Net mass loss from the Walrakei geothermal field, New Zealand

Abstract

A method of monitoring the net mass loss from a geothermal field under exploitation using a gravimeter is described. Measurements of the value of gravity at 50 benchmarks at Wairakei geothermal field, New Zealand, show that differences of up to 0.5 mgal have occurred between 1961 and 1967, and up to 0.1 mgal between 1967 and 1968. These differences, corrected for known changes in elevation, reflect the net mass of water lost from the aquifer. The net loss between 1961 and 1967 is determined as being about 2.9 × 10 4 g and hence only about 20% of the water drawn off was replaced, but between 1967 and 1968 there was little or no net loss. The gravity method can also give an indication of the area from which the water has been drawn.