Electromagnetic Surveys for 3-D Imaging of Subsurface Contaminants

Abstract : This report describes a demonstration sponsored and funded by the Department of Defense's (DoD) Environmental Security Technology Certification Program (ESTCP). GEHM Environmental Corporation and the Center for Environmental Technology at the University of Missouri (Columbia) were contracted by the Naval Facilities Engineering Service Ceiiter (NEESC) to investigate the use of quasi-static electromagnetic (EM) resistivity surveys to detect dense non-aqueous phase liquid (DNAPL) contamination in the subsurface at two U.S. DoD installations. This EM resistivity survey technique is a surface to borehole geophysical method that generates a three-dimensional (3-D) image of subsurface features based on their contrasting resistive properties. The two sites selected were the former Naval Air Station Alameda, renamed Alameda Point, and Tinker Air Force Base They were selected for this pilot study on the basis of having a previously well- documented DNAPL problem, coupled with the fact that they reside in two distinctly different types of geologic settings. These sites also have typical limitations with respect to drilling restrictions and with respect to the high degree of uncertainty in knowing where free-phase DNAPL currently occurs in the subsurface. Alameda Point's subsurface consists of saturated unconsolidated clastic sediments, while Tinker AFB consists of interbedded sands and shales.