Seismic migration in near-vertical and wide-angle reflection and refraction studies: towards a unified approach

Abstract

Conventional deep reflection profiles usually image upper to middle crustal levels quite well. The ability of this methodology to image the lower crust and transition to the mantle is often limited. We have made a step forward in developing a methodology for processing, interpreting and presenting near-vertical reflection, wide-angle reflection and refraction seismic data and velocity information derived from the three data sets within a unified approach. The key elements of this approach are: pre-stack depth migration of wide-angle reflection and refraction data; wave field analysis of the refraction and wide-angle reflection data prior to and after depth migration; and unified analysis of seismic velocity information. For the finite-difference continuation of the wave field we use a special oblique time-spatial grid. The time field calculation is based upon a finite-difference approximation of the eikonal equation and the grid used in this task is curvilinear. The coordinate lines are based on ray paths and wave fronts in the medium. Such a grid is particularly effective at large offsets. The overall configuration of the oblique grid used for the downward continuation of the time field is determined for the domain where the solution of the eikonal equation exists. The initial conditions are defined by the recorded travel times. We demonstrate how this approach works on an example from the Petrel sub-basin, Australian North West Shelf, where conventional CDP (near-vertical reflection) data were supplemented by the wide-angle reflection and refraction data recorded by ocean-bottom seismographs. Depth migration of refraction/wide-angle seismic data presents them in the same style as the conventional reflection data thus radically enhancing the seismic image of the lower crust where the CDP data lacks detail. Unlike previous studies in depth migration of refraction/wide-angle data did, we have dealt with refracting horizons in the deep crust. We have created seismic images for three (conventionally only one image is formed) refractors including the Moho discontinuity. To achieve this, we have processed the data recorded in the time-spatial zones where different seismic events interfere.