Particle Swarm Optimization-based Inversion of HVSR Measurement for Estimating Sediment Thickness in Paleovolcanoes around Bakauheni

Abstract In the geotechnical field, determining the thickness of the sediment layer is very important. The thickness of the sediment layer can provide invaluable information in the planning and design of building structures, infrastructure, and other construction projects. Bakauheni is an area that has many calderas and ancient volcanic deposits from the Pliocene - Holocene era. It is fascinating to study how thick the sediment layers are in the area. We used 64 Horizontal to Vertical Spectral Ratio (HVSR) measurement points to determine the thickness of the sediment layer and how it correlates with the presence of an ancient caldera in the Bakauheni area. Next, to obtain a 1D shear wave velocity model (Vs), an inversion process was carried out using the Particle Swarm Optimization (PSO) algorithm. The inversion results show that the average Vs value in the study area is around ~ 600 m/s. This value is relatively high for a sedimentary layer, which indicates that the sedimentary layer has started to become dense and compact or consists of a layer of weathered bedrock. Based on the results of calculating the average HVSR curve, a natural frequency (fo) value of 15.12 Hz was obtained. Assuming an average Vs of the sediment layer of 600 m/s, the sediment thickness is estimated at 9.92 meters. This result shows an excellent correlation with the median value of sediment thickness calculated at 64 measurement points, which is 10.55 meters. The minimum and maximum sediment thickness in the study area was 4.39 and 103.57 meters, with an average sediment thickness reaching 18.22 meters. From these results, we conclude that the thickness of the sediment layer in the Bakauheni area ranges from 10–18 meters. The thickest sediment layers (> 30 meters) are associated with the presence of calderas and low topography. It shows that a relatively thick layer of sediment covered the ancient caldera. Based on the results obtained, the HVSR method provides quite good results in determining sediment thickness to identify the presence of a caldera. However, it should be noted that Vs values may vary throughout the study area, depending on the nature and composition of the sedimentary rocks present. Therefore, further interpretation and research are needed to understand more deeply the nature and characteristics of the sediment layers in the study area.