An integrated modal approach to surface and volume scattering in ocean acoustic waveguides
Citable URI
https://hdl.handle.net/1912/5726Location
ArcticDOI
10.1575/1912/5726Keyword
Ocean waves; Water wavesAbstract
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.
Description
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 January 1996
Suggested Citation
Thesis: Tracey, Brian H., "An integrated modal approach to surface and volume scattering in ocean acoustic waveguides", 1996-01, DOI:10.1575/1912/5726, https://hdl.handle.net/1912/5726Related items
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