- Copyright © 2003 Society of Exploration Geophysicists
Recording and processing both the converted shear waves and the compressional waves from multicomponent ocean bottom seismic (OBS) data provide additional opportunities to understand the subsurface. The converted waves are generally used to provide complementary images and velocity information to support the conventional seismic model. However, they also provide a new opportunity to measure and understand shear-wave azimuthal anisotropy. This paper describes the results from an azimuthal anisotropy study on 3D 4C OBS data from Valhall Field.
The Valhall OBS survey was primarily designed to improve imaging of the crest of the Valhall structure where conventional seismic imaging is degraded (Rosland, 1999). The primary objective for this reprocessing project was to improve the seismic velocity information in the overburden where the seabed acquisition system and geometry were expected to have a number of effects on the shallow data. The geometry was designed for sufficient coverage at the target level but the 600 × 600 m cross-spread design has only limited fold and coverage in the shallow overburden and this constrains any velocity analysis methods. Due to fracturing in the crest of the reservoir structure we did expect to see shear-wave splitting; however, we were surprised by the striking results from the anisotropy analysis of the shallowest layer. Figure 1 shows the circular pattern of observed anisotropy and its correlation to the seafloor subsidence. Further azimuthal anisotropy analysis also showed shear-wave splitting in the deeper layers above the reservoir level, and compensating for these effects has significantly improved the quality of the converted wave image.
Although this project was finished in 2001, we believe the methods employed are still “state-of-the-art” as we …