Estimating seismic velocities below the sea‐bed using surface waves

Abstract

simulation of a land survey (i.e. application of horizontal sources and receivers) at the sea-bottom does not seem to be practical, even in very shallow water. An additional problem may arise when shallow subwater layers are composed of relatively soft material. In this case, the lack of sharp velocity contrasts in water-saturated rocks may hamper the use of P-wave refraction surveys for estimating V P below the sea-bed.In recent years, seismic methods based on the use of surface waves have been gaining a growing popularity, mainly for estimating near-surface V S distribution. The main advantage of the surface-wave methods is that they do not require a special acquisition technique since they utilize the data contained in the conventional (P-wave) seismic records. The methods are based on the analysis of the dispersion properties of surface waves, i.e. the dependence of the propagation velocity of different spectral components of the waves upon their frequency. This dependence (shown as dispersion curves) is closely related to the vertical distribution of V S in the subsurface. Thus, estimates of shear-wave velocities can be obtained by analysing the dispersion relationships derived from the surface waves contained in P-wave seismic records. Various modifications of the method have been successfully used for estimating V S in a variety of applications (Gabriels et al. ABSTRACTSeismic 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 V S below the water-bottom and can be inverted to estimate V S 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 V P in the shallow subwater layers can be estimated.Estimation of V S and V P 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. * vladi@gii.co.il

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