Development of an Integrated Geophysical Approach to Subsurface Imaging, 20-9210

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Principal Investigators
David A. Farrell
Chris L. Hackert
Brian J. Zook
Sitakanta Mohanty
Debra L. Hughson

Inclusive Dates: 08/01/00 - Current

Background - Adequate and cost-effective characterization of the subsurface, whether it be for environmental contaminants, geotechnical properties, or mineral resources, poses challenging problems. Traditional approaches to subsurface characterization for environmental contaminants rely primarily on borehole measurement networks from which physical and geochemical measurements are interpolated and the extent of contamination is delineated. Limitations of such approaches include 1) possible cross-contamination along poorly constructed boreholes rendering remedial designs ineffective or costly; 2) prohibitive cost associated with constructing an adequate number of monitoring and characterization boreholes at large complex sites; and 3) incomplete characterization of the subsurface. To minimize potential risks of cross-contamination and to clearly demonstrate compliance with regulatory standards at sites, cost-effective, surface-based, noninvasive methods are required. In recent years, minimally invasive geophysical methods have been increasingly used to provide detailed images of the subsurface in contaminated, environmentally sensitive areas. In recognition of the difficulties involved in imaging environmental processes in complex geologic media (e.g., subsurface contaminant migration), some investigators have employed multiple geophysical methods.

Approach - The primary goal of this work is to develop an easy-to-use geophysical data analysis tool that couples electrical resistivity and seismic tomography methods in an inverse scheme to produce high-resolution images of the subsurface. Accomplishing this objective involves 1) identification of appropriate software for seismic tomographic inversion and electrical resistivity tomographic inversion, 2) updating the tomographic software to improve computational efficiency and robustness, 3) developing an inversion framework that couples both inversion methodologies, 4) developing robust relationships that can be used to relate seismic velocity to electrical resistivity, 5) developing a suite of test problems to test the methodology, and 6) applying the methodology in the field to demonstrate its capabilities.

Accomplishments - The project has resulted in the development of a computationally efficient and flexible electrical resistivity tomography software, RESTOMO 6. This version represents a significant improvement over the previous version of the software, RESTOMO 5, developed in an earlier internal research project. The upgrade replaces the back-projection methodology with a robust, computationally efficient finite element formulation that solves Laplace's equation describing voltage distribution in the subsurface. The approach also utilizes an adjoint state sensitivity approach that takes advantage of the finite element formulation to determine the Jacobian matrix. The team has successfully demonstrated a two-dimensional version of the software and is extending the capabilities of the software to address three-dimensional problems. The research team has successfully coupled RESTOMO 6 with a seismic tomography code in an iterative inversion scheme. Using relationships between seismic velocity and electrical resistivity determined from borehole measurements, the team has shown that the iterative inversion scheme produces an image of the subsurface that significantly improves upon the resolution of those images created by independent application of each inversion method.

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