Publication Date:
2022-05-25
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 April 1983
Description:
Ocean acoustic tomography was proposed in 1978 by Munk
and Wunsch as a possible technique for monitoring the
evolution of temperature, density, and current fields over
large regions. In 1981, the Ocean Tomography Group
deployed four 224 Hz acoustic sources and five receivers in
an array which fit within a box 300 km. on a side centered
on 26°N, 70°W (southwest of Bermuda). The experiment was
intended both to demonstrate the practicality of tomography
as an observation tool and to extend the understanding of
mesoscale evolution in the low-energy region far from the
strong Gulf Stream recirculation.
The propagation of 224 Hz sound energy in the ocean
can be described as a set of rays traveling from source to
receiver, with each ray taking a different path through the
ocean in a vertical plane connecting the source and
receiver. The sources transmitted a phase-coded signal
which was processed at the receiver to produce a pulse at
the time of arrival of the signal. Rays can be
distinguished by their different pulse travel times, and
these travel times change in response to variations in
sound speed and current in the ocean through which the rays
passed.
In order to reconstruct the ocean variations from the
observed travel time changes, it is necessary to specify
models for both the variations and their effect on the
travel times. The dependence of travel time on the oceanic
sound speed and current fields can be calculated using ray
paths traced by computer. The vertical structure of the
sound speed and current fields in the ocean were modelled
as a combination of Empirical Orthogonal Functions (EOFs)
from MODE. The horizontal structure was continuous, but
was constrained to have a gaussian covariance with a 100
km. e- folding scale. The resulting estimator closely
resembles objective mapping as used in meteorology and
physical oceanography. The tomographic system has at
present only been used to estimate sound speed structure
for comparison with the traditional measurements,
especially the first two NOAA CTD surveys, but the method
provides means for estimating density, temperature or
velocity fields, and these will be produced in the future.
The sound speed estimates made using the tomographic
system match the traditional measurements to within the
associated error bars, and there are several possibilities
for improving the signal to noise ratio of the data. Given
high-precision data, tomographic systems can resolve ocean
structures at small scales, such as in the Gulf Stream, or
at large scales, over entire ocean basins. Work is in
progress to evaluate the usefulness of tomography as an
observation tool in these applications.
Description:
My support for the first 3 years came from an NSF
graduate fellowship, and I was then supported as a research
assistant by NSF Grant OCE-8017791.
Keywords:
Underwater acoustics
;
Sound transmission
Repository Name:
Woods Hole Open Access Server
Type:
Thesis
Format:
application/pdf
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