HR: 13:30h
AN: NS33A-01 INVITED     [Abstracts]
TI: Imaging 4-D hydrogeologic processes with geophysics: an example using crosswell electrical measurements to characterize a tracer plume
AU: * Singha, K
EM: ksingha@pangea.stanford.edu
AF: Stanford University, Building 320, Geology Corner, Stanford, CA 94305-2115 United States
AU: Gorelick, S M
EM: gorelick@ pangea.stanford.edu
AF: Stanford University, Building 320, Geology Corner, Stanford, CA 94305-2115 United States
AB: Geophysical methods provide an inexpensive way to collect spatially exhaustive data about hydrogeologic, mechanical or geochemical parameters. In the presence of heterogeneity over multiple scales of these parameters at most field sites, geophysical data can contribute greatly to our understanding about the subsurface by providing important data we would otherwise lack without extensive, and often expensive, direct sampling. Recent work has highlighted the use of time-lapse geophysical data to help characterize hydrogeologic processes. We investigate the potential for making quantitative assessments of sodium-chloride tracer transport using 4-D crosswell electrical resistivity tomography (ERT) in a sand and gravel aquifer at the Massachusetts Military Reservation on Cape Cod. Given information about the relation between electrical conductivity and tracer concentration, we can estimate spatial moments from the 3-D ERT inversions, which give us information about tracer mass, center of mass, and dispersivity through time. The accuracy of these integrated measurements of tracer plume behavior is dependent on spatially variable resolution. The ERT inversions display greater apparent dispersion than tracer plumes estimated by 3D advective-dispersive simulation. This behavior is attributed to reduced measurement sensitivity to electrical conductivity values with distance from the electrodes and differential smoothing from tomographic inversion. The latter is a problem common to overparameterized inverse problems, which often occur when real-world budget limitations preclude extensive well-drilling or additional data collection. These results prompt future work on intelligent methods for reparameterizing the inverse problem and coupling additional disparate data sets.
DE: 0694 Instrumentation and techniques
DE: 1832 Groundwater transport
DE: 3260 Inverse theory
DE: 5109 Magnetic and electrical properties
SC: Near-Surface Geophysics [NS]
MN: 2005 Joint Assembly