3D image-domain wavefield tomography using time-lag extended images

Image-domain wavefield tomography is a technique that reconstructs the velocity model by extracting information from migrated images. In time-lag extended images, velocity model accuracy can be evaluated by reflection focusing error, which represents the traveltime residual in the image domain. The model is updated by minimizing an objective function similar to the one used by wave-equation traveltime inversion. Unlike wave-equation traveltime inversion wherein traveltime residual is obtained from crosscorrelation of a single-shot data, the focusing error is extracted from time-lag gathers, which are wavefield crosscorrelations based on multiple experiments. Because the signal-to-noise ratio is higher in common-image gathers than that in common-shot gathers, our technique is able to measure focusing error more accurately in presence of noisy data and complex geologic structures. In addition, the imagedomain approach is robust as it does not suffer from cycleskipping, which is common in conventional data-domain fullwaveform inversion. We illustrate the method using both synthetic and field data. The North Sea 3D field data example demonstrates that the technique is effective in optimizing the velocity model and improving image quality. In addition, the method is efficient in 3D because only the time-lag extensions are computed and stored.