Shear Wave Velocity Analysis Based on Two Rock Layers Modeling Using Microtremor Data of Gresik and Surrounding Areas

Abstract

The lithology on the surface in the current condition is often found to be not fresh rocks, but all materials, be it alluvial, soil, backfill soil, or rocks with a very high degree of weathering. Therefore, this study was conducted to estimate the type of layer on the surface and its underlying layer based on the analysis of Vs (shear wave velocity) values in the surface lithological media and the underlying layer through the modeling of two rock layers. To complete this research, microtremor array data was collected by the Geological Survey Center in the Gresik area which was then processed using the SPAC (Spatial Autocorrelation) method. Based on the results of the study, the Gresik area has a surface Vs value that still shows soft soil values in the range of 65 - 170 m/s and its basement Vs already shows fresh rocks to older lithologies in the alluvial area in the range of 150 - 435 m/s, so it can be seen that some areas that should consist of a fairly hard lithology in the Gresik area have changed with a fairly high level of weathering.