Seismic Migration in Near-Vertical and Wide-Angle Reflection and Refraction Studies: Towards a Unified Approach

Conventional deep reflection profiles usually image upper to middle crustal levels quite well. The ability of this methodology toimage the lower crust and transition to the mantle is often limited. We have made a step forward in developing a methodology forprocessing, interpreting and presenting near-vertical reflection, wide-angle reflection and refraction seismic data and velocityinformation derived from the three data sets within a unifiedapproach. The key elements of this approach are: pre-stack depthmigration of wide-angle reflection and refraction data; wave fieldanalysis of the refraction and wide-angle reflection data prior toand after depth migration; and unified analysis of seismic velocityinformation. For the finite-difference continuation of the wavefield we use a special oblique time-spatial grid. The time fieldcalculation is based upon a finite-difference approximation of theeikonal equation and the grid used in this task is curvilinear. Thecoordinate lines are based on ray paths and wave fronts in themedium. Such a grid is particularly effective at large offsets. Theoverall configuration of the oblique grid used for the downwardcontinuation of the time field is determined for the domain wherethe solution of the eikonal equation exists. The initial conditionsare defined by the recorded travel times. We demonstrate how thisapproach works on an example from the Petrel sub-basin, Australian North West Shelf, where conventional CDP (nearverticalreflection) data were supplemented by the wide-anglereflection and refraction data recorded by ocean-bottomseismographs. Depth migration of refraction/wide-angle seismicdata presents them in the same style as the conventional reflectiondata thus radically enhancing the seismic image of the lower crustwhere the CDP data lacks detail. Unlike previous studies in depthmigration of refraction/wide-angle data did, we have dealt withrefracting horizons in the deep crust. We have created seismicimages for three (conventionally only one image is formed)refractors including the Moho discontinuity. To achieve this, wehave processed the data recorded in the time-spatial zones wheredifferent seismic events interfere.