Proceedings of the 11th SEGJ International Symposium, 2013 Magnitude Detection Threshold Map in MuaraLaboh Geothermal Field’s Seismic Network Plan and Development. Siti Fatimah (1) , Theodosius M. Irnaka (1) , Donata N.W. Wardani (1) , Julfi Hadi (2) , Wiwit Suryanto (1) (1) Geophysics Sub-Department of Physics, FMIPA, UGM, (2) PT. Supreme Energy MuaraLaboh Equity Tower 18 th floor, SCBD Lot, 9, Jl. Jendral Sudirman Kav, 52-53 Jakarta . ABSTRACT Fifteen seismometers have been set in MuaraLaboh geothermal field in order to monitor seismic activities. MuaraLaboh is in the South Solok Regency, 150 km south east of Padang, the capital city of West Sumatera Province, Indonesia. In geothermal system, we need to enhance water flow in a hot rock by injecting pressurized cold water to increase the permeability, and enhances the flow of water within the reservoir. The injection process will naturally cause fractures to be occurred. The opening of fracture usually can be detected at the surface by seismometer as micro- earthquake. Furthermore, a small fracture which is only detected by two or less seismic stations is not sufficient to obtain good hypocenter location. Therefore, an evaluation of geothermal seismic network design is very important aspect in order to optimize the detection threshold of micro-earthquake signal. In this paper, determination of magnitude threshold detection utilized MuaraLaboh geothermal field’s seismic network using fifteen short period seismometers. A local velocity model was used to simulate micro-earthquake signals. Ambient noise sample from every station was also taken into account to simulate the most realistic micro-earthquake model. The final result is magnitude detection threshold map for selected moment magnitude which will depict sensitivity of MuaraLaboh geothermal field’s seismic network towards certain micro-earthquake magnitude and distance. KEY WORDS: magnitude, threshold, geothermal, seismic, network INTRODUCTION There are two types of seismic activity considered in geothermal system, those are induced seismicity and natural seismicity. The presence of induced seismicity can lead to information regarding both the productivity and safety of a geothermal system. Natural seismicity occurs by natural means, for example movement along fault due to strain caused by plate tectonics or creation of hydraulic fracture due to water movement in the subsurface. Its presence also can lead to information regarding the monitoring of induced seismicity. Figure 1. MuaraLaboh location. Uppermost map shows fifteen seismic stations network location. Since the magnitude of micro earthquake in geothermal area is generally less than 2 MW, magnitude detection threshold should be determined to increase possibility of micro earthquake data recording by comparing amplitude of seismic synthetic data and ambient noise amplitude for each station. Ambient noise usually is in form of stationary signal with low amplitude and low frequency. Their appearance can be either quite periodic or quite random because of instrumental noise that produce transient