Geophysical Imaging of Subsurface ContaminantsEPA Grant Number: U914790
Title: Geophysical Imaging of Subsurface Contaminants
Investigators: Zhang, Jie
Institution: Massachusetts Institute of Technology
EPA Project Officer: Broadway, Virginia
Project Period: January 1, 1995 through January 1, 1996
Project Amount: $102,000
RFA: STAR Graduate Fellowships (1995) Recipients Lists
Research Category: Academic Fellowships , Ecological Indicators/Assessment/Restoration , Fellowship - Earth Sciences
The main objectives of this research project are to: (1) perform three-dimensional (3-D) geophysical tomographic studies and solve nonlinear inverse problems; these include 3-D direct-current resistivity tomography, 3-D seismic refraction traveltime tomography, and 3-D seismic reflection traveltime-migration tomography; (2) characterize the uncertainty associated with each tomographic method, and determine the accuracy of seismic velocity and resistivity information that the data provide; this will give a quantitative confidence on the 3-D geophysical means for certain environmental targets, and also will allow one to design an ideal field experiment; and (3) develop rapid and efficient algorithms for both forward modeling and inverse problems; this allows the application of Objectives 1 and 2 to environmental problems.
Because of the heterogeneity of the subsurface, using conventional 1-D geophysical methods to detect contamination remains a difficult problem. Advanced geophysical techniques for 2-D or 3-D interpretation are too expensive and time consuming for typical environmental studies. We seek a solution to this problem, because 88 of the top 100 common chemical contaminants on the U.S. Environmental Protection Agency Superfund list are organic contaminants, which may have small surface signatures (Lucius, et al., 1990). These contaminants may permeate the subsurface and the groundwater. It is imperative that we understand the interaction of these contaminants with the subsurface, and that we develop practical and effective means to noninvasively map and monitor this type of contamination on a real-time basis.
Many chemical contaminants are common and widespread, such as those used in dry-cleaning (perchloroethylene) or constituents of gasoline (toluene and benzene). However, most chemical contaminants also are known or suspected carcinogens that are regulated at the part per billion level (Olhoeft, 1992). This greatly challenges the capability of geophysical techniques for detecting them. For these problems, perhaps no single geophysical technique can uniquely reconstruct the image of contaminant from data collected on the surface. The complexity of the shallow geological structure can cause well-known depth ambiguity problems. For instance, the weathering layer right below the surface, which generally has low seismic velocity, low resistivity, unconsolidated material and may be the most variable of all layers. Using an incorrect weathering model in seismic reflection tomography or resistivity tomography can corrupt the image, possibly introducing a false structure into the deep subsurface (Zhang, et al., 1995a and b). It is necessary, therefore, to have geologically meaningful cross-constraints and obtain more reliable results from an integration of 3-D tomographic studies. In applying 3-D seismic refraction traveltime tomography, we are able to image shallow structural features such as weathering layer. After making residual static corrections for the shallow irregularities, 3-D seismic reflection traveltime-migration tomography can be used to precisely map deep geologic interfaces. 3-D d.c. resistivity tomography, with sufficient a priori information, can be applied to image contaminants that have anomalous electrical properties. However, can we perform a few different 3-D geophysical tomographic studies for environmental monitoring purposes that are accurate, rapid, and efficient? Can we image the contaminant, as well as characterize the uncertainty of the image in terms of resolution and variance?
So far I have accomplished about 60 percent of the project objectives. These include the implementation of a 3-D d.c. resistivity tomography program and applications to two real environmental problems (Zhang, et al., 1995a and b), a traveltime calculation for 3-D seismic refraction tomography, and a traveltime calculation and a prestack depth migration for 3-D reflection traveltime-migration tomography. It will take approximately 2 years to complete the remaining work.