| Title | Misinterpretation of Electrical Resistivity Data in Geothermal Prospecting: a Case Study from the Taupo Volcanic Zone |
|---|---|
| Authors | Bibby, H.M., Risk, G. F., Caldwell, T. G. and Bennie, S. L. |
| Year | 2005 |
| Conference | World Geothermal Congress |
| Keywords | Geothermal Prospecting, electrical resistivity, interpretation |
| 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 well-defined boundaries. The remarkable success of the method in the Taupo Volcanic Zone is linked to the very thick (500+ metres) cover of young (<500 ka), resistive, pyroclastic material that forms the upper layers of the region. However, the success has resulted in a lack of care with interpretation based on the belief that resistivity data can be simply interpreted using the apparent one-to-one correlation between low resistivity and the presence of geothermal fluids. The first example of misinterpretation of electrical data in the Taupo Volcanic Zone was the Horohoro (Matahana) prospect where a deep low resistivity layer was drilled for geothermal fluids in 1987 despite warnings of possible misinterpretation. This well was cold and the cause of the low resistivity has now been established as old ignimbrite layers (>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 wise guidance for future geothermal exploration. |