The impact of source type, source offset, and receiver spacing on experimental MASW data at soft-over-stiff sites

The focus of this study was to determine optimal field data collection parameters for in-situ evaluation of shear wave velocity at soft-over-stiff sites using the MASW method. The research was conducted at two soft-over-stiff sites located in the Fayetteville Arkansas area. At these two sites, a linear array of 24 receivers (4.5 Hz geophones) was used with various source types, source offset distances, and receiver spacings to determine the best combination for acquiring accurate dispersion data. The MASW data was analyzed using the frequency domain beamformer method. A downhole seismic survey was conducted at each site in existing boreholes to provide a reference shear wave velocity profile for comparison with the MASW results. It was determined that a 3 ft receiver spacing, a vibroseis source and a 30 ft source offset provided the smoothest and most accurate dispersion curve for each site. It was also determined that many of the MASW experimental dispersion curves contained significant higher mode behavior at long wavelengths (low frequencies) which appeared to be fundamental mode data. This higher mode data had to be neglected in the forward modeling process to accurately determine the shear wave velocity profile of the sites.