Underground survey to locate weathered bedrock depth using noninvasive microtremor measurements in Jurong sedimentary formation, Singapore

Abstract

Determination of bedrock depth is an essential part of design/alignment/initial planning of any infrastructure projects such as tunneling, foundations and underground utilities. Drilling a borehole to identify the depth of bedrock is often time consuming, costly and cumbersome process. In the present study, microtremor methods are used to identify the depth of weathered bedrock. Series of ambient microtremor array measurements (MAM) and 3-component microtremor methods are conducted in the weathered rock of Jurong formation in Singapore. Based on these surface wave measurements and available reference borelog information, three different approaches – (1) shear wave velocity (Vs) based approach; (2) wavelength based approach and (3) Horizontal to Vertical Spectral Ratio (HVSR) approach – are proposed to identify the depth of bedrock in the Jurong formation. For the Vs based approach, a specific reference value of Vs is suggested to estimate the depth of bedrock in the Jurong formation, which is readily determined by the Vs profile. In the wavelength based approach, an experimental dispersion curve in the form of wavelength and phase velocity is found able to identify a deviation point that can be used for predicting the corresponding bedrock depth. A relationship between the deviation point in wavelength and bedrock depth has been established. In the HVSR based approach, an empirical correlation model is formed between the natural frequency obtained from the HVSR profile and the bedrock depth information. To conclude, all three approaches can provide overall reasonable estimates in the depth of bedrock although the Vs based approach is slightly better in statistics, followed by HVSR based approach. In practice, the HVSR approach would be easy to implement in terms of field testing and analysis.