ABSTRACT

The Green’s function between two receiver locations can be estimated by crosscorrelating and summing the recorded Green’s functions from sources on a boundary that surrounds the receiver pair. We demonstrate that when two receivers are positioned far from the source boundary in a marine-type acquisition geometry, the crosscorrelations (the Green’s functions before summation over the source boundary) are dominated by reflected energy which can be used in a semblance analysis to determine the seismic velocity and thickness of the first layer. When these crosscorrelations are summed over the boundary of sources, the resulting Green’s function estimates along a receiver array contain nonphysical or spurious refracted energy. We illustrate that by using a further semblance analysis, the most prominent nonphysical refracted energy occurs prior to the direct arrival and determines the remaining refraction velocities of deeper layers (or interval velocities in the case of a subsurface with homogeneous layers). We demonstrate the velocity analysis procedure on a single layer over half-space model, a three layer over a half-space model, and a more realistic model based on a North Sea oil field.

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