Methods Of 3D VSP Multi-wave Velocity Analysis And NMO Correction

VSP is a measurement procedure that a seismic source on or near the earth's surface transmit energy to a geophone secured in the wall. Compared with surface seismic technology, since the receivers are placed within the earth, VSP has a higher S/N ratio, resolution and close to the reflector of interest, thus, VSP is a promising seismic survey technology. With the progresses of VSP exploration instrument and processing explanation methods, multi-wave VSP exploration is getting more and more common, so it's necessary to further study methods of calculating P-wave and S-wave velocity and NMO imaging with multi-component VSP date.This article has carried on summary to former methods of VSP P-wave and S-wave velocity analysis as well as NMO correction, in view of the insufficiency of former velocity analysis, proposes the method of velocity analysis under the shaft, this method is based on the travel-time curve of offset distance and the VSP reflection time. Infers hyperbolic curve equation by the P-P reflected wave's equation of higher degree similar to surface seismic, as first break time of down-going P-wave has been known, carries on P-wave velocity analysis and NMO correction under the shaft based on this equation. In view of the P-SV converted wave, proposes the similar double square root travel-time equation, as the p-wave velocity has been known, seeking higher accuracy S-wave velocity. After obtaining velocity parameter, this article uses digital NMO correction method similar to surface seismic, and carries on excision on the stretch distortion seriously record. Because the VSP ray path is asymmetrical, this article studied and summarized the method of calculating the position of spot in pressure wave and the converted wave, this is foundation of precisely imaging in reflector of interest , after calculating the position of reflection spot, obtains common-reflection point-gather on binning, then carries on NMO correction,stacks with changed fold number. The author processes the vertical well Walk-away VSP multi-wave record and deviated 3D-VSP theory record which were obtained by ray tracing scheme, has verified the validity of this method. Finally the practical data is analyzed and get a good effect. It proves the ability to solve practical problems.