CHARACTERIZATION OF GEOTHERMAL RESERVOIR IN DIENG FIELD THROUGH TOMOGRAPHY MODELING OF P AND S WAVE VELOCITY AND VP/VS RATIO

Abstract

The Dieng geothermal field, located in Wonosobo Regency, is classified as a high tempera-ture geothermal system with a predominance of fluids in the liquid phase. This study aims to analyze the distribution of microearthquake hypocenters and the characteristics of seismic wave velocities Vp, Vs, and the Vp/Vs ratio to characterize reservoir conditions. The data used were obtained from seismometer recordings over a period of 99 days between July and October 2020. Automatic phase picking was performed using EQTransformer, available in the SeisBench toolkit, following a waveform noise reduction (denoising) process. Initial hy-pocenter determination was carried out using the NonLinLoc method, followed by hypocen-ter relocation using the double-difference method. Tomographic modeling was performed using the SIMULPS12 software. The analysis of relocated hypocenter distribution reveals two main clusters associated with the active reservoir zones, Sileri in the north and Sikidang in the south. These clusters are located near surface manifestations such as solfataras and mud pools, and are associated with geological structures like faults, reflecting the presence of fluid migration pathways through fracture zones. The hypocenter depth distribution ranges between 1 and 2 km below the surface, consistent with reservoir depths reported in previous studies. Tomographic results indicate that water-saturated zones are characterized by low Vp and Vs values and a high Vp/Vs ratio, pressure decrease zones (caused by pressure reduction) are marked by high Vp and Vs values with a low Vp/Vs ratio, while vapor-saturated zones show low values in all three parameters. Gas saturated and pressure decrease zones are spa-tially associated with production well locations, and several gas-saturated zones are also as-sociated with injection wells. This indicates that production activities have caused pressure depletion, leading to fluid phase changes from liquid-dominated previously reported to gas-dominated. Understanding fluid distribution and reservoir characteristics is expected to support more effective and sustainable geothermal resource management.