Imaging the Roots of High-Temperature Geothermal Systems Using MT: Results From the Taupo Volcanic Zone, New Zealand

摘要

High-temperature, economic geothermal resources are currently restricted to drillable depths (typically less than 3 km). However, the roots of these geothermal resources originate at much greater depths, where fluids extract heat from high temperature sources (i.e. magma) and are transported to the surface. This convective process is not well understood; nor is the associated deep-seated permeability required. Besides sourcing hydrothermal systems from which electricity can now be generated, these deep-rooted geothermal fluids also represent untapped energy resources themselves, which in the coming years may well be exploited with advances in deep drilling technology. Thus a very good case can be made to expand our knowledge on the roots of geothermal systems to better extract the existing resource base as well as tap into new sources of geothermal energy, including sources beyond conventional extraction depths in the coming years. In this paper, we probe mechanisms of heat exchange and fluid transport beneath four high-temperature geothermal systems in the Taupo Volcanic Zone (TVZ) using one of the largest 3D magnetotelluric (MT) arrays ever assembled (259 MT soundings, 1250 km~2). We demonstrate the reliability of full tensor 3D MT modeling by applying a different data processing and 3D inversion algorithm to data analyzed by Bertrand et al. (2012), retrieving many of the same dominant resistivity features. We expand upon their study by using 90 additional MT soundings that cover a new area to the NW, and considering two geothermal systems not discussed in the previous work. According to our resistivity model, sustained convection cells extract energy and volatiles from quasi-plastic rock at ～5 - 7 km, elevating fluid density. Hot saline fluids migrate upward from this depth, potentially controlled by heavily fractured zones (i.e., fault accommodation zones), depressing resistivity as they are transported to the geothermal systems. Intrusive volcanics may also contribute to the low resistivity features imaged between 3 and 7 km.