Frisk, George V.

The development of an unmanned underwater vehicle at Florida Atlantic
University with onboard optical sensors has prompted the temporal and spatial optical
characterization of Port Everglades, with in-situ measurements of the turbidity,
conductivity, and temperature. Water samples were collected for laboratory analysis
where attenuation and absorption were measured with a bench top spectrometer. All of
the measurements showed a high degree of variability within the port on a temporal and
spatial basis. Correlations were researched between the measured properties as well as
tide and current. Temporal variations showed a high correlation to tidal height but no
relation was found between turbidity and current, or salinity. Spatial variations were
primarily determined by proximity to the port inlet. Proportionality constants were
discovered to relate turbidity to scattering and absorption coefficients. These constants
along with future turbidity measurements will allow the optimization of any underwater
camera system working within these waters.

A methodology for characterizing the acoustical properties of a port environment,
namely Port Everglades, has been proposed and carried out. This approach includes both
a port-wide analysis of how the basic oceanographic features within the port impact the
acoustic properties, and also a more focused sampling methodology within a small region
of Port Everglades, allowing for the acoustic characteristics, including ambient noise, and
an approximate signal absorption to be computed.
The results documented through the duration of this research indicate that the
temperature variation throughout the port is the principal contributor to the characteristics
of the sound velocity profile. Ambient noise measurements have revealed high levels of
background noise within the sub-5 kHz region, owing likely to consistent port traffic.
The calculation of absorption indicates that high frequency systems, i.e. > 100 kHz, may
encounter problems when transmitting over a considerable distance. These are important
factors for consideration when implementing a successful underwater acoustic system.

A three-dimensional parabolic equation (PE) and perturbation approach is used to
invert for the depth- and range-dependent geoacoustic characteristics of the seabed. The
model assumes that the sound speed profile is the superposition of a known
range-independent profile and an unknown depth- and range-dependent perturbation.
Using a Green’s function approach, the total measured pressure field in the water column
is decomposed into a background field, which is due to the range-independent profile, and
a scattered field, which is due to the range-dependent perturbation. When the Born
approximation is applied to the resulting integral equation, it can be solved for the
range-dependent profile using linear inverse theory. Although the method is focused on
inverting for the sound speed profile in the bottom, it can also invert for the sound speed
profile in the water column. For simplicity, the sound speed profile in the water column
was assumed to be known with a margin of error of ± 5 m/s. The range-dependent
perturbation is added to the index of refraction squared n2(r), rather than the sound speed profile c(ro). The method is implemented in both Cartesian (x,y,z) and cylindrical (r,q,z)
coordinates with the forward propagation of the field in x and r, respectively. Synthetic
data are used to demonstrate the validity of the method [1].
Two inversion methods were combined, a Monte Carlo like algorithm, responsible
for a starting approximation of the sound speed profile, and a steepest descent method, that
fine-tuned the results. In simulations, the inversion algorithm is capable of inverting for
the sound speed profile of a flat bottom. It was tested, for three different frequencies
(50 Hz, 75 Hz, and 100 Hz), in a Pekeris waveguide, a range-independent layered medium,
and a range-dependent medium, with errors in the inverted sound speed profile of less than
3%.
Keywords: Three-dimensional parabolic equation method, geoacoustic inversion,
range-dependent sound speed profile, linear inversion, Born approximation, Green’s
functions.