Identification of Micro-Earthquake Hypocenters using Geiger and Coupled Velocity-Hypocenters Methods

Abstract

The accuracy location of hypocenter is needed to determine the subsurface character beneath a geothermal area. The study used 73 micro-earthquake events; each micro-earthquake event was classified based on the time difference between the P and S waves (ts-tp) that had values ≤ 3seconds, the magnitude of micro-earthquake ≤ 3SR and each micro-earthquake event was recorded at least by 3 observer stations. We inverted selected P and S travel times from 11-unit seismic stations on X geothermal area. The initial hypocenter location was determined using Geiger method. The result of the Geiger method's initial hypocenter location was then used as the input to determine the accurate hypocenter location in the following method, Coupled Velocity-Hypocenters method. Other parameters were also used on this second method, including hypocenter location, 1-D velocity model, origin time, vp/vs ratio, zshift and the station correction. The distribution of hypocenter locations of micro-earthquakes obtained using the second method was better than the results from Geiger method. This result is supported by the Coupled Velocity-Hypocenters average RMS error value, which was smaller, only 0.19 seconds, compared to the Geiger method, which had an average RMS error of 0.74 seconds. The hypocenter location of the relocation was more clustered in the reservoir area, precisely in the production well, and in the heat source area. The hypocenter location in the production well area indicates fluid flow through the fracture from the permeable zone.

Delay Time (δt) and Polarization Direction (φ) Analysis Based on Shear Wave Splitting (SWS) Method

“…RAW data (waveform) recorded at the research location (geothermal field X) is a 3-component seismogram, consist of one vertical component (Z) and two horizontal components (N-S and E-W). This study's waveform selection to be analyzed and processed refers to the distribution of microearthquake hypocenter locations determined using the Geiger method carried out in a similar research area [15]. S wave polarization phenomenon can be analyzed by observing the horizontal component N-S and horizontal component E-W, where one component will show the appearance of S fast wave (Sfast) and S slow-wave (Sslow) in the other component [16].…”

“…Based on Table 1, it can be concluded that the polarization direction gained from each station has an angle directed towards the North and East. The previous research [15] determined the distribution of micro-earthquake hypocenter results using the Geiger method, at a depth of 2000 to -2000 m spread around the seismic station. Thus, slicing was carried out in the depth of 2000 to 0 m and 0 to -2000 m. In the depth of 0 to 2000 m shown in Table 2, deflection appears at each recording station on each captured layer.…”

“…Raypath value is the average length from one hypocenter to the recording station, which records the microearthquake event that is considered straight. In this research, hypocenter location coordinates is determined using the previous study [15]. After the value of the delay time and raypath length obtained, then the value of fracture intensity can be identified.…”

The Effect of Picking Uncertainty Window Interval on Hypocenter Micro-Earthquake (Meq) Location Using Geiger Method

“…Consistent timing of seismic wave arrivals (Primary and Secondary waves) is very important but has received little attention [1]. Primary waves arrival time (P) is one of the fundamental data in the inversion procedure, one of which is in determining the location of seismic sources.…”

“…Primary waves arrival time (P) is one of the fundamental data in the inversion procedure, one of which is in determining the location of seismic sources. Determination of the arrival time of high-quality seismic waves contributes more to the determination of the location of the seismic source (earthquake hypocenter) with a high degree of precision [2,3], so that it can provide a big picture that truly represents the dynamic conditions of an area [1,4,5,6].…”