ALBERT

Description: The detection and evaluation of geohazards continues to be an active and transcendental activity in applied geophysics. Geohazards are important not only to the hydrocarbon exploration community but also to near-surface studies in hydrology, agriculture, and civil engineering, to name a few. Modern geotechnical studies often include a good component of geophysical studies for soil mechanics and foundation studies. This special section of The Leading Edge considers two exceptional recent contributions to the study, anticipation, and appraisal of geohazards via geophysical means.

Description: In 1953, Richard Jolly used geophones coupled to the borehole wall to record seismic waves in the subsurface, and he highlighted the interpretive benefits of downhole recording. In the western world, the coupled downhole geophone was a first of its kind and was the ancestor to modern receiver tools (Figure 1) used in the vertical seismic profiling (VSP) method. The translation, by SEG, of Gal'perin's 1974 book on VSP from Russian into English increased awareness of the progress in borehole seismology made in Russia; the case studies outlined in the book gave further prominence to the benefits of the VSP method. Zero-offset VSP (meaning the source is relatively close to the borehole to provide 1D information) or ZVSP is now an essential part of the interpreter's toolkit that gives a seismic time-to-depth relationship, in-situ elastic properties, and multiple-reduced reflectivity. ZVSP continues to provide a key link between surface seismic images and geologic properties. In addition, Q measurement and the identification of multiple-generating and mode-changing horizons from near-offset VSP provide further interpretive insights. In-situ velocities also assist with building velocity models for migration and inversion.

Description: Downhole seismic monitoring has become an alternative to more conventional surface seismic in cases where the latter has low signal-to-noise ratio or poor repeatability, or when frequent monitoring of a local target is required. Downhole 3D VSP monitoring has been applied to both onshore (O'Brien et al., 2004; Kiyashchenko et al., 2011) and offshore environments (Wu et al., 2011). In onshore environments, surface seismic repeatability often suffers from surface noise and near-surface variations caused by water-table changes or surface facilities construction. For VSP surveys, the wavefield separation into downgoing and upgoing fields presents an opportunity to discriminate between near-surface and reservoir changes, thus making time-lapse applications more effective.

Description: Carbon capture and storage (CCS) is the process through which a nearly pure carbon dioxide (CO 2 ) stream is captured, separated from flue gas or other industrial processes, compressed, transported to an appropriate storage site, and injected deep underground into a geological formation where it can be safely stored for long-term geologic storage (Benson, 2005). Large sedimentary basins, such as the Illinois, Michigan, and Western Canadian sedimentary basins are good targets for CCS, as they are in close proximity to large CO 2 emitters and are composed of the appropriate saline formations and overlying nonpermeable formations. In 2003, the U.S. Department of Energy's National Energy Technology Laboratory (DOE-NETL) created a nationwide network of federal, state, and private sector partnerships to determine the most suitable technologies, regulations, and infrastructure for future CCS in different areas of the North America (Office of Fossil Energy, 2013).

Description: Vertical seismic profile surveys (VSP) provide a unique opportunity to obtain high-quality seismic images of the subsurface. Placing receivers downhole mitigates many near-surface and overburden challenges associated with surface seismic imaging and results in improved image quality. Unfortunately, these benefits are restricted by the limited area illuminated by the primary P-wave reflection wavefield (typically a relatively narrow cone that underlies the geophone array). The free-surface multiple wavefield recorded in a VSP survey illuminates a significantly larger area, both vertically and laterally, and so can provide an expanded imaging capability (Jiang et al, 2005; Jiang et al, 2007; Lou et al 2007). Subsurface imaging using the multiple wavefield has been demonstrated and is commonly used for imaging deep-water 3D node surveys, which are typically acquired using a sparse distribution of receivers. However, no case studies have been published yet, which present a critical analysis of the images provided by this technology when applied to VSP surveys.

Description: Shear (S)-wave velocities of near-surface materials can be derived from inverting the dispersive phase velocity of high-frequency (≥ 2 Hz) surface (Rayleigh and/or Love) waves (e.g., Song et al., 1989). Multichannel analysis of surface waves (MASW) uses phase information of high-frequency Rayleigh waves recorded on vertical component geophones to determine S-wave velocities (Miller et al., 1999). Multichannel analysis of Love waves (MALW) uses phase information of high-frequency Love waves recorded on horizontal (perpendicular to the direction of wave propagation) component geophones to determine S-wave velocities (Xia, 2012b). Both MASW and MALW possess stable and efficient inversion algorithms to invert phase velocities of surface waves but MALW has some attractive advantages: (1) Love-wave dispersion curves are simpler than those of Rayleigh waves; (2) dispersion images of Love-wave energy have a higher signal-to-noise ratio and are more focused than those generated from Rayleigh waves; and (3) inversion of Love-wave dispersion curves is less dependent on initial models and more stable than from Rayleigh waves.

Description: Seismic imaging need not be synonymous with or rely on the presence of reflectors in the Earth. Much can be gleaned from the nonreflected wavefield. For example, direct waves map out smooth velocity variations in crosswell seismic tomography, wide-angle refracted waves play a crucial role in full waveform inversion (Hole et al., 2005), and surface waves provide unmatched sensitivity to near-surface shear-wave velocity structure. Guided waves exist in both cracks (Korneev, 2008) and boreholes, the latter referred to as tube waves. The famous Biot slow wave is itself a guided-wave phenomenon akin to a tube wave (Norris, 1987). Surface-wave dispersion has a long history in seismology and was the first seismic characteristic to be subjected to an automated inversion procedure (Dorman and Ewing, 1962).

Description: In several domains of applied geophysics, surface, and guided waves are considered as a source of information for characterizing the near surface, which in a marine environment includes the seabed. By contrast, in exploration seismic surveys, these waves have traditionally been regarded as coherent noise that should be filtered out as soon as possible. The authors consider that surface and guided waves are not noise but a signal that can be lifted from the seismic record and exploited for a variety of well-established geophysical solutions. Surface and guided waves constitute a large part of the recorded energy and with proper acquisition, analysis, and inversion they can be used to characterize the near surface with surprisingly high resolution. In this role, they can provide valuable information for tasks such as perturbation correction—adjustment for near-surface traveltime distortions. They can also be used for velocity and geological modeling. In this article, the authors discuss a workflow for the analysis and joint inversion of surface and guided waves in both land and offshore seismic data.

Description: Multichannel anaylsis of surface waves (MASW) is a seismic surface-wave technique developed specifically for near-surface applications at depths usually shallower than a few tens of meters (Park et al., 1999). Since its introduction in the late 1990s, use of the technique has rapidly increased for two reasons: (1) it provides the shear-wave velocity ( V S ) of ground materials, which is one of the most important geotechnical parameters in civil engineering, and (2) it is easier to use than other common seismic approaches (e.g., refraction, reflection, and surface-wave surveys).

Description: Rayleigh-wave analysis is nowadays a standard tool for retrieving near-surface S-wave velocity models. The method, usually based on the inversion of surface-wave dispersion curves adopting a 1D forward operator, is most often applied to laterally varying sites and often on long and continuous seismic lines. The processing is performed using one of many available wavefield-transform techniques and results in several local dispersion curves estimated along the survey line. The dispersion curves are inverted to provide local S-wave velocity models which are merged to reconstruct 2D/3D structures.

Description: Multichannel analysis of surface waves (Park et al., 1999), commonly called MASW, is a seismic technique used to map the near-surface S-wave velocity structure. It has been applied to a range of geotechnical engineering problems, such as detection of cavities (Miller et al., 1999), the search for bedrock structure (Carnevale et al., 2005), examining water seepage (Ivanov et al., 2006), and monitoring ground improvement (Burke and Schofield, 2008). As a by-product of urban development, industrial and domestic refuge is amassed and deposited in various places, ranging from naturally low ground to abandoned quarries. These places are called landfill sites. As urban development progresses, the landfill sites reach their capacity and become unsuitable for further filling. At that point, a variety of approaches are considered as a means of increasing the capacity. When housing developments creep up to the fill site, more refuse is not welcomed and the land use is reconsidered.

Description: Common-offset gathers (COG) have been usefully employed in the processing of seismic reflection data (Fulton and Darr, 1984). COG methods have also been employed to a lesser extent with the processing of seismic refraction data (Coppens, 1985), but their wider application has yet to be fully exploited. This study describes novel COG adaptations of the generalized reciprocal method (GRM) (Palmer, 1981, 2010b). Initially, the COG GRM methods (Palmer, 2012) were developed as simple and convenient methods for the quality assurance of the large volumes of refraction data, which are characteristic of statics computations for high-fold CMP reflection data processing. However, their application to a variety of sets of data has demonstrated a number of additional unanticipated benefits.

Description: Empirical Green's function (EGF) retrieval and turning ambient noise into useful signal by crosscorrelation or seismic interferometry (Curtis et al., 2006) has been a popular topic in recent years in the seismological community. Many have discussed how the interstation distance or its equivalent affects the accuracy of the Green's function that can be retrieved by crosscorrelation of long-range noise between two stations (e.g., Snieder, 2004; Bensen et al., 2007; Halliday and Curtis, 2008; Tsai, 2009; Kimman and Trampert, 2010). It is generally accepted that the necessary or optimum interstation distance strongly depends on source distribution, length of records (and, hence, is naturally related to source distribution), and the duration of the Green's function to be retrieved. Noise generated by a surface source can be efficiently used to reconstruct the Green's function of surface waves if we focus on the accuracy of the phase, and not be too concerned with the amplitude accuracy of the retrieved Green's function (Halliday and Curtis, 2008; Kimman and Trampert, 2010).

Description: The joint analysis of refractions with surface waves (JARS) method offers an approach for finding solutions to the nonunique inverse refraction problem but, more specifically, to the inverse first-arrival traveltime problem (IFATP) because it includes the direct wave and excludes refractions that are not first arrivals. The inverse refraction problem is well known and clearly established to be nonunique (Slichter, 1932; Healy, 1963; Ackermann et al., 1986; Burger, 1992; Lay and Wallace, 1995). However, it wasn't until Ivanov et al. (2005) examined nonuniqueness from the perspective of solving inverse problems that it became clear that the objective function (the one minimizing the difference between the observed and the modeled data) did not have a global minimum (i.e., a unique solution), or only a few global minima, but a continuous range of minima (i.e., a valley of possible solutions). Insight into the significance of the problem was gained from experiments that maintained a constant number of parameters when solving the inverse problem (Ivanov et al., 2005). Furthermore, these observations were shown to apply even when dealing with a simple (very few parameters) three-layer model.

