Physical-Scale And Numerical Modeling Of The Azimuthal Electromagnetic Response In The Macroanisotropic Case

Abstract

Since 1995, we have collected electromagnetic (EM) field data using a rotating azimuthal geometry analogous to that described for resistivity surveys by other workers. These EM azimuthal resistivity data are relatively easy to acquire and can be collected in areas with high electrode contact resistance. As an aid to interpreting field results, including apparent paradox of anisotropy conditions, we have used physical-scale modeling, and have run a suite of numerical models. Numerical and physical-scale modeling results indicate that the primary factor affecting the orientation of the apparent resistivity ellipse relative to a macroanisotropic linear conductor is the depth to the target (relative to the transmitter-receiver coil separation). At target depths less than about 10% of the coil separation, an electromagnetic paradox of anisotropy is observed in the modeling tank, in the numerical model, and in our field data (i.e., high values of apparent resistivity along the strike of the target). While physical-scale modeling is limited by the availability of materials of appropriate conductivity, numerical modeling provides a sufficiently extended parameter space for these analyses. The numerical model also indicates that the conductance of the target does not effect the orientation of the apparent resistivity ellipse.