Mitigation of Near-Field Effects for Seismic Surface Wave Velocity Estimation with Cylindrical Beamformers

Analysis of seismic surface waves offers a noninvasive method to estimate the dynamic engineering properties of the near-surface earth. Traditional two-sensor spectral analysis of surface waves (SASW) techniques suffer from several limitations that impede their use in practice, including poor resolution, inability to properly account for multiple modes, and an imprecise understanding of near-field effects. Current multichannel and array analysis of surface waves resolve several of these limitations, but still suffer from a near-field model incompatibility, i.e., estimating plane wave parameters in a cylindrically spreading wave field. This paper introduces cylindrical beamforming, an array processing method to overcome the limitations of plane wave processing methods. To aide in motivating the cylindrical beamformer, the paper discusses the near-field effects and explains some of the underlying physical reasons for previous observations regarding the near-field. The cylindrical beamformer is then introduced to more accurately model the wave field, allowing better estimates of phase velocity from point sources. The results from measured data taken with a linear array on a site in Atlanta are used to compare the methods.