Month: May 2022

Variational picking of optimal surfaces

May 24, 2022 Examples No comments

A new paper is added to the collection of reproducible documents: A variational approach for picking optimal surfaces from semblance-like panels

We propose and demonstrate a variational method for determining optimal velocity fields from semblance-like volumes using continuation. The proposed approach finds a minimal-cost surface through a volume, which often corresponds to a velocity field within a semblance scan. This allows picked velocity fields to incorporate information from gathers that are spatially near the midpoint in question. The minimization process amounts to solving a nonlinear elliptic partial differential equation, which is accomplished by changing the elliptic problem to a parabolic one and solving it iteratively until it converges to a critical point which minimizes the cost functional. The continuation approach operates by using a variational framework to iteratively minimize the cost of a velocity surface through successively less-smoothed semblance scans. The method works because a global minimum for the velocity cost functional can only exist when the semblance scan varies smoothly in space and convexly in the parameter being scanned. Using a discretization of the functional with a limited-memory Broyden-Fletcher-Goldfarb-Shanno (L-BFGS) algorithm we illustrate how the continuation approach is able to avoid local minima that would typically capture the iterative solution of an optimal velocity field determined without continuation. Incorporating continuation enables us to find a lower cost final model which is used for seismic processing of a field data set from the Viking Graben. We then utilize a field data set from the Gulf of Mexico to show how the final velocity model determined by the method employing continuation is largely independent of the starting velocity model, producing something resembling a global minimum. Finally, we illustrate the versatility of the variational picking approach by demonstrating how it may be used for automatic interpretation of a seismic horizon from the Heidrun Field.

Wave-equation time migration

May 23, 2022 Examples No comments

A new paper is added to the collection of reproducible documents: Wave-equation time migration

Time migration, as opposed to depth migration, suffers from two well-known shortcomings: (1) approximate equations are used for computing Green’s functions inside the imaging operator; (2) in case of lateral velocity variations, the transformation between the image ray coordinates and the Cartesian coordinates is undefined in places where the image rays cross. We show that the first limitation can be removed entirely by formulating time migration through wave propagation in image-ray coordinates. The proposed approach constructs a time-migrated image without relying on any kind of traveltime approximation by formulating an appropriate geometrically accurate acoustic wave equation in the time-migration domain. The advantage of this approach is that the propagation velocity in image-ray coordinates does not require expensive model building and can be approximated by quantities that are estimated in conventional time-domain processing. Synthetic and field data examples demonstrate the effectiveness of the proposed approach and show that the proposed imaging workflow leads to a significant uplift in terms of image quality and can bridge the gap between time and depth migrations. The image obtained by the proposed algorithm is correctly focused and mapped to depth coordinates it is comparable to the image obtained by depth migration.