Reinjection Monitoring in the Larderello Geothermal Field Using Microgravity and Topographic Measurements

Abstract

In 1986 a geodetic network was set up to monitor the ground vertical movements and gravity changes of the Larderello geothermal field. Elevation measurements carried out in 1986 on benchmarks belonging to a precise leveling line of 1923 showed that the zone of Larderello that had been exploited for the longest time had experienced a maximum subsidence of approximately 170 cm in 63 years (an average of 2.7 cdyear). Systematic measurements performed after 1986 show that the subsidence is now less than 1 and that only the zones exploited most recently are subsiding, while the longest exploited zone appears to be relatively stable. Gravity variations in the time span 1986-1993 have been very small; practically with the same order of magnitude, or slightly higher, than the estimated .environmental noise. 15 This is in agreement with a quasi-equilibrium dynamic state of the fluid mass involved in geothermal exploitation. The gravity changes, although very small, have a good coherence in time and space. Therefore a rough evaluation of the masses that cause the most reliable gravity change features is possible. Two positive gravity change anomalies observed at the end of the 91 and periods, in the zone of maximum reinjection (west Valle Secolo zone), seem to indicate an accumulation of mass equal to approximately 14% of the water reinjected in the first period and 3% in the second period.