs on FCO2: during several weeks after the storm, oceanic pCO2 is reduced in response to vertical mixing, which systematically causes an influx anomaly. This implies that, contrary to previous estimates, TCs weakly affect the CO2 efflux when they blow over supersaturated areas because the instantaneous storm wind effect and post-storm mixing effect oppose with each other. In contrast, TCs increase the CO2 influx in undersaturated conditions because the two effects add up. These compensating effects result in a very weak contribution to global FCO2 and a very modest contribution to regional interannual variations (up to 10%).

Description: ABSTRACT More than 50 000 tons of CO 2 have been injected at Ketzin into the Stuttgart Formation, a saline aquifer, at approximately 620 m depth, as of summer 2011. We present here results from the 1 st repeat 3D seismic survey that was performed at the site in autumn 2009, after about 22 000 tons of CO 2 had been injected. We show here that rather complex time-lapse signatures of this CO 2 can be clearly observed within a radius of about 300 m from the injection well. The highly irregular amplitude response within this radius is attributed to the heterogeneity of the injection reservoir. Time delays to a reflection below the injection level are also observed. Petrophysical measurements on core samples and geophysical logging of CO 2 saturation levels allow an estimate of the total amount of CO 2 visible in the seismic data to be made. These estimates are somewhat lower than the actual amount of CO 2 injected at the time of the survey and they are dependent upon the choice of a number of parameters. In spite of some uncertainty, the close agreement between the amount injected and the amount observed is encouraging for quantitative monitoring of a CO 2 storage site using seismic methods.

Description: ABSTRACT We present a singular value decomposition (SVD) filtering method for the enhancement of coherent reflections and for attenuation of noise. The method is applied in two steps. First normal move-out (NMO) correction is applied to shot or CMP records, with the purpose of flattening the reflections. We use a spatial SVD filter with a short sliding window to enhance coherent horizontal events. Then the data are sorted in common-offset panels and the local dip is estimated for each panel. The next SVD filtering is performed on a small number of traces and a small number of time samples centred around the output sample position. Data in a local window are corrected for linear moveout corresponding to the dips before SVD. At the central time sample position, we sum over the dominant eigenimages of a few traces, corresponding to SVD dip filtering. We illustrate the method using land seismic data from the Tacutu basin, located in the north-east of Brazil. The results show that the proposed method is effective and is able to reveal reflections masked by ground-roll and other types of noise.

Description: The impact of low-frequency climate modes on the large-scale variability of chlorophyll-a (a proxy for phytoplankton biomass) concentration in the subtropics is little known. Here we examined the concurrent monthly chlorophyll-a and hydrographical records obtained at two subtropical time series stations (Bermuda Atlantic Time series Study, BATS and European Station for Time series in the Ocean Canary Islands, ESTOC) from the beginning of the time series (1989 at BATS and 1994 at ESTOC) to 2003, in order to analyze its seasonal and interannual variability and investigate the potential correlation with large-scale atmospheric oscillations. At BATS, Chl-a variations are mainly caused by differences in the convective mixing and mesoscale phenomena. Variability in winter mixing is a significant factor at ESTOC as exemplified by years with anomalously deep mixed layer depths. An additional nutrient source causing Chl-a variability at this station likely occurs due to nutrient advection driven by the baroclinic flow caused by the Trade Winds during summertime. We found that interannual variability in mean integrated total Chl-a (TChl-a) is significantly correlated with temperature and salinity anomalies at BATS. Chl-a also covaried with changes in temperature although the correlation was not significant at ESTOC. We could not find any direct correlation between TChl-a and NAO at BATS; the correlation improved between TChl-a and NAO +1 year but was still insignificant. However, significant correlations were found for ESTOC between TChl-a anomalies and ENSO (El Niño Southern Oscillation) +3 years and NAO +1 year time lag.

Description: ABSTRACT A validation experiment, carried out in a scaled field setting, was attempted for the long electrode electrical resistivity tomography method in order to demonstrate the performance of the technique in imaging a simple buried target. The experiment was an approximately 1/17 scale mock-up of a region encompassing a buried nuclear waste tank on the Hanford site. The target of focus was constructed by manually forming a simulated plume within the vadose zone using a tank waste simulant. The long electrode results were compared to results from conventional point electrodes on the surface and buried within the survey domain. Using a pole-pole array, both point and long electrode imaging techniques identified the lateral extents of the pre-formed plume with reasonable fidelity but the long electrode method was handicapped in reconstructing vertical boundaries. The pole-dipole and dipole-dipole arrays were also tested with the long electrode method and were shown to have the least favourable target properties, including the position of the reconstructed plume relative to the known plume and the intensity of false positive targets. The poor performance of the pole-dipole and dipole-dipole arrays was attributed to an inexhaustive and non-optimal coverage of data at key electrodes, as well as an increased noise for electrode combinations with high geometric factors. However, when comparing the model resolution matrix among the different acquisition strategies, the pole-dipole and dipole-dipole arrays using long electrodes were shown to have significantly higher average and maximum values within the matrix than any pole-pole array. The model resolution describes how well the inversion model resolves the subsurface. Given the model resolution performance of the pole-dipole and dipole-dipole arrays, it may be worth investing in tools to understand the optimum subset of randomly distributed electrode pairs to produce maximum performance from the inversion model.

Description: ABSTRACT Induced polarization (IP) is a geophysical method that is potentially sensitive to the presence of cracks in porous rocks and therefore to damage. We performed time-domain and frequency domain IP measurements at the Tournemire Underground Research Laboratory (URL, Aveyron, France) in areas where different types of cracks are observed. These cracks correspond to both tectonic fractures and new cracks associated with stress release and desiccation resulting from the excavation of a gallery. These measurements were performed both in eastern and northern galleries of the test site. The eastern gallery was excavated in 1996 while the northern gallery was excavated recently in 2008. This gives us the opportunity to study the electrical characteristics of the excavation damaged zone surrounding the galleries with respect to the age of the excavation. Longitudinal profiles were performed along the floor of the galleries with 48 Cu/CuSO 4 electrodes separated by a distance of 20 cm. Chargeability and resistivity were inverted using a Gauss-Newton iterative approach assuming an isotropic heterogeneous clay-rock material. The resulting IP tomograms show a correlation between high values of chargeability and the presence of calcite-filled tectonic fractures. X-ray analysis indicates that the presence of pyrite in these fractures is a potential source of the observed IP signals. The cracks associated with the mechanical damage of the formation exhibit low values of chargeability, on the same order of magnitude than the chargeability of the clay-rock matrix and are therefore hardly observable. A smaller IP response associated with the presence of these cracks is observed in the older gallery and this observation is qualitatively related to the desaturation process associated with these cracks. In a specific area of one of the galleries, the presence of calcareous nodules is observed to be an important source of anomalous chargeability. This signature seems to be associated with the presence of pyrite.

Description: ABSTRACT In seismic interferometry the response to a virtual source is created from responses to sequential transient or simultaneous noise sources. Most methods use crosscorrelation, but recently seismic interferometry by multidimensional deconvolution (MDD) has been proposed as well. In the simultaneous-source method (also known as blended acquisition), overlapping responses to sources with small time delays are recorded. The crosstalk that occurs in imaging of simultaneous-source data can be reduced by using phase-encoded sources or simultaneous noise sources, by randomizing the time interval between the shots, or by inverting the blending operator. Seismic interferometry and the simultaneous-source method are related. In this paper we make this relation explicit by deriving deblending as a form of seismic interferometry by MDD. Moreover, we discuss a deblending algorithm for blended data acquired at the surface.

