Near-Field Effects on the in situ Estimation of Shear-Wave Velocity and Damping Ratio from MASW Tests

Multichannel analysis of surface waves (MASW) is a viable approach for in situ estimation of shear-wave velocity and damping ratio. However, conventional processing techniques assume that the recorded wavefield consists only of surface waves, and the presence of body waves leads to detrimental consequences on the reliability of the estimate, referred to as “near-field effects”. The latter typically induce oscillations in the estimated phase velocity at low frequencies, while little is known about their effect on phase attenuation. This study investigates the influence of near-field effects on the estimated phase velocity and phase attenuation on a synthetic data set. The results show that the phase attenuation is far more sensitive to body waves than the phase velocity. Furthermore, the findings suggest that near-field effects can be mitigated by selecting the experimental data with normalized array center distance (NACD; i.e., the ratio between the average offset of the receivers and the Rayleigh wavelength) greater than 2-3. While phase velocity estimates are reliable at NACD values as low as 1, the requirement for NACD > 2-3 is mainly driven by phase attenuation and its higher sensitivity to near-field effects. However, this threshold should be adjusted based on the noise level, especially when few receivers are used. The proposed criterion has been applied to the characterization of a real site in Italy, demonstrating that the inclusion of data affected by near-field effects leads to an overestimation of the shear-wave damping ratio and significant variability in the estimated profiles.