Case History: Anisotropic dual-azimuth PSDM

Prestack depth migration of dual-azimuth surveys incorporating azimuthal P-wave anisotropy

The presence of strong azimuthal anisotropic effects can result in non-flattened PSDM gathers, dependent on the dominant azimuth of the data. This leads to poor quality structural images and data unsuitable for accurate AVO and inversion workflows.

In this example from the southern North Sea, the geophysical objective was to produce a prestack depth-migrated image using two orthogonal seismic surveys over the same prospect, but using a single velocity model.

Normally, this would require an anisotropic model showing orthorhombic symmetry, which requires five separate anisotropy parameters for definition. However, in this case, a simpler and more pragmatic approach was taken by assuming that in the presence of orthorhombic anisotropy, a transverse isotropic model would be correct for each azimuth. Therefore, a single P-wave velocity model could be used, but with anisotropic parameters (delta and epsilon) appropriate for each survey.

A technique using "butterfly gathers" was used where the PSDM gathers were displayed as "split spread" with each of the two survey azimuths on either side of the gather center. Using these, the independent anisotropy parameters can be derived empirically to produce flat gathers and an optimal image using a single P-wave velocity field.

This approach produced significantly better imaging results and provides an efficient technique for handling dual-azimuth surveys in areas exhibiting both VTI and azimuthal velocity anisotropy.