Demonstrating Three-Dimensional Electrical Resistivity Imaging in Fractured Rock to Characterize Fractures and Monitor Amendment Injections

Rutgers University, the U.S. Geological Survey, and Pacific Northwest National Laboratory are demonstrating the use of 3D cross-borehole electrical resistivity imaging (ERI) for improving the characterization of fractured rock aquifers and the monitoring of amendment treatments that biologically stimulate reductive dechlorination of chlorinated ethenes.  Seven, 4-inch diameter, boreholes have been drilled at the Naval Air Warfare Center (NAWC) in West Trenton, New Jersey, which is a mudstone fractured rock site contaminated with trichloroethylene, cis-1,2-dichloroethene, and vinyl chloride.  Interpretations of borehole geophysical logs including caliper, optical televiewer, acoustical televiewer, gamma, and ambient/transient heat pulse flowmeter determined local fracture locations, their hydraulic significance, and connectivity between boreholes.  Cross-hole hydraulic testing identified bedding-plane parting fracture zones and possible fault zones that are the dominant hydraulic pathways.  In a synthetic simulation based on the NAWC site, we found that constraining ERI inversions with this type of fracture zone information from geophysical logs and hydraulic testing substantially improved the resolution of fractures and the prediction of amendment transport away from the boreholes.  A field demonstration of ERI has been designed where 3D electrical resistance measurements were acquired from electrodes in all seven boreholes.  To test whether limiting the preferential current pathway introduced in a fluid-filled borehole will have a significant effect in resolving fracture zones in numerical modeling, highly resistive inflatable packers have been installed between two or more electrodes.  ERI measurements collected during a doublet tracer test will be used to: demonstrate the high-resolution capability of electrical resistivity in fractured rock, suggested in the synthetic studies, and to design an amendment injection for remediation.  Our goal is to demonstrate that ERI using structural constraints in numerical modeling improves characterization of fractures and spatial distribution of amendment treatments.