VSP Modeling With The Reflectivity Method And Semblance Processing

The present work deals with a specific detail of the VSP technique for estimating local compressional, 𝑣𝑃, and shear, 𝑣𝑆 , wave velocities focused on reservoir characterization and seismic interpretation in sedimentary basins. This investigation was based on the presence of fluids (water, oil, gas) in porous rocks and its relationship with the 𝛾 = 𝑣𝑆 𝑣𝑃 ratio, which mainly affects the shear wave velocity. The forward modeling was based on the reflectivity method to simulate seismic sections along depth in the borehole, considering the presence of a reservoir formation with an anomalous 𝛾 ratio value. The seismic processing of the vertical, transversal, and radial components produced by VSP modeling was based on band-pass filtering for upward and downward field separation, semblance picking, and NMO correction for estimating 𝑣𝑃 and 𝑣𝑆 wave velocities. Additionally, we compare different semblance measures to validate the NMO correction. The results show an accurate estimate of the 𝛾 ratio in the target layer, even considering receivers at different depths and high noise levels in the VSP sections. Besides that, the 2D numerical experiment reveals information about the sensitivity of the 𝛾 parameter in a layered medium with velocity variation, principally around regions with the potential presence of oil and gas.