Summary
Surface wave data were collected at a site in southern California to detect near-surface discontinuities. However, due to the abundance of faulted and fractured media throughout the site, clear fundamental mode Rayleigh wave dispersion patterns were difficult to obtain. To account for the apparent heterogeneity, three different processing techniques were applied to enhance the fundamental mode dispersion image and reduce higher mode interference: muting, high-resolution lateral radon transform, and dispersion-curve based FK filter. Although one method alone does not sufficently improve the fundamental mode disperion pattern in the presence of geologic heterogeneity, interpreting the enhanced dispersion images together guided picking for the fundamental mode.
Introduction
Multichannel analysis of surface waves (MASW) has become an increasingly popular method for estimating the average shear-wave velocity (Vs) quickly and efficiently (Park et al., 1999; Xia et al., 1999). Two-dimensional (2D) Vs profiles give an indication of material stiffness or rigidity, which aid geotechnical engineering investigations and understanding site effects (Penumandu and Park, 2005). Key to obtaining accurate velocity information is properly interpreting the surface wave dispersion pattern. Therefore, higher mode energy should be appropriately identified and not confused for the fundamental mode (Zhang and Chan, 2003). However, in complex and heterogeneous environments with large stiffness contrasts and velocity inversions, higher mode surface waves can dominate or interfere with the fundamental mode trend (O’Neill and Matsuoka et al., 2005). it can be difficult to identify higher mode dispersion patterns using basic processing procedures.
Alternatives for reducing higher mode interference include adjusting the survey geometry (Ivanov et al., 2008). Although it can be more desirable to use a longer spread length since it decreases the effects of aliasing, shortening the spread can help reduce higher mode contamination. However, for sites with increased heterogeneity, it becomes more difficult to account for all of these imaging tradeoffs. When adjusting the source offset or spread size does not sufficiently improve resolution of the fundamental mode, additional enhancement methods are required to improve interpretation and accurately pick the fundamental mode dispersion curve. Although it may be more desirable to use a longer spread length since it decreases the effects of aliasing, shortening the spread can help reduce higher mode contamination. However, for sites with increased heterogeneity, it becomes more difficult to account for all of these imaging tradeoffs.