Description: ABSTRACT In 2008, a wide azimuth simultaneous source (SimSrc) 3D data set was acquired over an area coinciding with an existing conventional wide azimuth survey. Using two (nearly) simultaneous sources provided twice the number of shots, an increased shot density and twice the fold for essentially the same acquisition time. Although the improved sampling for the SimSrc data was predicted to yield improvements in processes such as 3D multiple attenuation and migration, cross-talk between the sources was a concern. Cross-talk can be suppressed by known techniques such as separation algorithms that estimate the individual shots and thereby separate SimSrc records into their constituent shots. However, separation algorithms available at the time suffered from aliasing and were not effective for severely shot-aliased data such as those encountered in the experimental data set, which had a 75-m in-line shot spacing, half that typical for wide azimuth geometries. In this paper we present a new separation technique called the Alternating Projection Method (APM), which is demonstrated to be robust in the presence of aliasing. Through comparisons with conventional wide azimuth data we show that 3D multiple removal and migration are both aided by the increase in shots. Moreover, the high quality of the APM separated data indicates that prestack products such as migration angle gathers will have better quality than was achieved a few years ago using a passive separation approach. We found that by carefully selecting and tailoring the method's projection operators, excellent separation results can be achieved efficiently.

Description: ABSTRACT Forced imbibition was performed in reservoir sandstone by injecting water into a dry sample. The injection was monitored with X-ray computed tomography and acoustic acquisition to simultaneously visualize the displacement of the fluid and quantify its presence by calculating saturation and P-wave velocities. We observed a strong influence when changing the injection rates on the acoustic response. Upon decreasing the injection rate from 5 mL/h to 0.1 mL/h, P-wave velocities decreased sharply: 100 m/s in 1 h. This behaviour is related to the partially saturated conditions of the sample (76% of saturation) before decreasing the injection rate. The air that is still trapped is free to move due to a decrease of pore pressure that is no longer forced by the higher injection rate. After 1 hour, P-wave velocities started increasing with small changes in saturation. Stopping injection for 16 hrs decreased saturation by 8% and P-wave velocities by 100 m/s. Restarting injection at 5 mL/h increased saturation to 76% while P-wave velocities fluctuated considerably for 2 hrs until they stabilized at 2253 m/s. Through the computed tomography scans we observed a water front advancing through the sample and how its shape changed from a plane to a curve after decreasing the injection rate.

Description: ABSTRACT We present a singular value decomposition (SVD) filtering method for the enhancement of coherent reflections and for attenuation of noise. The method is applied in two steps. First normal move-out (NMO) correction is applied to shot or CMP records, with the purpose of flattening the reflections. We use a spatial SVD filter with a short sliding window to enhance coherent horizontal events. Then the data are sorted in common-offset panels and the local dip is estimated for each panel. The next SVD filtering is performed on a small number of traces and a small number of time samples centred around the output sample position. Data in a local window are corrected for linear moveout corresponding to the dips before SVD. At the central time sample position, we sum over the dominant eigenimages of a few traces, corresponding to SVD dip filtering. We illustrate the method using land seismic data from the Tacutu basin, located in the north-east of Brazil. The results show that the proposed method is effective and is able to reveal reflections masked by ground-roll and other types of noise.

Description: ABSTRACT More than 50 000 tons of CO 2 have been injected at Ketzin into the Stuttgart Formation, a saline aquifer, at approximately 620 m depth, as of summer 2011. We present here results from the 1 st repeat 3D seismic survey that was performed at the site in autumn 2009, after about 22 000 tons of CO 2 had been injected. We show here that rather complex time-lapse signatures of this CO 2 can be clearly observed within a radius of about 300 m from the injection well. The highly irregular amplitude response within this radius is attributed to the heterogeneity of the injection reservoir. Time delays to a reflection below the injection level are also observed. Petrophysical measurements on core samples and geophysical logging of CO 2 saturation levels allow an estimate of the total amount of CO 2 visible in the seismic data to be made. These estimates are somewhat lower than the actual amount of CO 2 injected at the time of the survey and they are dependent upon the choice of a number of parameters. In spite of some uncertainty, the close agreement between the amount injected and the amount observed is encouraging for quantitative monitoring of a CO 2 storage site using seismic methods.

Description: ABSTRACT Most amplitude versus offset (AVO) analysis and inversion techniques are based on the Zoeppritz equations for plane-wave reflection coefficients or their approximations. Real seismic surveys use localized sources that produce spherical waves, rather than plane waves. In the far-field, the AVO response for a spherical wave reflected from a plane interface can be well approximated by a plane-wave response. However this approximation breaks down in the vicinity of the critical angle. Conventional AVO analysis ignores this problem and always utilizes the plane-wave response. This approach is sufficiently accurate as long as the angles of incidence are much smaller than the critical angle. Such moderate angles are more than sufficient for the standard estimation of the AVO intercept and gradient. However, when independent estimation of the formation density is required, it may be important to use large incidence angles close to the critical angle, where spherical wave effects become important. For the amplitude of a spherical wave reflected from a plane fluid-fluid interface, an analytical approximation is known, which provides a correction to the plane-wave reflection coefficients for all angles. For the amplitude of a spherical wave reflected from a solid/solid interface, we propose a formula that combines this analytical approximation with the linearized plane-wave AVO equation. The proposed approximation shows reasonable agreement with numerical simulations for a range of frequencies. Using this solution, we constructed a two-layer three-parameter least-squares inversion algorithm. Application of this algorithm to synthetic data for a single plane interface shows an improvement compared to the use of plane-wave reflection coefficients.

Description: ABSTRACT We recently proposed an efficient hybrid scheme to absorb boundary reflections for acoustic wave modelling that could attain nearly perfect absorptions. This scheme uses weighted averaging of wavefields in a transition area, between the inner area and the model boundaries. In this paper we report on the extension of this scheme to 2D elastic wave modelling with displacement-stress formulations on staggered grids using explicit finite-difference, pseudo-implicit finite-difference and pseudo-spectral methods. Numerical modelling results of elastic wave equations with hybrid absorbing boundary conditions show great improvement for modelling stability and significant absorption for boundary reflections, compared with the conventional Higdon absorbing boundary conditions, demonstrating the effectiveness of this scheme for elastic wave modelling. The modelling results also show that the hybrid scheme works well in 2D rotated staggered-grid modelling for isotropic medium, 2D staggered-grid modelling for vertically transversely isotropic medium and 2D rotated staggered-grid modelling for tilted transversely isotropic medium.

Description: ABSTRACT The electric and magnetic fields generated by an individual horizontal current ring induced inside a homogeneous conductive half-space, originating from an external large circular loop source of current in the presence of a flat half-space, are deduced. A check of self-consistency for these expressions led to the known general functions for these fields due to the same external source in the presence of that medium. The current rings’ mutual coupling related to the magnetic field's radial component is thoroughly analysed and its specific members are presented. The existence of a relatively small zone inside the half-space responsible for the main contribution for the signal measured at the observation point, with the source and receiver on the ground surface, is made evident. For increasing values of frequency, at a given transmitter-receiver (T–R) configuration, this zone shrinks and its central point moves away from a maximum depth of about 30% and horizontal distance of nearly 85%, of the T–R separation, to a point very close to the receiver position. The coordinates of the central point of this zone of main contribution are provided as approximated functions in terms of the induction number .

