MULTICOMPONENT NEAR-SURFACE CORRECTION FOR LAND VSP DATA

Multicomponent seismic data collected using directional sources are de graded by the wave excitation process due to inaccurate control of the ground motion, unequal activation strengths or ground couplings betwe en differently oriented sources, and misalignment of the pad. These ac quisition uncertainties are exacerbated by the complicated near-surfac e scattering present in most seismic areas. Neither group of effects s hould be neglected in multicomponent analyses that make use of relativ e wavefield attributes derived from compressional and shear waves. The se effects prevent analysis of the direct and reflected waves using pr ocedures based on standard scalar techniques or a prima facia interpre tation of the vector wavefield properties, even for the seemingly stra ightforward case of a near-offset vertical seismic profile (VSP). Near -surface correction, using a simple matrix operator designed from the shallowest recordings, alleviates many of these interpretational diffi culties in near-offset VSP data. Results from application of this tech nique to direct waves from a nine-component VSP shot at the Conoco tes t-site facility, Oklahoma, are encouraging. The technique corrects for unexpected compressional-wave energy from shear-wave vibrators and co llapses near-surface multiples, thus facilitating further processing f or the upgoing wavefield. The method provides a simple and effective p rocessing step for routine application to near-offset VSP analyses.