Experimental study of near-field effects in multichannel array-based surface wave velocity measurements

摘要

This paper examines the influence of source offset distance on surface wave phase velocity values determined from frequency-wavenumber (f-k) processing of multi-channel data. Experimental surface wave data were collected over a broad range of frequencies at eleven deep soil sites in the Mississippi embayment of the central United States. Using analyses of multiple array configurations at each site, near-field phase velocity values (determined with the source close to the array) were compared to far-field velocity values. The source offset distance was expressed as a normalized value calculated as the distance from the source to the centre of the array, divided by the wavelength. The results from these field measurements showed that the influence of near-field effects became evident when the normalized source offset distance was 0.5 or less, a value that is less restrictive than values determined from a recent study of near-field effects using numerical simulations and experimental data. A possible reason for this discrepancy is the high Poisson's ratio values in this study due to shallow water tables at the field test sites, a condition that was not examined in the previous study. Last, the effectiveness of f-k processing using cylindrical beamforming for mitigating near-field effects is also examined in this study and shown to provide a small improvement in velocity estimates in the near-field.