A new investigative approach to understanding heat migration pathways within the shallow subsurface at Orakei Korako, New Zealand

Abstract

A heat flow survey of Orakei Korako, located within the Taupo Volcanic Zone, New Zealand was undertaken in 2012 using the new multi-technique approach of sinter analyses, infrared imaging, downhole temperature measurements, Ground Penetrating Radar and surface heat loss calculations. The survey established sites of elevated temperature, not only at the surface but also in the shallow subsurface ( < 5 m depth). Sinter mineralogy and morphology identified acidic steam condensate, post-depositional overprinting at sites of elevated temperature in the subsurface, and unaltered sinter at the surface even though there were elevated subsurface temperatures. Ground Penetrating Radar (GPR) imaged strong amplitude reflections over unaltered sinter deposits and weak amplitude reflections where the sinter had been altered via acidic steam condensate. GPR also imaged subsurface conduits for ascending steam, which were confirmed by measured downhole temperatures of > 90°C. Infrared imaging of individual sites within the geothermal field and heat loss calculations of accessible hot springs and pools provided further information on the heat flow at Orakei Korako. This new multi-method approach identified areas of heat in the subsurface where there is no present-day evidence of heat at the surface, and areas of sustained heat in the subsurface as evidenced by the high level of acidic steam condensate overprinting of surface sinter deposits. Repeated surveys will identify if the surface and shallow subsurface heat flow mapped in 2012 shifts around Orakei Korako, and map areas of the field that are heating up or cooling down with time.