Differentiating anisotropy and lateral effects using azimuthal resistivity offset Wenner soundings

Abstract

Abstract Azimuthal resistivity surveys increasingly are being used by hydrogeologists in the identification and characterization of fractured rocks. In these investigations, electrical resistivity is measured as a function of azimuth about a fixed central point. In most recent published examples, any observed change in apparent resistivity with azimuth is interpreted as being indicative of fracture anisotropy. However, interpretation of rotational sounding data is actually more complicated, as azimuthal variations in apparent resistivity are also produced by the presence of dipping stratigraphy and other lateral changes in formation resistivity. Such effects are generally overlooked because the field techniques normally employed are incapable of detecting them. Consequently, it is quite probable that the results of many published surveys have been wrongly interpreted. An alternative field procedure and interpretation methodology has been developed to differentiate anisotropy, dipping layers, and lateral effects. This approach makes use of the offset Wenner technique and examines the different responses of the individual Wenner resistances above different geological structures. Analysis of data obtained using the azimuthal offset Wenner technique from sites in Britain and Ireland have successfully identified subsurface structures and determined the anisotropy where it is present.