Description: Distributed acoustic sensing (DAS) is a relatively recent development in the use of fiber-optic cable for measurement of ground motion. Discrete fiber-optic sensors, typically using a Bragg diffraction grating, have been in research and development and field testing for more than 15 years with geophysical applications at least 12 years old (Bostick, 2000, and summary in Keul et al., 2005). However, developments in recent years have sought to remove the need for point sensors by using the fiber cable itself as a sensor (Mestayer et al., 2011; Miller et al., 2012).

Description: This is the last column of my two-year term as the coordinator for Bright Spots. I would like thank you for tracking the column and tracking new developments in G eophysics . I thank the editors for identifying Bright Spot papers, and thank Editor Tamas Nemeth for this enjoyable assignment. You might recall that we featured in the September-October column a paper by Gallardo et al. on integration of multiple geophysical data types (e.g., seismic and EM) using structure-coupled joint inversions. The authors applied a so-called "cross-gradient constraint" in the inversion. Such a constraint favors structural consistencies among the multiple types of model parameters. In the current issue of G eophysics , Lochbühler et al. present another application of structure-coupled inversion. Their twist is to impose a "structural similarity constraint" among different model parameter types for the joint inversion of geophysical (GPR) and hydrological data.

Description: This article summarizes a passive surface-wave method that uses only two sensors and its application to the estimation of deep S-wave velocity structure. Three-dimensional S-wave velocity structure to a depth of several kilometers has a large effect on long-period ground motion in tectonic basins, such as the Los Angeles (LA) Basin. Recent studies of long-period ground motion in the LA Basin (e.g., Hatayama and Kalkan, 2012) show that observed ground motion in some areas cannot be explained by the S-wave velocity models in current use. Most studies of basin velocity structure rely on geologic information, surface and borehole geophysical data, and observed earthquake records to deduce or measure seismic velocities. Geophysical data and seismic stations commonly used for velocity analysis are sparsely distributed and most well data are too shallow to characterize deep S-wave velocity structure. To establish more accurate basin velocity structure, there is a need for more densely distributed deep S-wave velocity data.

Description: With the rapid increase in prospecting for unconventional oil and gas, a large part of which remains on land, the demand for land seismic data processing has increased substantially and is expected to further increase in the future. It is known that land seismic data are often of much poorer quality than marine seismic data. This is to a large extent caused by the presence of unconsolidated rock in the near surface with often complex velocity structure, which is absent in many marine settings. Such near-surface variations cause the wavefield to scatter or even lose its coherence as it propagates. This makes it difficult to accurately image the deeper-lying targets in land seismic data.

Description: Surface-wave interferometry on local scale usually aims at recovering Rayleigh waves. This is because of the predominant use of vertical component geophones in exploration seismology and the fact that Rayleigh waves occur for any given subsurface structure. On the other hand, Love waves are present only in layered media and require horizontal component geophones for their observation. As they depend on shear-wave velocity structure and density only, the analysis of Love waves provides a potentially powerful supplement to Rayleigh wave inversion. Perhaps surprisingly, recent studies show that low-frequency Love waves (0.05–0.1 Hz) excited by the interaction of ocean waves with the ocean floor (the Earth's microseism) can be recovered by interferometry, and that their S/N is high compared to Rayleigh waves (Lin et al., 2008). On a regional scale, Jay et al. (2012) analyzed the ambient noise field in a volcanic region and found that Love waves with frequencies of about 0.3 Hz are observed more clearly than corresponding Rayleigh waves. In this article, we show that Love waves in the frequency band of 1.5 to 5 Hz can be obtained from local noise interferometry, and that they are of comparable S/N as Rayleigh waves. Thus they may also be used to constrain the near-surface structure.

Description: Following the 2012 SEG Annual Meeting in Las Vegas, the SEG Research Committee sponsored a post-convention research workshop on subsea technologies, in general, and on seafloor characterization in particular. The goal of the workshop was to share experiences in acquisition, processing and applications of geotechnical and geophysical measurements for seafloor property characterization. This includes: To help geophysicists in better understanding geotechnical seafloor measurements, e.g., when and how they are collected as well as their actual field applications; To help geotechnical specialists in better understanding geophysical seafloor measurements, how they are derived, and their importance for accurate seismic waveform modeling and inversion; To discuss technology and application trends, and how the geophysical community can participate in the fast-growing market for subsea operations in the oil and gas industry.

Description: Geohazards have a direct impact on the drilling and completion of wells; they present safety risks and are costly. They can be caused by formation properties such as overpressure and can be associated with geological structures such as faults and salt bodies. Critical to drilling success, seismic data provide information used to construct an Earth model consisting of 3D structural depth images showing geological targets or hazards and formation properties relevant to drilling such as pore pressure. However, predrill estimates of structural depth images and formation pressures typically have large uncertainties, which elevate safety concerns and drilling risks and could increase the cost of wells. This risk is especially problematic in challenging environments such as deep water, where rig rates are high and continue to increase.

Description: 3D vertical seismic profile (VSP) data offer far more than just the opportunity to generate images. The 3D VSP geometry offers a rich data set suitable for examining many issues faced by interpreters and processors of geophysical data. In-situ measurements of velocity, anisotropy, Q, multiple generators, stress, and fracture orientation are all possible. This is in addition to the basic products provided by 3D VSP, namely high-resolution images of the subsurface.

Description: A central purpose of this article is to bring an alternative voice, perspective, and understanding to the latest geophysical stampede, technical bubble, and self-proclaimed seismic cure-all, the so-called "full-waveform inversion" or FWI. If you think this is exaggerated, I refer to the advertisement/announcement of the 2013 SEG Workshop on FWI whose opening line is, "Full-waveform inversion has emerged as the final and ultimate solution to the Earth resolution and imaging objective."

Description: Over the last decade or two, digital micro-electro-mechanical system (MEMS) accelerometers have been proposed as a replacement for analog electromagnetic coiled geophones. Although many positive results have been reported in articles and at conferences, other authors have claimed that there are no obvious differences between them. This article compares the analog geophone and MEMS accelerometer and our results show that the MEMS accelerometer has made some improvements on electric specifications. However, these improvements are so slight that identifying the enhanced signals obtained by the MEMS accelerometer is extremely challenging, if not impossible. Whether these improvements can evolve into significant acquisition benefits depends on many other factors involved in a seismic survey and which may contaminate the weak signal and effectively cancel most of the accelerometer's improvement.

Description: 3D VSP has long been viewed as conceptually attractive for illuminating targets under complex overburden, both for exploration purposes and for time-lapse monitoring of reservoirs. However, the widespread use of 3D VSP has been hindered by the cost and risk of deploying geophones in a borehole, and by the limited availability of accessible wells. These hurdles are largely removed when acquiring downhole seismic with a new measurement called distributed acoustic sensing (DAS).

Description: A series of ocean-bottom cable (OBC) surveys has been conducted in the Bohai Sea in China in recent years to overcome difficulties experienced with streamer surveys in shallow water, such as strong currents, missing near offsets, and obstacles. The main challenges in OBC data imaging include steeply dipping structures, serious multiples in the shallow-water environment, large lateral velocity variations, fault shadow effects, and low signal-to-noise ratio (S/N). To obtain optimal images, advanced processing technologies have been developed and applied to OBC data which involve effective PZ summation and shallow-water demultiple, a high-fidelity beam migration in the wide-azimuth domain, and accurate velocity-model building in 3D tilted-transverse-isotropy (TTI) media. The PZ summation and shallow-water demultiple methods aim to effectively eliminate shallow-water ghosts to achieve broadband seismic data. Furthermore, high-fidelity controlled-beam migration (CBM) and TTI velocity-model updates greatly enhance steep dip imaging, improve S/N, and reduce turnaround time. Through the combination of these technologies, OBC data processing provides high-quality images with well-defined steeply dipping structures to reduce exploration risk in the Bohai area.

Description: Geophysics students taking their first seismic data processing course during their freshman year often wonder how we seem to be perfectly happy with single-component data in a three-dimensional world, the single component being either vertical geophones on land or hydrophones in water. Some students ask the question but then are happy to accept an answer that multicomponent is not covered in the freshman course. They quickly get busy learning what material is in the course and will be on the final exam. Deconvolution, NMO, and migration — all applied to single-component data — will determine their grade. Childhood innocence lost! Years later, sometimes at an SEG honorary lecture, it comes back. It is a relief. They always knew that one-component data could not be complete. They are glad that somebody else cares. It is then usually assumed that in a three-dimensional world, three-component data would be complete. Right? No. Seabed systems record four components, not just three. Why do we need four components in a 3D world? If four components are more complete than 3C, then would 5C be more complete than 4C? How about 6C? Where will we stop ... at 7C?

Description: Over the last decade, seismic inversion has acquired the status of being a routine interpretation step during exploration as well as development of fields. For detailed reservoir characterization, prestack inversion is desirable; however, poststack inversion is a reasonably effective tool. The inversion starts with calibration of the well with seismic data. The synthetic seismogram is tied with stacked seismic data on the basic premise that the seismic data (full stack) mimics the zero-offset data and therefore should match with the zero-offset synthetic seismogram. Generally, a good match is obtained. However, during the course of many inversion exercises, it is observed that the synthetic-to-seismic match is not good in parts of the drilled interval. There could be many reasons for this partial mismatch. One of the important reasons is that the well data represent one-dimensional data, whereas the seismic data are stacked over an angle range. In other words, the well data are equivalent to zero-offset data, whereas seismic is not. The poor calibration at the well location leads to lack of confidence in inversion results. The possible solution, such as simultaneous or full-waveform inversion, involves a lot of computation time and hardware requirements. An alternative approach is introduced which uses elastic impedance instead of normal P-impedance for synthetic match and inversion of stacked data. The synthetic seismic generated from elastic impedance shows an improved match with seismic data. The approach is quick, reliable, easy, and not heavy on computational resources. The result is inverted volumes with better hydrocarbon delineation and characterization ability.

Description: Determination of moment and moment magnitude for seismic events located by stacking has been an unsolved problem because current methodologies cannot be applied. To solve this problem, we have developed a methodology for moment magnitude $${M}_{w}$$ estimation of (micro) seismic events. Our method was designed for stacking waveforms. We found that magnitude can be determined from the stack of amplitudes corrected for radiation pattern and propagation effects such as geometric spreading, attenuation, and free-surface boundary. From the stack of corrected waveforms, we found the low-frequency limit corresponding to the stack of particle displacement and calculated seismic moment. We found that an unstacked method of estimating moment magnitude on each trace and averaging these estimates was consistent with the method we developed although it was limited to higher magnitude events and more sensitive to noise. We benchmarked our methodology with a case study and found improvement in $${M}_{w}$$ estimation resulting in determination of magnitude for weaker events when unstacked methodology cannot be used.

