Detection of near-surface anisotropy in a weathered metamorphic schist using time-domain electromagnetics



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Texas A&M University


Controlled-source, azimuthal, time-domain, electromagnetic (TDEM) surveys were conducted over a schist formation with uniformly striking, nearly vertical foliation. Direct current electrical resistivity and seismic refraction surveys provided additional independent assessment of the field site. Quantitative interpretation of the TDEM survey used a theoretical electromagnetic model of a vertical transverse anisotropic conducting half-space. The combination of forward modeling and azimuthal acquisition geometry provides an innovative geophysical technique useful for mapping poorly exposed metamorphic terrains, and possibly determining fracture system orientations and assessing anisotropic hydraulic conductivity.
Metamorphic rocks may exhibit transverse electrical anisotropy detectable by time-domain electromagnetics due to the characteristics of foliated rocks. For this reason, the field site was chosen within the Packsaddle Schist exposed in Mason County, Texas. Foliation of the Packsaddle Schist at the survey site strikes 146? and dips 82? NE. Polar plots of early-time, TDEM voltages, measured at large transmitter-receiver separations (> 40m) exhibit a symmetric two-lobed curve that agrees with theoretical model responses calculated for a vertical transverse anisotropic half space. The long axis of the symmetric two lobe response function is oriented 137?, which is nearly parallel to schist foliation of 146?. A best-fit forward model to the data indicates the electrical conductivity parallel and perpendicular to foliation are 0.015 S/m and 0.0012 S/m, respectively. Small transmitter-receiver separations (< 40m) exhibit azimuthal responses typical of an isotropic half space, which indicates the presence of a layer overlying the schist probably produced by weathering.
An additional independent azimuthal Wenner resistivity survey exhibits apparent resistivity in the form of an ellipse with the major axis (direction of maximum conductivity) oriented 149?, which is nearly parallel to schist foliation of 146?. Analysis of data indicates the apparent electrical conductivity parallel and perpendicular to foliation are 0.0163 S/m and 0.0094 S/m, respectively. Results of TDEM and direct current resistivity closely match in both orientation and electrical conductivity values. Preliminary seismic refraction data were compatible with the TDEM data and also indicated anisotropy, but were not as conclusive.