Description: ABSTRACT In this paper, we propose a nearly-analytic central difference method, which is an improved version of the central difference method. The new method is fourth-order accurate with respect to both space and time but uses only three grid points in spatial directions. The stability criteria and numerical dispersion for the new scheme are analysed in detail. We also apply the nearly-analytic central difference method to 1D and 2D cases to compute synthetic seismograms. For comparison, the fourth-order Lax-Wendroff correction scheme and the fourth-order staggered-grid finite-difference method are used to model acoustic wavefields. Numerical results indicate that the nearly-analytic central difference method can be used to solve large-scale problems because it effectively suppresses numerical dispersion caused by discretizing the scalar wave equation when too coarse grids are used. Meanwhile, numerical results show that the minimum sampling rate of the nearly-analytic central difference method is about 2.5 points per minimal wavelength for eliminating numerical dispersion, resulting that the nearly-analytic central difference method can save greatly both computational costs and storage space as contrasted to other high-order finite-difference methods such as the fourth-order Lax-Wendroff correction scheme and the fourth-order staggered-grid finite-difference method.

Description: ABSTRACT Since natural fractures in petroleum reservoirs play an important role in determining fluid flow during production, knowledge of the orientation and density of fractures is required to optimize production. This paper outlines the underlying theory and implementation of a fast and efficient algorithm for upscaling a Discrete Fracture Network (DFN) to predict the fluid flow, elastic and seismic properties of fractured rocks. Potential applications for this approach are numerous and include the prediction of fluid flow, elastic and seismic properties for fractured reservoirs, model-based inversion of seismic Amplitude Versus Offset and Azimuth (AVOA) data and the optimal placement and orientation of infill wells to maximize production. Given that a single fracture network may comprise hundreds of thousands of individual fractures, the sheer size of typical DFNs has tended to limit their practical applications. This paper demonstrates that with efficient algorithms, the utility of Discrete Fracture Networks can be extended far beyond mere visualization.

Description: ABSTRACT For some acquisition geometries, the cost of Full Waveform Inversion (FWI) can be considerably reduced by inverting simultaneously encoded shots. Encoded-shot strategies have the undesirable effect of leaving crosstalk noise in the final result. For FWI, changing the coding sequence periodically mitigates this effect. Another alternative is to use preconditioning, whereby the gradient is smoothed at every iteration along predefined directions. Preconditioning steers the solution towards accurate models while attenuating crosstalk artefacts. It also increases convergence speed and robustness to noise present in the data.

Description: ABSTRACT Seismic wave propagation in reservoir rocks is often strongly affected by fractures and micropores. Elastic properties of fractured reservoirs are studied using a fractured porous rock model, in which fractures are considered to be embedded in a homogeneous porous background. The paper presents an equivalent media model for fractured porous rocks. Fractures are described in a stress-strain relationship in terms of fracture-induced anisotropy. The equations of poroelasticity are used to describe the background porous matrix and the contents of the fractures are inserted into a matrix. Based on the fractured equivalent-medium theory and Biot's equations of poroelasticity, two sets of porosity are considered in a constitutive equation. The porous matrix permeability and fracture permeability are analysed by using the continuum media seepage theory in equations of motion. We then design a fractured porous equivalent medium and derive the modified effective constants for low-frequency elastic constants due to the presence of fractures. The expressions of elastic constants are concise and are directly related to the properties of the main porous matrix, the inserted fractures and the pore fluid. The phase velocity and attenuation of the fractured porous equivalent media are investigated based on this model. Numerical simulations are performed. We show that the fractures and pores strongly influence wave propagation, induce anisotropy and cause poroelastic behaviour in the wavefields. We observe that the presence of fractures gives rise to changes in phase velocity and attenuation, especially for the slow P-wave in the direction parallel to the fracture plane.

Description: ABSTRACT Wide-angle seismic data acquired by use of air-guns and ocean bottom seismometers (OBS) contain strong direct water arrivals and multiples, generally considered as noise and thus not included in the modelling. However, a recent study showed that standard ray-tracing modelling of the water multiples recorded off the Bear Island, North Atlantic, provided a reliable estimate of the velocity distribution in the water layer. Here, we demonstrate that including the amplitudes in the modelling provide valuable information about the V P contrast at the seafloor, as well as the V P /V S ratio and attenuation ( Q P ) of the uppermost sediments. The V P contrast at the seafloor is estimated at about 250 m/s, within a precision of approximately ±30 m/s. The V P /V S ratio in the uppermost sedimentary layer is modelled in the range 2.25–2.50 and the Q P factor is estimated at 1000 for the water, 30–50 for the uppermost layer and 40–50 for the second sedimentary layer. The values obtained for the sediments suggest a lithology dominated by silty clays, with porosity below average.

Description: ABSTRACT We developed a frequency-domain acoustic-elastic coupled waveform inversion based on the Gauss-Newton conjugate gradient method. Despite the use of a high-performance computer system and a state-of-the-art parallel computation algorithm, it remained computationally prohibitive to calculate the approximate Hessian explicitly for a large-scale inverse problem. Therefore, we adopted the conjugate gradient least-squares algorithm, which is frequently used for geophysical inverse problems, to implement the Gauss-Newton method so that the approximate Hessian is calculated implicitly. Thus, there was no need to store the Hessian matrix. By simultaneously back-propagating multi-components consisting of the pressure and displacements, we could efficiently extract information on the subsurface structures. To verify our algorithm, we applied it to synthetic data sets generated from the Marmousi-2 model and the modified SEG/EAGE salt model. We also extended our algorithm to the ocean-bottom cable environment and verified it using ocean-bottom cable data generated from the Marmousi-2 model. With the assumption of a hard seafloor, we recovered both the P-wave velocity of complicated subsurface structures as well as the S-wave velocity. Although the inversion of the S-wave velocity is not feasible for the high Poisson's ratios used to simulate a soft seafloor, several strategies exist to treat this problem. Our example using multi-component data showed some promise in mitigating the soft seafloor effect. However, this issue still remains open.

Description: ABSTRACT A new, adaptive multi-criteria method for accurate estimation of three-component three-dimensional vertical seismic profiling of first breaks is proposed. Initially, we manually pick first breaks for the first gather of the three-dimensional borehole set and adjust several coefficients to approximate the first breaks wave-shape parameters. We then predict the first breaks for the next source point using the previous one, assuming the same average velocity. We follow this by calculating an objective function for a moving trace window to minimize it with respect to time shift and slope. This function combines four main properties that characterize first breaks on three-component borehole data: linear polarization, signal/noise ratio, similarity in wave shapes for close shots and their stability in the time interval after the first break. We then adjust the coefficients by combining current and previous values. This approach uses adaptive parameters to follow smooth wave-shape changes. Finally, we average the first breaks after they are determined in the overlapping windows. The method utilizes three components to calculate the objective function for the direct compressional wave projection. An adaptive multi-criteria optimization approach with multi three-component traces makes this method very robust, even for data contaminated with high noise. An example using actual data demonstrates the stability of this method.

Description: ABSTRACT The seismic vibrator has become a very important source for land data acquisition and there have been dramatic improvements in recent times in the application of the vibroseis technique. These improvements have led to much increased productivity and in many cases much denser source sampling. At the same time, the vibrator itself has seen little improvement over the last couple of decades. There are needs in a few areas where an improvement in the vibrator itself can bring benefits to the quality of the seismic data acquired. This paper describes progress in four such areas, low-frequency performance, high-frequency performance, an improved estimate of the vibrator groundforce and source signature consistency over variable ground conditions. Each of these vibrator characteristics will be discussed in turn. Meanwhile, two field test results in which the performance of two different vibrators in these four areas are compared.