Description: Conventional methods of prestack depth imaging aim at producing a structural image that delineates the interfaces of the geologic variations or the reflectivity of the earth. However, it is the underlying impedance and velocity changes that generate this reflectivity that are of more interest for characterizing the reservoir. Indeed, the need to generate a better product for geologic interpretation leads to the subsequent application of traditional seismic-inversion techniques to the reflectivity sections that come from typical depth-imaging processes. The drawback here is that these seismic-inversion techniques use additional information, e.g., from well logs or velocity models, to fill the low frequencies missing in traditional seismic data due to the free-surface ghost in marine acquisition. We found that with the help of broadband acquisition and processing techniques, the bandwidth gap between the depth-imaging world and seismic inversion world is reducing. We outlined a theory that shows how angle-domain common-image gathers produced by an amplitude-preserving reverse time migration can estimate impedance and velocity perturbations. The near-angle stacked image provides the impedance perturbation estimate whereas the far-angle image can be used to estimate the velocity perturbation. In the context of marine acquisition and exploration, our method can, together with a ghost compensation technique, be a useful tool for seismic inversion, and it is also adaptable to a full-waveform inversion framework. We developed synthetic and real data examples to test that the method is reliable and provides additional information for interpreting geologic structures and rock properties.

Description: Frequency-domain methods, which are typically applied to 3D magnetotelluric (MT) modeling, require solving a system of linear equations for every frequency of interest. This is memory and computationally intensive. We developed a finite-difference time-domain algorithm to perform 3D MT modeling in a marine environment in which Maxwell’s equations are solved in a so-called fictitious-wave domain. Boundary conditions are efficiently treated via convolutional perfectly matched layers, for which we evaluated optimized parameter values obtained by testing over a large number of models. In comparison to the typically applied frequency-domain methods, two advantages of the finite-difference time-domain method are (1) that it is an explicit, low-memory method that entirely avoids the solution of systems of linear equations and (2) that it allows the computation of the electromagnetic field unknowns at all frequencies of interest in a single simulation. We derive a design criterion for vertical node spacing in a nonuniform grid using dispersion analysis as a starting point. Modeling results obtained using our finite-difference time-domain algorithm are compared with results obtained using an integral equation method. The agreement was found to be very good. We also discuss a real data inversion example in which MT modeling was done with our algorithm.

Description: We have investigated the value of isotropic seismic converted-wave (i.e., PS) data for reservoir parameter estimation using stochastic approaches based on a floating-grain rock-physics model. We first performed statistical analysis on a simple two-layer model built on actual borehole logs and compared the relative value of PS data versus amplitude-variation-with-offset (AVO) gradient data for estimating the floating-grain fraction. We found that PS data were significantly more informative than AVO gradient data in terms of likelihood functions, and the combination of PS and AVO gradient data together with PP data provided the maximal value for the reservoir parameter estimation. To evaluate the value of PS data under complex situations, we developed a hierarchical Bayesian model to combine seismic PP and PS data and their associated time registration. We extended a model-based Bayesian method developed previously for inverting seismic PP data only, by including PS responses and time registration as additional data and PS traveltime and reflectivity as additional variables. We applied the method to a synthetic six-layer model that closely mimics real field scenarios. We found that PS data provided more information than AVO gradient data for estimating the floating-grain fraction, porosity, net-to-gross, and layer thicknesses when their corresponding priors were weak.

Description: Methods for wavefield injection are used in, for instance, reverse time extrapolation of shot gathers in reverse time migration. For correct injection of recorded data without any ambiguity of the propagation direction, the wavefield-injection methodology requires pressure and particle velocity data such as multicomponent towed marine or seabed seismic recordings. We discovered that by carefully considering the models (medium parameters and boundary conditions) for injection, wavefield injection of multicomponent data can also be used to solve several long-standing challenges in marine seismic data processing by means of conventional time-space-domain finite-difference propagators. We outlined and demonstrated several of these important applications including up-down separation of wavefields (deghosting), direct-wave removal, source-signature estimation, multiple removal, and imaging using primaries and multiples. Only acoustic models are considered, but the concepts are straightforward to generalize to elastodynamic and electromagnetic models.

Description: Understanding the effects of induced-polarization (IP) effects on time-domain electromagnetic data requires the ability to simulate common survey techniques when taking chargeability into account. Most existing techniques preform this modeling in the frequency domain prior to transforming their results to the time domain. Even though this technique can allow for chargeable material to be easily incorporated, its application for some problems can be computationally limiting. We developed a new technique for forward modeling the time-domain electromagnetic response of chargeable materials in three dimensions. The frequency dependence of Ohms’ law translates to an ordinary differential equation when considered in the time domain. The system of ordinary-partial differential equations was then discretized using an implicit time-stepping algorithm, that yielded absolute stability. This approach allowed us to operate directly in the time domain and avoid frequency to time-domain transformations. Although this approach can be applied directly to materials exhibiting Debye dispersions, other Cole-Cole dispersions resulted in fractional derivatives in time. To overcome this difficulty, Padé approximations were used to represent the frequency dependence as a rational series of integer order terms. The resulting method was then simplified to generate a reduced time-domain model that can be used to forward model the IP decay curves in the absence of any electromagnetic coupling. We found numerical examples in which the method produced accurate results. The potential application of the method was demonstrated by modeling the full time-domain electromagnetic response of a gradient array IP survey, and the occurrence of negative transients in airborne time-domain electromagnetic data.

Description: We developed a novel parallel domain-decomposition approach for 3D large-scale electromagnetic induction modeling in the earth. We used the edge-based finite-element method and unstructured meshes. Unstructured meshes were divided into sets of nonoverlapping subdomains. We used the curl-curl electric field equation to carry out the analysis. In each subdomain, the electric field was discretized by first-order vector shape functions along the edges of tetrahedral elements. The tangential components of the magnetic field on the interfaces of the subdomains were defined as a set of Lagrange multipliers. The unknown Lagrange multipliers were solved from an interface problem defined on the interfaces of the subdomains. With the availability of the Lagrange multipliers, the electric field values in each subdomain were solved independently. Three synthetic examples were evaluated to verify our code. Excellent agreement with previously published solutions was obtained. Synthetic examples revealed that our domain decomposition technique is scalable with respect to the number of subdomains and robust with regard to frequency and the heterogeneous distribution of material parameters, i.e., electric conductivity, electric permittivity, and magnetic permeability.

Description: Strong subsurface attenuation leads to distortion of amplitudes and phases of seismic waves propagating inside the earth. Conventional acoustic reverse time migration (RTM) and least-squares reverse time migration (LSRTM) do not account for this distortion, which can lead to defocusing of migration images in highly attenuative geologic environments. To correct for this distortion, we used a linearized inversion method, denoted as $${Q}_{p}$$ -LSRTM. During the least-squares iterations, we used a linearized viscoacoustic modeling operator for forward modeling. The adjoint equations were derived using the adjoint-state method for back propagating the residual wavefields. The merit of this approach compared with conventional RTM and LSRTM was that $${Q}_{p}$$ -LSRTM compensated for the amplitude loss due to attenuation and could produce images with better balanced amplitudes and more resolution below highly attenuative layers. Numerical tests on synthetic and field data illustrated the advantages of $${Q}_{p}$$ -LSRTM over RTM and LSRTM when the recorded data had strong attenuation effects. Similar to standard LSRTM, the sensitivity tests for background velocity and $${Q}_{p}$$ errors revealed that the liability of this method is the requirement for smooth and accurate migration velocity and attenuation models.

Description: In the covered karst of west-central Florida, USA, sinkholes (sandy collapse conduits) provide locally concentrated recharge to underlying aquifers. For water management, it would be beneficial to understand the rates at which collapse conduits recharge an underlying aquifer. Self-potential (SP) monitoring has promise as a noninvasive, relatively low-cost method for assessing temporal variability in flow. Previous studies suggested that negative SP anomalies over collapse conduits correspond to downward-flowing groundwater; however, before SP surveys can be reliable indicators of conduit flow, SP from ET, soil conductivity changes, redox, electrode effects, and cultural noise must be better understood. A year of continuous SP monitoring with two grids of approximately 15 Pb/PbCl/KCl nonpolarizing electrodes each was combined with high-resolution ground-penetrating radar surveys and intermittent water table monitoring over two small covered-karst conduits in Tampa, Florida, USA. Although variations in SP resulting from changes in cultural noise, soil conductivity, ET, redox, and rainfall were evident, separate and unrelated positive and negative SP anomalies episodically manifested over conduits, which suggested that conduit flow could be dynamic, not static. Three flow regimes in conduits were postulated: conduit permeability higher than in surrounding surficial sediments, conduit permeability lower than in surrounding surface sediments, conduit permeability matched surrounding surface sediments. Numerical steady-state 2D simulations in Comsol created the three postulated flow regimes and revealed that a different SP polarity could result from different rates of flow: positive SP corresponded to higher permeability conduits, negative SP corresponded to lower permeability conduits, no or minimal SP appeared when conduits and surrounding sediments had equal permeability. In these models, downward flow was not responsible for generating negative SP. To assess the hydrology of a conduit, it appears that SP should be monitored continuously. Further monitoring of field sites with hydrologic sensors is needed.

Description: A classical split perfectly matched layer (PML) method has recently been applied to the scalar arbitrarily wide-angle wave equation (AWWE) in terms of displacement. However, the classical split PML obviously increases computational cost and cannot efficiently absorb waves propagating into the absorbing layer at grazing incidence. Our goal was to improve the computational efficiency of AWWE and to enhance the suppression of edge reflections by applying a convolutional PML (CPML). We reformulated the original AWWE as a first-order formulation and incorporated the CPML with a general complex frequency shifted stretching operator into the renewed formulation. A staggered-grid finite-difference (FD) method was adopted to discretize the first-order equation system. For wavefield depth continuation, the first-order AWWE with the CPML saved memory compared with the original second-order AWWE with the conventional split PML. With the help of numerical examples, we verified the correctness of the staggered-grid FD method and concluded that the CPML can efficiently absorb evanescent and propagating waves.

