The fluid bottom approximation and its implications for sound attenuation in underwater waveguides

Abstract

The fluid-bottom model makes forward and inverse problems more amenable to theoretical analysis and numerical simulations but has some well-known limitations. The accuracy and weaknesses of the fluid approximation are elucidated in this paper by quantifying the effects of weak shear rigidity on acoustic normal modes. Mathematically, appearance of finite shear rigidity and attendant slow shear waves is a singular perturbation of the fluid-bottom model, which standard perturbation theories fail to capture correctly. The problem is addressed here by combination of an asymptotic technique and an analysis of exact solutions for idealized seabed models and placed in the context of earlier studies of the effects of shear rigidity. Due to coupling between compressional and shear waves at the seafloor and within the seabed, shear rigidity affects phase and group speeds of normal modes. By transferring the energy from compressional to shear waves, wave coupling makes a significant contribution to sound attenuation from low up to mid-frequencies. It is found that the effects of shear rigidity on the acoustic field in water are magnified by seabed stratification and especially by density variations. Implications of the results for seabed parameterization in geoacoustic inversions will be discussed. [Work supported by ONR.]