RAYLEIGH-WAVE DIFFRACTIONS DUE TO NEAR-SURFACE FEATURES

Detection of near-surface features such as voids and faults is challenging due to the complexity of near-surface materials and limitations on the resolution of geophysical methods. Although multichannel high-frequency surface-wave techniques can provide reliable shear (S)-wave velocities in different geological settings, they are not suitable to detect near-surface voids/faults directly based on anomalies of the S-wave velocity because of limitations on the resolution of S-wave velocity profiles inverted from surface-wave phase velocities. We studied the feasibility of detecting near-surface features with surface-wave diffractions and presented a simple method in the time-space domain to detect near-surface features based a traveltime equation of surface-wave diffractions. The Rayleigh-wave diffraction traveltime equation due to a void in the homogeneous half space was verified by two-dimensional numerical modeling. This equation was also applied to a void in a layered half space and a vertical fault. The phase velocity of diffracted Rayleigh waves is predominately determined by surrounding materials of a void/fault. The modeling results demonstrate that the Rayleigh-wave diffraction traveltime equation due to a void in the homogeneous half space can be applied to the case of a void in the layered half space and of a vertical fault. Rayleigh-wave diffractions were recognizable for all these models after removing direct surface waves and body waves by F-K filtering. In practice, only two diffraction times are necessary to define the depth to the top of a void and the average Rayleigh-wave velocity that generates the diffraction curve. Encouraging results of directly detecting a void from Rayleigh-wave diffractions were presented. One real-world example was presented to show how to utilize the derived equation of surface-wave diffractions. Both synthetic and real-world examples demonstrated that it is feasible to directly detect a void in a shot gather. Because surface-wave diffractions are relatively weak, as the synthetic examples demonstrate, F-K filtering is necessary to remove the direct surface/body waves before analyzing diffracted waves.