Underwater acoustics--Measurement

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 computer-efficient model of the underwater acoustic propagation m a
shallow, three-dimensional duct closed at one end has been developed using
the method of images. Presented in this research is the development of this
three-dimensional method of images analysis for a rectangular duct. Using
this analysis, a model of the impulse response of the acoustic channel is
constructed. Also presented in this work is the actual impulse response
collected during field experimentation in the south turning basin of Port
Everglades in Fort Lauderdale, Florida. The results demonstrate that the
impulse response is modeled with a relative echo magnitude error of 1.62 dB
at worst, and a relative echo location error varying between 0% and 4% when
averaged across multiple measurements and sensor locations.

A daily study spanning a month of the shallow water acoustic channel was conducted to
estimate the environmental influence on performance of an underwater acoustic communications
system. An automated acoustic modem transmitted phase-coherent modulated sequences of
identical data with 186 dB re IpPa source level, at coded rates from 4000 to 16000 bits/s with
4 or 8 kHz symbol bandwidth, three times daily for a month. A 64 channel Mills-Cross receiver
array was used with horizontal and vertical beams each containing 32 and 33 elements
respectively, spaced 0.03 meters apart, with a sampling frequency of 72 kHz. Source and
receiver were deployed at depths of 20 meters respectively, with a 720 meter separation range.
Environmental measurements of wind velocity and direction, surface wave activity, current and
sound velocity profiles, and tidal measurements were performed. Results demonstrate reliable
achievement of high data-rate shallow water acoustic communications using phase-coherent
modulation techniques.

A theoretical model has been developed to compute the vertical array directional response for surface generated ambient noise in a shallow water environment. The cross spectrum function is based on a normal mode solution to the wave equation in which the effective depth approximation is used to yield closed form solutions for two distinct mode types. The effective depth modes encompass the shallowest grazing angles where the bottom reacts as a pressure release surface to the incident plane waves. The rigid bottom solution takes over as the grazing angle increases and attenuation becomes significant. The computed vertical array beam output was compared to other models including a fast field wavenumber integration method and a multipath eigenray method with mixed results. The results indicated good agreement for both comparisons with the realization that the effective depth model is sensitive to the approximation discontinuity at the mode transition point.

A new method is proposed to infer the geotechnical properties of the sea floor from its response to the frequency-modulated pulses emitted by the subbottom profiler called Chirp Sonar. The environment is assumed to be a multilayered medium, composed of homogeneous layers, or an inhomogeneous half-space with depth-dependent properties. The acoustic response of the sediment is computed using the Biot-Stoll theory. The Levenberg-Marquardt method is applied to fit the synthetic response to the experimental response of an homogeneous layer overlying the sea floor. The porosity, the permeability, the mean grain diameter, the mass density, the bulk modulus and the shear modulus within this sediment layer can be estimated. A multilayered medium with depth-dependent properties could be applied to this inversion technique in the future.

A theoretical model accounting for underwater ambient noise due to whitecap-generated bubbles and drop-impacts is developed to investigate the possible mechanisms of ocean noise generation. A general analysis is laid down, leading to expressions of the noise spectrum in an undefined environment. Subsequently the cases of isovelocity and stratified deep oceans are considered, and specific expressions are derived. The stratified deep ocean estimations of the directional spectrum are then compared to experimental data and a good agreement is found. Finally the absolute noise levels due to bubbles and drop impacts are discussed, and bubble noise is found to be larger than drop impact noise. The collective bubble oscillation hypothesis is also discussed, and although clues are given for and against this hypothesis no rigorous justification for this has been found.

This research investigates the validity of an acoustic propagation model by comparing theoretical reflection coefficients, function of frequency, to FAU chirp sonar measurements (chirp sub-bottom profiler). An acoustic model has been implemented to estimate the spectrum of energy reflected from sandy sediments in the presence of surface scattering. The surface roughness being the dominant reverberation part, the volume scattering has been neglected in this model. A laser scanning system involving an image-processing algorithm has been designed to measure the seafloor bottom roughness using 1D Fourier transforms. In the case of anisotropic roughness, an estimation of the sand ripples dominant direction is provided involving 2D Fourier transforms. Measurements of acoustic data using a chirp sonar and estimation of bottom roughness from video data of the scanner over an artificial bottom are provided to compare the reflection coefficients obtained from the data actually measured with those from the acoustical model.

A thesis investigates the measured and theoretical pressure reflection coefficients of the seabed at normal incidence. The theoretical reflection coefficient is calculated using a physics-based model developed by Maurice Biot. The model describes sound propagation in saturated porous media and interrelationships between the acoustic properties of the media and the physical properties of the pore fluid and the porous solid. Stoll modified the Biot model for the case of ocean sediments and developed an expression for calculating the reflection coefficient as a function of frequency. This thesis tests the model by comparing the reflection coefficient measured with chirp sonar to the reflection coefficient calculated using the Biot model. An experiment was conducted off Fort Walton Beach, Florida where chirp sonar transmitted FM pulses at normal incidence to a sandy seabed. Sediment properties measured during SAX-99 are used to calculate the theoretical reflection coefficient using the Biot-Stoll model. The agreement of the measured reflection coefficients with the theoretical calculations over the band of 1500 to 16000 Hz varies as much as 70%. The plotted results show a reduction of the reflection coefficient with frequency but the large deviations from the trend prevent any further conclusions.

The impact of depth-dependent geophysical parameters on the acoustic pressure reflection coefficient is studied at normal incidence using the Biot-Stoll theory in porous marine sediments. The seabed is modeled as a sediment layer with depth-dependent properties on top of a homogeneous half-space, as originally proposed by Stern. There is no discontinuity in sediment properties between the layer and the half-space. The reflection coefficient is determined by the evaluation of boundary conditions at the water-sediment layer interface and the sediment layer-half-space interface. Results are obtained for different types of sediment, from medium size sands to silty clay, and different porosity profiles vs. depth.

Experimental measurements have been conducted to investigate the effects of a three dimensional bathymetry on ocean acoustic propagation and our abilities to use array processing for localizing sources. This work is unique because it uses laboratory scale measurements to isolate the effects of the bottom bathymetry. Previous investigations using laboratory scale measurements have only used simplistic bottom profiles. In addition, experiments which have investigated the effects of the bottom bathymetry at sea have encountered difficulties isolating these effects due to range dependent sound speed profiles and the uncertainties of ocean acoustic experiments. The first part of this dissertation investigates the tracking of an acoustic source in a three dimensional shallow water environment. This work is comprised of two studies. The first study uses matched field processing for identifying the trajectory of a source. The second investigation uses experimental measurements and theoretical predictions to evaluate the beating angle bias caused by the use of plane-wave beamforming in the presence of bathymetric refraction. The second part of this dissertation uses laboratory scale measurements to analyze two and three dimensional propagation over a realistic bottom bathymetry. This series of investigations uses an inverse approach based on normal mode theory. The inversion algorithm is used to extract the normal mode amplitudes for the purpose of analyzing the measurements for two dimensional mode coupling and bathymetric refraction. The results of this investigation show that the bathymetry has a strong influence on the three dimensional acoustic field. Analysis of the experimental measurements identify that mode coupling and bathymetric refraction are important for propagation over the laboratory scale model and these effects adversely influence our abilities to localize sources in three dimensional shallow water environments. It is also shown that by incorporating three dimensional propagation models into the signal replica used by the array processor a significant improvement in performance can be achieved.