Description: ABSTRACT We use numerically modelled data sets to investigate the sensitivity of electromagnetic interferometry by multidimensional deconvolution to spatial receiver sampling. Interferometry by multidimensional deconvolution retrieves the reflection response below the receivers after decomposition of the fields into upward and downward decaying fields and deconvolving the upward decaying field by the downward decaying field. Thereby the medium above the receiver level is replaced with a homogeneous half-space, the sources are redatumed to the receiver level and the direct field is removed. Consequently, in a marine setting the retrieved reflection response is independent of any effect of the water layer and the air above. A drawback of interferometry by multidimensional deconvolution is a possibly unstable matrix inversion, which is necessary to retrieve the reflection response. Additionally, in order to correctly separate the upward and the downward decaying fields, the electromagnetic fields need to be sampled properly. We show that the largest possible receiver spacing depends on two parameters: the vertical distance between the source and the receivers and the length of the source. The receiver spacing should not exceed the larger of these two parameters. Besides these two parameters, the presence of inhomogeneities close to the receivers may also require a dense receiver sampling. We show that by using the synthetic aperture concept, an elongated source can be created from conventionally acquired data in order to overcome these strict sampling criteria. Finally, we show that interferometry may work under real-world conditions with random noise and receiver orientation and positioning errors.

Description: ABSTRACT To provide a guide for future deep (〈1.5 km) seismic mineral exploration and to better understand the nature of reflections imaged by surface reflection seismic data in two mining camps and a carbonatite complex of Sweden, more than 50 rock and ore samples were collected and measured for their seismic velocities. The samples are geographically from the northern and central parts of Sweden, ranging from metallic ore deposits, meta-volcanic and meta-intrusive rocks to deformed and metamorphosed rocks. First, ultrasonic measurements of P- and S-wave velocities at both atmospheric and elevated pressures, using 0.5 MHz P- and S-wave transducers were conducted. The ultrasonic measurements suggest that most of the measured velocities show positive correlation with the density of the samples with an exception of a massive sulphide ore sample that shows significant low P- and S-wave velocities. The low P- and S-wave velocities are attributed to the mineral texture of the sample and partly lower pyrite content in comparison with a similar type sample obtained from Norway, which shows significantly higher P- and S-wave velocities. Later, an iron ore sample from the central part of Sweden was measured using a low-frequency (0.1–50 Hz) apparatus to provide comparison with the ultrasonic velocity measurements. The low-frequency measurements indicate that the iron ore sample has minimal dispersion and attenuation. The iron ore sample shows the highest acoustic impedance among our samples suggesting that these deposits are favourable targets for seismic methods. This is further demonstrated by a real seismic section acquired over an iron ore mine in the central part of Sweden. Finally, a laser-interferometer device was used to analyse elastic anisotropy of five rock samples taken from a major deformation zone in order to provide insights into the nature of reflections observed from the deformation zone. Up to 10% velocity-anisotropy is estimated and demonstrated to be present for the samples taken from the deformation zone using the laser-interferometery measurements. However, the origin of the reflections from the major deformation zone is attributed to a combination of anisotropy and amphibolite lenses within the deformation zone.

Description: ABSTRACT In this paper, we describe a non-linear constrained inversion technique for 2D interpretation of high resolution magnetic field data along flight lines using a simple dike model. We first estimate the strike direction of a quasi 2D structure based on the eigenvector corresponding to the minimum eigenvalue of the pseudogravity gradient tensor derived from gridded, low-pass filtered magnetic field anomalies, assuming that the magnetization direction is known. Then the measured magnetic field can be transformed into the strike coordinate system and all magnetic dike parameters – horizontal position, depth to the top, dip angle, width and susceptibility contrast – can be estimated by non-linear least squares inversion of the high resolution magnetic field data along the flight lines. We use the Levenberg-Marquardt algorithm together with the trust-region-reflective method enabling users to define inequality constraints on model parameters such that the estimated parameters are always in a trust region. Assuming that the maximum of the calculated  g zz  (vertical gradient of the pseudogravity field) is approximately located above the causative body, data points enclosed by a window, along the profile, centred at the maximum of  g zz  are used in the inversion scheme for estimating the dike parameters. The size of the window is increased until it exceeds a predefined limit. Then the solution corresponding to the minimum data fit error is chosen as the most reliable one. Using synthetic data we study the effect of random noise and interfering sources on the estimated models and we apply our method to a new aeromagnetic data set from the Särna area, west central Sweden including constraints from laboratory measurements on rock samples from the area.

Description: ABSTRACT We present laboratory ultrasonic measurements of shear-wave splitting from two synthetic silica cemented sandstones. The manufacturing process, which enabled silica cementation of quartz sand grains, was found to produce realistic sandstones of average porosity 29.7 ± 0.5% and average permeability 29.4 ± 11.3 mD. One sample was made with a regular distribution of aligned, penny-shaped voids to simulate meso-scale fractures in reservoir rocks, while the other was left blank. Ultrasonic shear waves were measured with a propagation direction of 90° to the coincident bedding plane and fracture normal. In the water saturated blank sample, shear-wave splitting, the percentage velocity difference between the fast and slow shear waves, of 〈0.5% was measured due to the bedding planes (or layering) introduced during sample preparation. In the fractured sample, shear-wave splitting (corrected for layering anisotropy) of 2.72 ± 0.58% for water, 2.80 ± 0.58% for air and 3.21 ± 0.58% for glycerin saturation at a net pressure of 40 MPa was measured. Analysis of X-ray CT scan images was used to determine a fracture density of 0.0298 ± 0.077 in the fractured sample. This supports theoretical predictions that shear-wave splitting (SWS) can be used as a good estimate for fracture density in porous rocks (i.e., SWS = 100 ε f , where ε f is fracture density) regardless of pore fluid type, for wave propagation at 90° to the fracture normal.

Description: ABSTRACT We invert prestack seismic amplitude data to find rock properties of a vertical profile of the earth. In particular we focus on lithology, porosity and fluid. Our model includes vertical dependencies of the rock properties. This allows us to compute quantities valid for the full profile such as the probability that the vertical profile contains hydrocarbons and volume distributions of hydrocarbons. In a standard point wise approach, these quantities can not be assessed. We formulate the problem in a Bayesian framework, and model the vertical dependency using spatial statistics. The relation between rock properties and elastic parameters is established through a stochastic rock model, and a convolutional model links the reflectivity to the seismic. A Markov chain Monte Carlo (MCMC) algorithm is used to generate multiple realizations that honours both the seismic data and the prior beliefs and respects the additional constraints imposed by the vertical dependencies. Convergence plots are used to provide quality check of the algorithm and to compare it with a similar method. The implementation has been tested on three different data sets offshore Norway, among these one profile has well control. For all test cases the MCMC algorithm provides reliable estimates with uncertainty quantification within three hours. The inversion result is consistent with the observed well data. In the case example we show that the seismic amplitudes make a significant impact on the inversion result even if the data have a moderate well tie, and that this is due to the vertical dependency imposed on the lithology fluid classes in our model. The vertical correlation in elastic parameters mainly influences the upside potential of the volume distribution. The approach is best suited to evaluate a few selected vertical profiles since the MCMC algorithm is computer demanding.

