Abstract

Gravity changes (corrected for subsidence) of up to −1 000 (±300) μGal have occurred in the 1 km2 area of the production bore field at Wairakei, and smaller decreases extend over a 50 km2 surrounding area. The largest part of these decreases occurred during the 1960s; since then the net gravity change for the whole field has been zero, indicating mass flow equilibrium. The principal causes of gravity change have been deep liquid pressure drawdown which resulted in formation of a steam zone, subsequent saturation changes in the steam zone, liquid temperature decline, and groundwater level changes. Gravity models suggest saturation of the steam zone was 0.7 (±0.1) in 1962 and decreased to 0.6 by 1972. Gravity increases in the northern and eastern bore field since the early 1970s are attributed to cool water invading the steam zone.

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