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Bistatic sonar and a novel form of variable depth sonar
This thesis relates to the sonar system research undertaken for a Naval Requirement for proposals to
improve the cost effectiveness of the defence of shipping against submarine attacks. Defence systems evolved as a function of the developing technology of the opposition which in this instance is the ability of a submarine to remain undetected below the sea surface while searching for, tracking and attacking its targets. An inherent problem for underwater detection with Escort Ships hull type sonars is its location on the air-sea interface with the need of a two-way propagation path to access the
depth-range volume available to a submarine. As the power of an Escort's sonar is increased so is the size of ship, 5000 tons and more, to accommodate the optimum size of transducers required.Sonar system research at all times is a multi-discipline task and in this particular case was further broadened with a requirement to review the possibilities for sources of energy other than underwater
acoustics. The research confirms the dominance of sonar for underwater detection and establishes the feasibility of a Bistatic Sonar concept which replaces the two-way propagation path of a hull type sonar with a one-way path, source-target-receiver with a variable depth directive towed line receiver on a small ship as a distant receiver. A second objective which became feasible with the development of an adequate towed source was a variable depth sonar which has now been produced by British Aerospace for a world market. A summary of the thesis is provided as an introduction to the subject matter of the different sections
Moored acoustic travel time (ATT) current meters : evolution, performance, and future designs
New laboratory measurements and numeric model studies show
the present folded-path ATT current meters are stable and
sensitive, but are not well suited for mean flow observations
in surface gravity waves. Alternate designs which reduce
unwanted wake effects are proposed. ATT flowmeter history,
principles of acoustic flow sensors, mean flow near cylinders,
and the need for linear flow sensors are reviewed.Prepared for the Office of Naval Research under
Contract Number N00014-76-C-0197; NR083-400 to
the Woods Hole Oceanographic Institution
Effects of internal waves on low frequency, long range, acoustic propagation in the deep ocean
Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution September 2007This thesis covers a comprehensive analysis of long-range, deep-ocean, low-frequency, sound
propagation experimental results obtained from the North Pacific Ocean. The statistics of
acoustic fields after propagation through internal-wave-induced sound-speed fluctuations
are explored experimentally and theoretically.
The thesis starts with the investigation of the North Pacific Acoustic Laboratory 98-99
data by exploring the space-time scales of ocean sound speed variability and the contributions from different frequency bands. The validity of the Garret & Munk internal-wave
model is checked in the upper ocean of the eastern North Pacific. All these results impose hard bounds on the strength and characteristic scales of sound speed fluctuations one
might expect in this region of the North Pacific for both internal-wave band fluctuations
and mesoscale band fluctuations.
The thesis then presents a detailed analysis of the low frequency, broadband sound
arrivals obtained in the North Pacific Ocean. The observed acoustic variability is compared with acoustic predictions based on the weak fluctuation theory of Rytov, and direct
parabolic equation Monte Carlo simulations. The comparisons show that a resonance condition exists between the local acoustic ray and the internal wave field such that only the
internal-waves whose crests are parallel to the local ray path will contribute to acoustic
scattering: This effect leads to an important filtering of the acoustic spectra relative to
the internal-wave spectra. We believe that this is the first observational evidence for the
acoustic ray and internal wave resonance.
Finally, the thesis examined the evolution with distance, of the acoustic arrival pattern
of the off-axis sound source transmissions in the Long-range Ocean Acoustic Propagation
EXperiment. The observations of mean intensity time-fronts are compared to the deterministic ray, parabolic equation (with/without internal waves) and (one-way coupled) normal
mode calculations. It is found the diffraction effect is dominant in the shorter-range transmission. In the longer range, the (internal wave) scattering effect smears the energy in both
the spatial and temporal scales and thus has a dominant role in the finale region.The funding that made this research possible came from the Office of Naval Research,
and the WHOI Academic Programs Office