SEISMIC VELOCITY STRUCTURE VP, VS, AND VP/VS RATIO ANALYSIS OF “AS” GEOTHERMAL FIELD USING SEISMIC DELAY TIME TOMOGRAPHY METHOD

• Main Author: Azfar Badaruddin, Alif
• Format: Final Project
• Language: Indonesia
• Online Access: https://digilib.itb.ac.id/gdl/view/43941
• Institution: Institut Teknologi Bandung

Currently, one of the most widely developed unconventional-renewable energy source is geothermal energy. Indonesia has 127 volcanoes since it is nearly located on plates subduction zone, which means it has a high enough geothermal potential. Geothermal’s prospect sites are often associated with micro-earthquake (M<3) activity. In this study, these microearthquake activities used to analyze the subsurface structures based on its seismic velocity structure of Vp, Vs, and Vp/Vs ratio. The data used in this research is from the catalog of micro-earthquake activities (ts-tp < 6 seconds) at “AS” Geothermal Field in the span of January-July 2013. 1.065 micro-earthquake events were recorded by 50 seismometers that were distributed on the research’s site. The hypocenter’s locations are determined using the grid search method with the aid of the software NonLinLoc. The results of determining the hypocenter’s locations shows that micro-earthquake activities are located at three main clusters. Events used for hypocenter relocation and updating velocity model are reduced based on azimuthal gap value, resulting in only 416 events. Updated hypocenters data are used to be input for delay time tomography using software SIMULPS12, including starting velocity model, stations coordinate, and inversion control parameters. The grid parameterization used are 2km x 2km x 0.5km. The updated velocity model and hypocenters relocation process giveas more precise hypocenters based on its relatively small residual travel time values. Hypocenters location are associated with reservoir location, proving the existence of seismic activities caused by production and injection activities. The 3D seismic tomography inversion results for Vp, Vs, and Vp/Vs ratio shows the existence of geothermal components such as cap rock, vapor-dominated reservoir, liquiddominated reservoir, near surface fluid-saturated zone, and pressure decrease zone. Interpretation results and conceptual model conform with geologic model of “AS” Geothermal Field done in previous researches.