USGS Groundwater Information: Branch of Geophysics
Figure 1. Sample image showing percent change between subsurface electrical resistivity on day 5 after injection of saline tracer, and subsurface resistivity prior to injection of tracer. Electrical resistivity values are inverted from data collected during an electrical resistivity tomography (ERT) survey. Blue regions indicate zone of decreased resistivity, representing the transport of the saline tracer. (Singha and others, 2003)
As part of its applied research initiatives, the USGS Office of Ground Water, Branch of Geophysics (OGW BG) conducted research on the use of geophysical methods to map aquifer heterogeneity at the USGS Toxic Substances Hydrology Program Massachusetts Military Reservation (MMR) research site, Cape Cod, Massachusetts.
Accurate characterization of flow and solute transport in groundwater systems is a critical problem in hydrology. Given the large volume of data required to develop an accurate model of subsurface flow and the cost of direct sampling, the use of geophysical methods can contribute significantly to information about the subsurface.
The goal of the project was to provide images of aquifer properties through integrated analysis of multiple data types in order to improve modeling of three-dimensional (3D) flow and transport in the study area. Work was part of doctoral research by Kamini Singha (Pennsylvania State University) on the use of electrical resistivity tomography (ERT) surveys to monitor chloride tracers in porous media in order to map subsurface flow paths and flow barriers and to create a forward flow and transport model for the site.
Field geophysical and hydraulic experiments were conducted at MMR over several field seasons. Prior to the field experiments, three-dimensional computer simulations were performed to design the experiments, and a tank-scale laboratory experiment was conducted to provide a simplified representation of conditions at the field site.
Field experiments and geophysical surveys included the following:
Time-lapse ERT was used to monitor saline tracer tests at the MMR research site. The electrically conductive, saline tracer served to illuminate flow paths in a heterogeneous porous media for ERT monitoring. The series of time-lapse ERT "snapshots" revealed the three-dimensional distribution of the tracer plume through time, thus providing higher resolution detail about subsurface flow paths and flow barriers than would be possible with only conventional, hydrologic sampling. The tomograms indicate movement of the saline tracer consistent with measured saline concentration data. Therefore, the resistivity tomograms serve as an appropriate surrogate for concentration maps that are otherwise impossible to obtain, and under reasonable assumptions, estimates of groundwater velocity and hydraulic conductivity can be obtained by tracking the tracer. A flow and transport model for the site was developed and calibrated to the combined dataset.
This research was conducted by Kamini Singha of Pennsylvania State University Department of Geosciences - Hydrogeology Program as part of her doctoral research at Stanford University Department of Geological and Environmental Sciences. Additional collaborators included:
Singha, K., Binley, A.M., Lane, J.W., Jr., and Gorelick, S.M., 2003, Electrical imaging of tracer migration at the Massachusetts Military Reservation, Cape Cod, in Symposium on the Application of Geophysics to Engineering and Environmental Problems (SAGEEP), April 6-10, 2003, San Antonio, Texas, Proceedings: Denver, Colorado, Environmental and Engineering Geophysics Society, CD-ROM, 11 p.
Singha, K. and Gorelick, S.M., 2006, Effects of spatially variable resolution on field-scale estimates of tracer concentration from electrical inversions using Archie's law: Geophysics, v. 71, no. 3, p. G83-G91.
Singha, K. and Gorelick, S.M., 2006, Hydrogeophysical tracking of 3D tracer migration—use of apparent petrophysical models: Water Resources Research, v. 42, 9 p.,W06422, doi:10.1029/2005WR004568,2006.
Singha, K. and Gorelick, S.M., 2005, Saline tracer visualized with electrical resistivity tomography—field scale spatial moment analysis. Water Resources Research, v. 41, W05023, doi:10.1029/2004WR003460.
For more information on this project, please contact Kamini Singha (Pennsylvania State University) or John W. Lane, Jr. (Chief, USGS OGW Branch of Geophysics), or call the Branch of Geophysics at (860)487-7402.