Rapid automated determination of shallow velocity?depth structure using first breaks and the generalised reciprocal method

Abstract

The generalised reciprocal method (GRM) is a convenient and substantial enhancement to the capability of the refraction method in dealing with irregular refractor interfaces with lateral changes in refractor velocity. Processing approaches have been developed which implement a fast automated GRM-based approach and are capable of handling multifold refraction data such as that from long seismic reflection survey lines. A method is proposed for determining the resolution achievable with the velocity analysis technique as a function of the detector separation. This provides a means of optimising the resolution with spatially under-sampled data for most shallow exploration applications. There are three major stages in this approach: the recognition through stacking coherency of individual layers within the multifold data, and the production of a single composite travel-time graph; the determination of the seismic velocities within each refractor using an automatic curve recognition technique, using the velocity analysis function to obtain the optimum geophone migration separation and to locate the lateral velocity variations within a refractor; and depth inversion in which a modified depth conversion factor is proposed to control the trade off between the vertical resolution of velocity layers and changes in layer thickness. This factor reduces spurious details in inverted depths layers and is stable when a layer appears or disappears in the section. The approach has been tested successfully on both synthetic data and real data from the Kalgoorlie deep seismic reflection traverse. Results show that application of the techniques achieves better resolution than conventional manual interpretation, and provides a fast and direct inversion that appears to discount spurious detail.