HR: 14:40h
AN: NS33A-05    [Abstracts]
TI: Cone-based Electrical Resistivity Tomography
AU: * Pidlisecky, A
EM: apid@pangea.stanford.edu
AF: Geophysics Deparment, Stanford University, 2215 Mitchell Bldg, Stanford, CA 94305-2215 United States
AU: Knight, R
EM: rknight@pangea.stanford.edu
AF: Geophysics Deparment, Stanford University, 2215 Mitchell Bldg, Stanford, CA 94305-2215 United States
AU: Haber, E
EM: haber@mathcs.emory.edu
AF: Deparment of Mathematics and Computer Science, Emory University, Suite W401 400 Dowman Drive, Atlanta, GA 30322 United States
AB: Determining the 3D spatial distribution of subsurface properties is a critical part of managing the clean-up of contaminated sites. Most standard hydrologic methods sample small regions immediately adjacent to wells or testing devices. This provides data which are not representative of the entire region of interest. Furthermore, at many contaminated sites invasive methods are not acceptable, due to the risks associated with contacting and spreading the contaminants. To address these issues, we have developed a minimally invasive technology that provides information about the 3D distribution of electrical conductivity. This new technique, cone-based electrical resistivity tomography (C-bert), involves placing several permanent current electrodes in the subsurface and using electrodes mounted on a cone penetrometer to measure the resultant potential field while advancing the cone into the subsurface. In addition to potential field measurements, we obtain the standard suite of cone-penetration measurements, including high resolution resistivity logs; these data can then be used to constrain the inversion of the potential field data. A major challenge of working with these data is that the cone penetrometer is highly conductive, and thus presents a large local perturbation around the measurement location. As the cone is very small (approximately 30mm in diameter) with respect to the total model space, explicitly modeling the cone is computationally demanding. We developed a method for solving the forward model that reduces computational time by an order of magnitude. This solution method, iteratively determined boundary conditions, makes it possible to correct for the cone effect before inversion of the data. Results from synthetic experiments suggest that the C-bert method of data acquisition can result in high quality electrical conductivity images of the subsurface. We tested the practicality of this technique by performing a field test of the C-bert system to image a salt water intrusion in a fresh water aquifer in Vancouver, British Columbia. A total of nine current electrodes were emplaced at the site and five C-bert profiles were obtained, resulting in approximately 6500 data points. We conclude, from this first field test, that this method is a promising new way to image the subsurface.
DE: 0644 Numerical methods
DE: 0915 Downhole methods
SC: Near-Surface Geophysics [NS]
MN: 2005 Joint Assembly