ALBERT

Description: Seismic waveform-inversion offers opportunities for detailed characterization of the subsurface. However, its full potential can only be exploited when any systematic source and receiver effects are either carefully avoided or appropriately accounted for during the inversions. Repeated crosshole measurements in the Mont Terri (Switzerland) underground laboratory have revealed that receiver coupling may significantly affect the seismic waveforms. More seriously, coupling conditions may vary during the course of a monitoring experiment. To address this problem, we have developed a novel scheme that estimates medium properties, frequency-dependent source functions, and frequency-dependent receiver-coupling factors. We demonstrate the efficacy of the new scheme via a synthetic 2D crosshole experiment in which realistic receiver-coupling factors are incorporated. Because determination of medium parameters and estimation of source functions and receiver-coupling factors are largely separated, the method can be easily adapted to any other waveform-inversion problem, including elastic, anisotropic, 2.5D, or 3D situations.

Description: Increases in the emissions and associated atmospheric deposition of nitrogen (N) have the potential to cause significant changes to the structure and function of N-limited ecosystems. Here we present the results of a long-term (13 year) experiment assessing the impacts of N addition (30 kg ha −1 yr −1 ) on a UK lowland heathland under a wide range of environmental conditions, including the occurrence of prolonged natural drought episodes and a severe summer fire. Our findings indicate that elevated N deposition results in large, persistent effects on Calluna growth, phenology and chemistry, severe suppression of understorey lichen flora and changes in soil biogeochemistry. Growing season rainfall was found to be a strong driver of inter-annual variation in Calluna growth and, although interactions between N and rainfall for shoot growth were not significant until the later phase of the experiment, N addition exacerbated the extent of drought injury to Calluna shoots following naturally occurring droughts in 2003 and 2009. Following a severe wildfire at the experimental site in 2006, heathland regeneration dynamics were significantly affected by N, with a greater abundance of pioneering moss species and suppression of the lichen flora in plots receiving N additions. Significant interactions between climate and N were also apparent post-fire, with the characteristic stimulation in Calluna growth in +N plots supressed during dry years. Carbon (C) and N budgets demonstrate large increases in both above- and below-ground stocks of these elements in N-treated plots prior to the fire, despite higher levels of microbial activity and organic matter turnover. Although much of the organic material was removed during the fire, pre-existing treatment differences were still evident following the burn. Post-fire accumulation of below-ground C and N stocks was increased rapidly in N-treated plots, highlighting the role of N deposition in ecosystem C sequestration.

Description: Surface nuclear magnetic resonance (NMR) is a noninvasive geophysical method that is primarily used in hydrological investigations of shallow aquifers. An important parameter in surface-NMR experiments is the relaxation time T1. Information on pore structure and even hydraulic permeability/conductivity may be inferred from accurate estimates of this parameter. Estimates of T1 are usually obtained by evaluating the spin response of groundwater molecules to excitation by two sequential electromagnetic pulses, the second of which is delayed and phase-shifted by p relative to the first. We have discovered that variations of the excitation field with distance from the transmitter and common imperfections in the transmitted pulses introduce considerable bias in estimates of T1 (e.g., errors as large as 50%). We assess the significance of these problems via numerical simulations based on the Bloch equation. As a result of this assessment, we propose a novel yet simple modification to the T1 acquisition method that resolves the identified problems. Our new scheme involves applying two types of double-pulse sequence, one in which the second pulse is phase-shifted by p relative to the first (i.e., the current procedure) and one in which the second pulse is in phase with the first. Subtracting the voltage responses measured after each of the two double-pulse experiments eliminates the bias, thus allowing T1 to be reliably estimated under general surface-NMR conditions.

