How does the shear-wave structure of the seabed affect the seismic wavefield?

Abstract

Much modelling of the seismic wavefield is undertaken with an acoustic approximation in which the influence of shear waves is neglected. Although such calculations can predict the correct traveltimes for compressional-(P-) wave propagation, they can be very misleading with respect to the distribution of seismic amplitudes, especially at larger offsets. At the seabed, 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 below the sea-floor is taken into account, P waves in the sea water incident beyond the critical angle can give rise to transmitted S waves, with a consequent major change in the propagation pattern. Such effects are very important for areas with high velocities at the sea-floor, as commonly occurs in tropical waters, such as the north-west shelf of Australia. The character of the water-borne noise in these conditions depends on whether the shear wavespeed at the seabed 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. Further, the conversion of energy to S waves reduces the energy available for P-wave multiples and dramatically reduces the influence of waterbottom multiples compared with a purely acoustic situation. Synthetic seismogram calculations for large offsets and equivalent calculations in the slowness-time domain, 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.