Tracey
Brian H.
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An integrated modal approach to surface and volume scattering in ocean acoustic waveguides
Design and testing of an acoustic ultra-short baseline navigation system
Tracey
Brian H.
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Thesis
Acoustic propagation in the ocean can be strongly affected by small random variations in
ocean properties, including rough surfaces and volume fluctuations in the ocean or seabed.
Such inhomogeneities scatter part of the incident acoustic field, stripping energy from the
coherent part of the field. This scattered energy, or reverberation, propagates further in the
modes of the ocean waveguide. The distribution of energy among modes is changed and
the coherence of the acoustic field is reduced.
This thesis introduces several models which describe scattering of low-frequency sound.
First, the rough surface scattering theory of Kuperman and Schmidt is reformulated in terms
of normal modes. Scattering from rough fluid-fluid interfaces and rough elastic halfspaces is
modeled, and statistics of the acoustic field are calculated. Numerical results show the modal
formulation agrees well with Kuperman and Schmidt's model, while reducing computation
times by several orders of magnitude for the scenarios considered.
Next, a perturbation theory describing scattering from sound speed and density fluctuations
in acoustic media is developed. The theory is used to find the scattered field generated
by volume fluctuations in sediment bottoms. Modal attenuations due to sediment volume
scattering are calculated, and agreement is demonstrated with previous work.
The surface and volume scattering theories are implemented in a unified modal reverberation
code and used to study bottom scattering in shallow water. Numerical examples
are used to demonstrate the relationship between volume and surface scattering. Energy
distribution among scattered field modes is found to be a complicated function of the scattering
mechanism, the scatterer statistics, and the acoustic environment. In particular, the
bottom properties strongly influence the coherence of the acoustic field. Examples show
that excitation of fluid-elastic interface waves is a potentially important scattering path.
Cross-modal coherences are calculated and used to study the loss of signal coherence with
range.
Finally, earlier work on scattering from the Arctic ice sheet is extended. Simulations of
long-range transmissions are compared with data from the April 1994 trans-Arctic propagation
test. The results show modal attenuations and group speeds can be predicted
reasonably well, indicating that acoustic monitoring of Arctic climate is feasible.
Thesis
An ultra-short baseline acoustic navigation system has been developed which is
capable of determining bearing to a sound source with an error of less than 1° in typical
operational conditions. The system has a demonstrated ability to operate in an environment
in which multipath interference is significant. A DSP microprocessor is used to process
signals received by two hydrophones from a 26 kHz toneburst sound source. This
processing power is used to implement features not commonly available with commercial
systems. The system has the capability to make an on-line measurement of the signal-tonoise
ratio, which can be used to estimate the confidence which should be placed in the
data. Estimates of phase difference and signal power are generated many times within each
received pulse, so the effects of multi path interference throughout the pulse can be
observed. Results of tests at several ranges are presented, and compared to performance
models developed in the thesis. System performance is quantified, and an effort is made to
understand the effects of multipath arrivals.