Knowledge of the reflectivity of the sediment-covered seabed is of
significant importance to marine seismic data acquisition and interpretation
as it governs the generation of reverberations in the water layer.
In this context pertinent, but largely unresolved, questions concern
the importance of the typically very prominent vertical seismic velocity
gradients as well as the potential presence and magnitude of anisotropy
in soft surficial seabed sediments. To address these issues, we explore
the seismic properties of granulometric end-member-type clastic sedimentary
seabed models consisting of sand, silt, and clay as well as scale-invariant
stochastic layer sequences of these components characterized by realistic
vertical gradients of the P- and S-wave velocities. Using effective
media theory, we then assess the nature and magnitude of seismic
anisotropy associated with these models. Our results indicate that
anisotropy is rather benign for P-waves, and that the S-wave velocities
in the axial directions differ only slightly. Because of the very
high P- to S-wave velocity ratios in the vicinity of the seabed our
models nevertheless suggest that S-wave triplications may occur at
very small incidence angles. To numerically evaluate the P-wave reflection
coefficient of our seabed models, we apply a frequency-slowness technique
to the corresponding synthetic seismic wavefields. Comparison with
analytical plane-wave reflection coefficients calculated for corresponding
isotropic elastic half-space models shows that the differences tend
to be most pronounced in the vicinity of the elastic equivalent of
the critical angle as well as in the post-critical range. We also
find that the presence of intrinsic anisotropy in the clay component
of our layered models tends to dramatically reduce the overall magnitude
of the P-wave reflection coefficient as well as its variation with
incidence angle.