In recent years, the topic of noise in the sea and its effects on marine mammals has attracted considerable attention from both the scientific community and the general public. Since marine mammals rely heavily on acoustics as a primary means of communicating, navigating, and foraging in the ocean, any change in their acoustic environment may have an impact on their behavior. Specifically, a growing body of literature suggests that low-frequency, ambient noise levels in the open ocean increased approximately 3.3 dB per decade during the period 1950–2007. Here we show that this increase can be attributed primarily to commercial shipping activity, which in turn, can be linked to global economic growth. As a corollary, we conclude that ambient noise levels can be directly related to global economic conditions. We provide experimental evidence supporting this theory and discuss its implications for predicting future noise levels based on global economic trends.
Three perturbative inverse methods for obtaining bottom geoacoustic parameters as a function of depth in shallow water are described. The required input data are the trapped mode eigenvalues for one or more frequencies, the group velocity dispersion curves for one or more modes, or the cw pressure field versus range. In each case, a Fredholm integral equation arises that can be solved using linear inverse theory, and for which resolution and variance estimates of the solution can be readily made. Attention is focused primarily on the modal eigenvalue inverse problem for which the theory for determining the compressional wave speed, compressional wave attenuation, and density is developed in detail. Properties of this technique are studied using synthetic data and include investigations of the dependence of the results on acoustic frequency, number of modes excited, and partial a priori knowledge of the bottom. The method is demonstrated on experimental data obtained in Nantucket Sound at 140 and 220 Hz. Directions of future research on these techniques are indicated.
A technique for acoustically characterizing shallow water waveguides is presented. For a horizontally stratified ocean and bottom, the method consists of measuring the magnitude and phase versus range of the pressure field due to a cw point source and numerically Hankel transforming these data to obtain the depth-dependent Green’s function versus horizontal wavenumber. It is shown that, in the context of normal mode theory, the Green’s function contains information about the nature of the discrete and continuous modal spectra as well as the plane-wave reflection coefficients of the waveguide boundaries. Inversions are performed using pressure field data generated synthetically over reasonable experimental apertures (1–5 km) to obtain Green’s functions for the cases of an isovelocity water column overlying both hard and fast isovelocity bottoms (Pekeris waveguide). The Green’s function results show excellent agreement with theory, while the subsequently calculated reflection coefficients of the bottom are of somewhat lower quality. It is shown that features of the Green’s function itself can be used to extract modal properties and characteristics of the bottom. The effects of sediment attenuation and shear in the Pekeris case are discussed, and a comparison of this method with conventional phased array mode resolution techniques is made.
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