Use of the azimuthal offset Wenner technique to characterise a single dipping interface

摘要

Previous work has demonstrated that azimuthal soundings collected using a symmetrical four-electrode array over anisotropic ground are ambiguous and may be confused with similar results obtained over dipping interfaces and lateral changes in resistivity. However, use of a five-electrode asymmetrical offset Wenner technique enables discrimination between the different earth models. Subsequent detailed analysis of apparent resistivity patterns recorded by azimuthal Wenner arrays highlights the similarity of response generated by dipping interfaces and truly anisotropic ground. For example, it is possible to reproduce multiple peaked ellipses, often attributed to multiple fracture sets, in azimuthal data recorded above a single dipping interface. Analytical modelling of azimuthal offset Wenner data for a wide range of interface depths, dip angles and resistivity contrasts identifier key stages in azimuthal sounding, demonstrates the sensitivity of the response to differing ground conditions, and enables estimates of the dip and resistivity contrast to be made.