Prediksi Zona UAP dan Air di Dalam Reservoir Panas Bumi Berdasarkan Analisa Kecepatan Gelombang Seismik Hasil dari Implementasi Tomografi Double Difference (Studi Kasus: Lapangan Panas Bumi Wayang Windu)
A geothermal knowledge about reservoir condition is one of the key factors for sustainable production in a geothermal field. Monitoring of geothermal reservoir by using micro-earthquake method is implemented recently in several fields in the world. Double difference seismic tomography is one of the tool that can help to monitor reservoir using seismic velocity analysis (Vp, Vs, Vp/Vs ratio). Seismic tomography is a geophysical tool that could be used for monitoring geothermal reservoir condition through P-wave and S-wave velocities and also the ratio Vp/Vs.
Double difference uses two types of data, i.e absolute and differential catalog data, which could gives better tomogram. Besides that, this method has a capabilities to relocate hypocenter based on event pairs that are determined with a certain criteria, such as spesific distance or similarity of waveform. As implication, double difference tomography has several advantages compared to the conventional tomography since it uses bigger size of data. One of the ability double difference tomography is delineate the steam and water zone.
This research used micro-earthquake data that was recorded within two months, i.e from May 19 until July 19 2005 by 17 seismometers deployed in the field. Catalog data was created from re-picked of waveform data in order to determined P and S wave arrival times. Micro-earthquake event are classified if the difference time between S and P arrival times (ts-tp) is less than or equal to three seconds. Totally, 353 micro-earthquake events had been successfully identified and
picked(2502 P-wave and 2579 S-wave arrival time phases).
Initial location was calculated using Geigers method and two layers 1-D initial velocity model as input. Hypocenter positions determined from Geigers method was recalculated by using VELEST software in order to obtain update velocity model and hypocenter positions. Totally, 197 events which have a gap value of less than 180o between epicenter and station were used for input data of double difference tomography.
By observing horizontal slices of tomogram, fault structures that are observed in the surface are coincide well with the boundaries between positive and negative velocities anomalies inside the 3-D tomogram. Vp and Vs low anomaly values of -20 % to -10 % from background velocity model are concentrated in the northern part of research area. The existence of those Vp and Vs anomalies and the appearance of low Vp/Vs ratio of 1.6 to 1.7 at the elevation of +0.5 km to 1.0 km is interpreted as steam zone. High Vp anomaly values of 0 % to 10 % from background velocity model and low Vs anomaly values of -15 % to 0 % followed by high Vp/Vs ratio of 1.8 to 2.0 is interpreted as brine zone. High Vp and Vs anomaly values of +10 % until +30 % with high ratio higher than 2.00 below Bedil and Wayang mountain is interpreted as old magma degassing body or intrusion.