Fracture anisotropy characterization in crystalline bedrock using field-scale azimuthal self potential gradient

Abstract

The potential for the geophysical characterization of fractured rock bulk anisotropy using azimuthal self potential gradient (ASPG) method was investigated at four fractured rock field sites in the New Jersey Highlands (NJH) province. A visual (from polar plots) and empirical (from linear regression of cross plots) correlation exists between ASPG measurements and dominant fracture strike orientations mapped at three of the four study sites, with the magnitude of the SP signal positively correlated to the number of fractures (N) along the same azimuth. This result is consistent with recent electrokinetic theory that invokes a dependence of the streaming potential coupling coefficient on microgeometry in addition to hydraulic gradient. Based on local surface drainage patterns and regional fracture orientations, the direction of the positive lobe of the ASPG signal appears indicative of the likely flow direction within fracture sets. Three repeat measurements at one site over a one year period consistently reproduced the ASPG lobes along major and minor fracture sets, although the magnitude of the self potential varied along the major strike set possibly due to variable rates of groundwater flow. Whereas ASPG captured hydraulic-anisotropy at three of four sites, resistivity anisotropy only dominated azimuthal resistivity surveys (ARS) data at one site. Our results suggest that ASPG is a surprisingly simple, effective tool for characterization of fracture-dominated media. In addition to defining the fracture-induced hydraulic anisotropy, the direction of fracture-controlled flow is evident from the polarity of the SP signal.