Assessment of precise surface-gravity measurements for monitoring the response of a geothermal reservoir to exploitation

Recommendations for carrying out surveys which achieve 15, 10 and 5 microgal precisions are presented. Achieving the smaller standard deviations will require more field effort and will be more costly. For a 60 station survey, at commercial rates in 1981, typical costs are estimated to be $20,000, $26,000 and $35,000 respectively, for data collection, reduction and interpretation. These figures exclude instrument purchase or rental. Twenty geothermal areas in the western United States which might be suitable for precise repetitive gravity monitoring were evaluated. The evaluation criteria included capability for subsidence on a geological basis, estimated electrical production, environmental impact, and anticipation of production in the near future. It is felt that the most promising areas in order of priority are: (1) the Salton Sea field, California; (2) Valles Caldera, New Mexico; (3) The Geysers-Clear Lake; and (4) Westmorland, California; (5) Roosevelt Hot Springs, Utah; and (6) Heber; (7) Brawley; and (8) Long Valley, California.