Description: We have developed an efficient numerical scheme for fast multimodel 3D electromagnetic simulations by applying a Schur complement approach to a frequency-domain finite-difference method. The scheme is based on direct solvers and developed with constrained inversion algorithms in view. Such algorithms normally need many forward modeling jobs with different resistivities for the target zone and/or background formation. We geometrically divide the computational domain into two subdomains: an anomalous subdomain, the resistivities of which were permitted to change, and a background subdomain, having fixed resistivities. The system matrix is partially factorized by precomputing a Schur complement to eliminate unknowns associated with the background subdomain. The Schur complement system is then solved to compute fields inside the anomalous subdomain. Finally, the background subdomain fields are computed using inexpensive local substitutions. For each successive simulation, only the relatively small Schur complement system has to be solved, which results in significant computational savings. We applied this approach to two moderately sized 3D problems in marine controlled-source electromagnetic modeling: (1) a deepwater model in which the resistivities of the seawater and the air layer were kept fixed and (2) a model in which focused inversion was performed in a scenario in which the resistivities of the background formation, the air layer, and the seawater were known. We found a significant reduction of the modeling time in inversion that depended on the relative sizes of the constrained and unconstrained volumes: the smaller the unconstrained volume, the larger the savings. Specifically, for a focused inversion of the Troll oil field in the North Sea, the gain amounted up to 80% of the total modeling time.

Description: We have developed a new and simple method for deghosting of conventional hydrophone streamer data towed at arbitrary variable depths. The method uses a time-space domain finite-difference (FD) solution to the wave equation with pressure field boundary conditions to predict and remove ghosts. Because it operates in the time domain, our method is unaffected by any number of notches in the frequency spectrum of the data and therefore will deghost "through notches." Apart from the acquired hydrophone data, the method relies on the depth profile of the streamer recording the data beneath a sea surface with a known reflection coefficient as well as the propagation velocity in the water above the streamer. The method was applied to simple and more complex synthetic data, which illustrated its ability to deal with complex data and any acquisition geometry.

Description: Surface nuclear magnetic resonance (NMR) is a geophysical technique that provides the ability to noninvasively image water content in the subsurface. To improve the ability of this method to produce images representative of the true subsurface structure, we require high spatial resolution. We derive a method to provide improved spatial resolution through the use of novel excitation strategies designed to enhance and exploit the information content within the quadrature component of the NMR signal. In a traditional surface NMR experiment, the frequency of the perturbing magnetic field ( $${\mathbf{\boldsymbol{B}}}_{\mathrm{1}}$$ ) is chosen to equal the Larmor frequency of the hydrogen nuclei in the subsurface. In this case, it is assumed that the signal phase is determined entirely by the conductivity structure of the subsurface. Several studies have found that modeling the signal phase accurately and inverting a complex-valued NMR signal, can improve the spatial resolution of the surface NMR water content images. We propose alternative excitation schemes designed to generate a complex-valued signal, where the quadrature component can be controlled experimentally and was larger than that generated by the conductivity effects. This allowed a single excitation to provide two samplings of the subsurface properties, one stored in the real component and another in the quadrature component. To test if the alternative sampling strategies can provide improved spatial resolution in surface NMR, we evaluated a synthetic study contrasting the performance of three techniques. We contrasted two techniques designed to generate a complex-valued NMR signal during excitation, called off-resonance excitation and composite pulse excitation , to a traditional on-resonance excitation. We demonstrated that our proposed excitation schemes were able to better resolve boundaries between layers with contrasting properties, and we produced images with improved spatial resolution.

Description: Laboratory physical modeling and laser-based experiments are frequently proposed to tackle theoretical and methodological issues related to seismic prospecting, e.g., when experimental validations of processing or inversion techniques are required. Lasers are mainly used to simulate typical field acquisition setups on homogeneous and consolidated materials assembled into laboratory-scale physical models (PMs) of various earth structures. We suggested the use of granular materials to study seismic-wave propagation in unconsolidated and porous media and target near-surface exploration and hydrogeologic applications. We designed and tested the reproducibility of an experimental procedure to build and probe PMs consisting of micrometric glass beads (GBs). A mechanical source and a laser-Doppler vibrometer were used to record small-scale seismic lines at the surface of three GBs models. When guided surface acoustic mode theory should prevail in such unconsolidated granular packed structure under gravity, we only considered elastic-wave propagation in stratified media to interpret recorded data. Thanks to basic seismic processing and inversion methods (first arrivals and dispersion analyses), we were able to correctly retrieve the gradients of pressure- and shear-wave velocities in our models. A 3D elastic finite difference simulation of the experiment offered, despite significant differences in terms of amplitudes, a supplementary validation of our approximation, as far as elastic properties of the medium were concerned.

Description: The abundance and growth stages of bacteria in subsurface porous media affect the concentrations and distributions of charged species within the solid-solution interfaces. Therefore, spectral induced polarization (SIP) measurements can be used to monitor changes in bacterial biomass and growth stage. Our goal was to gain a better understanding of the SIP response of bacteria present in a porous material. Bacterial cell surfaces possess an electric double layer and therefore become polarized in an electric field. We performed SIP measurements over the frequency range of 0.1–1 kHz on cell suspensions alone and cell suspensions mixed with sand at four pore water conductivities. We used Zymomonas mobilis at four different cell densities (including the background). The quadrature conductivity spectra exhibited two peaks, one around 0.05–0.10 Hz and the other around 1–10 Hz. Because SIP measurements on bacterial suspensions are typically made at frequencies greater than 1 Hz, these peaks have not been previously reported. In the bacterial suspensions in growth medium, the quadrature conductivity at peak I was linearly proportional to the density of the bacteria. For the case of the suspensions mixed with sands, we observed that peak II presented a smaller increase in the quadrature conductivity with the cell density. A comparison of the experiments with and without sand grains illustrated the effect of the porous medium on the overall quadrature conductivity response (decrease in the amplitude and shift of the peaks to the lower frequencies). Our results indicate that for a given porous medium, time-lapse SIP has potential for monitoring changes in bacterial abundance within porous media.

Description: The problem of predicting the change in seismic velocities (P-wave and S-wave) upon the change in pore-fill material properties is commonly known as substitution . For isotropic rocks, P- and S-wave velocities are fundamentally linked to the effective P-wave and shear moduli. The change in the S-wave velocity or shear modulus upon fluid substitution can be predicted with Gassmann’s equations starting only with the initial S-wave velocity. However, predicting changes in P-wave velocity or the P-wave modulus requires knowledge of the initial P- and S-wave velocities. We initiated a rigorous derivation of the P-wave modulus for fluid and solid substitution in monomineralic isotropic rocks for cases in which an estimate of the S-wave velocity or shear modulus is not available. For the general case of solid substitution, the exact equation for the P-wave modulus depends on parameters that are usually unknown. However, for fluid substitution, fewer parameters are required. As Poisson’s ratio increases for the mineral in the rock frame, the dependence of exact substitution on these unknown parameters decreases. As a result, in the absence of shear velocity, P-wave modulus fluid substitution can, for example, be performed with higher confidence for rocks with a calcite or dolomite frame than it can for rocks with quartz frame. We evaluated a recipe for applying the new P-wave modulus fluid substitution. This improves on existing work and is recommended for practice.

Description: Relationships between the compaction state and effective stresses are the basis for most quantitative pore-pressure and stress estimates. Common practice uses only a single element of the stress tensor, the vertical stress, for these calculations; mean stress formulations also exist, although they are less widely applied. Using simple models and field data from two distinct stress regimes, we examined the validity and limitations of the vertical-stress approach as well as a mean-stress approach, showing that in complex stress settings, both can perform very poorly. We evaluated a method for incorporating shear stresses into compaction relations by using state boundary surface (SBS) formulations from soil mechanics and demonstrated how the resulting model may be calibrated and applied to field data. This approach was found to perform much better in the complex stress environment, providing more stable calibration behavior and more reliably extrapolating to stress states beyond those present in the calibration data. Although vertical and mean stress compaction models may work well in simple stress environments, we discovered that incorporation of shear stress is necessary for models in complex stress settings. Although the addition of shear stress significantly improves agreement with field data, it also increases the complexity of the model as well as the requirements for calibration data. We therefore evaluated the settings in which each of these three approaches — vertical stress, mean stress, and SBS — may be most appropriate.

Description: The key computational kernel of most advanced 3D seismic imaging and inversion algorithms used in exploration seismology involves calculating solutions of the 3D acoustic wave equation, most commonly with a finite-difference time-domain (FDTD) methodology. Although well suited for regularly sampled rectilinear computational domains, FDTD methods seemingly have limited applicability in scenarios involving irregular 3D domain boundary surfaces and mesh interiors best described by non-Cartesian geometry (e.g., surface topography). Using coordinate mapping relationships and differential geometry, an FDTD approach can be developed for generating solutions to the 3D acoustic wave equation that is applicable to generalized 3D coordinate systems and (quadrilateral-faced hexahedral) structured meshes. The developed numerical implementation is similar to the established Cartesian approaches, save for a necessary introduction of weighted first- and mixed second-order partial-derivative operators that account for spatially varying geometry. The approach was validated on three different types of computational meshes: (1) an "internal boundary" mesh conforming to a dipping water bottom layer, (2) analytic "semiorthogonal cylindrical" coordinates, and (3) analytic semiorthogonal and numerically specified "topographic" coordinate meshes. Impulse response tests and numerical analysis demonstrated the viability of the approach for kernel computations for 3D seismic imaging and inversion experiments for non-Cartesian geometry scenarios.

Description: The frequency-domain electromagnetic response of a confined conductor buried in a resistive host has received much attention, particularly in the context of mineral exploration. In contrast, the problem of the electromagnetic response of a confined resistor buried in a conductive host has been less thoroughly studied. However, resistive targets are important in geotechnical and hydrologic studies, archaeological prospecting, and, more recently, offshore hydrocarbon exploration. I analytically address the problem of the electromagnetic response of a completely resistive cylindrical cavity buried in a conductive host in the presence of a simplified 2D electric dipole source. In contrast to the confined conductor, which channels and induces current systems, the confined resistor deflects current and produces additional eddy current systems in the conductive host. I apply this theory to model the response of a grounded electric dipole-dipole system operating over a range of frequencies from 0 Hz to 10 kHz, in the presence of a horizontal 5-m radius insulating cylinder located 1-m beneath the surface of a uniform earth. This represents a common hazard encountered during mining and civil engineering operations. Results show that such an insulating cavity increases the recorded electric field amplitude and phase delay at all transmitted frequencies. These observations suggest that a broadband electromagnetic prospecting system may provide additional information about the location and extent of a void, over and above a standard dipole-dipole resistivity survey. When the host skin depth is much larger than all other length scales, the response can be approximated by an equivalent single dipole unless the cylinder’s radius is much larger than its distance from the transmitter. This result provids a useful rule of thumb to determine the acceptable range over which a resistive target can be modeled by a distribution of dipoles.

