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The oil and gas industry employs powerful, low-frequency impulsive sources for exploration during seismic surveys. These sources, known as seismic air gun arrays, have the ability to propagate long distances. Seismic surveys can last for weeks and happen simultaneously at multiple locations around the world. These operations have the potential to disturb and displace marine mammals and other forms of marine life. There is a number of mitigation strategies that can be used to reduce the impact and exposure of marine animals to noise. For seismic surveys, the most cost-effective method consists in implementing a real-time monitoring plan around a predefined exclusion zone. If a marine mammal enters that area, the operations stop as requested by the Marine Mammal Observer (MMO) or Passive Acoustic Monitoring (PAM) technician who detected and identified the animal. Nowadays, these exclusion zones are calculated from sound field measurements or models, or a combination of both. For the purpose of sound source verification (SSV), a model of the sound field is required before the survey starts. The pre-survey simulation of the sound field is obtained as a combination of an air gun model, which simulates the acoustic output of the seismic source, and a propagation model, which simulates the pressure attenuation with distance. The pre-survey simulations are used to calculate the exclusion zone that is to be monitored before the sound mapping survey can gather the data necessary to verify the model. Once the data has been collected, a report is issued within 24-72 h. The report summarises the results of the model validation and provides an updated exclusion zone. In this context, there are three essential tools needed for noise impact assessment: a source model and propagation model for the pre-survey sound field simulations, and a software package that can generate a map of sound pressure levels from raw audio and position data. However, the tools currently available have some fundamental limitations: (1) commercial source models of air gun arrays are costly and are not designed for noise impact assessment, and (2) sound field processing is typically carried out with nonstandardised software routines developed by the companies providing the SSV services. In this thesis we present the design and implementation of two pieces of software: (1) a package for the computation of the sound field from raw audio and position data, and (2) a calibrated air gun array model that can compete with commercial software. The purpose of these tools is to provide efficient, accurate and free-access routines that can be used for the characterisation of the acoustic source, the sound field, and the propagation environment in the context of seismic surveys.

This thesis is the culmination of work carried out on a number of different UAS, in order to determine the feasibility of using UAS to deliver sensor payloads to the marine environment. A number of experiments involving said UAS were conducted, and future work detailed. Chapter 2 provides a brief overview of literature in acoustics and UAS, including UAS use around marine mammals. Chapter 3 looks into the noise produced by a gimbal, which relates the noise levels seen to literature and determines if the acoustic signal emitted could interfere with wildlife. Chapter 4 delves into the acoustics generated by the motors and propellers, with a comparison of these results with published literature. Chapter 5 shows the work conducted and implemented on a custom data logging system with in-house designed underwater housing, for use on a Splashdrone UAS, which is used in Chapter 6 for an acoustic survey of the marine environment. Chapter 7 uses on-board accelerometers inside of a flight controller to monitor displacement experienced, for use in monitoring surface heave of a water body. Chapter 8 concludes this thesis with a review on key findings, and a summary of future work that is to be conducted. Overall, this thesis determines that it is feasible to deliver sensor payloads to marine environments to an extent; a number of issues remain, which are detailed throughout this thesis.

Passive acoustic methods were used to study the effect of mid-frequency active sonar (MFAS) on a population of Blainville's beaked whales (Mesoplodon densirostris, Md) at the U.S. Navy Atlantic Undersea Test and Evaluation Centre (AUTEC), Bahamas. AUTEC contains an array of bottom-mounted hydrophones that can detect Md echolocation clicks. Methods to estimate abundance, the risk of behavioural disruption, and the population level effect of repeated MFAS exposure are presented. A passive acoustic abundance estimation method, a parametric equation that predicts the probability of foraging dive disruption as a function of MFAS received level and an Md bioenergetics model were developed. The effect of changes in energy flow on the demographic characteristics of an Md population were explored. Passive acoustic data from AUTEC were used to estimate the behavioural disturbance resulting from sonar operations; combined with the bioenergetic model, this suggested that the effect of sonar operations could cause an increase in a female's age at maturity, a longer inter-calf-interval, calf survival rate and probability of giving birth that could in turn result in a declining population. Model results were also compared to those from an expert elicitation study. A power analysis for predicting abundance via passive acoustic and visual monitoring were used to inform recommendations for a long-term monitoring plan that includes passive acoustic monitoring of Md abundance combined with a photo-identification study at both AUTEC and a reference site (Abaco) to improve demographic rate estimates.

