Gravity Changes in the Wairakei-Tauhara Geothermal Field: 1983-1991

Abstract

During the period 1983- 1991about 375Mt (375 x kg) of fluid was extracted the Wairakei borefield and it estimated a further 40 Mt was discharged naturally. In precise gravity measurements were made at 88 benchmarks in and around the field to enable the gravity changes since 1983 to be determined. The data show that, for the period 1983- 1991,gravity increased by 0 - microgals over most of the part of the field, and increased by - 400 microgals in the eastern part of the borefield. Only in a area, northwest of Thermal has gravity decreased, and here most of the changes have been less than 50 microgal. The gravity increases probably reflect a decrease of up to 30 in the thickness of the steam zone, or an increase in liquid saturation of up to 0.05 in the steam zone, over most of the Wairakei part of the field. In the Tauhara part of the field there were gravity increases of 0 - 50 microgal in the northern and western areas, but elsewhere there were no significant changes. Calculations, using Gauss's indicate a field-wide net mass of about 45 Mt.