Description: ABSTRACT For a 4D seismic operation to be successful, it is important to know what kind of 4D signal we expect to observe, as well as its magnitude. Normally, in a 4D feasibility study, we use rock physics models to quantify the effect of fluid or pressure changes within the reservoir and calculate the corresponding effects to the seismogram. However, to find if the predicted changes are actually observable at a given field, a dedicated calibration procedure might give valuable insight. One such procedure for marine seismics is to gradually change the source strength by varying the firing pressure in order to detect the sensitivity threshold for a given subsurface reflection. This procedure would be practical and feasible if the change of the source signature changes linearly with the source pressure. However, non-linear effects will lead to minor changes in the later arrivals of the source signature, the so-called bubble. By investigating these introduced errors for a reasonable air-gun array we conclude that the method is still feasible since we find it possible to control and diminish the impact of the introduced errors.

Description: ABSTRACT A compact representation is obtained for the separation of a scalar wavefield on a closed surface into parts due to internal and external sources. The formula assumes that the total field and its gradients are known on the surface, as is the exact Green function of the medium. The derivation involves four rather straightforward applications of Green’s theorem or the representation theorem, though it is a remarkable result in that waves from either source that traverse the boundary many times are appropriately separated. The intermediate results at the four steps of the derivation also shed light on the possibility of acoustic shielding from unwanted sources without knowledge of the Green function for the medium.

Description: ABSTRACT A detailed investigation on the location of magmatic intrusions in the Carboniferous strata of the Qinggelidi area, north-eastern Junggar Basin, is presented based on the interpretation of gravity and magnetic data constrained by petrophysical data, seismics and surface geology. The wavelet multi-resolution analysis based on the discrete wavelet transform is adopted to the regional-residual separation of gravity and magnetic anomalies. A power spectrum analysis is applied to estimate the source depths corresponding to different scales. A comparative analysis on the characteristics of local gravity and magnetic anomalies improved our understanding of volcanic rock distribution in the Carboniferous strata. Generally speaking, in total 75 anomalies are recognized, among which 23 are inferred to be the responses of basalts, diabases and andesites with high density and strong magnetization. Twelve anomalies are assumed to be caused by andesites, rhyolites and volcanic breccias with medium-low density and high magnetization. There are still five anomalies that are believed to be generated by volcanic tuffs with low density and weak magnetization. Lastly, four cross-sections in 3D gravity and magnetic modelling are displayed to provide a more thorough image of volcanic rocks in our study area.

Description: ABSTRACT One hundred and fifty five years ago, Kelvin published the first part of a fundamental analysis of the elastic tensor, in which he proposed a coordinate-free representation through its eigensystem. His thoughts were apparently far ahead of his time, since it took 125 years before the paper elicited a positive reaction (it is now accessible through several modern reviews). Science not only lost track for 125 years of the original paper but also lost the ideas Kelvin might have proposed in the second part, a publication that was never put to paper, presumably in view of the lack of appreciation of the first part. In an attempt to establish what might have been on Kelvin's mind for a second part, one has to ‘forget’ the progress of mathematical physics in the intervening time and base all arguments strictly on the content of the first part and on the state of science in the second half of the 19 th century. The theory of elasticity would certainly have developed faster, had Kelvin's paper peen appreciated by his ‘peers’. But a theory based on Kelvin's ideas would be fruitful even today.

Description: Deuterium-excess (d) in water is a combination of the oxygen (δ18O) and hydrogen (δD) isotope ratios, and its variability is thought to indicate the location and environmental conditions of the marine moisture source. In this study, we analyze d of water vapor (dv) from six sites, all between 37 and 44°N to examine patterns in the atmospheric surface layer and identify the main drivers of variability. Two sites are in urban settings (New Haven, CT, USA and Beijing, China), two sites are in agricultural settings (Rosemount, MN, USA and Luancheng, China), and two sites are in natural ecosystems, a forest (Borden Forest, Ontario, Canada) and a grassland (Duolun, China). We found a robust diurnal cycle in dv at all sites with maximum values during mid-day. Isotopic land surface model simulations suggest that plant transpiration is one mechanism underlying the diurnal pattern. An isotopic large-eddy simulation model shows that entrainment of the free atmosphere into the boundary layer can also produce high dv values in mid-day. Daily mid-day means of dv were negatively correlated with local mid-day relative humidity and positively correlated with planetary boundary layer height at the North American sites, but not the Chinese sites. The mechanism for these differences is still undetermined. These results demonstrate that within the diurnal time scale, dv of the surface air at continental locations can be significantly altered by local processes, and is therefore not a conserved tracer of humidity from the marine moisture source region as has previously been assumed.

Description: Climate change and human activities are expected to have a major impact on the structure and functioning of marine ecosystems and the biogeochemical cycles they mediate in the coming years. Here we describe time series measurements of biogenic bromocarbons (CHBr3 and CH2Br2) collected in coastal waters of the western Antarctic Peninsula which is one of the world's most rapidly changing marine environments. Our measurements spanned a period of changing sea-ice dynamics and phytoplankton community structure driven by climatic forcing. Specifically, the occurrence of high chlorophyll a concentrations (≥5 μg L−1) and dominance of the largest phytoplankton size fraction (≥20 μm) indicating diatom bloom conditions was reduced following winter periods with a relatively short winter sea-ice duration (

Description: While much of the dissolved organic carbon (DOC) within rivers is destined for mineralization to CO2, a substantial fraction of riverine bicarbonate (HCO3−) flux represents a CO2 sink, as a result of weathering processes that sequester CO2 as HCO3−. We explored landscape-level controls on DOC and HCO3− flux in subcatchments of the boreal, with a specific focus on the effect of permafrost on riverine dissolved C flux. To do this, we undertook a multivariate analysis that partitioned the variance attributable to known, key regulators of dissolved C flux (runoff, lithology, and vegetation) prior to examining the effect of permafrost, using riverine biogeochemistry data from a suite of subcatchments drawn from the Mackenzie, Yukon, East, and West Siberian regions of the circumboreal. Across the diverse catchments that we study, controls on HCO3− flux were near-universal: runoff and an increased carbonate rock contribution to weathering (assessed as riverwater Ca:Na) increased HCO3− yields, while increasing permafrost extent was associated with decreases in HCO3−. In contrast, permafrost had contrasting and region-specific effects on DOC yield, even after the variation caused by other key drivers of its flux had been accounted for. We used ionic ratios and SO4 yields to calculate the potential range of CO2 sequestered via weathering across these boreal subcatchments, and show that decreasing permafrost extent is associated with increases in weathering-mediated CO2 fixation across broad spatial scales, an effect that could counterbalance some of the organic C mineralization that is predicted with declining permafrost.

Description: ABSTRACT Blended or simultaneous sources have been the focus of a good deal of interest recently. In conventional acquisition the time intervals between successive sources are large enough to avoid interference in time. In blended acquisition, temporal overlap between source responses is allowed. The procedure of retrieving data as if they were acquired in the conventional way is called deblending. This is an essential step if standard processing flows are to be applied. Several inversion techniques have been proposed for solving this ill-posed problem. We study the properties of an iterative estimation and subtraction algorithm that integrates a coherency-pass filter in a dedicated iteration. We begin by stating the problem we wish to solve and develop a new, more general, interpretation of the method. We then apply an algebraic analysis of the iteration to establish the convergence characteristics of the algorithm. In order to facilitate this analysis, the notion of leakage subspace is introduced, i.e., a subspace where energy that cannot be uniquely assigned to one of the sources resides. We find that a unique solution exists, if, and only if, there is no leakage subspace. If a unique solution does not exist, then the iteration converges to a least-norm solution contaminated by the projection of the initial guess onto the leakage subspace. The insights gained by this analysis lead us to the development of a simple tool that can provide valuable information during the design of a blended survey. Finally we present results from the application of this method to real blended marine data and then draw our conclusions.

