ABSTRACT
Geothermal fluid has been extracted for electricity generation from the Taupo Volcanic Zone in the central North Island of New Zealand since the mid-1950s. This has induced regional subsidence of more than 1 m; but there are a few localized bowls with much greater subsidence. A comprehensive geotechnical investigation with recovery of undisturbed samples from depths of up to 774 m was undertaken with testing of core samples from within and outside subsidence bowls to determine material properties. More than 130 Ko triaxial compression tests were done. Given the volcanic origins of most of the material, the observed wide variation in the measured constrained modulus values is not surprising. Scanning electron microscope images (SEM) reveal differences between the texture of the soft materials and the very stiff materials, these correlate with the constrained modulus values. It was SEM imaging that provided a window into the complex nature of these subsurface environments and identified processes driving the physio-chemical conditions acting and either hardening or softening rocks at specific depths. As geothermal environments are constantly changing and each environment leaves a footprint which is preserved within the rock, SEM can be used to track these changes placing them into a spatial and temporal context. While compressibility measurements identified soft zones within the formations, SEM observations documented subsurface processes responsible for weakening the various lithologic units; these changes in the subsurface conditions are ultimately responsible for the subsidence visible at the surface.
