The 3D Seismic Velocity Model and Hypocenter Relocation Results of Patuha Geothermal Field

Geothermal reservoir monitoring using micro seismic survey has been implemented recently in several geothermal fields in Indonesia. Micro seismic survey has been conducted from January 1st until later April 1st 2018 in Patuha Geothermal Field using 9 seismometers that are placed near well locations and fault distribution. 654 micro seismic events (ts-tp less than 3 second) and 640 regional (ts-tp more than 5 second) was identified in this period, but only 491 micro seismic events have clear onset of P and S waves arrival time. Initial location is determined using NonLinLoc algorithm and will be used as input for Velest program to obtain 1D velocity model. Double difference seismic tomography is applied to jointly determine relocation hypocenter and 3D seismic velocity model (Vp, Vs, & Vp/Vs). The aim 3D seismic velocity model determination is to infer reservoir condition, because Vp/Vs ratio is sensitive with sub-surface variations in rock and fluid properties. The results show hypocenter location (depth 0-500 masl) is correlated with existing production zone with high permeability and main fault direction. It is expected to support well targeting for the next development stage of Patuha geothermal field. 1. GEOLOGY PATUHA The Patuha field is situated within a northwest-trending volcanic mountain range, including the nearby peaks of North Patuha (2414 m); South Patuha (2390 m); Urug (2201 m) and Walang (2178m) (Layman et.al., 2003). The structure in Patuha Field is dominated by NE-SW (Alamendah Fault, Cibuni Crater Fault, Ciwidey Crater Fault, Tiis Fault, and Dewata Fault) and NW-SE (Rancabali Fault, Patenggang Fault, Ciwidey Crater Fault, Urug Fault, Tiis Fault, and Dewata Fault) in line with Meratus and Sumatera trend (Figure 1). The type of lithology in Patuha geothermal field consist of lavas, tuff, and breccia. Diorite or micro-diorite intrusive cut the volcanic section in some wells. Surface manifestations in Patuha consist of fumaroles, thermal springs, and cold gas discharges. These include the three fumarole areas at Kawahs Cibuni, Putih and Ciwidey. Thermal springs have been identified at lower elevations on the south, west and northwest flank of the volcanic highland. An area of cold gas discharge is present between Kawah Ciwidey and Kawah Putih, and also on the south flank of the volcanic highland. Figure 1: Fault distribution, production & injection well, and manifestation in Patuha Geothermal Field (Elfina, 2017) 2. DATA & INSTRUMENTATION We have deployed 6 (six) MEQ station as agreed in Terms of Reference (Term of Reference). The 3 (three) spare station were also deployed as additional stations, so that 9 (nine) station were successfully deployed on 4 January 2018. On 4 February 2018, 2 (two) additional instruments (surface type) were also installed in the designated place as a additional station. Therefore, a total of 11 (eleven) stations was deployed in the survey area. Several parameters were set to fulfill the survey specification in terms of reference: sampling frequency was set to 200Hz, the high gain mode was used, and the raw data were set to have 1 hour of record length. The parameter of the recording could be seen in Table.