Subsidence in the geothermal fields of the Taupo Volcanic Zone, New Zealand from 1996 to 2005 measured by InSAR

Abstract

A number of the geothermal systems in the Taupo Volcanic Zone (TVZ), New Zealand, have been utilised on a large scale to provide heat and to generate electricity, in some cases causing areas of localised subsidence. Subsidence monitoring using field-based surveys is practically constrained by limits to available resources so we have investigated the use of satellite differential radar interferometry (InSAR) for this purpose. Using ERS and Envisat radar data spanning 1996 to 2005, we have mapped the deformation at five of the heavily utilised geothermal fields of the TVZ. Subsidence signals were identified at the Ohaaki geothermal field from 1999–2004, and at Wairakei–Tauhara from 1996–2005, where our measurements compare well with coeval levelling data across the wider deformation field. Subsidence was also measured at Rotorua from 1996–2000. In favourable conditions, the InSAR measurements provide a relatively dense spatial coverage of the deformation field that extends well beyond the boundaries of the geothermal systems and beyond the scope of the networks of levelling benchmarks. In the case of the Wairakei–Tauhara geothermal field, using InSAR it is now possible to improve the spatial resolution near the field margins and to interpret the subsidence signals in the context of the wider, more regional, deformation. Our data also provide new insights into possible fault motion at the Mokai geothermal field occurring around the time of the commissioning of its first power station. We note, however, that the InSAR technique is not without limitations. High gradient subsidence features are poorly represented, although this can be resolved to some extent via a trade-off in data processing. Temporal decorrelation, a well known problem for this technique, is also an issue for TVZ geothermal fields. Therefore, we find that it is possible to provide fortuitous snapshots of the deformation at the TVZ geothermal fields, but operational monitoring using InSAR would be difficult as the proportion of suitable interferograms is low.