Description: Single-well imaging has been a technique increasingly used in the detection of near-borehole geologic structures. The azimuth of a geologic structure, however, cannot be uniquely determined with acoustic signals recorded in the borehole alone, due to the azimuth ambiguity existing in current imaging techniques. We eliminated such ambiguity by revealing the relevant acoustic principle underlying the P-wave reflection behavior. When a P-wave excited by a transducer in the logging tool impinges upon a planar interface, the P-wave reflection coefficient (RC) of the displacement is opposite in sign to that of the normal stress or fluid pressure, regardless of the incident angle and the parameters of the media on the two sides. The derived relation about signs of RCs was validated by finite-difference solutions for reflected waves from a near-borehole plane fault. With this newly discovered reflection principle, one can eliminate the azimuth ambiguity of any interface outside a borehole by checking if the waveforms of pressure and the displacement component are both changed in polarity after reflection. Furthermore, because the pressure and displacement are observable quantities and the waveform of the acoustic source is known in acoustic logging, it is convenient to implement the data acquisition for this technique, which is a major advantage over other techniques. We expounded and exemplified our new technique by numerical simulation.

Description: P-wave amplitude anomalies below reservoir zones can be used as hydrocarbon markers. Some of those anomalies are considerably delayed relatively to the reflections from the reservoir zone. High P-wave attenuation and velocity dispersion of the observed P-waves cannot justify such delays. The hypothesis that these amplitude anomalies are caused by wave propagation through a layered permeable gaseous reservoir is evaluated. The wave propagation through highly interbedded reservoirs suggest an anomalous amount of mode conversions between fast and slow P-waves. The converted P-waves, which propagated even a short distance as slow P-waves, should be significantly delayed and attenuated comparatively, with the fast P-wave reflections. The amplitudes and arrival time variations of conventional and converted P-wave reflections at low seismic frequencies were evaluated by means of an asymptotic analysis. The calculations confirmed that the amplitude anomalies due to converted P-waves are noticeably delayed in time relatively to fast P-wave reflections. However, the amplitudes of the modeled converted P-waves were much lower than the amplitude anomalies observed from exploration cases.

Description: Africa has long been a place for successful mineral and oil and gas exploration; however, the diversity of geology, technical innovation, and opportunity make it a unique place. A single special section cannot hope to capture the breadth of the geophysical activity going on today somewhere on the continent. But we hope that with this issue we at least make a dent in the subject by offering a good variety of papers from both hard rock and hydrocarbon exploration.

Description: Characterization and quantification of physical properties and contents of geomaterials and rocks is the subject of various conventional and nonconventional analyses.

Description: Data acquisition projects that cost significant sums of money should always be justified based on derived benefits. Time-lapse seismic data requirements need to be driven by the impact of the new data upon the field devel-opment plan (FDP) and integrated reservoir management. This article discusses the business impact of time-lapse seismic data using both a lookahead and a lookback value of information (VOI) analysis in an offshore oil field in Nigeria.

Description: Over the past 25 years, airborne geophysics has become an effective technology for mapping various natural resources targets and signatures on the Earth's surface and subsurface.

Description: This paper examines how the understanding of the prospectivity of the Liberia-Sierra Leone Basin has developed over the last 40 years largely due to improvements in seismic data over this period. A series of basins developed between major transform fault zones associated with the opening of the South Atlantic Ocean from the early Cretaceous onward. The Liberia-Sierra Leone Basin forms part of the West African Transform Margin that extends from Sierra Leone to Benin. Early hydrocarbon exploration (1972-1985) was targeted at shelfal structural synrift traps of Lower Cretaceous age. None of the wells drilled in this period was regarded as a discovery. In 2001, a new regional 2D seismic survey allowed a new play, deepwater Upper Cretaceous channel fans, to be identified. More recent 3D seismic using the latest data acquisition and processing technologies, including AVO, has enabled drillable prospects to be identified. Wells drilled in 2009-2010 offshore Sierra Leone have established that a working hydrocarbon system exists in this basin.

Description: Mineral deposits occur in different geological envi-ronments than oil and gas deposits, and are usually at very shallow depths (from surface outcrop, to hundreds of meters). Because of internal heterogeneities within most deposit types, extensive use of borehole sampling is required to satisfy resource and reserve reporting requirements. While borehole sampling can be designed to satisfy the geostatistical variability of a deposit, it is usually insufficient to adequately sample the structural continuity of deposits to a level required for accurate mine planning and production scheduling. For this reason, 2D and 3D seismic surveys are increasingly used in conjunction with borehole drilling and other geophysical methods such as aeromagnetic surveys to map the geological structure of known deposits. This paper will examine some challenges and results from Anglo American's use of seismic surveying in African coal and platinum deposits.

Description: Reservoir geometry and internal architecture in the Niger Delta can vary over short distances with rapid lateral and vertical changes in lithology and porosity. Understanding such variations is critical to designing an optimum development strategy for prospects in this basin. It was in order to fully understand the variations in reservoir facies and internal architecture over Okari oil field in the Niger Delta that this study was undertaken. Conventional seismic interpretation, attribute analyses, and subsequent drilling had located a stack of reservoirs in a rollover anticline. To unravel the paleo-stratigraphy of the field and fully populate the field with log properties, we used a multilayered feed-forward neural network (MLFN) to predict shale volume and porosity from seismic and well-log data sets. Earlier, rock physics analyses had been undertaken to understand litho-fluid facies associations in the field and assist in further quantitative interpretation and calibration of neural-network predictions of res-ervoir property distribution.

Description: High-pressure sediments in which oil and gas are generated and accumulate in traps are proven in many operating areas along the entire West African margin. Although classic areas for high pressure are found in Tertiary deltas, such as the Niger Delta, other types of basins in which young clastic sediments have accumulated also create the environment for high pressure and drilling challenges. The BP Macondo oil spill in the Gulf of Mexico has highlighted the technical challenge of drilling for oil in deep water, and the high pressures there added complexity to the control incident and the high volumes of fluids which blew out to the seabed.

Description: The Bushveld Complex consists of three roughly coeval components, namely the Lebowa Granite Suite, the Rashoop Granophyre, and the Rustenburrg Layered Suite (RLS), the latter being the only economically important unit. The RLS underlies an area of [~]65,000 km2 in the northeast sector of South Africa, and is the oldest and largest mafic-layered complex in the world (Figure 1). It is also the largest repository of magmatic ore deposits and hosts, inter alia, 80% of the world's platinum-group-metal (PGM) resources and most of the chromitite resources. Over an east-west distance of roughly 350 km, the RLS outcrops as three apparently discrete, crescent-shaped lobes (the Western, Eastern and Northern limbs) and lies below cover rocks along the southeastern limb. The far Western Limb is currently not of economic significance. Lithologies range across mafic and ultramafic cumulates plus anorthosites. Pseudo-stratigraphic layering exhibits dips of 8-20{degrees} toward the center of the complex. The UG2 and Merensky "reefs" constitute the major PGM-bearing horizons of mining interest and fall close to the top of the 1500-m thick Critical Zone, an assemblage of alternating leuconorite, anorthosite, and pyroxene rocks outcropping midway through the RLS succession.

Description: A major feature of the habitat of petroleum in the Niger Delta Basin is the association of petroleum traps with growth faults. Because of the significant role of these faults in hydrocarbon accumulation and redistribution in the basin, a good understanding of the timing of fault motion has now been shown to be vital for successful exploration of fault-bounded prospects. In most petroleum habitats, structural elements such as fault patterns, their kinematics, geometry, timing, and size of the structures control the distribution of hydrocarbons in adjacent fault blocks. The success or otherwise of an exploration well in such areas depends on the location of such a well relative to the structural closure interpreted from the seismic data. Experience has shown that detailed structural analysis of prospective fields can provide a reliable kinematic and growth history upon which risks associated with fault movement, trap integrity and structure, geometry/size modification can be evaluated before deciding on the drilling location.

Description: Unconventional resources such as shale gas are becoming increasingly important exploration and production targets. To understand the geophysical responses of shale-gas plays, we use a rock physics relationship, which is constrained with geology and formation-evaluation analysis, to calculate effective properties such as impedance and VP/VS. Numerical studies suggest that in-situ rock para-meters such as mineral composition (e.g., clay, quartz, and calcite) and TOC, as well as the interaction among them, can significantly influence the geophysical responses of the organic-rich rocks, thus providing the basis for the geophysical characterization of shale-gas plays.

Description: Neither introduction nor motivation are required for this special section of The Leading Edge dedicated to the SEG Foundation's program Geoscientists Without Borders (GWB). Natural disasters such as earthquakes, tsunamis, volcanic eruptions, killer storms, landslides, and other natural disasters occur all too frequently. People across the planet struggle to provide themselves the basic necessities of life such as water, food, and energy. While we, as geophysicists, cannot control nature or eliminate the human condition, it is possible for us to play a role in mitigating these problems and improving human lives. We can all contribute as individuals, but the goal of GWB is larger: to effect long-term and sustainable aid to people in need through technology transfer and training involving the next generation of geoscientists.

Description: Kingston, Jamaica, the capital of the Caribbean island nation of Jamaica, is prone to infrequent but devastating earthquakes and tsunamis, yet the locations of the faults responsible for generating these geohazards are poorly known. The city rests precariously at the western terminus of the Enriquillo Plantain Garden Fault (EPGF)--the same fault that ruptured during the 12 January 2010 Haiti earthquake, destroying Port-au-Prince and killing about 250,000 people (Figure 1 inset). Like Haiti, Jamaica has experienced a significant earthquake every few hundred years; however, the exact frequency and location of large earthquakes across Jamaica remain unclear. In the past 300 years, Jamaica has experienced at least two earthquakes (in 1692 and 1907) comparable to the 2010 Haiti earthquake and, like Haiti, these earthquakes caused significant loss of life, triggered tsunamis, slope failure, and caused widespread ground liquefaction (e.g., Sloane, 1694; Tabor, 1920). The 1907 earthquake killed [~]1000 people in Kingston. The 1692 earthquake completely destroyed Port Royal, a city then notorious as a haven for privateers and as the Western Hemisphere's center for slaving operations.

Description: Hydraulic fracturing is an important tool that helps extract fluids from the subsurface. It is critical in applications ranging from enhanced oil recovery to geothermal energy production. As the goal of fracturing is to increase flow rates within the reservoir volume, and because the reservoir is typically heterogeneous, several fractures are often created. Because of confining stresses, most fractures that have been created and remain open are nearly vertical (Zoback et al., 2003). Creating a set of almost parallel fractures is quite common in situations with smoothly varying stress (Figure 1).

