Abstract

The near-surface is usually defined as the depth interval below the topography, composed of a low-velocity soil column and weathered rock layer. In contrast with the near-surface, the subsurface is composed of relatively higher velocity rock layers. This means that the interface between the near-surface and the subsurface often gives rise to a strong shallow reflection. The image-based method described in this paper makes use of the strong reflection at the base of the near-surface to estimate a model for the near-surface for statics corrections. The method is based on construction of constant-velocity migration volume by prestack time migration of shot records. The near-surface rms velocities associated with the strong reflector at the base of the near-surface are picked from the semblance spectra to create the near-surface rms velocity field. This laterally varying, but vertically invariant, velocity field actually is equivalent to the near-surface interval velocity field, which can be used to perform prestack depth migration of shot records to obtain a shallow seismic image of the near-surface. Finally, the depth horizon associated with the base of the near-surface is delineated from this shallow seismic image. The image-based near-surface model is formed by combining this depth horizon with the interval velocity field. This image-based model for the near-surface yields essentially the same statics that one calculates from a more complicated model for the near-surface that may be estimated from inversion methods.

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