Reflection seismic survey in crystalline rock is much more difficult than in sedimentary basins because of the irregular and complex reflector geometry and low S/N across many geological boundaries due to the long period of geological process. In particular, shallow reflections in land seismic data are overwhelmed by random and coherent noises. Furthermore, time shifts caused by rapid variations in the thickness of low-velocities weathering layer and change in elevation of the location with shot points and receivers generate long-wavelength traveltime deviations.
Such long wavelength statics have been solved by weathering correction(refraction statics). Two representative refraction-statics techniques of GRM(conventional method) and IRS(recent method) are discussed in terms of continuities of the refrators and horizontal resolution of the underlying reflectors, especially in the application to crystalline-rock seismic data. The first data set includes the field data from crystalline rock terrane for GRM-based mapping, whereas the second one from Cheongu granitic bodies for IRS-based imaging.
A built-in-house Excel-GRM routine is found to be effective for mapping the shallow refractor, with simply employing a pair of forward and reverse shots. Regarding the deeper refractor mapping, velocity analysis function and time-depth function are computed and presented for a range of values from zero to 50 considerably larger than the optimum value. The increment in values of (distance between surface emergences of forward and reverse rays) was taken being equivalent to the geophone spacing of 10. The bedrock refractor is successfully mapped in 25~35 depth ranges with three values of refractor velocity (decreasing north-eastwards), using the selected optimum value of =10. Optimum value was determined by considering the simple graph and detailed graph for velocity analysis function and time-depth function, respectively.
IRS technique utilizes the first arrival signal instead of first break picking and therefore it is required to remove the shallow noises as much as possible prior to the application, by AGC(automatic gain control) for compensation of decreased amplitude, surgical muting for erasing the dirty-signal zone, and f-k filtering for rejecting the ground-roll. It is designed with optimum parameters of interferometry offset, deconvolution prewhitening, offset reduction stack, smoothing, etc. We tested the parameters and applied IRS to the synthetic data for deep and shallow targets provided by seismic mapping(refraction, 3-component geophone, MASW) and SPS logging from the Cheongju granite area.
The unresolved long-wavelength statics are much resolved by IRS weathering correction and a desired refractor is incidentally and distinctly mapped in the refraction convolution stack(RCS). IRS approach is found to be better than conventional first-break picking method for removing the hyperbolic traveltime deviations in the shot gather, and improving the resolution and continuities of the reflection events in the stack section.