Characterizing near-surface features of shallow shear wave velocity in the Way Ratai geothermal field

Passive seismic activity measurements in the Way Ratai geothermal field are needed to identify near-surface features. In addition to the ease of measurement, passive seismic methods can be an initial solution in characterizing geological conditions. This study aims to characterize near-surface features using the dominant frequency ( f_0 ) pattern and 3D modeling of the shear wave velocity ( V_s ). A total of 70 measurement points with intervals between points of approximately 500 m are scattered in the southern area of Mount Ratai. The f_0 curve was obtained using the horizontal-to-vertical spectral ratio (HVSR) method, while 3D modeling uses OpenHVSR. The characteristics of the surface rock layer identified by f_0 pattern is in the range of 0.2–2 Hz. Furthermore, the Kali Tiga and Margodadi fields show thick soft rock patterns near the surface, with an f_0 values between 0.6 and 0.8 Hz. The cross-section of the 3D model at a depth of 10 and 25 m is dominated by stiff soil layers, while at a depth of 35 m, it becomes very dense soil and soft rock. The characteristic of shear wave velocity ( V_s ) in the geothermal field influenced by the clay alteration near the surface, ranging from 175 to 350 m/s. These results can explain the possibility of a thick cap rock layer feature in the Way Ratai geothermal field.