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

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