Description: ABSTRACT Least-squares migration has been shown to improve image quality compared to the conventional migration method, but its computational cost is often too high to be practical. In this paper, we develop two numerical schemes to implement least-squares migration with the reverse time migration method and the blended source processing technique to increase computation efficiency. By iterative migration of supergathers, which consist in a sum of many phase-encoded shots, the image quality is enhanced and the crosstalk noise associated with the encoded shots is reduced. Numerical tests on 2D HESS VTI data show that the multisource least-squares reverse time migration (LSRTM) algorithm suppresses migration artefacts, balances the amplitudes, improves image resolution and reduces crosstalk noise associated with the blended shot gathers. For this example, the multisource LSRTM is about three times faster than the conventional RTM method. For the 3D example of the SEG/EAGE salt model, with a comparable computational cost, multisource LSRTM produces images with more accurate amplitudes, better spatial resolution and fewer migration artefacts compared to conventional RTM. The empirical results suggest that multisource LSRTM can produce more accurate reflectivity images than conventional RTM does with a similar or less computational cost. The caveat is that the LSRTM image is sensitive to large errors in the migration velocity model.

Description: The organic carbon stock in permafrost is of increasing interest in environmental research, because during the late Quaternary a large pool of organic carbon accumulated in the sedimentary deposits of arctic permafrost. Because of its potential to degrade and release organic carbon, the organic-matter inventory of Yedoma Ice Complex deposits is relevant to current concerns about the effects of global warming. In this context, it is essential to improve the understanding of preserved carbon quantities and characteristics. The paper aims to clarify the Yedoma Ice Complex origin, and to develop an approach for volumetric organic-matter quantification. Therefore, we analyzed the grain size and the organic-matter characteristics of the deposits exposed at the stratigraphic key site Duvanny Yar (lower Kolyma River, northeast Siberia). A distinct bimodal grain-size distribution confirms a polygenetic origin of the frozen sediments from a floodplain environment. The total organic-carbon content averages 1.5 ± 1.4 wt% while the volumetric organic-carbon content averages 14 ± 8 kg/m3. However, large-scale extrapolations for Yedoma Ice Complex deposits in general are not reasonable yet because of their rather unclear spatial distribution. We conclude that Yedoma Ice Complex formation at Duvanny Yar was dominated by water-related (alluvial/fluvial/lacustrine) as well as aeolian processes. The total organic-carbon content of the studied deposits is low if compared to other profiles, but it is still a significant pool.

Description: Aeolian dust transport from the Saharan/Sahel desert regions is considered the dominant external input of iron (Fe) to the surface waters of the eastern (sub-) tropical North Atlantic Ocean. To test this hypothesis, we investigated the sources of dissolved Fe (DFe) and quantified DFe fluxes to the surface ocean in this region. In winter 2008, surface water DFe concentrations varied between 1.5 nM) correlated positively with apparent oxygen utilization (AOU) and showed the importance of organic matter remineralization as an DFe source. As a consequence, vertical diffusive mixing formed an important Fe flux to the surface ocean in this region, even surpassing that of a major dust event.

Description: ABSTRACT The attenuation of coherent and random noise still poses technical challenges in seismic data processing, especially in onshore environments. Multichannel Singular Spectrum Analysis (MSSA) is an existing and effective technique for random-noise reduction. By incorporating a randomizing operator into MSSA, this modification creates a new and powerful filtering method that can attenuate both coherent and random noise simultaneously. The key of the randomizing operator exploits the fact that primary events after NMO are relatively horizontal. The randomizing operator randomly rearranges the order of input data and reorganizes coherent noise into incoherent noise but has a minimal effect on nearly horizontal primary reflections. The randomizing process enables MSSA to suppress both coherent and random noise simultaneously. This new filter, MSSARD (MSSA in the randomized domain) also resembles a combination of eigenimage and Cadzow filters. I start with a synthetic data set to illustrate the basic concept and apply MSSARD filtering on a 3D cross-spread data set that was severely contaminated with ground roll and scattered noise. MSSARD filtering gives superior results when compared with a conventional 3D f-k filter. For a random-noise example, the application of MSSARD filtering on time-migrated offset-vector-tile (OVT) gathers also produces images with higher signal-to-noise ratios than a conventional f-xy deconvolution filter.

Description: We assess the global balance of calcite export through the water column and burial in sediments as it varies regionally. We first drive a comprehensive 1-D model for sediment calcite preservation with globally gridded field observations and satellite-based syntheses. We then reformulate this model into a simpler five-parameter box model, and combine it with algorithms for surface calcite export and water column dissolution for a single expression for the vertical calcite balance. The resulting metamodel is optimized to fit the observed distributions of calcite burial flux. We quantify the degree to which calcite export, saturation state, organic carbon respiration, and lithogenic sedimentation modulate the burial of calcite. We find that 46% of burial and 88% of dissolution occurs in sediments overlain by undersaturated bottom water with sediment calcite burial strongly modulated by surface export. Relative to organic carbon export, we find surface calcite export skewed geographically toward relatively warm, oligotrophic areas dominated by small, prokaryotic phytoplankton. We assess century-scale projected impacts of warming and acidification on calcite export, finding high sensitive to inferred saturation state controls. With respect to long-term glacial cycling, our analysis supports the hypothesis that strong glacial abyssal stratification drives the lysocline toward much closer correspondence with the saturation horizon. Our analysis suggests that, over the transition from interglacial to glacial ocean, a resulting ∼0.029 PgC a−1 decrease in deep Atlantic, Indian and Southern Ocean calcite burial leads to slow increase in ocean alkalinity until Pacific mid-depth calcite burial increases to compensate.

Description: Terrestrial ecosystem models (TEMs) contain the coupling of many biogeochemical processes with a large number of parameters involved. In many cases those parameters are highly uncertain. In order to reduce those uncertainties, parameter estimation methods can be applied, which allow the model to be constrained against observations. We compare the performance and results of two such parameter estimation techniques - the Metropolis algorithm (MA) which is a Markov Chain Monte Carlo (MCMC) method and the adjoint approach as it is used in the Carbon Cycle Data Assimilation System (CCDAS). Both techniques are applied here to derive the posterior probability density function (PDF) for 19 parameters of the Biosphere Energy Transfer and Hydrology (BETHY) scheme. We also use the MA to sample the posterior parameter distribution from the adjoint inversion. This allows us to assess if the commonly made assumption in variational data assimilation, that everything is normally distributed, holds. The comparison of the posterior parameter PDF derived by both methods shows that in most cases an approximation of the PDF by a normal distribution as used by the adjoint approach is a valid assumption. The results also indicate that the global minimum has been identified by both methods for the given set up. However, the adjoint approach outperforms the MA by several orders of magnitude in terms of computational time. Both methods show good agreement in the PDF of estimated net carbon fluxes for the decades of the 1980s and 1990s.

Description: Primary production in large areas of the open ocean is limited by low iron concentrations. Rivers are potential sources of iron to the ocean, however, riverine iron is prone to intense flocculation and sedimentation in the estuarine mixing zone. Here we report the detection of iron-rich nanoparticles in a typical peatland-draining creek which are resistant against salt-induced flocculation i.e., their behavior is in sharp contrast to the well-known behavior of Fe colloids in river waters. Sample fractionation by AsFlFFF (Asymmetric Flow Field Flow Fractionation) revealed that these powerful iron carriers are in the size range of only 0.5–3.0 nm hydrodynamic diameter. They were isolated from the water phase using solid phase extraction/gel permeation chromatography, and analyzed by a CuO oxidation/GC-MS method. Our results suggest that the particles consist mainly of lignin catabolites and that gymnosperm as well as angiosperm tissues are contributors to the seawater-resistant iron-bearing DOM. Lignin phenols, which have no autochthonous source in the ocean, have been nevertheless found in low concentrations throughout the entire Arctic, Atlantic, and Pacific oceans. It is therefore tempting to speculate that peatland-derived iron-bearing lignin particles may have a sufficiently long half-life in ocean waters to sustain iron concentration in extended regions of the ocean.

