Estimation of Three-dimensional Soft Ground Layer Distribution by Combining 3D Microtremor Array and H/V Spectral Ratio

Summary 3D microtremor array explorations can estimate three-dimensional S-wave velocity structure. However, the result depends on the initial model selected for the inverse analysis. When we selected a smooth structure in the depth direction for the initial model, the inverted result may deviate from the structure with huge velocity contrasts. On the other hand, when there are significant velocity contrasts, differences in velocity layer boundaries can be easily understood using the H/V spectrum, although it is generally difficult to determine the S-wave velocity structure. Therefore, by combining both approaches, it is possible to efficiently capture the three-dimensional ground structure with extreme velocity contrasts, such as soft ground distributed over a basement layer. As a case study, we report the results of estimating the three-dimensional S-wave velocity structure in an area where the thickness of the soft ground layers varies in three dimensions. This was accomplished through a 3D microtremor array survey using a grid array of 49 one-component geophones, supplemented by 18 three-component geophones placed at the center of the bin. By leveraging the advantages of both measurement methods, we successfully estimated the three-dimensional S-wave velocity structure.