Description: In time-lapse controlled-source electromagnetics, it is crucial that the source and the receivers are positioned at exactly the same location at all times of measurement. We use interferometry by multidimensional deconvolution (MDD) to overcome problems in repeatability of the source location. Interferometry by MDD redatums the source to a receiver location and replaces the medium above the receivers with a homogeneous half-space. In this way, changes in the source position and changes of the conductivity in the water-layer become irrelevant. The only remaining critical parameter to ensure a good repeatability of a controlled-source electro-magnetic measurement is the receiver position.

Description: By demonstrating offshore ambient-noise surface-wave tomography (ANSWT) at reservoir scale, we add a method to the commercially usable geophysical methods. Analysis of ambient-noise records at 126 locations above a hydrocarbon reservoir offshore Norway proves that the marine environment provides good conditions for 3D estimation of shear-wave velocities at frequencies above 0.1 Hz. The presented results are used to discuss potential application areas.

Description: Seismic interferometry based on cross-correlation of traces recorded at different positions makes it possible to recover new Green's functions and to obtain signals as if sources were placed at the receiver positions. This case history describes results obtained using sea-bottom hydrophone signals recorded in shallow water below the ice plate in an Alaska seismic survey. The aim was to simulate new shots at the sea-bottom receiver positions and to redatum the surface-generated seismic signals below the ice layer. The application was performed and the method investigated to attenuate the strong noise due to dispersive wave modes (mainly flexural waves) which propagate in the ice layer.

Description: Refraction surveys are a well-established method of imaging subsurface velocities, both in terms of the deep crustal structure at global scales and in the shallow near surface. These surveys generally involve deploying an array of receivers on the surface (or water bottom) and recording arrivals from a seismic source initiated at or near the surface.

Description: Crossplots are commonly used in the geosciences to gain qualitative insight about relationships between different variables, typically three (for two-dimensional colored crossplots). On rare occasions, the relationships among four variables are explored by using three-dimensional colored crossplots. The variable used to color the crossplot is usually related to the property of interest, sand or pay for instance. In these cases, crossplots can be used in a quantitative sense by selecting (drawing) a region in the crossplot where most of the property of interest "lives." Drawing a polygon in a 2D crossplot to separate "good" from "bad" areas is the extension to 2D of simple cutoffs commonly applied to 1D well-log data to separate scenarios of interest. One drawback of this approach is that it works best only when there is no overlap between the region occupied by the property of interest and the region occupied by the background. Another drawback is that it is difficult to extend to three-dimensional crossplots and impossible to apply for dimensions higher than three.

Description: Generally, seismic interferometry refers to a method of extracting the wavefield responses between two receivers as if we had replaced one of those receivers by a source (van Manen et al., 2005; Wapenaar and Fokkema, 2006). This is done by a process of cross-correlation and summation of wavefields observed at those receivers. These recorded wavefields can be excited by active sources (earthquakes, dynamite, air guns, vibroseis, and others) or passive sources (oceanic microseisms, traffic on roads, industrial machinery, and others). When applying seismic interferometry using surface sources and surface receivers, the recovered inter-receiver response is dominated by surface waves. This phenomenon is observed in both global and regional seismology where passive seismic energy is used, and in exploration seismology, where active sources are used.

Description: If some ten years ago, one approached a highly skilled geophysicist randomly picked from our community and asked "How can you turn signals recorded at two receivers into what would be recorded if one acts as a source?", or "How can we use random noise from unknown sources for imaging?", or "How do we turn an earthquake into a buried virtual receiver?", the answer most likely would lie somewhere between "Uh, I don't know," to "What?! You can't do that!" However, in recent years these questions as well as others (possibly even stranger) have been answered due to a boom of creative scientific work in interferometry.

Description: The conditions under which seismic interferometry (SI) leads to the exact Green's function (GF) are rarely met in practice. As a result, we generally recover only estimates of the true GF. This raises the questions: How good an approximation to the GF can SI give? Can we improve this estimated GF?

Description: Welcome new friend or old friend to another edition of Bright Spots, your friendly GEOPHYSICS spotlighter. The spotlighted articles in this month's column are recommendations from the editorial staff of GEOPHYSICS. They spotlighted 10 (2 letters, 1 tutorial, and 7 technical papers) of the 59 articles in the May-June issue of GEOPHYSICS. I hope my short summaries of their choices motivate you to check out these articles or, perhaps, other GEOPHYSICS articles equally worthy, but not on the radar. As always, GEOPHYSICS is worth your effort.

Description: Major oil companies generally hold a large investment portfolio, including several development projects with some worth hundreds of millions of US dollars. It is the geophysicist's task to assist in gathering the information required to optimize field development, including the positioning of infill drilling and determining the best oil recovery strategy. The final goal is to achieve an improved recovery factor at a reduced cost of drilling.

Description: Relative sea level rise (RSLR) has driven large increases in annual water level exceedances (duration and frequency) above minor (nuisance level) coastal flooding elevation thresholds established by the National Weather Service (NWS) at U.S. tide gauges over the last half-century. For threshold levels below 0.5 m above high tide, the rates of annual exceedances are accelerating along the U.S. East and Gulf Coasts, primarily from evolution of tidal water level distributions to higher elevations impinging on the flood threshold. These accelerations are quantified in terms of the local RSLR rate and tidal range through multiple regression analysis. Along the U.S. West Coast, annual exceedance rates are linearly increasing, complicated by sharp punctuations in RSLR anomalies during El Niño Southern Oscillation (ENSO) phases, and we account for annual exceedance variability along the U.S. West and East Coasts from ENSO forcing. Projections of annual exceedances above local NWS nuisance levels at U.S. tide gauges are estimated by shifting probability estimates of daily maximum water levels over a contemporary 5-year period following probabilistic RSLR projections of Kopp et al. (2014) for representative concentration pathways (RCP) 2.6, 4.5, and 8.5. We suggest a tipping point for coastal inundation (30 days/per year with a threshold exceedance) based on the evolution of exceedance probabilities. Under forcing associated with the local-median projections of RSLR, the majority of locations surpass the tipping point over the next several decades regardless of specific RCP.

Description: We have combined modeling and gravity inversion to estimate an overall smooth but locally discontinuous basement relief of a sedimentary basin. Through an initial global smoothness solution, the method indicates fault positions that may be accepted or modified. Then, the interpreter specifies the fault dip and a tentative fault slip, so the fault plane is fixed on the estimated relief. The segments of the current interpreted relief that are not accepted faults are estimated through smoothness inversion, and the response of the current estimated relief is computed. The slip of the fault being incorporated in the estimated relief is then modified interactively until the solution presents no side lobes about the fault extremes. The process is repeated for all faults. The iteration stops when the tentative solution becomes compatible with the geologic knowledge of the area and produces an acceptable data fit. The method was applied to data produced by a simulated graben defined by step faults dipping 60º and by inclined and arcuate terraces. The solution virtually coincided with the true source. The method’s ability in testing geologic hypotheses was demonstrated in gravity profiles across the Büyük Menderes Valley in west Turkey and the San Jacinto graben in southern California.

Description: The Morro do Leme laterite nickel deposit lies inside the western border of the Parecis Basin (Brazil). This deposit is characterized by high concentrations of lateritic Ni (about 1.8%) and anomalous contents of Pd, Au, Cu, Na, Co, Zn, and Pt in a peridotite and dunite layered intrusion. Besides the existence of geochemical and drilling data, the 3D distribution in the subsurface of this layered intrusion is still unknown. An airborne magnetic survey revealed three east–west elongated magnetic anomalies, characterized by a significant remanent magnetization. The sources of these anomalies were delimitated laterally and had their depths estimated between 90 and 150 m, using techniques that use derivatives. Further, the total magnetization direction was obtained from a distortion analysis of the magnetic anomalies. All these data were united in an initial model for the 3D inversion of the magnetic data. The total and induced magnetization directions were attributed to the inverted model of 0.12 (SI) susceptibility, allowing indirect estimation of the remanence. The model, defined by the depth, the inversion, and the remanence estimates, linked the intrusion to analogue events in the Rondonian-San Ignácio Province. The results indicated that to explore for laterite Ni, the best locations are the southern part of the main anomaly and in the cover above the two smaller anomalies, whereas to explore for Pd, Au, Cu, Na, Co, Zn, and/or Pt, the indicated region is the central portion of the main anomaly.

Description: With higher capacity recording systems, long-offset surveys are becoming common in seismic exploration plays. Long offsets provide leverage against multiples, have greater sensitivity to anisotropy, and are key to accurate inversion for shear impedance and density. There are two main issues associated with preserving the data fidelity contained in the large offsets: (1) nonhyperbolic velocity analysis and (2) mitigating the migration/NMO stretch. Current nonhyperbolic velocity analysis workflows first estimate moveout velocity $${V}_{\mathrm{nmo}}$$ based on the offset-limited gathers, then pick an effective anellipticity $${\eta }_{\mathrm{eff}}$$ using the full-offset gathers. Unfortunately, estimating $${V}_{\mathrm{nmo}}$$ at small aperture may be inaccurate, with picking errors in $${V}_{\mathrm{nmo}}$$ introducing errors in the subsequent analysis of effective anellipticity. We have developed an automated algorithm to simultaneously estimate the nonhyperbolic parameters. Instead of directly seeking an effective stacking model, the algorithm finds an interval model that gives the most powerful stack. The searching procedure for the best interval model was conducted using a direct, global optimization algorithm called differential evolutionary . Next, we applied an antistretch workflow to minimize stretch at a far offset after obtaining the optimal effective model. The automated velocity analysis and antistretch workflow were tested on the data volume acquired over the Fort Worth Basin, USA. The results provided noticeable improvement on the prestack gathers and on the stacked data volume.

Description: This paper reports a comparison of three different rheological models used to characterize receiver coupling to the seafloor. We used a finite-element simulation tool to simulate the mechanical receiver coupling to the seafloor as a viscoelastic system with a combination of linear elastic springs and linear viscous dashpots, known as rheological models . Three models cover most of all mechanic coupling systems, the most commonly applied Kelvin-Voigt model (KVM), the Maxwell model (MM), and the standard linear solid (SLS) model. The models differ in behavior for different coupling aspects such as oscillation, creeping, stress relaxation, and their combinations. We tested these models’ ability and relevance for use in modeling seismic receiver coupling to the seafloor. For that purpose, we used an optimized mathematical approach to simulate coupling behavior under various coupling conditions. We found how receiver coupling will affect P- and S-waves for all three models and provided some insight into which model is most suitable to describe coupling under different circumstances. We found that the SLS model represents a general description of most of the coupling effects to the seafloor and should be used when the coupling acts as a viscoelastic system. The KVM and MM are applicable in extreme cases, such as for elastic waves in consolidated sediments (KVM) and dominant creeping effects, as in very soft biosediment (MM).