Description: The global tropospheric budget of gaseous and particulate non-methane organic matter (OM) is re-examined to provide a holistic view of the role that OM plays in transporting the essential nutrients nitrogen and phosphorus to the ocean. A global 3-dimensional chemistry-transport model was used to construct the first global picture of atmospheric transport and deposition of the organic nitrogen (ON) and organic phosphorus (OP) that are associated with OM, focusing on the soluble fractions of these nutrients. Model simulations agree with observations within an order of magnitude. Depending on location, the observed water soluble ON fraction ranges from ∼3% to 90% (median of ∼35%) of total soluble N in rainwater; soluble OP ranges from ∼20–83% (median of ∼35%) of total soluble phosphorus. The simulations suggest that the global ON cycle has a strong anthropogenic component with ∼45% of the overall atmospheric source (primary and secondary) associated with anthropogenic activities. In contrast, only 10% of atmospheric OP is emitted from human activities. The model-derived present-day soluble ON and OP deposition to the global ocean is estimated to be ∼16 Tg-N/yr and ∼0.35 Tg-P/yr respectively with an order of magnitude uncertainty. Of these amounts ∼40% and ∼6%, respectively, are associated with anthropogenic activities, and 33% and 90% are recycled oceanic materials. Therefore, anthropogenic emissions are having a greater impact on the ON cycle than the OP cycle; consequently increasing emissions may increase P-limitation in the oligotrophic regions of the world's ocean that rely on atmospheric deposition as an important nutrient source.

Description: Particulate nitrogen (PN) dynamics in the oligotrophic northern South China Sea (around the SouthEast Asian Time-series Study (SEATS) station) was explored by examining the isotopic compositions of suspended PN in the top 200 m over 3 years and sinking PN collected by sediment traps. The PN inventory (IPN) in the upper 100 m is larger than in the lower 100 m, exhibiting stronger seasonality. Both layers reveal significant seasonality in mean δ15NPN, yet, the mean in the upper 100 m (2.0 to 5.3‰) is consistently smaller than that in the lower 100 m, implying the occurrence of vertical biological fractionation and/or an addition of 15N-depleted N from the atmosphere. The δ15NPN surges in winter, when the mixed layer is deeper, indicate an intensified nitrate supply from thermocline, during which relatively stronger downward transfer efficiency was inferred by a small IPN gradient. The largest vertical gradient in IPN appeared during intermonsoon periods, corresponding with weak vertical mixing, low δ15NPN, and high N* values. N fixation is likely the cause for the intermonsoon δ15NPN lows. The δ15NPN values of trapped material at 374 m and 447 m range from 3.3 to 7.3‰ with a flux-weighted mean of 5.6‰ resembling the δ15NO3 of upwelled sources. By using a mass-isotope balance model under the assumption of no atmospheric N deposition, we obtained an N fixation input of ∼20 ± 26 mmol N m−2 yr−1. This value accounts for only ∼5–10% of the new production on an annual basis.

Description: Seawater dissolved iron isotope ratios (δ56Fe) have been measured in the North Atlantic near Bermuda. In a full-depth profile, seawater dissolved δ56Fe is isotopically heavy compared to crustal values throughout the water column (δ56FeIRMM-014 = +0.30‰ to +0.71‰). Iron isotope ratios are relatively homogenous in the upper water column (between +0.30‰ to +0.45‰ above 1500 m), and δ56Fe increases below this to a maximum of +0.71‰ at 2500 m, decreasing again to +0.35‰ at 4200 m. The δ56Fe profile is very different from the iron concentration profile; in the upper water column [Fe] is variable while δ56Fe is relatively constant, and in the deeper water column δ56Fe varies while [Fe] remains relatively constant. The δ56Fe profile is also not well correlated with other hydrographic tracers in the North Atlantic such as temperature, salinity, or the concentrations of oxygen, phosphate, silica, and CFC-11. The dissimilarity between δ56Fe profiles and profiles of [Fe] and other hydrographic tracers shows that Fe isotope ratios provide a unique sort of information about ocean chemistry, and they suggest that Fe isotopes may therefore be a valuable new tool for tracing the global sources, sinks, and biogeochemical cycling of Fe.

Description: In the past 800 thousand years and before industrialization, the largest variations in atmospheric CO2 concentration (pCO2) occurred in connection with the glacial cycles that characterized Earth's climate over this period. One curious feature of at least the last four glacial-interglacial cycles is that atmospheric pCO2 reached about the same upper limit of 280 ppm during peak interglacial periods and about the same lower limit of 180 ppm during peak glacial periods. Here, we show using a numerical model of earth system that enhanced shelf sediment weathering during glacial sea level lowstands tends to raise pCO2 even after carbonate compensation and thus stabilize pCO2 from further reduction. This is because not all nutrients from weathering will be utilized by biology but more importantly because the spatial distributions of carbon and phosphorus from weathering become decoupled in such a way that carbon is preferentially stored in the upper ocean and phosphorus in the deep ocean. In addition, the C:P ratios in continental margin sediments are generally much higher than the Redfield ratio due to preferential remineralization of phosphorus in shelf sediment diagenesis. When these factors are accounted for in our model, the input of organic matter, which corresponds to the observed negative shift in ocean δ13C during glacial periods, raises pCO2 by approximately 14 ppm. The same mechanisms operating in the opposite directions during interglacial highstand tend to lower pCO2 and stabilize it from further increase. The impact of sea level-driven continental shelf exposure and submersion of CO2 is therefore a negative feedback that may have contributed to limiting the variation of Pleistocene pCO2 to the observed 100 ppm range.

Description: A great deal of attention, both negative and positive, has been directed at the potential of large-scale iron fertilization schemes to sequester carbon by inducing phytoplankton blooms that would, in theory, result in significant export of organic carbon to the deep ocean in high nitrogen - low chlorophyll regions. A suite of iron manipulation or ‘patch’ experiments has been performed over length-scales of 10s of km. Here, we use a physical-ecological-chemical model, with prognostic nitrogen, silica and iron dynamics, to study one of the most successful of these experiments, the Subarctic Ecosystem Response to Iron Enrichment Study (SERIES), focusing on the vertical export of organic material, which is difficult to observe in the field. The implications of large-scale fertilization, i.e. increasing patch size, are investigated. Our results agree with the general conclusions obtained from the field experiments. Only a modest export of organic carbon occurs (less than 25% of carbon uptake by phytoplankton) at the base of the mixed layer. Furthermore, we show that lateral and vertical supply of silicic acid is necessary to fuel a sustained phytoplankton bloom. Increasing patch size results in less lateral nutrient supply relative to patch area and so a decrease, not only in total production (per unit area), but in the contribution by large phytoplankton due to silica limitation. Most importantly, the export of organic carbon (per unit area) decreases substantially, by nearly an order of magnitude, as scales of 1000 km are approached.

