
Allis, R.G.; Webber, S. 1984 Shallow temperature measurements at Wairakei and Broadlands fields. [s.l.]: [s.n.]. Report / Geophysics Division 197 27 p
Abstract: One metre depth temperatures have been surveyed over a 2 sq. km area containing the Wairakei production borefield, and over a 16 sq km area covering most of Broadlands field. Both surveys were intended to provide a data set which could be compared with future surveys to determine the changes caused by the withdrawal of geothermal fluid from production depths. A comparison with similar surveys made at Wairakei field in the late 1950's and during mid 1960's shows substantial differences. Significant cooling has occurred in the northern and eastern part of the production borefield since the mid 1960s. One thermal anomaly which contained over 1000 sq. m of ground with a temperature greater than or equal to 50 deg. C at 1 m depth, has disappeared. Measurements in several groundwater wells in this area indicate that the groundwater level has fallen by over 10 m, but the temperature at the water level has increased since the mid 1960s. One metre depth ground temperatures appear to be much more sensitive to groundwater depth changes than to groundwater temperature changes. This may be due to the movement of water vapour in a 10-20 m thick zone above a boiling water surface. Once the water surface descends below about 20 m depth, the shallow thermal anomalies become small and may be masked by other perturbing factors such as infiltrating rainwater. In areas with a falling groundwater surface, thermal anomalies may persist only if the groundwater is strongly boiling, and the steam flow is sufficient to reach the surface before significant condensation