Estimating seismic velocities below the sea-bed using surface waves
Seismic data acquired in shallow offshore surveys often display well-defined dispersion patterns related to two types of surface wave, propagating in shallow subwater layers. The waves of the first type propagate as normal modes and are represented by a low-velocity, low-frequency wavetrain identified with Scholte waves. The phase velocities of Scholte waves are related to the shear-wave velocities VS below the water-bottom and can be inverted to estimate VS in the subwater layers. The waves of the second type propagate as leaking modes and are characterized by a number of distinctive features: their dispersion patterns have a resonant frequency-tuned appearance, they have relatively high cut-off frequencies and their phase velocities exceed the velocity in the water. When the subwater layers are composed of relatively soft saturated rocks with high Poisson’s ratio, the leaking modes can be closely approximated by acoustic waves. By inverting the approximating dispersion curves, the vertical distribution of the compressional-wave velocity VP in the shallow subwater layers can be estimated. Estimation of VS and VP below the sea-bottom using the two types of surface wave is illustrated by examples from shallow offshore surveys conducted at several sites in the eastern Mediterranean area.