Underwater sound propagation over a layered seabed with weak shear rigiditya)

The shear wave speed is often small compared to the compressional wave speed in the top part of the seabed, where acoustic normal modes penetrate. In sediments with weak but finite shear rigidity, the strongest conversion from compressional to shear waves occurs at interfaces within the sediment. Shear wave generation at such interfaces and interference within sediment layers lead to first-order perturbations in the normal mode phase speed and contributions to sound attenuation, which vary rapidly with frequency. Weak shear rigidity is shown to lead to unexpectedly strong mode group speed perturbations that retain finite magnitudes for very small shear speeds in range-independent waveguides. Variation of the waveguide parameters with range affects shear-induced attenuation and mode travel time perturbations in a different manner, depending on whether shear wave interference conditions vary appreciably along the propagation path. In horizontally inhomogeneous ocean, weak shear magnifies the horizontal refraction of adiabatic normal modes due to sloping intra-sediment interfaces. In contrast to normal modes, attenuation of lateral waves with range is insensitive to weak shear. Concurrent measurements of normal mode and lateral wave attenuation can be potentially used to identify and separate the contributions of dissipation and shear waves into observed sound attenuation.