On the determination of modal attenuation coefficients and compressional wave attenuation profiles in a range-dependent environment in Nantucket Sound
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Techniques for determining modal attenuation coefficients and the compressional wave attenuation profile of the bottom in shallow water are presented. The input data consist of spatially well-sampled measurements of the pressure field versus range due to a monochromatic point source, which can be provided by either real or synthetic aperture horizontal arrays. Several methods are described for obtaining modal attenuation coefficients from the pressure field or its Hankel transform. The bottom attenuation profile is related to the modal attenuation coefficients through an integral equation that is solved using linear inverse theory. The methods are demonstrated using both noise-free and noisy synthetic data. The results of inverting experimental data from Nantucket Sound at 140 Hz and 220 Hz are presented, including the resolution and variance estimates of the inferred bottom attenuation profiles. Separation of the contributions of other attenuating mechanisms that can be confused with compressional wave attenuation (shear, rough surface scattering) is also discussed.