SGP-TR-165 EVALUATION OF MICROGRAVITY BACKGROUND AT THE UNDISTURBED OGUNI GEOTHERMAL FIELD, JAPAN

Repetitive microgravity monitoring surveys are planned to help characterize reservoir behavior after electric power generation begins at Oguni in 2002. To prepare for this monitoring program, background microgravity has already been surveyed numerous times at Oguni. Seasonal changes in gravity have been observed. The observed background temporal changes in microgravity are believed to be due primarily to variations in the depth of the shallow ground water table. To separate the effects of water table fluctuations from those caused by events in the deep reservoir, we first tried to correlate both gravity changes and water table level changes with precipitation at a particular benchmark where the water level and microgravity histories were both known. Next, the resulting correlation was used to compute the effect of precipitation on microgravity elsewhere in the field, and the results were compared with measurements. Observed excursions in microgravity are somewhat greater than those estimated in this manner