Levels of underwater noise in the open ocean have been increasing since at least the 1960s due to growth in global shipping traffic and the speed and propulsion power of vessels. This rise in noise levels reduces the range over which vocal marine species can communicate, and can induce physiological stress and behavioural responses, which may ultimately have population-level consequences. Although long-term noise trends have been studied at some open-ocean sites, in shallower coastal regions the high spatiotemporal variability of noise levels presents a substantial methodological challenge, and trends in these areas are poorly understood. This thesis addresses this challenge by introducing new techniques which combine multiple data sources for ship noise assessment in coastal waters. These data include Automatic Identification System (AIS) ship-tracking data, shore-based time-lapse footage, meteorological data, and tidal data. Two studies are presented: in the first, AIS data and acoustic recordings from Falmouth Bay in the western English Channel are combined using an adaptive threshold, which separates ship passages from background noise in the acoustic data. These passages are then cross-referenced with AIS vessel tracks, and the noise exposure associated with shipping activity is then determined. The second study, at a site in the Moray Firth, Scotland, expanded the method to include shore-based time-lapse footage, which enables visual corroboration of vessel identifications and the production of videos integrating the various data sources. Two further studies examine and enhance basic analysis techniques for ambient noise monitoring. The first study examines averaging metrics and their applicability to the assessment of noise from shipping. Long-term data from the VENUS observatory are empirically assessed for different averaging times and in the presence of outliers. It is concluded that the mean sound pressure level averaged in linear space is most appropriate, in terms of both standardization and relevance to impacts on marine fauna. In the second study, a new technique for the statistical analysis of long-term passive acoustic datasets, termed spectral probability density (SPD), is introduced. It is shown that the SPD can reveal characteristics such as multimodality, outlier influence, and persistent self-noise, which are not apparent using conventional techniques. This helps to interpret long-term datasets, and can indicate whether an instrument’s dynamic range is appropriate to field conditions. Taken together, the contributions presented in this thesis help to establish a stronger methodological basis for the assessment of shipping noise. These methods can help to inform emerging policy initiatives, efforts to standardise underwater noise measurements, and investigation into the effects of shipping noise on marine life.

The zooplankton communities off the north east coast of Scotland, Wee Bankie sand-banks, were studied with traditional net samples and for the first time, for this area, with acoustic methods. A series of zooplankton samples were collected during June 2010 and March/June 2011 together with a set of physical parameters of the water column. Taxonomic analysis of the collected samples enabled the characterisation of zooplankton communities. Neritic communities with oceanic influence were found along the north east coast of Scotland and over the Wee Bankie with spatial variability associated with higher densities of zooplankton inshore. Temporal variability was associated with the expected succession of dominant copepods from spring to summer. An appropriate scattering model, Distorted Wave Born Approximation, was used to predict the target strength of the major zooplanktonic groups and so solve the forward problem to verify a plankton separation algorithm. The algorithm isolated weak scattering targets of zooplankton using three frequencies (38, 120 and 200 kHz) and was used to study the zooplankton spatial distribution during June 2008-2011 in relation to the physical characteristics of the water column. Model predicted target strength for the major zooplanktonic groups was sensitive to orientation for large organisms and material properties for both copepods and euphausiids. The Wee Bankie waters were thermally stratified, with June 2011 the least stratified of all years. Depths of observed maximum backscatter varied in relation to the pycnocline depth and tended to be deeper in offshore stations. The estimated biomass from the observed backscatter increased from 2008 to 2011 but spatial patterns varied from year to year. Although, estimated biomass across years reduced with increasing maximum fluorescence and surface to bottom temperature difference, the same relationship was not evident within year.

