Repeat Sp Measurements at the Sumikawa Geothermal Field, Japan

Abstract

Baseline self-potential (SP) surveys at the Sumikawa geothermal field were carried out in 1983. Similar SP surveys were done in 1993, two years before the startup of the Sumikawa geothermal power plant, and repeated in 1996, 1997 and 1998. The result of 1997 survey shows a negative anomaly of about -100mV in the central part of the exploitation area. Unfortunately, no data free from artificial noises are available from the 1993, 1996 and 1998 surveys in most of the central area. Comparing the result taken in 1997 with the observation in 1983, we found remarkable reduction in SP over the wellfield. This change in SP is thought to be caused by production-induced fluid flows in the reservoir through electrokinetic coupling. The pressure decline due to production causes the expansion of the two-phase zone within the upper part of the reservoir. Downward flow of the liquid phase in the two-phase region transports positive charge with it and induces negative and positive sources of conduction current at shallower and deeper part of the two-phase zone, respectively. Since both of the current sources have the same magnitude, the effect of the shallower negative one dominates on the ground surface. In addition to this effect, negative SP anomalies expected for the reinjection region, which are also caused through electrokinetic coupling, might be responsible for the observed SP changes.