The influence of the shear-wave structure of the sea bed on the seismic wavefield

Abstract

Much modelling of the seismic wave field is undertaken with an acoustic approximation in which the influence of shear waves is neglected. Although such calculations can predict the correct travel times for compressional (P) wave propagation, they can be very misleading with respect to the distribution of seismic amplitudes, especially at larger offsets. At the sea bed, the acoustic approximation predicts total reflection for P waves incident beyond the critical angle. However, once the presence of shear waves in the solid material of the sea floor is taken into account, P waves in the water can give rise to transmitted S waves with a major change in the propagation pattern. The effects are very important for hard sea bottoms such as those on the Northwest Shelf. The character of the water-borne noise in these conditions depends on whether the shear-wave speed at the sea bed lies above or below the P-wave velocity in the sea water above. For high shear velocities, two distinct sets of critically reflected multiples can be produced to give a very energetic noise train trapped in the water column. Conversion of P to S in transmission at the sea floor can often be important and give rise to significant arrivals on the outer traces from long marine cables. Synthetic seismogram calculations for large offsets, with selective control of the level of multiples and conversion at each interface, provide a convenient tool for characterizing the expected water-borne energy and the influence of converted shear waves on the pressure field recorded in the water.