Passive acoustic monitoring has been used to investigate many aspects of marine mammal ecology, although methods to estimate absolute abundance and density using acoustic data have only been developed in recent years. The instrument configuration in an acoustic survey determines which abundance estimation methods can be used. Sparsely distributed arrays of instruments are useful because wide geographic areas can be covered. However, instrument spacing in sparse arrays is such that the same vocalisation will not be detected on multiple instruments, excluding the use of some abundance estimation methods. The aim of this thesis was to explore cetacean abundance and density estimation using novel sparse array datasets, applying existing methods where possible, or developing new approaches. The wealth of data collected by sparse arrays was demonstrated by analysing a 10-year dataset collected by the U.S. Navy's Sound Surveillance System in the north-east Atlantic. Spatial and temporal patterns of blue (Balaenoptera musculus) and fin whale (Balaenoptera physalus) vocal activity were investigated using generalised additive models. Distance sampling-based methods were applied to fin whale calls recorded by an array of Ocean Bottom Seismometers in the north-east Atlantic. Estimated call density was 993 calls/1000 km².hr⁻¹ (CV: 0.39). Animal density could not be estimated because the call rate was unknown. Further development of the call localisation method is required so the current density estimate may be biased. Furthermore, analysing a single day of data resulted in a high variance estimate. Finally, a new simulation-based method developed to estimate density from single hydrophones was applied to blue whale calls recorded in the northern Indian Ocean. Estimated call density was 3 calls/1000 km².hr⁻¹ (CV: 0.17). Again, density of whales could not be estimated as the vocalisation rate was unknown. Lack of biological knowledge poses the greatest limitation to abundance and density estimation using acoustic data.

Acoustic survey techniques are widely used to quantify abundance and distribution of a variety of pelagic fish such as herring (Clupea harengus). The information provided is becoming increasingly important for stock assessment and ecosystem studies, however, the data collected are used as relative indices rather than absolute measures, due to the uncertainty of target strength (TS) estimates. A fish’s TS is a measure of its capacity to reflect sound and, therefore, the TS value will directly influence the estimate of abundance from an acoustic survey. The TS is a stochastic variable, dependent on a range of factors such as fish size, orientation, shape, physiology, and acoustic frequency. However, estimates of mean TS, used to convert echo energy data from acoustic surveys into numbers of fish, are conveniently derived from a single metric - the fish length (L). The TS used for herring is based on TS-L relationships derived from a variety of experiments on dead and caged fish, conducted 25-30 years ago. Recently, theoretical models for fish backscatter have been proposed to provide an alternative basis for exploring fish TS. Another problem encountered during acoustic surveys is the identification of insonified organisms. Trawl samples are commonly collected for identification purposes, however, there are several selectivity issues associated with this method that may translate directly into biased acoustic abundance estimates. The use of different acoustic frequencies has been recognised as a useful tool to distinguish between different species, based on their sound reflection properties at low and high frequencies. In this study I developed theoretical models to describe the backscatter of herring at multiple frequencies. Data collected at four frequencies (18, 38, 120 and 200 kHz) during standard acoustic surveys for herring in the North Sea were examined and compared to model results. Multifrequency backscattering characteristics of herring were described and compared to those of Norway pout, a species also present in the survey area. Species discrimination was attempted based on differences in backscatter at the different frequencies. I examined swimbladder morphology data of Baltic and Atlantic herring and sprat from the Baltic Sea. Based on these data, I modelled the acoustic backscatter of both herring stocks and attempted to explain differences previously observed in empirical data. I investigated the change in swimbladder shape of herring, when exposed to increased water pressures at deeper depths, by producing true shapes of swimbladders from MRI scans of herring under pressure. The swimbladder morphology representations in 3-D were used to model the acoustic backscatter at a range of frequencies and water pressures. I developed a probabilistic TS model of herring in a Bayesian framework to account for uncertainty associated with TS. Most likely distributions of model parameters were determined by fitting the model to in situ data. The resulting probabilistic TS was used to produce distributions of absolute abundance and biomass estimates, which were compared to official results from ICES North Sea herring stock assessment. Modelled backscatter levels of herring from the Baltic Sea were on average 2.3 dB higher than those from herring living in northeast Atlantic waters. This was attributed to differences in swimbladder sizes between the two herring stocks due to the lower salinity Baltic Sea compared to Atlantic waters. Swimbladders of Baltic herring need to be bigger to achieve a certain degree of buoyancy. Morphological swimbladder dimensions of Baltic herring and sprat were found to be different. Herring had a significantly larger swimbladder height at a given length compared to sprat, resulting in a modelled TS that was on average 1.2 dB stronger. Water depth, and therefore the increase in ambient pressure, was found to have a considerable effect on the size and shape of the herring swimbladder. Modelled TS values were found to be around 3 dB weaker at a depth of 50 m compared to surface waters. At 200 m, this difference was estimated to be about 5 dB. The Bayesian model predicted mean abundances and biomass were 23 and 55% higher, respectively, than the ICES estimates. The discrepancy was linked to the depth-dependency of the TS model and the particular size-dependent bathymetric distribution of herring in the survey area.

