Imaging of VSP Data

Abstract

Migration of VSP data can be effectively accomplished using a focused-lag sum imaging technique, similar to the Kirchoff 'diffraction stack' method. The imaging procedure is sufficiently flexible to handle the irregular source-receiver geometry associated with walkaway-source and deviated hole VSP. Signal dispersion is readily accommodated by admitting both group and phase velocity parameters. The migration can be performed on transmission data (downgoing wave) to yield a velocity map or on reflection data (upgoing wave) to produce a reflector map. The accuracy of both reflector mapping and velocity mapping depends on the nature and form of the signal. Pulse compression and multiple suppression are desirable pre-processing operations. The technique has been tested on synthetic VSP data and successfully applied to shallow VSP data collected in the Sydney Basin coalfields. For the real data, average velocities of the coal measure sequence and blurred images of the main seams were obtained. An equivalent section migration technique, based on leastsquares theory, is formulated in the Appendix.