Diving-wave Refraction Tomography

Robust near-surface velocity modeling using turning waves

In areas displaying weak shallow seismic reflections, multiple contamination, and strong near-surface velocity variations, reflection tomography often produces an inadequate and poorly resolved velocity model.

WesternGeco offers diving-wave refraction tomography to provide a reliable near-surface velocity model for reflection tomography iterations in a depth migration workflow, or static corrections for time processing.

Turning-ray tomography uses refracted first arrivals to compute a near-surface earth model by minimizing the difference between calculated and observed travel times. Because diving waves sample the near surface with more redundancy and with a greater angular range than reflected waves, we can converge on a more robust model using fewer iterations. This approach can be used in land, marine, or OBC environments.

In a recent offshore example, the presence of shallow gas created several problems for a conventional depth imaging approach; the dataset was contaminated by strong surface and interbed multiples, as well as significant scattering of the wavefield and lower overall bandwidth. Initial attempts using reflection tomography failed to resolve these short-wavelength features, and there were added concerns that the lack of fold in the shallow data was causing the reflection tomography approach to converge on an incorrect geological model.

WesternGeco applied a full imaging workflow incorporating diving-wave refraction tomography to provide an initial shallow velocity model that rapidly converged on a more reliable solution for estimating near-surface velocities, resulting in fewer reflection tomography updates to complete the velocity model.