This thesis is concerned with long-range sound propagation in deep water.  The main area of interest is the stability of acoustic ray paths in wave guides in which there is a transition from single to double duct sound speed profiles, or vice-versa.  Sound propagation is modelled within a ray theoretical framework, which facilitates a dynamical systems approach of understanding long-range propagation phenomena, and the use of its tools of analysis. Alternative reduction techniques to the Poincaré sections are presented, by which the stability of acoustic rays can be graphically determined.  Beyond periodic driving, these techniques prove to be useful in case of the simplest quasiperiodic driving of the ray equations.  One of the techniques facilitates a special representation of ray trajectories for periodic driving. Namely, the space of sectioned trajectories is partitioned into nonintersecting regular and chaotic regions as with the Poincaré sections, when quasiperiodic and chaotic trajectories are represented by curve segments and area filling points, respectively.  In case of the simplest quasiperiodic driving – speaking about the same technique – regular trajectories are represented by curves similar to Lissajous curves, which are opened or closed depending on whether the two driving frequencies involved make relative primes or not. It is confirmed for a perturbed canonical profile that the background sound speed structure controls ray stability. It is also demonstrated for a particular double duct profile, when the singularity of the nonlinearity parameter for the homoclinic trajectory associated with this profile refers to the strong instability of corresponding perturbed trajectories.

Understanding what psycho-physiological and behavioural factors influence aversiveness of sound in marine mammals is important for conservation and practical applications. The aim of this study was to determine predictors for impact of anthropogenic noise and to develop a target-specific predator deterrence system for use on fish farms. Three classes of stimuli were tested: 1.) grey seal underwater communication calls expected to be used in territorial defence, 2.) high duty-cycle moderately loud artificial sounds (some of which were based on models of unpleasantness for humans), 3.) brief, intense pulses designed to elicit the acoustic startle reflex. Communication calls had no deterrence effect but instead caused attraction responses. Tests with high duty-cycle artificial sounds showed that food-motivated animals habituate quickly, although sound exposure caused subtle changes in diving patterns over a longer time. Field trials using the same stimuli were used to determine avoidance thresholds but also indicated that sound features like ‘roughness’ play a role. The startle eliciting stimuli, however, had the most dramatic effects. To this stimulus most seals exhibited rapid flight responses, hauled out, sensitised and showed signs of fear conditioning. Startle thresholds were found to be 80-85 dB above the assumed hearing threshold. The data showed that startle thresholds are a crucial predictor for the occurrence of strong avoidance behaviour and suggests that the startle response evolved to increase an animal’s propensity for flight. Finally, a prototype predator deterrence system based on the startle sounds was developed to repel seals whilst not affecting toothed whales. In fish farm trials, seals were deterred at close ranges but local abundance of cetaceans did not change showing that it is possible to cause differential responses between species based on differences in their audiograms. The results are used to develop noise exposure criteria and to elucidate acoustic parameters that can be used to predict responses to anthropogenic noise.

This thesis is concerned with the problem of estimating the acoustic properties of a seabed from measurements of the sound field made in a shallow ocean. It studies existing methods for performing geoacoustic inversion in shallow water waveguides, and purposes two new techniques. The sound field in the water column of a laboratory tank is predicted with a Fast Field Program (FFP) and compared in phase and magnitude, with measurements made in both range and depth. Head waves' arrival times are measured and used to infer the shear and compressional wave speeds of the tank base. The density and wave speeds of the tank base are also estimated by Matched Field Inversion using both a single frequency and an incoherent multi-frequency Bartlett processor. The FFP is used to compute the replica acoustic fields. A numerical sensitivity analysis confirms that the wave speed estimates are very dependent on the accuracy of the water depth measurement. A method for estimating the angle dependent plane-wave reflection coefficient due to Frisk et al. [J. Acoust. Soc. Am. 68, 602-612, (1980)] is shown to be too sensitive to errors in the depth-dependent Green function to be of any practical use. An investigation, using synthetic data, aims to determine the most accurate Hankel transform (HT) routine for use in shallow water acoustics. The asymptotic routine is recommended because of its speed. Both the HT and Prony's method are used to calculate modal dispersion curves from measured pressure data. A new inversion method, Matched Dispersion Curve Inversion is proposed, which is robust to error in the measurements of source, receiver and water depth. Another novel inversion method based on model dispersion curves is also presented. Measured model dispersion curves and their gradients are used to invert for the phase of the frequency and angle dependent plane-wave reflection coefficient. Both new techniques are demonstrated using the measured dispersion curves.

