| Abstract |
Explotation has caused over 9m of subsidence at Wairakei since 1950, up to 30 cm at Broadlands between 1968 and 1974, and up to 25 cm at Kawerau since 1970. Despite these differences and large differences in the rate and amount of mass withdrawal, there are similarities in the pattern of subsidence at all 3 fields. In each field, pressure drawdown in the production zone can be identified as a nearly circular area of consolidation centred on the production borefield. The circularshape suggests that the predominant fault direction may not be the main factor controlling horizontal permeability. In addition to production zone consolidation, each field has a small area of relatively intense subsidence originating fran shallow depth. The area of shallow consolidation is near to the natural outflow zone where geothermal water originally rose to nearsurface and dispersed in highly compressible formations. Early identification of such areas in other fields is important because the high rate of consolidation at shallow depth cancause large horizontal strain and tilt of the ground surface. There is also evidence of cool groundwater inflow near the area of shallow consolidation in all three fields. Shallow reinjection into these areas may mitigate the effects of subsidence and groundwater invasion. |