Abstract
SUMMARY A method for using PS converted waves to detect and characterize azimuthal seismic anisotropy is developed, and applied to four-component ocean-bottom seismometer data acquired near the base of the continental slope west of Svalbard. The method determines the distribution of coherent signal vibrating perpendicular to the sagittal plane (the transverse direction), and employs an optimized rotation technique to separate the effects of anisotropy from other contributions to the seismic amplitude. The magnitude of the PS-wave transverse amplitude, when normalized by its radial counterpart, is shown to correspond to the degree of ellipticity of particle motions and is used to constrain the anisotropic symmetry class in the near-surface sediments. Results indicate the presence of orthorhombic or horizontal, transversely isotropic symmetry that is consistent with the presence of near-vertical, aligned cracks. From its orientation, the observed anisotropy is associated with: (1) gravitational, downslope stresses in the sediment sequence, which dips by approximately two degrees to the SW and (2) tectonic stresses associated with the segmented spreading centres of the northern Knipovich Ridge, which spread in the NW–SE direction. Aligned cracks are likely to influence the local distribution of gas hydrate and free gas in these sediments. An investigation of the effect of seismic noise on optimized rotation shows that the vectorial combination of independent geophone records has variable sensitivity to source-related noise. This sensitivity peaks where the sagittal azimuth bisects the deployment orientation of the geophones on the seabed. A method to reduce the sensitivity to directional noise is developed and shown to improve the transverse amplitude plots. This approach to noise reduction may be used to improve the quality of seismic images derived from multicomponent data.
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