Description: Surface nuclear magnetic resonance (NMR) is a noninvasive geophysical tool used to investigate groundwater reservoirs. The relevant physical process in surface NMR is the nuclear spin of hydrogen protons in liquid water. Standard single-pulse surface NMR experiments provide estimates of water content in the shallow subsurface. Under favorable conditions, pore-structure and even hydraulic-conductivity information can be extracted from double-pulse surface NMR data. One crucial issue in surface NMR experiments is the resonance condition: the frequency of the excitation field should closely match the Larmor frequency of the protons, which is controlled by the local magnitude of the earth's magnetic field. Although the earth's field can be measured accurately by an on-site magnetometer, several effects impede perfect matching of the frequencies. These include temporal variations of the earth's field, instrumental imperfections, and the magnetic susceptibility of the underlying rocks. We assess the impact of violating the resonance condition on surface NMR experiments. Our investigation involves numerical simulations and measurements using a sample-scale earth-field NMR device and a surface NMR acquisition system. For frequency offsets up to 5 Hz, we have find that relatively standard single-pulse surface NMR recording procedures are likely to produce reliable water-content estimates as long as the pulse moments are small to moderate or the aquifer is relatively deep. If strong pulse moments are required or shallow aquifers are probed, off-resonance conditions can lead to anomalous increases in recorded amplitudes that can be mistakenly interpreted in terms of deepwater occurrences. Double-pulse surface NMR experiments are particularly sensitive to off-resonance effects, such that the results may be highly biased even for the small-frequency offsets commonly encountered in field situations.

Description: The hydrogeological properties and hydrological responses of a productive aquifer in northeastern Switzerland are investigated. For this purpose, 3D crosshole electrical resistivity tomography (ERT) is used to define the main lithological structures within the aquifer (through static inversion) and to monitor the water infiltration from an adjacent river. During precipitation events and subsequent river flooding, the river water resistivity increases. As a consequence, the electrical characteristics of the infiltrating water can be used as a natural tracer to delineate preferential flow paths and flow velocities. The focus is primarily on the experiment installation, data collection strategy, and the structural characterization of the site and a brief overview of the ERT monitoring results. The monitoring system comprises 18 boreholes each equipped with 10 electrodes straddling the entire thickness of the gravel aquifer. A multichannel resistivity system programmed to cycle through various four-point electrode configurations of the 180 electrodes in a rolling sequence allows for the measurement of approximately 15,500 apparent resistivity values every 7 h on a continuous basis. The 3D static ERT inversion of data acquired under stable hydrological conditions provides a base model for future time-lapse inversion studies and the means to investigate the resolving capability of our acquisition scheme. In particular, it enables definition of the main lithological structures within the aquifer. The final ERT static model delineates a relatively high-resistivity, low-porosity, intermediate-depth layer throughout the investigated aquifer volume that is consistent with results from well logging and seismic and radar tomography models. The next step will be to define and implement an appropriate time-lapse ERT inversion scheme using the river water as a natural tracer. The main challenge will be to separate the superposed time-varying effects of water table height, temperature, and salinity variations associated with the infiltrating water.

Description: In an attempt to understand the origin of the enigmatic Chessel-Noville hills in the Rhone River plain of western Switzerland, we have recorded two and three-dimensional ground-penetrating radar (georadar) data and drilled six shallow boreholes. Cross-sections and maps of horizons extracted from the georadar data reveal normal fault zones under the hill flanks and minor grabens and domes below the hill crests. Comparable graben-like structures with normal step faults converging with depth are observed at a nearby outcrop. Fine sand and silt within some of the exposed faults may have been dragged into the fault planes or injected as clastic dikes under high pore pressures. A silt unit encountered in two of the boreholes correlates with a strong georadar reflection. Although angular calcareous boulders are scattered across the surface, the early suggestion that the hills are remnants of a large historic rockfall is not compatible with the subsurface lithologies and structures. Neither is the frequently referenced hypothesis that they are moraines. Moreover, the absence of significant thrust faulting and lack of strong structural trends in the georadar data and at outcrops are inconsistent with the more recent interpretation of the hills as glaciotectonic ridges. Instead, our results indicate that the hills and their internal structures represent vertically uplifted and deformed fluvio-deltaic sediments. Diapirism offers a plausible explanation for the observed uplift and deformation. Saturated fine-grained sediments underlying the former Rhone Delta may have been compacted and overpressured during burial by pro-grading deltaic sands and thickening sequences of fluvial sands and gravels. Under these conditions, mud diapirs may develop. Actual diapiric uplift and deformation may have been triggered by a large historic earthquake (e. g., Tauredunum event of 563 A.D.) that would have induced additional over-pressurization, liquefaction and upward mobilization of the fine-grained sediments. Such an earthquake may also have triggered the rockfall responsible for the scattered calcareous boulders.