Subsurface characterization of Diwak-Derekan geothermal field by HVSR analysis method based on mictrotremor data

Abstract

The research aims to analyze the subsurface structures that are manifested in geothermal represented by using the Horizontal to Vertical Spectral Ratio (HVSR) method. The data is obtained by microtremor recording, which obtains the dominant frequency value and amplification factor that will give the value of the wave velocity P (Vp) and the wave velocity S (Vs). The results of the study at 52 measurement points were processed using Geopsy and Surfer 13. The results of microtremor data processing using the HVSR method obtained the distribution of dominant frequency values around 0.08 to 9.29 Hz, the distribution of amplification factor values around 0.03 to 7.07, the distribution of Vp values around 390 m/s to 4993 m/s and the distribution of Vs values around 190 m/s to 3054 m/s. Manifestation area ofDiwak-Derekan is located on the weak zone, based on the subsurface structural analysis, a possibility of normal fault is also identified, it causes manifestation of hot water springs on the surface. The Poisson's Ratio shows subsurface lithology mostly in the form of sandy loam, and a small portion of silt and saturated clay. The analysis of Poisson's Ratio shows the geothermal manifestation area is possibly a geothermal system that the fluid type is dominated by vapor/steam. The result explains that microtremor analysis by HVSR method can be utilized to subsurface interpretation for characterization purpose of the study area.