USING RAPID AZIMUTHAL RESISTIVITY SURVEYING FOR FRACTURE DETECTION AND ORIENTATION ANALYSES IN GROUNDWATER STUDIES

Using azimuthal electrical resistivity to estimate fracture-related permeability is not a new concept. Many previous studies have been conducted using traditional galvantically-coupled resistivity (GCR) meters for this purpose. However, the GCR meter requires a significant amount of time to set an array (for multi-electrode units) or to take multiple resistivity measurements (for single channel units). As an alternative for large study areas or for detailed work that requires a large number of resistivity measurements, a capacitively-coupled resistivity (CCR) meter may be used. The advantage of the CCR meter is that it has no electrodes to plant--instead the transmitter and receiver consist of coaxial cables that can be easily and quickly moved on the surface, requiring no more than a few seconds per reading.

With the CCR meter, it becomes logistically feasible to collect more data in a relatively short period of time (e.g., 8 n-spacings for each 10-degree increment of azimuth including setup time in about three hours). This type of data is used to construct 1-D depth models of apparent resistivity for each azimuth and estimate depth-dependent variations in fracture orientation and density (i.e., permeability). The theoretical basis for this approach and preliminary field data are presented.