GEOELECTRICAL ADVANTAGES OF HYDRAULIC ANISOTROPY CHARACTERIZATION OF FRACTURED ROCK FROM THE AZIMUTHAL SELF POTENTIAL GRADIENT

Azimuthal resistivity surveys (ARS) have been used to characterize hydraulic anisotropy in rock fractures. Previous researchers have inferred that the correlation between electrical resistivity anisotropy and dominant fracture-strike orientations are equivalent to the hydraulic conductivity anisotropy of an aquifer. Electrical resistivity anisotropy is sensitive to the electrical conductivity of fractures, but bears no empirical relation to the velocity, magnitude, or directional fluid potential energy during the movement of groundwater through geologic media (pores and fractures). For example, whereas both water-filled and clay-lined fractures can generate preferential flow of electrical current, only the former can generate preferential flow of groundwater. A combination of azimuthal geoelectrical measurements was used to assess the anisotropic characteristics of aquifers on (1) a physical-scale fracture block model (an analogue of the Wisconsin-Niagaran Dolomite Formation), (2) bedrock fractures in the New Jersey Highlands Province, and (3) mechanically-induced fractures in unconsolidated sediments. Despite the basic analogy that can be made between the flow of groundwater and the flow of electric current in the earth, the hydraulic properties of an aquifer are not readily extractable from electrical resistivity measurements. We argue that the geophysical estimation of hydraulic anisotropy (direction of preferential groundwater flow) is better diagnosed by measuring the natural electrical field generated by the flow of water through the fracture network i.e. the azimuthal self potential gradient (ASPG). A comparison of polar plots of self potential and resistivity data clearly distinguish between hydraulic anisotropy and electrical anisotropy responses imposed on fracture strike sets. Our findings indicate the polarity of the self potential signal also defines the direction of groundwater flow within the fractures and the magnitude is proportional to the flow velocity induced within the fractures. We suggest a combination of azimuthal geoelectrical measurements should be used to examine the anisotropic characteristics of aquifers, but that the azimuthal self potential gradient may be the obvious candidate method for examining anisotropy in groundwater flow.