Investigation and Use of Surface-wave Characteristics for Near-surface Applications

High-frequency surface-wave methods can provide reliable near-surface shear-wave (S-wave) velocity, which is a key parameter in many shallow-engineering applications, groundwater and environmental studies, and petroleum exploration. Recent research and key accomplishments at the China University of Geosciences at Wuhan into nearfield effects on surface-wave analysis provide not only insight into minimum-source geophone offsets required for generating high-quality surface-wave images but also provide a better understanding of the propagation characteristics of seismic wavefields through near-surface materials. New numerical modeling and dispersion-analysis algorithms are key tools used routinely in those studies. The modeling results illustrate very different energy-partitioning characteristics for Rayleigh and Love waves. Using a high-resolution linear Radon transform produces dispersion images with much better resolution and therefore represents a tool for more accurate separation and determination of phase velocities for different modes. Mode separation results in wavefield components that individually possess great potential for increasing horizontal resolution of S-wave velocity-field determinations. Amplitude corrections can significantly improve the accuracy of phase-velocity estimates from mixed-modal wavefields. Results from two simple models demonstrate how dramatic topographic changes can distort wavefields. This finding was the catalyst for suggesting that a topographic correction should be considered for surface-wave data acquired on a rugged ground surface. Phase-velocity inversion is an ill-posed problem. Rayleigh-wave sensitivity analysis reveals the difficulty in estimating S-wave velocities for a model with a low-velocity layer. Constraints in the model space are therefore necessary. Approximating cutoffs could help build a better initial model and provide critical information about the subsurface when higher modes are present.