Three-dimensional magnetotelluric modelling of the Taupo Volcanic Zone, New Zealand

Abstract

The Taupo Volcanic Zone (TVZ) is an important resource for New Zealand’s energy sector as it hosts the majority of New Zealand’s geothermal systems. To be able to increase the power generation in this area significantly over the next few decades, higher temperature fluids from greater depths will need to be harnessed. For this to be achievable a better understanding of the structures that control fluid flow is required. The conductive hydrothermal alteration minerals and fluids associated with high-temperature geothermal systems in an otherwise resistive volcanic environment provide a distinct contrast in electrical resistivity. This contrast makes the geophysical method of magnetotellurics (MT) an ideal tool for the imaging of subsurface structures in the TVZ. This thesis discusses the 3D modelling and interpretation of a large MT dataset from the central TVZ. Although MT is an important tool in geophysical exploration and 3D inversions are becoming increasingly popular for the modelling of MT datasets, there are still many issues associated with this geophysical method and its analysis. Chapter 4 discusses the issues associated with the distortion of MT data caused by electric fences, which is a common problem with measurements taken on farmland in New Zealand. It shows on one hand that not all electric fences distort the MT signal and on the other hand that low level noise from electric fences recorded at remote reference stations can lead to significant systematic errors in calculated electrical impedances. Chapters 6 and 7 address the issues associated with smoothing parameters used to regularise data inversion and workflow of 3D inversion using the 3D MT inversion algorithm ModEM. They discuss selection criteria for the ideal smoothing parameters such as a frequency-dependent RMS and L-curves, and show the advantages of a guided development of the inversion result starting from a homogeneous halfspace. These extensive analyses improved the 3D resistivity model of the deeper structures in the central TVZ compared to the preliminary model used in the study of the Reporoa geothermal system, as discussed in Chapter 5, and especially compared to a resistivity model using default inversion parameters. A joint interpretation of this new resistivity model and pre-existing geophysical and geological data, as discussed in Chapter 8, provided new insights on the location and melt fraction of the heat source, as well as the heat and mass transport in the Taupo-Reporoa Basin and surrounding area.