Bedrock depth evaluation using microtremor measurement: empirical guidelines at weathered granite formation in Singapore

Abstract

Detection of bedrock depth is one of the critical site investigation procedures for seismic hazard analysis and underground developments that may encounter varying rock formation. The most common practice to detect bedrock is to directly drill boreholes. However, the intrusive borehole-based site investigation process is often expensive and time consuming and provide limited information from discrete boreholes. In the present study, a surface wave based technique, microtremor array measurement (MAM) and microtremor measurement (MM) is used to find the depth of bedrock at Bukit Timah granite formation in Singapore. Based on a series of MAM and/or MM surveys, four interpretation approaches, i.e., (1) Bilinear intersection method; (2) Preselected shear wave velocity (VS) based approach; (3) Normalized phase velocity approach; and (4) Horizontal to vertical spectral ratio (HVSR) analysis, are proposed for bedrock depth detection. The first three approaches are based on surface wave inverted VS profile using a vertical component of MAM. The fourth approach is to utilize the natural frequency of the ground through HVSR using three components microtremor measurements (MM). By compiling experimental data at nine different testing sites, it was demonstrated that non-invasive surface wave-based approaches can be effectively used for bedrock detection. Especially, a promising empirical correlation between the natural frequency of ground and the depth of bedrock is subsequently proposed.