New insights from analysis and 3D inversion of legacy geophysical data at the Ngawha geothermal field, New Zealand

We present a revised resistivity boundary of the Ngawha geothermal field, New Zealand, based on the re-processing, inversion and interpretation of legacy direct current (DC) Schlumberger, multiple-source bipole-dipole resistivity, gravity and magnetic measurements. Three-dimensional (3-D) resistivity inversion of the combined electrical datasets indicates there is a deep (c. 2 km) low resistivity zone c. 4 km northwest of Ngawha Springs. This is interpreted as an area where hot geothermal fluids rise from beneath Late Miocene to Pleistocene volcanics and migrate southward through permeable Waipapa Group greywacke rock to Ngawha Springs. A gravity low with comparable geometry to the region of low resistivity is modelled as a zone of lower density Waipapa Group greywacke rock (-120 kg/m³), corresponding to c. 7% porosity, and is likely due to a network of joints and macrofractures that form the geothermal reservoir. Aeromagnetic data modelling suggests the presence of a deep, weakly magnetic pluton in the northwest part of the geothermal system. This, together with the occurrence of gas seeps, warm springs and relic sinters deposits suggests a geothermal heat source in this area. Our findings demonstrate that modern analytical and modelling techniques can extract new information from legacy geophysical data leading to new insights into the structure of the Ngawha geothermal system. Legacy data from geothermal fields globally, warrants reinterpretation with modern methods.