AZIMUTHAL GEOELECRIC CHARACTERIZATION OF FRACTURE FLOW IN THE NEW JERSEY HIGHLANDS BEDROCK

Fracture anisotropy and interconnectivity control the flow and transport of fluids in fractured rocks. The analysis of flow in fractured rock is complex, as anisotropy may be masked by heterogeneity at the field scale. Collinear azimuthal resistivity surveys (ARS) have been used by several previous authors to characterize hydraulic anisotropy along principal fracture strike orientations based on the analogy between hydraulic conductivity and electrical conductivity in rock aquifers. Consequently, the delineation of groundwater flow paths from ARS investigations performed on fractured rocks at the field scale is often uncertain. Electrical resistivity is related to electrical conductivity in rocks, whereas the self potential method is closely coupled to groundwater flow by the presence of a natural electrical field and is also proportional to the applied hydraulic gradient (but not necessarily the volume of flow). We propose that the ARS technique be combined with the azimuthal self potential (ASP) to increase the capability to predict hydraulic anisotropy by delineating groundwater flow paths in fractured rocks compared to the sole use of ARS that results in the inherent ambiguity of the geologic interpretation.

We performed geoelectrical investigations at eight sites in the New Jersey Highlands using non-linear (asymmetric) azimuthal arrays (self potential (SP), electrical resistivity (ER), and induced polarization (IP)) to (1) improve the characterization of hydraulic anisotropy associated with the strike of dominant fractures sets and (2) distinguish anisotropy from structural heterogeneity in the subsurface. An evaluation of the agreement between the geophysical data and the strike of fracture sets mapped at some sites indicated that hydraulic and resistivity anisotropy were not coincident with the principal fracture strike as a result of structural heterogeneities present in the subsurface. Finally, we evaluate the implications of this integrated approach for understanding groundwater flow in regional-scale fracture systems and the advancement of fracture anisotropy characterization.