Problems Associated with the Multiple Source Dipole-Quadripole Array in Locating a Concealed Geothermal Fracture Zone Reservoir

Abstract

The multiple source bipole-quadripole resistivity array has been used successfully in New Zealand to delineate the boundary structure of high temperature prospects in the Taupo Volcanic Zone (NZ). Recently, this array was tested to assess the electrical anisotropy of greywacke basement rocks exposed in two and to delineate the structure of an inferred low temperature system (Whitford prospect, S. Auckland) which occurs in fractured greywacke rocks concealed by a conductive layer of Tertiary sediments. Results of these tests are presented which show that the computed tensor invariants are disturbed by topography effects of the quarry walls; the method appears to be unsuitable for measuring electrical anisotropy of basement rocks in such a setting. The method can be used to delineate low resistivity zones of a concealed low temperature reservoir with small resistivity contrasts (factorof about 2 to 3) where the terrain is smooth and moderate. The observed topography effect has not previously been described.