Integrated Approach To Interpretation Of Magnetotelluric Study at Wairakei, New Zealand

Abstract

Contact Energy Ltd (Contact) undertook a magnetotelluric (MT) survey at the Wairakei-Tauhara geothermal system during early 2010. This study presents MT interpretations for the Wairakei system. Both one-dimensional (1D) and three-dimensional (3D) MT models were developed to maximize confidence in MT data interpretations. While the 3D model is useful to study field wide variations of resistivity, the 1D model allows a more detailed examination of the shallow conductive layers. The availability of drill-hole data and geophysical information (e.g. magnetism) has allowed an integrated interpretation of MT in the context of stratigraphy, structure, hydrothermal alteration and temperature distribution. Some features that stand out in the Wairakei system are the: 1. Correlation between the base of conductive layers as defined by MT, the base of swelling clay zone (as indicated by Methylene Blue tests) and the 200°C isotherm; 2. Lack of conventional, doming shape for conductive layer at upflow area (Te Mihi), possibly due to presence of impermeable rhyolite bodies; 3. Deep outfield (Poihipi West) and infield (Karapiti South) conductors indicating relict alteration, locally in agreement with aeromagnetic anomalies; 4. Deep conductive layer within infield injection area (Otupu)