Correcting for Effects of Ground Subsidence in Microgravity Monitoring

Abstract

The correction for the gravity effects of ground subsidence in microgravity monitoring depends on the vertical gravity gradient (VGG) and the amount of subsidence. Measurements show that the mean values for VGG in areas of subsidence are: -302 (+13) microgal/m at Wairakei, -300 &14) at Tauhara, and -312 (+13) at Ohaaki fields. The topography within about a hundred metres of the measurement point can have a significant effect on the VGG, but in all previously reported gravity data in these fields the correction has been valid. Calculations show that, at Wairakei, the effects on the VGG of changes in shallow groundwater level, fluid movement associated with the subsidence, and net mass changes in the reservoir can be neglected.