Thesis (D.S.)--Massachusetts Institute of Technology and Woods Hole Oceanographic Institution, 1986. / Includes bibliographical references (p. 291-296).

Thesis (M.S.)--Joint Program in Oceanography/ Applied Ocean Science and Engineering, Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution, 2005. / Bibliography: p.49-50.

Thesis (M.S.)--Massachusetts Institute of Technology and Woods Hole Oceanographic Institution, 1979. / Contract no.: N00014-77-C-096; NR 083-004. Includes bibliographical references (p. 132-133).

Thesis (Ph. D.)--University of Canterbury, 2006. / Typescript (photocopy). "June 2006." Includes bibliographical references (p. [169]-182) and index. Also available via the World Wide Web.

No description available.

In passive sonar systems, knowledge of low-frequency shipping noise is an important factor for target detection performance. However, an accurate model for the shipping noise structure is difficult to obtain, due to the varying distributions of ships and complicated underwater environment. This work characterizes low-frequency distant shipping noise observed in deep water environments as a function of receiver depth and vertical arrival structure for the case of a receiver below the conjugate depth. Surface shipping noise is examined using Monte Carlo simulations using a normal mode propagation model based on random distribution of ships and realistic parameters. The depth dependence of the simulated distant shipping noise is in agreement with published experimental measurements. A Vertical Line Array (VLA) is used to produce vertical beams that isolate the surface interference from nearby targets. Simulation results quantifying the beamformer output as a function of ocean environment, receiver aperture, and frequency are presented for both conventional and adaptive beamformers. The results suggest that conventional beamforming could detect the noisy target from both direct arrival and bottom bounce in the presence of distant shipping interferers and wind noise. However, the beamwidth of conventional beamforming is wider than that of adaptive beamforming. Once the motion effects of nearby ship interferences are considered, the adaptive beamforming using diagonal loading provides better detection performance. Preliminary adaptive beamforming results corresponding to different snapshot times show that motion effects can be minimized by using short observation times.

A design for a deep ocean hydrophone is proposed here. The hydrophone's sound sensing element is comprised of a capped end, piezoelectric cylinder. This sound sensing element is encased in an acoustic coupling fluid filled elastomeric boot. A small diameter tube communicates hydrostatic pressure from the coupling fluid to the interior of the sound sensing element for hydrostatic pressure compensation. The theoretical free field voltage sensitivity, the ratio of open circuit voltage to incident acoustic pressure, is predicted from mathematical model of the sound sensing element.

Acoustical interactions with ocean sediments effect a wide range of sonar applications in littoral environments. An important factor in understanding the acoustical behavior of the ocean bottom is how the sediment density changes with depth. Although there are existing techniques for obtaining information about sediment gradients, these methods are unable to provide direct measurements of the sediment density gradient without significantly disrupting the test site and requiring significant diver support for installation and implementation. The proposed X-Ray Attenuation Measurement (XRAM) device aims to improve upon these existing techniques with the goal of being a portable diver operated device that can perform direct in situ measurements of sediment density gradients without significant disruption of the ocean bottom. To accomplish this, the XRAM utilizes the attenuation of x-rays passing through the sediment to measure the density as a function of depth, and is arranged in a compact, portable design that can be deployed and operated by a single diver. The layout and basic design of the XRAM device is discussed, and a physical model of its operation is developed. Results of experimental testing on homogeneous liquid samples and liquid/solid mixtures to evaluate the effectiveness of the XRAM device in measuring density gradients are presented. Based on the analysis of these results, recommendations of improved performance for future development are given. / text

Three-dimensional acoustic propagation in shallow water waveguides is studied using the longitudinally invariant finite element method. This technique is appropriate for environments with lateral variations that occur in only one dimension. In this method, a transform is applied to the three-dimensional Helmholtz equation to remove the range-independent dimension. The finite element method is employed to solve the transformed Helmholtz equation for each out-of-plane wavenumber. Finally, the inverse transform is used to transform the pressure field back to three-dimensional spatial coordinates. Due to the oscillatory nature of the inverse transform, two integration techniques are developed. The first is a Riemann sum combined with a wavenumber sampling method that efficiently captures the essential components of the integrand. The other is a modified adaptive Clenshaw-Curtis quadrature. Three-dimensional transmission loss is computed for a Pekeris waveguide, underwater wedge, and Gaussian canyon. For each waveguide, the two integration schemes are compared in terms of accuracy and efficiency. / text

Approved for public release; distribution is unlimited / A simple low-noise hydrophone design with internal preamplifier is presented. This design is similar to published designs and is a variation of the design developed in the NPS thesis by Miguel Alvarado [2003], except that several improvement features are included. These include a simplification of the structure and its modes of vibration, a large reduction in package diameter and the effect its acoustic diffraction has on the sensitivity, and an extended upper frequency response of 42 kHz resulting from the simplified structure and reduced diameter. Furthermore, the modified geometry along with its orientation in the water should produce a very omni-directional response in the horizontal plane at the higher frequencies. Finally, an new feedback preamplifier design developed by Hofler and Alvarado was discovered to have some subtle but serious performance problems. These problems were resolved in this research and the resulting preamplifier performance was tested and documented herein. / Lieutenant, Hellenic Navy