USGS Groundwater Information: Branch of Geophysics
by Carole D. Johnson, Eric A. White, Julian Ivanov, and John W. Lane, Jr.
Surface-geophysical methods are rapid, minimally invasive ways to characterize the geologic framework at contaminated fractured rock sites. Two-dimensional (2D) direct-current (DC) electrical resistivity profiling and multi-channel analysis of surface-wave (MASW) seismic methods were conducted in open land east of the former Naval Air Warfare Center (NAWC) in West Trenton, NJ. The surface-geophysical methods proved effective for delineating large-scale geologic structures, including fault characteristics and lithologic changes. Because the two geophysical methods used for this study measure different physical properties, (electrical resistivity and seismic shear-wave velocity, respectively), their combined use to characterize a site can improve interpretational confidence.
The 2D resistivity profiling data were acquired using a multi-channel, 56-electrode resistivity system and Dipole-Dipole, Schlumberger, and Wenner arrays. The resistivity field data were collected over a single day, using inter-electrode spacings ranging from 2 to 8 meters. The resistivity field data were inverted to generate cross sectional electrical resistivity models of the subsurface to depths of about 70 m below land surface. Interpretation of the resistivity results was aided by electrical resistivity forward modeling of the NAWC site geology.
The MASW seismic data were acquired over 1.5-days using an accelerated weight-drop seismic source and a 30-geophone towed land streamer. The MASW seismic data were inverted to generate shear-wave velocity models of the subsurface to depths of about 20 m below land surface.
Although collected and processed independently, the 2D resistivity and MASW seismic method interpretations were generally consistent. Both methods identified the depth to bedrock, shallow northward dipping stratigraphic layers, and a steep southward dipping fault whose trend projects toward the NAWC site. Overall, the MASW method better resolved the bedrock surface than the 2D resistivity method. However, the 2D resistivity imaged deeper into the subsurface, thus improving interpretation of the fault dip. This work provides an example of the benefit of using surface geophysical methods to characterize geologic structure and stratigraphy, and their potential for identifying fluid and chemical migration pathways, refining site conceptual models, and for optimizing the location of characterization and sampling boreholes.
Final copy as submitted to the Geological Society of America for publication as: Johnson, C.D., White, E.A., Ivanov, J., and Lane, J.W., Jr., 2006, Use of electrical resistivity and surface-wave seismic imaging methods to characterize the geologic framework of the NAWC site, West Trenton, New Jersey [abs.]: Geological Society of America Abstracts with Programs, v. 38, no. 7, p. 528, Paper no. 219-9.
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