Description: We used the discrete element method (DEM) to understand the underlying attenuation mechanism in granular media, with special applicability to the measurements of the so-called effective mass developed earlier. We considered that the particles interacted via Hertz-Mindlin elastic contact forces and that the damping was describable as a force proportional to the velocity difference of contacting grains. We determined the behavior of the complex-valued normal-mode frequencies using (1) DEM, (2) direct diagonalization of the relevant matrix, and (3) a numerical search for the zeros of the relevant determinant. All three methods were in strong agreement with each other. The real and the imaginary parts of each normal-mode frequency characterized the elastic and the dissipative properties, respectively, of the granular medium. We found that as the interparticle damping $$\xi $$ increased, the normal modes exhibited nearly circular trajectories in the complex frequency plane, and that for a given value of $$\xi $$ , they all lay on or near a circle of radius $$R$$ centered on the point $$-iR$$ in the complex plane, where $$R\propto 1/\xi $$ . We found that each normal mode became critically damped at a value of the damping parameter $$\xi \approx 1/{\omega }_{n}^{0}$$ , where $${\omega }_{n}^{0}$$ was the (real-valued) frequency when there was no damping. The strong indication was that these conclusions carried over to the properties of real granular media whose dissipation was dominated by the relative motion of contacting grains. For example, P- or S-waves in unconsolidated dry sediments can be expected to become overdamped beyond a critical frequency, depending upon the strength of the intergranular damping constant.

Description: We have developed a new imaging technique of subsurface heterogeneities that uses Sp-waves from natural earthquakes. This technique can be used as a first screening tool in frontier exploration areas before conventional active exploration. Analyzing Sp-waves from 28 earthquakes ( $${M}_{\mathrm{j}}$$ 2.0 to 4.2) recorded by two permanent seismic stations, we built an image of the distributions of velocity discontinuities in southeastern offshore Hokkaido, Japan, where intraplate earthquakes in the Pacific plate frequently occur. Our results indicated the presence of three horizontally continuous, distinct discontinuities corresponding to geologic boundaries estimated in a previous study. We also derived the frequency-dependent quality factor $$Q$$ for P- and S-waves and use it as a method of characterizing physical properties of subsurface structure. The waveform traces with coherent Sp-phases in the southern part of the study area generally show a constant $${Q}_{\mathrm{S}}/{Q}_{\mathrm{P}}$$ ratio, and the waveform traces with randomly distributed phases in the northern part show a large variation of the $${Q}_{\mathrm{S}}/{Q}_{\mathrm{P}}$$ ratio (including several high values).

Description: We have developed a simple iterative gravity-inversion approach to map the basement and Moho surfaces of a rift basin simultaneously. Gravity anomalies in rift basins commonly consist of interfering broad, positive crustal-thinning anomalies and narrow, negative sedimentary-basin anomalies. In our model, we assumed that the Moho and basement surfaces are in Airy isostatic equilibrium. An initial plane-layered model was iterated to fit the gravity data. We applied the process to a model in which the inverted basement and Moho surfaces matched the model surfaces well and to a gravity profile across the Kosti Basin in Sudan.

Description: Quasistatic deformation experiments in the laboratory are key to determining the poroelastic moduli of rocks. For microinhomogeneous porous rocks, it is a challenge to determine a complete set of poroelastic parameters. This is because an additional parameter is required that quantifies the effect of microinhomogeneities because then the unjacketed bulk and pore moduli are no longer the same as the bulk modulus of the solid phase. We found that measurements for the drained and unjacketed bulk moduli together with Skempton’s pore-pressure build-up coefficient were sufficient to determine the solid-phase bulk modulus and the microinhomogeneity parameter. The latter served as a direct measure for the deviation from Biot-Gassmann prediction for the undrained bulk modulus. We applied the results to a set of measured poroelastic moduli in which microinhomogeneities have been made responsible for a non-Gassmann rock behavior. Accordingly, our estimate for the microinhomogeneity parameter quantified the deviation from the Biot-Gassmann prediction.

Description: The estimation of the structural index and of the depth to the source is the main task of many popular methods used to analyze potential field data, such as Euler deconvolution. However, these estimates are unstable even in the presence of a weak amount of noise, and Euler deconvolution of noisy data leads to an underestimation of structural index and depth. We have studied how the structural index and depth estimates are affected by applying low-pass filtering to the data. Physically based low-pass filters, such as upward continuation and integration, have been shown to be the best choice over a range of altitudes (upward continuation) or orders (integration filters), mainly because their outputs have a well-defined physical meaning. In contrast, mathematical low-pass filters require that the filter parameters be tuned carefully by means of several trial tests to produce optimally smoothed fields. The C-norm criterion is a reliable strategy to produce a stabilized vertical derivative, and we discourage Butterworth filters because they tend to a vertical integral filter, for a high cutoff wavenumber, thus complicating the interpretation of the estimated structural index. We found that the estimated structural index and depth to source increase proportionally with the amount of smoothing, unless in the case of overfiltering. In that case, the severe distortion of the original field may cause a decrease of the estimated structural index and depth to source.

Description: We present a reformulation of reduction to the pole (RTP) of magnetic data at low latitudes and the equator using equivalent sources. The proposed method addresses both the theoretical difficulty of low-latitude instability and the practical issue of computational cost. We prove that a positive equivalent source exists when the magnetic data are produced by normal induced magnetization, and we show that the positivity is sufficient to overcome the low-latitude instability in the space domain. We further apply a regularization term directly to the recovered RTP field to improve the solution. The use of equivalent source also naturally enables the processing of data acquired on uneven surface. The result is a practical algorithm that is effective at the equatorial region and can process large-scale data sets with uneven observation heights.

Description: Surface waves are advantageous for mapping seismic structures of permafrost, in which irregular velocity gradients are common and thus the effectiveness of refraction methods are limited. Nevertheless, the complex velocity structures that are common in permafrost environments often yield unusual dispersion spectra, in which higher-order and leaky modes are dominant. Such unusual dispersion spectra were prevalent in the multichannel surface-wave data acquired from our permafrost study site at Barrow, Alaska. Owing to the difficulties in picking and identifying dispersion curves from these dispersion spectra, conventional surface-wave inversion methods become problematic to apply. To overcome these difficulties, we adopted a full-wavefield method to invert for velocity models that can best fit the dispersion spectra instead of the dispersion curves. The inferred velocity models were consistent with collocated electric resistivity results and with subsequent confirmation cores, which indicated the reliability of the recovered seismic structures. The results revealed embedded low-velocity zones underlying the ice-rich permafrost at our study site — an unexpected feature considering the low ground temperatures of $$-10^\circ \mathrm{C}$$ to $$-8^\circ \mathrm{C}$$ . The low velocities in these zones ( $$\sim 70\%\mbox{--}80\%$$ lower than the overlying ice-rich permafrost) were most likely caused by saline pore-waters that prevent the ground from freezing, and the resultant velocity structures are vivid examples of complex subsurface properties in permafrost terrain. We determined that full-wavefield inversion of surface waves, although carrying higher computational costs than conventional methods, can be an effective tool for delineating the seismic structures of permafrost.

Description: The Intellectual Property department in each issue of Geophysics provides U.S. patent abstracts, published patent abstracts of U.S. patent applications, and published abstracts filed under the Patent Cooperation Treaty (PCT).

Description: Streaming potentials can be generated when geologic porous media are subjected to pumping tests. For a homogeneous medium, theory predicts that input and output points for water circulation generate field responses in the form of electric potentials that are equivalent to those produced by current sources that are externally driven by a power source. We evaluated tank experiments showing that this assumption is valid for common geophysical scenarios and can be used to determine charge density for porous geologic media, a key parameter in interpreting electrokinetic and interfacial properties in hydrogeophysics. We also determined that when water circulation encompasses a heterogeneity, the equivalence with single current poles is lost, and this can be used as a field criterion to detect inhomogeneities near a well. Our experimental results were analyzed with finite-element modeling of water and charge flow, showing that an interfacial distribution of currents must be expected as the cause of distortions in self-potential fields. We developed a procedure that used the background resistivity model to better image the distribution of currents onto media interfaces, pointing out advances still needed and challenges still remaining to improve source imaging.

Description: Inconsistent horizontal receiver coupling to the seafloor causes measured signal differences on both horizontal receiver components. To explain this inconsistency, we considered distinct coupling parameters, the damping ratio and resonance frequency, for the receiver inline and crossline directions. Our approach combined these coupling parameters with the azimuth angle between an airgun shot and the receiver geometrically and used two visualization methods to show spatially dependent receiver coupling, based on correlation and root-mean-square amplitudes. We developed finite-element method simulation results together with field data from one ocean bottom cable (OBC) in very soft biosediment. The simulations provided an insight to the difference between perfectly coupled ideal receiver response and poor coupling. From the field data, we compared OBC receiver coupling for trenched and untrenched cable. Our results revealed that the field data had an azimuth-dependent response pattern with amplitude decay and time shift on the untrenched inline component, which we can reproduce with our simulations. Azimuth-dependent receiver coupling indicated that the inline and crossline receiver components were connected by the direction of the traveling wave, and trenching the cable will reduce the azimuth-dependent coupling effects.

Description: Conventional moveout analysis stretches and squeezes traces to increase the coherence of reflected amplitudes in prestack seismic gathers. Higher order residual moveouts require increasingly difficult scans of semblances with extra dimensions or picking from correlations with many local minima. Alternatively, we can model our data with an adaptive convolution that assumes consistent reflectivities at all offsets (or angles). Short, convolutional wavelets can adjust residual moveouts arbitrarily with offset, but slowly with time (or depth). A Gauss-Newton optimization easily inverts this transform by minimizing a least-squares objective function. With estimated and normalized wavelets, we deconvolved the original data to remove phase and spectral distortions that affected more than one reflection. By constraining how slowly wavelets adapt, we retained phase and amplitude changes distinctive to individual reflections. Deconvolution also avoided any explicit smoothing or mixing of amplitudes among traces. Estimated wavelets captured residual coherence and were easier to track visually than individually weak reflections. By adjusting the length and number of independent dynamic wavelets, we can adjust the resolution to the redundancy supported by the data.

