Near surface velocity models using a combined inversion of surface and refracted P waves

In addition to conventional P-wave refraction tomography, we propose a workflow based on the complementary use of Rayleigh waves. We develop a surface wave processing sequence to determine the S-wave near surface velocity field, which can be used as a constraint for P-wave tomography and statics determination. Rayleigh waves are processed in three steps. The first one consists of an accurate frequency-dependent travel-time measurement for each selected source-receiver pair, in which the phase difference between two adjacent traces is used to derive the phase velocity. Then, a frequency-dependent surface-wave velocity tomography is performed, in order to merge information coming from multi-azimuth and multi-offset picking. Finally, after surface wave tomography, the frequency-dependent phase velocity volume is inverted to deliver an S-wave near-surface velocity field. This model is then used as a constraint to the first arrivals P-wave tomography. To illustrate our method a conventional 3D Narrow-Azimuth land dataset is used, acquired around a river surrounded by hills. Strong lateral velocity variations are expected in the shallow part, with slow velocities around the unconsolidated sediments of the river bed, and faster velocities in the consolidated sediments of the surrounding hills. A combined first arrivals tomography using the depth S-wave velocity, the refracted P-wave velocity and their first arrivals is used to obtain a more accurate refracted P-waves model in the shallow part. Hence, the application of primary statics derived from this constrained refracted P-wave velocity model results in a much better image in the shallowest part with better focusing and continuity of thin layers.