โ Scribed by Trevor Hunt; Mituhiko Sugihara; Tatsuya Sato; Tomoyuki Takemura
No coin nor oath required. For personal study only.
Changes in mass resulting from production and reinjection in geothermal fields can be monitored using repeat microgravity measurements. The measured changes in gravity, however, need to be corrected for the effects of any exploitation-induced ground subsidence. The correction required at each measurement point is the product of the amount of ground subsidence and the vertical gravity gradient at that point. Measurements of the gravity gradient, made using a portable tower, show that it varies from place to place depending mainly on the local topography. Measured gradients, in areas of high subsidence rate, range between ร276 and ร339 mgal/m at 30 sites in the Wairakei-Tauhara field and between ร296 and ร321 mgal/m at nine sites in the Ohaaki field (New Zealand). These values are similar to those used previously. At 37 sites measured in the Yanaizu-Nishiyama field (Japan) the gradient varied between ร244 and ร352 mgal/m. Measurements at six sites showed no significant change in gravity gradient with height above the ground surface; a single value for the gradient can therefore be used at each site to correct for the gravitational effect of any ground movement. Calculations show the effects on the vertical gravity gradient of groundwater level variations, mass changes associated with the ground subsidence, and reservoir mass changes are negligible at Wairakei.
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