Three-Dimensional Magnetotelluric Imaging of the Reporoa / Waiotapu Geothermal System, New Zealand

Abstract

Broadband magnetotelluric (MT) measurements at 230 sites have been used to create a 3D resistivity model of the Taupo Volcanic Zone (TVZ). This paper focuses on the northeastern part of this model that gives an image of the structures at the Reporoa / Waiotapu geothermal systems down to several kilometres depth. Preliminary interpretation suggests the presence of a thin, low resistivity, hydrothermally altered layer at 200-500 m depth. Between 1-2 km a more extensive low resistivity zone, which infills the Reporoa caldera, was also identified. This feature was attributed to hydrothermal alteration by past and present convecting geothermal fluids. A plumelike structure near the northern boundary of the caldera located between the Reporoa and Waiotapu systems was attributed to the upflow of the geothermal system heated from below by a magmatic intrusion in the 6-10 km depth range.