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Using tomograms for quantitative interpretation of processes and properties

F.D. Day-Lewis (daylewis@usgs.gov)
U.S. Geological Survey, 11 Sherman Place, Unit 5015, Storrs, CT 06269, United States

T.C. Johnson (timothy.johnson@inl.gov)
Idaho National Laboratory, P.O. Box 1625, Idaho Falls, ID 83415, United States

K. Singha (ksingha@psu.edu)
The Pennsylania State University, 311 Deike Building, University Park, PA 16802, United States

R.D. Henderson (rhenders@usgs.gov)
U.S. Geological Survey, 11 Sherman Place, Unit 5015, Storrs, CT 06269, United States

J.W. Lane, Jr. (jwlane@usgs.gov)
U.S. Geological Survey, 11 Sherman Place, Unit 5015, Storrs, CT 06269, United States

Abstract

Changes in subsurface hydrologic conditions that result from remediation or natural processes increasingly are monitored with near-surface geophysical imaging. In particular, time-lapse electrical resistivity tomography (ERT) and radar tomography (RT) are well suited to monitoring fluid-conductivity changes associated with injection of amendments for biostimulation, freshwater/saltwater dynamics in coastal aquifers, and aquifer-storage and recovery (ASR). Quantifying results from tomograms is complicated by limited resolution and estimation uncertainty that are inherent to geophysical inverse problems. Hydrologic and engineering parameters are difficult to estimate because survey geometry, measurement physics, experimental errors, regularization strategy, prior information, and parameterization also affect interpretation of geophysical measurements. Here, we review recent work to address the problem of "correlation loss" and to directly extract information about transport processes from geophysical time series. Both synthetic and field-experimental examples are presented. Case studies include applications of (1) time-lapse ERT and RT to monitor biostimulation injections at a former Department of Defense site in Brandywine, MD; (2) time-lapse ERT to monitor submarine ground-water discharge at Cape Cod, MA; and (3) time-lapse ERT to monitor an ASR experiment in Charleston, SC. In the first study, a spatially variable calibration between ERT-estimated bulk and sampled fluid conductivity was developed and applied to three-dimensional tomograms to infer the evolution of fluid conductivity after injections of amendments and pH adjustment. In the second study, the ability to resolve subsurface salinity contrasts is shown to vary strongly as a function of tide level. In the third study, electrical methods are used to monitor rate-limited mass transfer, and hydrologic parameters are estimated from the temporal moments of fluid and ERT-estimated conductivity; however, the applicability of this approach is shown to be limited by spatial resolution, temporal resolution, and the relative rates of advection and mass transfer.

Final copy as submitted to American Geophysical Union for publication as: Day-Lewis, F.D., Johnson, T.C., Singha, Kamini, Henderson, R.D., and Lane, J.W., Jr., 2008, Using tomograms for quantitative interpretation of processes and properties [abs.]: EOS Transactions, American Geophysical Union, v. 89, no. 53, Fall Meeting Supplement, abstract IN51C-1171 (Invited).