Description: The locations of seismic events are used to infer reservoir properties and to guide future production activity, as well as to determine and understand the stress field. Thus, locating seismic events with uncertainty quantification remains an important problem. Using Bayesian analysis, a joint probability density function of all event locations was constructed from prior information about picking errors in kinematic data and explicitly quantified velocity model uncertainty. Simultaneous location of all seismic events captured the absolute event locations and the relative locations of some events with respect to others, along with their associated uncertainties. We found that the influence of an uncertain velocity model on location uncertainty under many realistic scenarios can be significantly reduced by jointly locating events. Many quantities of interest that are estimated from multiple event locations, such as fault sizes and fracture spacing or orientation, can be better estimated in practice using the proposed approach.

Description: Due to incomplete aperture coverage and complex overburden structures, the migration process cannot provide a true-amplitude image even though a true-amplitude propagator is used. Amplitude compensation based on source-side illumination ignores the aperture effects on the receiver side, and it may fail to recover the true-reflection/scattering strength of a geologic structure from the image. The structural dip largely controls if the wave incident on the structure can be reflected back and received by the acquisition aperture, so it should be taken into account in removing the acquisition effects from the migration image. We derived a dip-angle domain amplitude correctionfrom the resolution theory. The stacked migration image created by reverse time migration was decomposed into common dip images, which were compensated individually by the corresponding amplitude correction factor. Then, we summed up the corrected images to form a final image. To construct the amplitude correction factor, we generated a monofrequency Green’s function at the shot/geophone location and further decomposed it into incident/scattered plane waves. They were combined based on Snell’s law to construct correction factors for different dips. The final amplitude correction factor was formed by visiting all the shot-geophone pairs in the observation system. We devised efficient algorithms to make the amplitude correction more practical. We evaluated two numerical experiments, a five-layer model and the SEG/EAGE salt model, in which the amplitude correction led to a scalar/pressure image with an amplitude better matching the true impedance contrasts of subsurface structures, especially in areas with steep dips and in subsalt regions.

Description: A method for static correction of time-lapse differences in reflection arrival times of time-lapse prestack seismic data is presented. These arrival-time differences are typically caused by changes in the near-surface velocities between the acquisitions and had a detrimental impact on time-lapse seismic imaging. Trace-to-trace time shifts of the data sets from different vintages are determined by crosscorrelations. The time shifts are decomposed in a surface-consistent manner, which yields static corrections that tie the repeat data to the baseline data. Hence, this approach implies that new refraction static corrections for the repeat data sets are unnecessary. The approach is demonstrated on a 4D seismic data set from the Ketzin $${\mathrm{CO}}_{2}$$ pilot storage site, Germany, and is compared with the result of an initial processing that was based on separate refraction static corrections. It is shown that the time-lapse difference static correction approach reduces 4D noise more effectively than separate refraction static corrections and is significantly less labor intensive.

Description: An airborne electromagnetic (AEM) survey often covers hundreds of square kilometers. Huge amounts of survey data make 2D/3D data inversion very difficult. However, due to the compact configurations of AEM systems, the sensitive area for each single survey station is much smaller than the whole survey area, which makes it possible to only invert partial survey data. The sensitive area is called the footprint . Based on "moving-footprint" technology, the entire survey can be divided into subareas and the data are first inverted individually and then combined to form the inversions of the entire survey area, so that the cost for forward and inverse modeling can be vastly reduced. Contrary to previous electromagnetic (EM) footprints defined only for an EM transmitter or for a perfectly conductive earth, we defined the frequency-domain AEM footprint by considering a complete AEM transmitter-receiver system over an earth with limited conductivity. We used the tensor Green’s function to calculate the secondary magnetic field from the induced underground current and evaluate the EM footprint as the volume in which the induced current contributes 90% to the total secondary magnetic field at the EM receiver. Numerical experiments for horizontal coplanar and vertical coaxial coil configurations revealed that among all influence factors on the AEM footprint, the flight altitude was dominant, with a high flight altitude corresponding to a large EM footprint, whereas the transmitting frequency and earth resistivity played a secondary role and in a combined way of induction number, with the low frequency or high earth resistivity (small induction number) corresponding to large EM footprints.

Description: Full-waveform inversion (FWI) of Rayleigh waves is attractive for shallow geotechnical investigations due to the high sensitivity of Rayleigh waves to the S-wave velocity structure of the subsurface. In shallow-seismic field data, the effects of anelastic damping are significant. Dissipation results in a low-pass effect as well as frequency-dependent decay with offset. We found this by comparing recorded waveforms with elastic and viscoelastic wave simulation. The effects of anelastic damping must be considered in FWI of shallow-seismic Rayleigh waves. FWI using elastic simulation of wave propagation failed in synthetic inversion tests in which we tried to reconstruct the S-wave velocity in a viscoelastic model. To overcome this, $$Q$$ -values can be estimated from the recordings to quantify viscoelasticity. Waveform simulation in the FWI then uses these a priori values when inferring seismic velocities and density. A source-wavelet correction, which is inevitable in FWI of field data, can compensate a significant fraction of the residuals between elastically and viscoelastically simulated data by narrowing the signals’ bandwidth. This way, elastic simulation becomes applicable in FWI of data from anelastic media. This approach, however, was not able to produce a frequency-dependent amplitude decay with offset. Reconstruction, therefore, was more accurate when using appropriate viscoelastic modeling in FWI of shallow-seismic Rayleigh waves. We found this by synthetic inversion tests using elastic forward simulation as well as viscoelastic simulation with different a priori values for $$Q$$ .

Description: Multicomponent seismic images are composed of different combinations of downgoing and upgoing wavefields. Each wave mode has different propagation velocity and polarization direction and thus carries unique, direction-dependent information about the subsurface. Differences in propagation velocity cause events in converted-wave or PS images to appear at later times than the P-wave or PP image counterparts. Reflectivities are different for each wave mode, and therefore, multicomponent images are not related simply by time shifts. These complications historically required that the alignment, also called registration , of corresponding image features be done manually, which is a tedious process. To register PP and PS images automatically, we used a smooth dynamic image-warping algorithm that can be accurate with respect to problems unrelated to time shifts, such as differences in noise and reflection waveforms. Interval $${V}_{\mathrm{P}}/{V}_{\mathrm{S}}$$ ratios can be estimated from derivatives of vertical shifts that align reflections in PP and PS images. To optimize accuracy of estimated time shifts and $${V}_{\mathrm{P}}/{V}_{\mathrm{S}}$$ ratios, we automatically constructed a coarse lattice of points located on reflections with high amplitudes and then estimated time shifts at only those image samples. By adjusting the coarseness of the lattice, we trade off resolution of changes in $${V}_{\mathrm{P}}/{V}_{\mathrm{S}}$$ with increased accuracy in $${V}_{\mathrm{P}}/{V}_{\mathrm{S}}$$ estimates. By processing 3D PP and PS images, we learned that our estimates of $${V}_{\mathrm{P}}/{V}_{\mathrm{S}}$$ cannot be obtained by smoothing time shifts estimated at every image sample.

Description: We examined the sensitivity of the electrochemical spectral induced polarization (SIP) model developed by Wong to the oxidation extent of pyrite and pyrrhotite minerals disseminated in silica sand. The sensitivity of this model to the oxidation of sulfide minerals was mainly related to the model parameters defining the ratio of the active to the inactive passive ions $$({c}_{2}/{c}_{o})$$ dissolved in the pore water, and the variation of the current reaction parameters $$\alpha $$ and $$\beta $$ . The increase in these parameters as well as in the associated exchange current densities, $${i}_{o}(\alpha )$$ and $${i}_{o}(\beta )$$ was consistent with an increase in the activation of the charge transfer at the metal-electrolyte interface, resulting in the decrease in polarization of such an interface, which was reflected by a decrease in the SIP phase response as previously argued by Wong. Under this premise, the model described fairly well measurements below 500 Hz from a laboratory experiment, being consistent with the depletion of the SIP phase response associated with the oxidation degree promoted on the disseminate sulfides analyzed here. This suggested that electrochemical modeling of SIP measurements can provide information to assess the oxidation state of sulfides and also to infer the formation of passivating layers coating the metal minerals during oxidation-dissolution processes. Our results suggested a possible alternative for the monitoring of mine waste deposits producing acid mine drainage and the stability of sequestered harmful metals during remedial treatments by means of the SIP method.

Description: Free-surface-related multiples are usually regarded as noise in conventional seismic processing. However, they can provide extra illumination of the subsurface and thus have been used in migration procedures, e.g., in one- and two-way wave-equation migrations. The disadvantage of the migration of multiples is the migration artifacts generated by the crosscorrelation of different seismic events, e.g., primaries and second-order free-surface-related multiples, so the effective elimination of migration artifacts is crucial for migration of multiples. The angle domain common image gather (ADCIG) is a suitable domain for testing the correctness of a migration velocity model. When the migration velocity model is correct, all the events in ADCIGs should be flat, and this provides a criterion for removing the migration artifacts. Our approach first obtains ADCIGs during reverse time migration and then applies a high-resolution parabolic Radon transform to all ADCIGs. By doing so, most migration artifacts will reside in the nonzero curvature regions in the Radon domain, and then a muting procedure can be implemented to remove the data components outside the vicinity of zero curvature. After the application of an adjoint Radon transform, the filtered ADCIGs are obtained and the final denoised migration result is generated by stacking all filtered ADCIGs. A three-flat-layer velocity model and the Marmousi synthetic data set are used for numerical experiments. The numerical results revealed that the proposed approach can eliminate most artifacts generated by migration of multiples when the migration velocity model is correct.

Description: We present location results for a group of $$\sim 200$$ microearthquakes that occurred in 2012 in a region of Oklahoma hosting ongoing exploration activities. Using a local passive surface seismic monitoring network of 15 broadband stations, we applied two modern location techniques that use fundamentally different approaches. The first is a pick-based double-difference relocation method with waveform crosscorrelation. Multiple-event location techniques such as these are generally regarded as the best approach for obtaining high-precision locations from pick data. The second approach is an automated waveform migration stacking method. These types of methods are becoming increasingly common due to increasing network station density and computer power. The results from the two methods show excellent agreement and provide similar results for the interpreter. Both methods reveal spatial and temporal patterns in the locations that are not visible in results obtained using a more traditional pick-based approach. We performed two statistical uncertainty tests to assess the effects of data quality and quantity on the two methods. We show that the uncertainties for both methods are comparable, but that the stack-based locations are less sensitive to station geometry, likely due to the different treatment of outliers and the beneficial inclusion of noisier data. Finally, we discuss the favorable conditions in which to apply each method and argue that for small aperture surface arrays where accurate velocity information exists, such as in this study, the stack-based method is preferable due to the higher degree of automation. Under these conditions, stack-based methods better allow for rapid and precise determination of microearthquake locations, facilitating improved interpretations of seismogenic processes.