Geo-electrical anisotropy corrections derived from square array data to improve Earth resistivity models of the Shale Hills' critical zone

Geo-electric anisotropy in the shallow subsurface, often expressed as the “paradox of anisotropy”, is seldom accounted for in colinear array resistivity models of the critical zone. The paradox of anisotropy can be observed in areas of inclined bedding planes or fractures where longitudinal apparent resistivities are greater than transverse apparent resistivities, which is opposite of the true resistivities. In this study, we illustrate the paradox of anisotropy for fractured shale bedrock in Pennsylvania's Susquehanna Shale Hills Critical Zone Observatory (Shale Hills) using square and Wenner array data, and derive an anisotropy correction from the square array data, which is not influenced by the anisotropy. Square array measurements were made around a center point near the middle of a planar slope on the north-facing side of the Shale Hills watershed at four a-spacings (5, 10, 25, and 50 m), each rotated at 15° increments until a full rotation (0° to 165°) of measurements were collected. Coefficients of anisotropy, estimated from the square array data, were then applied to Wenner array data collected at the same location in the watershed to correct the anisotropic flow of current affecting the colinear (i.e., Wenner) array data. Correction of Wenner array data for profiles parallel and orthogonal to bedding strike yield favorable model results when compared to expected resistivity ranges with respect to bedding strike. We further demonstrate that application of the anisotropy corrections to several Wenner array profiles, followed by a pseudo 3D inversion of the profiles, yields a resistivity model of the subsurface providing new insights into groundwater flow through the Shale Hills' critical zone.