Resistivity structure of the Waimangu, Waiotapu, Waikite and Reporoa geothermal areas, New Zealand

Abstract

Electrical resistivity measurements have been used to investigate a 650 km 2 region containing four separate areas of geothermal surface activity: Waimangu, Waiotapu, Waikite and Reporoa. Measurements were made using the Schlumberger array with fixed spacings (AB/2) of 500 m (1167 measurements) and 1000 m (720 measurements). The data outline an area of over 100 km 2 where near surface electrical resistivity is less than 30 Ωm; the most extensive low resistivity region known in the Taupo Volcanic Zone (TVZ). Within this large area, three smaller zones of resistivity less than 10 Ωm can be distinguished which contain the geothermal activity of Waimangu, Waiotapu and Reporoa. A fourth area of surface geothermal activity, Waikite, lies about 5 km to the west of the Waiotapu Springs, and is separated from them by the high elevation Paeroa Range. Interpretation of the resistivity measurements suggests a single deep source for the geothermal fluid discharged at both Waiotapu and Waikite, that lies beneath the high elevation ground. This Waiotapu-Waikite system straddles a northwest-southeast drainage divide, and thus subsurface movement of geothermal waters occurs to both the north and south, masking the northern and southern extent of the deeper parts of the system. The Waimangu Geothermal Field, as outlined by the resistivity data, is much more extensive than indicated by the distribution of surface features. The field is characterised by abrupt resistivity discontinuities to the north, west and east, with the northern boundary crossing Lake Rotomahana. It is not possible to identify a southern boundary to Waimangu because of the masking effects of the near surface movement of geothermal fluids from Waiotapu. The resistivity signature of the Reporoa thermal area is not consistent with the commonly accepted model in which the Reporoa thermal waters originate as groundwater drainage from Waiotapu. The sharp boundaries to the resistivity anomaly, and the resistivity variation with depth at Reporoa, are quite different from the region between the Reporoa and Waiotapu. These characteristics, observed in many geothermal systems in the TVZ, suggest that the resistivity anomaly marks an independent geothermal system at Reporoa.