An analytic model is developed for scattering from random inhomogeneities in range-dependent ocean waveguides using the Rayleigh-Born approximation to Green's theorem. The expected scattered intensity depends on statistical moments of fractional changes in compressibility and density, which scatter as monopoles and dipoles, respectively, and the coherence volume of the inhomogeneities. The model is calibrated for ocean bottom scattering using data acquired by instantaneous wide-area ocean acoustic waveguide remote sensing (OAWRS) and geophysical surveys of the ONR Geoclutter Program. The scattering strength of the seafloor on the New Jersey shelf, a typical continental shelf environment, is found to depend on wave number k, medium coherence volume V(c), and seabed depth penetration factor F(p) following a 10 log(10)(F(p)V(c)k(4)) dependence. A computationally efficient numerical approach is developed to rapidly compute bottom reverberation over wide areas using the parabolic equation by exploiting correlation between monopole and dipole scattering terms and introducing seafloor depth penetration factors. An approach is also developed for distinguishing moving clutter from statistically stationary background reverberation by tracking temporal and spatial fluctuations in OAWRS intensity images.
An analytic model is developed for the time-dependent ultrasound field reflected off a randomly rough vibrating surface for a continuously scanning ultrasound vibrometer system in bistatic configuration. Kirchhoff's approximation to Green's theorem is applied to model the three-dimensional scattering interaction of the ultrasound wave field with the vibrating rough surface. The model incorporates the beam patterns of both the transmitting and receiving ultrasound transducers and the statistical properties of the rough surface. Two methods are applied to the ultrasound system for estimating displacement and velocity amplitudes of an oscillating surface: incoherent Doppler shift spectra and coherent interferometry. Motion of the vibrometer over the randomly rough surface leads to time-dependent scattering noise that causes a randomization of the received signal spectrum. Simulations with the model indicate that surface displacement and velocity estimation are highly dependent upon the scan velocity and projected wavelength of the ultrasound vibrometer relative to the roughness height standard deviation and correlation length scales of the rough surface. The model is applied to determine limiting scan speeds for ultrasound vibrometer measuring ground displacements arising from acoustic or seismic excitation to be used in acoustic landmine confirmation sensing.
Fish populations in continental shelf environments can be continuously imaged over thousands of square kilometers using acoustic waveguide remote sensing techniques [Makris et al., Science, Feb. (2006)]. A calibrated range-dependent scattering and reverberation model [Ratilal et al., J. Acoust. Soc. Am. 114, 2302 (2003)] based on the parabolic equation has been applied to assess population densities of fish by inverting long-range acoustic data collected on the New Jersey continental shelf. This model is now applied to predict the types of fish species and zooplankton that are detectable in a general range-dependent continental shelf environment, including the resolution and accuracy that can be expected in estimating fish population densities and for differentiating fish species. We consider different geometries of the source and receiving array to enhance biological detection and reduce background reverberation in highly range-dependent environments. Using multiple source frequencies, the possibility of distinguishing fish species based on their differing scattering characteristics and resonance frequencies will be examined.
The statistically stationary background reverberation in long-range sonar data acquired in the New Jersey Continental shelf during the ONR 2003 Main Acoustic Clutter Experiment is mainly due to seafloor scattering. We compare three mechanisms or models for scattering from the sea bottom in a range-dependent waveguide with the experimental data. They are the Rayleigh-Born volume scattering model, a rough surface scattering model, and an empirical Lambertian model. Each of these models has a different decay rate as a function of range and different frequency dependence. These models are compared to the measured background reverberation as a function of range from the sonar system and at various frequency bandwidths from 300 to 1500 Hz to determine which mechanism or model for seafloor scattering best describes the data.
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