An analysis of the bipole-dipole method of resistivity surveying

Abstract

Bipole-dipole (B-D) resistivity mapping has been widely used as a reconnaissance method in geothermal exploration. In this technique, apparent resistivities are plotted at roving dipole receiver locations and the current source (bipole) is left fixed. Interpretation to date has been in terms of simple, layered, dike, vertical contact, or sphere models. In the case of more complicated two-dimensional models the interpretation is much more ambiguous and the detection of buried conductors depends very much on the choice of transmitter location. Since apparent resistivities taken on a line collinear with the dipole are roughly equivalent to the apparent resistivities for one sounding in a dipole-dipole (D-D) pseudo-section, the two methods have been compared for several two-dimensional models. A buried quarter space and a buried horizontal cylinder of rectangular cross section, with or without an overburden layer, have been used in the comparison. Unless the target is very shallow or close to the bipole or dipole, the resolution of the horizontal position or depth extent for the B-D method is very poor. Conductive overburden worsens the situation for both methods but the effect is more drastic for the B-D method. The spatial patterns for these models is complex for the B-D method and in fact for certain transmitter positions only subtle differences exist for the buried cylinder and buried quarter space models. Multiple sources improve the resolution of the B-D method, but many sources coupled with the high sampling density of receivers required to define the spatial patterns would greatly reduce the cost effectiveness claimed for this method. Changing the bipole orientation with respect to the strike of the models contributes little if anything to the resolution of the models. A further experiment of calculating a residual map by subtracting the half-space or layered half-space response from the response of the buried models was also unsuccessful in improving the interpretability of the B-D method. Finally, a model representative of a typical Basin-and-Range valley with and without a hypothesized geothermal reservoir shows that in more complex models the B-D map would not, in a practical survey, reveal the reservoir. From these model studies it is clear that, except for some simple geologic situations, the B-D method is not effective for subsurface mapping. Selected D-D lines would be far more useful and more cost effective.