Determining Fracture Geometry From Azimuthal Resistivity Data

This paper presents the results of a series of three-dimensional finite difference models conducted to<br>determine the response for Wenner array azimuthal resistivity surveys run over conductive fracture zones in resistive<br>bedrock beneath conductive overburden. Microanisotropic and macroanisotropic fracture systems with varying<br>overburden thicknesses were simulated. Microanisotropic models were simulated by applying a uniform anisotropy<br>factor to the bedrock layers. Macroanisotropic models were simulated by representing fracture zones as conductive<br>vertical dikes. The model results demonstrated that for both microanistropic and macroanistropic cases, the shape<br>and magnitude of the apparent resistivity ellipse varies significantly as a function of overburden and “a” spacing.<br>The magnitude of the variations is sufficient to cause significant misinterpretations of azimuthal resistivity field data.<br>Several important factors must be considered when interpreting fracture patterns from field data. In summary, these<br>are: 1) the choice of a macroanisotropic or microanisotropic conceptual model, 2) the thickness of overburden<br>relative to the “a” spacing, 3) the position of macroanisotropic fracture zones relative to the center of the array, 4)<br>the width of macroanisotropic fracture zone relative to the “a’ spacing and thickness of overburden, 5) the presence<br>of parallel or intersecting macroanisotropic fracture zones, and 6) the degree to which the field conditions are purely<br>microanisotropic or macroanisotropic, rather than a mixed model.