Fault Trace Analysis Using Dipole-Dipole Resistivity and Downhole Geophysics in an LNAPL Contaminated Fractured Aquifer

Abstract

A series of significant hydrocarbon releases occurred at a former Ingersoll Rand facility in Phillipsburg, New Jersey between the 1940’s and 1960’s causing an extensive light non-aqueous phase liquid (LNAPL) contamination presence in an aquifer which supplies water for human use. LNAPL migration is difficult to monitor, as its non-aqueous behavior causes migration to occur in the direction of groundwater flow or along less confining features such as fractures or faults. Non-invasive geophysical techniques hold promise for LNAPL characterization at fractured bedrock sites such as this one, given their ability to provide information on a variety of physical earth properties. The primary objective of this research is to test the hypothesis that a thrust fault has influenced LNAPL migration at the site. Geophysical field models were generated from dipole-dipole Electrical Resistivity Imaging (ERI) data which displayed a roughly linear, significantly low-resistivity zone that has been interpreted as a shallow, near vertical expression of a thrust fault which had an apparent influence on LNAPL migration. To complement the ERI survey, optical televiewer, caliper, formation resistivity, temperature, gamma, fluid resistivity, and heat pulse flow meter data were collected in two bedrock wells which

provided insight as to the general strike and dip of fractures and bedding features, minor variations in lithology, and vertical flow conditions as they relate to LNAPL migration. This research also tests the hypothesis that direct detection of LNAPL contamination through the use of ERI measurements is not likely based on predictive forward models constructed prior to the field study. A forward model was generated in the lab using predetermined parameters of known and anticipated geologic conditions. Both the forward model prediction and actual field models revealed that direct detection was not possible as a function of loss of resolution of ERI measurements with increasing depth and the lack of contrast between the measured ERI responses of the LNAPL plume and surrounding geologic material. The geophysical techniques applied in this research show a promising outcome for use at other sites because structural features contributing to contaminant transport were readily identified in the modeled ERI data sets, and the forward model and field models both suggest the direct detection of LNAPL contamination would not be possible under similar conditions.