Detailed Refraction Statics with the GRM and the RCS

Abstract

We derive refraction statics for seismic data recorded in a hard rock terrain, in which the statics corrections range from less than 10 ms to in excess of 70 ms, over distances as short as 12 receiver intervals or 480 m. We compare statics values computed with a simple model of the weathering using the generalized reciprocal method (GRM) and the refraction convolution section (RCS) with those computed with a more complex model of the weathering using least-mean-square inversion with the conjugate gradient algorithm (Taner et al, 1998). The differences between the GRM model and that of Taner et al (1998), systematically vary from an average of 2 ms to 4 ms over a distance of 8.8 km. The differences between these two refraction models and the final statics model which includes the automatic residual values, are generally less than 5 ms. The residuals for the GRM model are frequently less than those for the model of Taner et al (1998). The RCS statics are picked approximately 10 ms later, but their relative accuracy is comparable to that of the GRM statics. The residual statics values show a general correlation with the refraction statics values, and they can be reduced in magnitude by using a lower average seismic velocity in the weathering. These results suggest that inaccurate average seismic velocities in the weathered layer may often be a source of short wavelength statics, rather than any shortcomings with the inversion algorithms in determining averaged delay times from the traveltimes. The significance of these results is that the RCS achieves improved accuracy through stacking to improve signal-to-noise ratios prior to the measurement of any traveltimes. Therefore, the RCS offers a new approach to determining more accurate statics with second arrivals such as shear waves, for which signal-to-noise ratios on shot records can be much less than those of the first arrival compressional waves.