Near-field effects on site characterization using MASW technique

The application of Multichannel Analysis of Surface Wave (MASW) technique is increasing in geotechnical engineering to characterize near surface materials. The dispersion property of Rayleigh wave is utilized in MASW method. MASW often suffers from near-field effects which may result in either underestimation or overestimation of Rayleigh wave phase velocity due to the body waves contamination near to the source. In this paper, a detailed numerical study has been carried out to examine the near-field effects considering three different types of typical S-wave velocity models with four different impedance scenarios in each case. The study shows that the impedance contrast between the half-space and overlying soil layer is having a considerable effect on the underestimation of phase velocity. These near-field effects are also found to be influenced by the type of the S-wave velocity model as well as far and near offset distances. With the increase of impedance contrast, the level of underestimation seems to increase at lower normalized array centre distance due to mode jump. However, such jump can not be observed with limited far offset distances generally used in usual practice due to poor resolution in the dispersion spectra. Significant near-field effect are observed for lower far offset distances and inversely dispersive S-wave velocity models. Underestimation of Rayleigh wave phase velocity is quantified in terms of two normalized parameters. Finally, a field study is also conducted to verify our numerical findings.