Misinterpretation of Electrical Resistivity Data in Geothermal Prospecting: a Case Study from the Taupo Volcanic Zone

Abstract

Perhaps the most successful application of electrical resistivity techniques for delineating geothermal systems has been in the early exploration of the Taupo Volcanic Zone of New Zealand. Systematic mapping using the Schlumberger resistivity arrays with fixed spacings has identified 23 individual geothermal systems with welldefined boundaries. The remarkable success of the method in the Taupo Volcanic Zone is linked to the very thick (500+ metres) cover of young ( 1 Ma) which have undergone low-temperature diagenetic alteration to form highly conductive, connected clay paths within the rock matrix. The older, conductive ignimbrites are extensive and continuous around the edges of the TVZ where they are underlain by resistive basement rocks. Within the TVZ, structural investigations using magnetotelluric show these conductors to be present at depth beneath much of the area. We outline here both the history and the lessons of the past, in the hope that they will provide guidance for future geothermal explorations.