Differentiating anisotropy and lateral effects using azimuthal resistivityoffset Wenner soundings

Azimuthal resistivity surveys increasingly are being used by hydrogeologist s in the identification and characterization of fractured rocks. In these i nvestigations, electrical resistivity is measured as a function of azimuth about a fixed central point. In most recent published examples, any observe d change in apparent resistivity with azimuth is interpreted as being indic ative of fracture anisotropy. However, interpretation of rotational soundin g data is actually more complicated, as azimuthal variations in apparent re sistivity are also produced by the presence of dipping stratigraphy and oth er lateral changes in formation resistivity. Such effects are generally ove rlooked because the field techniques normally employed are incapable of det ecting them. Consequently, it is quite probable that the results of many pu blished surveys have been wrongly interpreted. An alternative field procedure and interpretation methodology has been deve loped to differentiate anisotropy, dipping layers, and lateral effects. Thi s approach makes use of the offset Wenner technique and examines the differ ent responses of the individual Wenner resistances above different geologic al structures. Analysis of data obtained using the azimuthal offset Wenner technique from sites in Britain and Ireland have successfully identified su bsurface structures and determined the anisotropy where it is present.