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 (V-P and V-S, 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 V-P and V-S images, which fit their respective datasets within the noise level, were smooth with no clear anomalous structure at the pipe locations. In the V-P/V-S ratio image, however, the pipe locations appear as zones with high ratio values (> 1.8). A nearby zone of low (< 1.5) V-P/V-S 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.