Self potential improves characterization of hydraulically‐active fractures from azimuthal geoelectrical measurements

We examined the sensitivity of azimuthal self potential (ASP) and azimuthal resistivity surveys (ARS) to hydraulic anisotropy in a laboratory fracture block model. Both ASP and ARS measurements are influenced by anisotropy (due to the strike of major fracture sets) and heterogeneity (due to variable fracture density) of the block model. The polarity of the self potential defines the flow direction and the self potential magnitude within a single fracture set is observed to increase with flow rate. Whereas the ARS anisotropy is primarily controlled by fracture density/connectivity (and hence presumably hydraulic conductivity) ASP anisotropy appears diagnostic of (1) hydraulic gradient driving flow within fracture sets, and (2) fracture density (presumably controlling the strength of the streaming potential coefficient). Preliminary field data from the New Jersey Highlands illustrate that ASP surveys can define hydraulic anisotropy in fractured rock environments. Our results suggest that combined interpretation of ASP and ARS can lead to improved geoelectrical characterization of fracture rock anisotropy.