2018 Volume 40 Issue 3 Pages 159-173
Since 2013, an engineered / enhanced geothermal system (EGS) experiment has been conducted at the Okuaizu geothermal field with the goal of recovering steam and stabilizing steam production. In this experiment, river water is injected near the geothermal reservoir to make up for a lack of natural recharge. We conducted repeated audio-frequency magnetotelluric (AMT) measurements to monitor the resistivity variation of the geothermal reservoir and nearby hot springs aquifer at the initial stage of the water injection. The initial AMT data before the water injection were collected along a line crossing the injection point in November 2013. The water injection was started in June 2015. A second set of AMT measurements at the same location was obtained at the end of August 2015. The total amount of water injected by that time was 1.2 × 105 t. The precise resistivity sections to a depth of about 1.5 km were analyzed from the two-dimensional inversion using the AMT data for both periods. The findings were consistent with the resistivity logging data of nearby wells. A comparison of the two sections indicates that the resistivity of the reservoir, including the caprock, increased between the two AMT measurements. The second section showed the resistive features of the reservoir and its caprock. The rise in resistivity suggests that the superheated steam progressed at depth and the effect of water injection had not yet appeared. AMT is an effective technique for monitoring geothermal reservoirs, although more precise measurements will be required to recognize the micro-resistivity variation associated with water injection.