VSP Traveltime Inversion: Near-Surface Issues

P-wave velocity information obtained from vertical seismic profiles (VSPs) can be useful in imaging subsurface structure, either by directly detecting changes in the subsurface or as an aid to the interpretation of seismic reflection data. In the shallow subsurface, P-wave velocity can change by nearly an order of magnitude over a short distance, so curved rays are needed to accurately model VSP traveltimes. We used a curved-ray inversion to estimate the velocity profile and the discrepancy principle to estimate the data noise level and to choose the optimum regularization parameter. The curved-ray routine performed better than a straight-ray inversion for synthetic models containing high-velocity contrasts. The application of the inversion to field data produced a velocity model that agreed well with prior information. These results show that curved-ray inversion should be used to obtain velocity information from VSPs in the shallow subsurface.