[ 1 ] The self-potential (SP) method has long been used for av ariety of geophysical applications because of its ease of acquisition, but has suffered from difficulty in interpretation of the data. Self-potential signals result from as ource term that is coupled with the earth resistivity and appropriate boundary conditions. This work describes an inversion methodology for determining the self-potential sources from measured SP and resistivity data. The SP source inversion is al inear problem, though it is complicated by nonuniqueness that is common to potential-field problems. The linear operator is also poorly conditioned because of the limited set of measurements, which are often constrained to the earth'ssurface. Our approach utilizes model regularization that selects a class of solutions which fit the data with sources that are spatially compact. Large variations in sensitivity due to distance and resistivity structure throughout the model are addressed through the use of ascaling term derived from the Green'sfunctions that define the linear operator.Asignificant benefit of these methods is the resolution of targets at depth from surface measurements alone. This inversion technique is first illustrated with a simple synthetic data set. In as econd example we apply this approach to af ield data set taken from previously published literature and investigate the effects of different resistivity structure assumptions on the inversion results. The spatial distribution of sources provides useful information that can subsequently be interpreted in terms of physical processes that generate the SP data.
[1] Self-potential (SP) data are collected over an area of approximately 250 m 2 at the Department of Energy Savannah River Site where there is known subsurface dense nonaqueous phase liquid (DNAPL) contamination. Nonpolarizing electrodes are used to measure the SP signal on a two-dimensional (2-D) surface grid with 2-m spacing in both horizontal directions, and four borehole arrays with 3.7-m electrode spacing and 25.6-m total depth. The primary contaminants, tetrachloroethylene (PCE) and trichloroethylene (TCE), are known to undergo redox reactions in the environment. Variations in the subsurface redox conditions are proposed as an electrochemical source for the SP signals measured in this investigation. A 3-D self-potential inversion algorithm is used to find an electrical current source model, taking into account the resistivity structure derived from a 3-D spectral induced polarization survey at the same field location. The sources and sinks of electrical current can be related to the zones of relative high or low redox potential and are therefore interpreted in the context of contaminated areas. These results are reasonably correlated with DNAPL concentration data obtained from several ground-truth well measurements, indicating that the SP sources can be an indicator of contaminated areas where electrochemical source mechanisms are active. In several cases, however, the SP sources and contaminant concentrations are not correlated, reflecting the spatial variability of biogeochemical parameters in the Earth that control the SP response in addition to concentration. More extensive geochemical ground-truth information is therefore needed to validate the self-potential source inversion methodology and develop it as a predictive tool in the context of contaminated sites.
Several laboratory and scaled model investigations suggest that organic contaminants affect the surface electrical properties of exposed soils/rocks and therefore produce measurable induced polarization (IP) signatures. However, there is little field evidence of an IP methodology for contaminant mapping. A 2D time-domain IP method is developed for mapping the FS-12 contaminant plume at the Massachusetts Military Reservation (MMR) located in Cape Cod, Massachusetts. The FS-12 plume consists of approximately [Formula: see text] of fuel that erupted from a broken underground pipeline in the early 1970s. Benzene and ethylene dibromide (EDB) are the primary contaminants at FS-12, with concentrations exceeding the allowed maximum concentration levels (MCL), while other constituents of the plume did not exceed their MCL. Therefore, the contaminants of interest are benzene and EDB, partly because of their health risk and partly because they present the highest concentrations (2400 and [Formula: see text], respectively) among the plume constituents and are therefore more likely to be related to the polarization source. IP data were acquired along a survey line that partially transects the plume extending over contaminated and uncontaminated zones and were inverted to give 2D resistivity and chargeability plots to [Formula: see text] depth and a horizontal extent of [Formula: see text]. By separately inverting IP data derived from time windows located at short and long decay times, a time-domain gross (spectral) chargeability difference is produced. Both the chargeability and gross spectral chargeability difference show good agreement with the known location of the plume from monitoring wells, with the IP chargeability section suggesting contaminant distribution detail that cannot otherwise be inferred from the sparse borehole distribution.
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