Description: ABSTRACT Microseismic monitoring, particularly the monitoring of hydraulic fracturing in gas- and oil-bearing shales, has developed significantly over the last ten years. Early work focused on the location of microseismic events but more recently there have been attempts to extract more of the information afforded by this rich data source. In particular, the recovery of the frequency-magnitude distribution, which is expected to follow a Gutenberg-Richter distribution, may provide insights into the prevailing effective stress regime in the vicinity of the events. This stress regime varies with distance from the hydraulic fracturing: at the propagating fracture one expects conditions for tensile or shear failure, away from the fracture one may broadly expect microseismicity associated with pre-existing weakness in the rock, occurring at effective stress conditions close to the conditions existing prior to the treatment. All geophysical experiments are detection limited and the microseismic monitoring case does not differ in this regard. In constructing a statistical indicator such as the distribution of moment magnitudes we would like the estimate to be robust and use as much of the data as possible. In analysing earthquake catalogues the predominant practise is to determine a magnitude of completeness denoting the detection limit of the catalogue. This approach defines a minimum magnitude above which all events are thought to have been reliably recorded. In effect, this imposes an artificial, conservative detection limit to replace the unknown detection limit of the catalogue. We present the case of an arbitrary detection limit and introduce an approach from astronomy that is particularly suited to the single-well observing geometry most prevalent in hydraulic fracture monitoring. We calculate b-values for a set of event magnitudes from the Barnett Shale formation, where multiple stimulation treatments were applied in a pair of wells (‘zipper frac’) followed by a four-stage treatment in a third well and find significant variations in the b-value between the pumped stages.

Description: ABSTRACT We generalize the classical theory of acoustoelasticity to the porous case (one fluid and a solid frame) and finite deformations. A unified treatment of non-linear acoustoelasticity of finite strains in fluid-saturated porous rocks is developed on the basis of Biot’s theory. A strain-energy function, formed with eleven terms, combined with Biot’s kinetic and dissipation energies, yields Lagrange’s equations and consequently the wave equation of the medium. The velocities and dissipation factors of the P- and S-waves are obtained as a function of the 2nd- and 3rd-order elastic constants for hydrostatic and uniaxial loading. The theory yields the limit to the classical theory if the fluid is replaced with a solid with the same properties of the frame. We consider sandstone and obtain results for open-pore jacketed and closed-pore jacketed ‘gedanken’ experiments. Finally, we compare the theoretical results with experimental data.

Description: ABSTRACT Randomized source-encoding has recently been proposed as a way to dramatically reduce the costs of full waveform inversion. The main idea is to replace all sequential sources by a small number of simultaneous sources. This introduces random cross-talk in model updates and special stochastic optimization strategies are required to deal with this. Two problems arise with this approach: i) source-encoding can only be applied to fixed-spread acquisition setups and ii) stochastic optimization methods tend to converge very slowly, relying on averaging to suppress the cross-talk. Although the slow convergence is partly off-set by a low iteration cost, we show that conventional optimization strategies are bound to outperform stochastic methods in the long run. In this paper we argue that we do not need randomized source-encoding to reap the benefits of stochastic optimization and we review an optimization strategy that combines the benefits of both conventional and stochastic optimization. The method uses a gradually increasing batch of sources. Thus, iterations are initially very cheap and this allows the method to make fast progress in the beginning. As the batch-size grows, the method behaves like conventional optimization, allowing for fast convergence. Stylized numerical examples suggest that the stochastic and hybrid methods perform equally well with and without source-encoding and that the hybrid method outperforms both conventional and stochastic optimization. The method does not rely on source-encoding techniques and can thus be applied to marine data. We illustrate this on a realistic synthetic model.

Description: ABSTRACT In shallow water the frequency domain controlled source electromagnetic method is subject to airwave saturation that strongly limits the sensitivity to resistive hydrocarbon targets at depth. It has been suggested that time-domain CSEM may offer an improved sensitivity and resolution of these deep targets in the presence of the airwave. In order to examine and test these claims, this work presents a side-by-side investigation of both methods with a main focus on practical considerations, and how these effect the resolution of a hydrocarbon reservoir. Synthetic noisy data for both time-domain and frequency domain methods are simulated using a realistic frequency dependent noise model and frequency dependent scaling for representative source waveforms. The synthetic data studied here include the frequency domain response from a compact broadband waveform, the time-domain step-response from a low-frequency square wave and the time-domain impulse response obtained from pseudo-random binary sequences. These data are used in a systematic resolution study of each method as a function of water-depth, relative noise and stacking length. The results indicate that the broadband frequency domain data have the best resolution for a given stacking time, whereas the time-domain data require prohibitively longer stacking times to achieve similar resolution.

Description: ABSTRACT Simulation of induction logging responses in formations with large conductivity contrasts is an important but challenging problem due to the singularity of a linear system caused by large contrasts. Also, three-dimensional (3D) analysis of complex geophysical structures usually encounters high computational demands. In this paper, a pre-corrected fast Fourier transform (pFFT)-accelerated integral equation method is applied to overcome these difficulties. In the approach, the entire formation is included in the solution domain. The volume integral equation is set up in the region based on the fact that the total field is the summation of the excitation field and the secondary field. The emitted field by the transmitter coil (treated as a magnetic dipole) is regarded as the excitation of the system. Then the method of moments (MoM) is used to solve the integral equation. To reduce the high computational requirements of the MoM, the pFFT method is used to speed up the solution of the matrix equation and reduce the memory requirement as well. The resultant method is capable of computing induction logging problems involving large and complex formations. For problems with high conductivity contrasts, the solution of the matrix equation usually converges very slow or even fails to converge due to the large condition number of the coefficient matrix. To overcome this difficulty, an incomplete LU pre-conditioner is used to significantly speed up the convergence of the matrix equation, thus further reducing the computation time. Numerical results show that the present method is efficient and flexible for 3D simulation of induction logging and is specifically superior for problems with high conductivity contrasts.

Description: ABSTRACT The fast generalized Fourier transform algorithm is extended to two-dimensional data cases. The algorithm provides a fast and non-redundant alternative for the simultaneous time-frequency and space-wavenumber analysis of data with time-space dependencies. The transform decomposes data based on local slope information and therefore making it possible to extract the weight function based on dominant dips from alias-free low frequencies. By projecting the extracted weight function to alias-contaminated high frequencies and utilizing a least-squares fitting algorithm, a beyond-alias interpolation method is accomplished. Synthetic and real data examples are provided to examine the performance of the proposed interpolation method.

Description: ABSTRACT Survey sinking migration downward continues the entire surface observed multi-shot data to the subsurface step by step recursively. Reflected energy from reflectors at current depth appear at zero time and zero offset in the extrapolated wavefield. The data (seismic records) of t 〉 0 at this depth are equivalent to the data acquired by a survey system deployed at this depth. This is the reason to name the process ‘survey sinking’. The records of negative time need not to be further propagated since they carry no information to image structures beneath the new survey system. In this paper, we combine survey sinking with dreamlet migration. The dreamlet migration method decomposes the seismic wavefield and one-way wave propagator by complete time-space localized bases. The localization on time gives flexibility on time-varying operations during depth extrapolation. In dreamlet survey sinking migration, it only keeps the data for imaging the structures beneath the sunk survey system and gets rid of the data already used to image structures above it. The deeper the depth is, the shorter is the valid time records of the remaining data and less computation is needed for one depth step continuation. For data decomposition, in addition to time axis, dreamlet survey sinking also decomposes the data for source and receiver gathers, which is a fully localized decomposition of prestack seismic data. A three-scatter model is first used to demonstrate the computational feature and principle of this method. Tests on the two-dimensional SEG/EAGE salt model show that with reduced data sets the proposed method can still obtain good imaging quality on complex geology structures and a strong velocity contrast environment.