Tailoring Reflection Seismic Experiments to Geothermal Exploration Targets in Indonesia - Wayang Windu Geothermal Field Case Study

Abstract

Reflection seismic exploration in volcanic areas is still a scientific challenge and requires major efforts to develop imaging workflows capable of an economic utilization, e.g., for geothermal exploration. The SESaR (Seismic Exploration and Safety Risk study for decentral geothermal plants in Indonesia) project therefore used both active P-wave and S-wave seismic to test site-specific exploration procedures in different tectonic and lithological regimes to compare imaging conditions. Based on the results of a small-scale, active seismic pre-site survey in the area of the Wayang Windu geothermal field in November 2012, an additional medium-scale active seismic experiment using P-waves was carried out in August 2013. Thus, for the first time in the area, a powerful, hydraulically driven seismic mini-vibrator device (LIAG’s mini-vibrator MHV2.7) was used as seismic source instead of hammer blow applied in former field surveys. Aiming at acquiring parameter test and production data southwest of the Wayang Windu geothermal power plant, 48-channels were used in a high-resolution configuration, with receiver group intervals of 5 m and source intervals of 10 m. Thereby, we acquired a 630 m long profile. In general, we observe the successful applicability of the vibroseis method for such a difficult seismic acquisition environment. Taking into account the local conditions at Wayang Windu, the method is superior to the common seismic explosive source techniques, both with respect to production rate as well as resolution and data quality. Source offset was the key strategy to prevent surface wave noise. Further, source signal frequencies of 20-80 Hz are most efficient for the attempted depth penetration, even though influenced by the dry subsurface conditions during the experiment. Depth penetration ranges between 0.5-1 km, and a supposed fault could be located and traced to depth. Based on these new experimental data, processing workflows can be tested the first time for adapted imaging strategies. This will not only allow to focus on larger exploration depths covering the geothermal reservoir at the Wayang Windu power plant site itself, but also opens the possibility to transfer the gained knowledge to other sites.