Tilt monitoring at wairakei geothermal field

Abstract

The withdrawal of fluids from the Wairakei geothermal field has induced substantial deformation of the ground surface, with subsidence rates as great as 400 mm/yr. Substantial changes to pressures within the upper steam zone of the reservoir would be expected to be recognised in corresponding changes in the deformation rate and in particular in the tilting. Two types of tiltmeter have been tested for this purpose at Wairakei geothermal field. A 5 month test of borehole tiltmeters showed short duration tilt differences as great as 35 μrad (microradians) between meters 5 m apart. This effect is believed to be caused by differences in local response to regional subsidence. Over 5 months of testing the tilt rates from two borehole meters were in good agreement with surface tilt rates derived from a local levelling network (520 μrad/yr). The short-term test of a two-component bubble tiltmeter mounted on a horizontal concrete block gave results that were dominated by a large amplitude (150 μrad) diurnal tilt variation attributed to the response of the block to air temperature variations. However the repeatability of the diurnal response suggests that the tilt rate could be determined from as little as 3 days' observations provided temperatures of the site and instrument were also monitored. In the absence of site temperature data, daily temperature extremes (measured at about 1 km from the site) were used to eliminate the temperature-dependent response. The resulting secular tilt rate (measured over 36 days) of 950 ± 130 μrad/yr, is larger than the long-term estimate determined from precise levelling surveys.