MASW Investigations on Sites with Shallow Bedrock

Abstract

Multi channel analysis of surface waves (MASW) method is used to measure shear wave velocity profiles on sites comprising shallow overburden overlying bedrock. Field observations show unusual trends in time traces, dispersion curves and inverted velocity profiles. Various velocity trends can be traced in the time records, dispersion curves are generally contaminated with higher modes, higher modes are observed at low frequencies rather than higher frequencies, and low wave velocities are measured below bedrock surface. Considering the aforementioned issues the results of MASW tests at sites with shallow overburden overlying bedrock may be questionaable. In this paper the results of various field investigations and numerical models are presented. To obtain more reliable field data a multi geometry approach is used. In this approach data are collected along the same line with multiple geophone spacings and multiple offset distances. Further the results of a series of multi geometry tests with both forward and reverse offsets are presented. In all cases borehole data are obtained at various locations to confirm the depth to bedrock and quality of surficial bedrock. Comparisons are made between the obtained time traces and the calculated dispersion curves using different set ups. It is observed that in complex geologies the results can significantly vary depending on the array set up. Numerical models are developed to generate synthetic data under controlled conditions. The trends observed in the field and synthetic data confirms that the penetration depth relations commonly used in MASW inversions are not accurate in complex geologies. Further, it is recommended to use a multi geometry approach at sites with shallow bedrock to improve the reliability of the field data. This approach reduces the risk of interference of various modes at low frequencies. Therefore, fundamental modes of surface waves can be selected more confidently.