A CASE EXAMPLE OF NEAR-SURFACE CORRECTION FOR MULTICOMPONENT VSPS

A deterministic near-surface correction procedure is developed for mul ticomponent VSP data, shot using directional sources and recorded usin g three-component receivers. The method is capable of removing unwante d effects of acquisition such as unequal source strengths or misorient ations, but may also remove near-surface multiples and anisotropy. Thi s is of considerable benefit for obtaining accurate and consistent est imates of subsurface anisotropy from different source combinations. Ap plication of the technique is illustrated using a dataset from the Rom ashkino reservoir in Russia, where three or four different directional sources are used at the same source locations. The technique corrects for the large discrepancies which exist between the estimates obtaine d using different source combinations. Application of the technique to three wells in the survey region reveals a nearly isotropic subsurfac e, except for a few isolated zones of moderate to high (2 to 8%) aniso tropy which lie close to the expected depth for the reservoir. Althoug h there is no significant correlation with the production figures for each well, the qS1 polarization azimuth within the reservoir does vary at each well location, suggesting that this may be a more sensitive i ndicator of reservoir process.