Near Surface Characterization Using VP/VS and Poisson's Ratio from Seismic Refractions

This paper shows that the ratio of compressional- and shear-wave velocities (VP and VS, respectively) better characterize the near-surface than individual velocity models. In a proof-of-concept experiment, vertical and tangential data were acquired along a 23 m, 2D profile containing two well-documented utility pipes with 0.76 m diameter and 1.5 m burial depth. First arrivals of both datasets were inverted using a regularized tomography algorithm. The inverted VP and VS images, which fit their respective datasets within the noise level, were smooth with no clear anomalous structure at the pipe locations. In the VP/VS ratio image, however, the pipe locations appear as zones with high ratio values (>1.8). A nearby zone of low (<1.5) VP/VS ratio was interpreted as a backfilled void. These results suggested that carefully conducted P- and S-refraction tomography can provide quick and effective near-surface reconnaissance.