Multi Geometry Approach for MASW Survey‐Field and Synthetic Data Results

Abstract

The geophone spacing and offset distances for MASW field investigations are commonly chosen based on common rules of thumb and experience. This method of survey design usually can not guarantee the adequacy of the final results in terms of penetration depth and resolution. Further, in complex geologies existence of higher modes of surface waves will make the interpretation of collected data difficult. One possible reason for these shortcomings is that the commonly used rules of thumb are empirical, do not have sufficient accuracy, and are not costumed for various geological conditions. To overcome the existing uncertainties, the authors previously proposed the multi geometry approach in which field data are collected using multiple geophone spacings along the same line, with active shots at multiple offset locations. Then the obtained dispersion images are overlaid to obtain one combined image with a better quality than the individual dispersion images. Field and numerical data are presented to show the efficiency of the multi geometry approach. Field data are complemented by borehole information. Comparisons are made between the curves obtained from different set ups. Field data shows that dispersion curves usually show different trends than the trends predicted by empirical rules of thumb. Specifically, the penetration depth to array length ratio depends on site conditions, the effect of array length on the quality and frequency bandwidth of the dispersion curve is not well understood and relations between offset distances and dispersion curve quality is not well known. Further, in complex geologies the observation of different modes depends on the array geometry. Synthetic data from numerical models are used to generate dispersion curves for various layering systems. Relations between penetration depths and array length are investigated. It is confirmed that due to the dependency of the field data quality to the site condition a multi geometry approach is required to reduce the risk of misinterpretations.