Sound production is widespread among teleosts and is usually observed in social contexts. Many fish, such as triglids, produce sounds by contracting a pair of specialised striated muscles attached to both sides of the swimbladder wall, yielding low frequency sounds made up of repeated brief pulses. Temporal features of acoustic emissions can be controlled by the rate of muscle contraction, thus providing a rich means of conveying information. In this study the sound features of the grey (EutrigJa gurnardus), the streaked (Trigloporus lastoviza), the red (Aspitrigla cuculus) and the tub (Trigla Jucerna) gurnards were compared. The grey and the red gurnards emitted 3 different sound types, knocks, grunts and growls, whereas the streaked and the tub gurnards only emitted one sound type, growls and grunts respectively. Interspecific differences of calls were marked and based on the temporal patterning and on the grouping of the pulses. In the grey gurnard, ontogenetic changes in sound production were found. The sound production rate, the proportion of emitted sound type and the physical features of sounds varied with fish size. A study of diel and seasonal variations of sound production in the grey gurnard showed that more sounds were uttered during the day than at night and that grunts were more important and intense during the SpringSummer period. The sonic apparatus was examined in the species mentioned above and also in the large-scaled gurnard (Lepidotrigla cavillone) and the piper (Trigla lyra). The swimbladder and the sonic muscles grew throughout life in all species. Variations in the sonic apparatus with fish gender or time of the year were not detected. This suggests that the ability to vocalise is similar in both male and female gurnards, probably even during courtship. The shape of the swimbladder was species-specific. All species possessed a pair of intrinsic sonic muscles except for the piper whose sonic muscles were extrinsic. The pair of intrinsic sonic muscles of the grey gurnard contracted synchronously and each contraction generated a pulse of sound. The biological Significance of sound production was studied in the grey, the streaked and the tub gurnards. Few other studies have made a comparison of the behavioural context of sound production in different species of fish of the same family. Competitive feeding interactions were examined and compared between species. The different sound types emitted were also correlated with different behavioural categories. Typical sequences of behaviour were found in each species. Disputes over food items were settled by either reaching food first or by being aggressive, but never involved fighting escalation. The knocks of the grey gurnard and the growls of the streaked gurnard were associated with feeding and low levels of aggressive behaviour, and the grunts of grey and tub gurnards were associated with agonistic behaviour, such as approach, chase and frontal displays. The grey gurnard was the most vocal and aggressive species during competitive feeding. Smaller grey gurnards interacted more frequently than did larger fish, and a larger proportion of their interactions were accompanied by grunt calls. The metabolic costs of sound production in fish were calculated theoretically for different types of acoustic activity. If the assumptions made are valid then it seems that producing acoustic signals in fish is cheap. The biological implications of these low energetic costs were discussed. Finally, the acoustic repertoire of several non-triglid species of fish was examined. Sounds from 7 species were described for the first time. These species uttered sounds during territorial defence and courtship; contexts quite different from those studied for triglids.

In situ measurements of geoacoustic properties provide direct characterization of the seabed at near ambient conditions. The Acoustic Coring System (ACS) is a gravity corer equipped with acoustic probes that obtains in situ compressional wave (30–200 kHz) and shear wave (400--1200 Hz) measurements as the corer penetrates the seabed. During the April 2022 R/V Endeavor coring survey, the ACS was deployed at 36 locations within the New England Mud Patch (NEMP) and New England Shelf Break areas. Data from these experiments were analyzed to characterize the depth-dependent structure of the seabed geoacoustic properties as well as their spatial variability. The in situ measurements are interpreted in the context of stratigraphic layering measured by a seismic survey. Depth-dependent profiles of compressional wave speed from a subset of these deployments in the NEMP are compared to profiles previously collected at nearby locations in 2016. In situ compressional wave records from both areas are compared with ex situ sediment core measurements, including data collected from core loggers and laboratory analyses. Additionally, novel in situ shear wave speed measurements from the NEMP and surrounding areas are introduced.

Abstract Acoustic recordings of underwater sounds produced by marine mammals present an attractive alternative to costly and logistically complex ship based visual surveys for collecting population data for various species. The first reported use of underwater acoustic recordings in the long-term monitoring of sperm whale populations was by Ackleh et al. (Ackleh et al., 2012). The paper describes counting sperm whale clicks at different locations to track population changes over time. Analysis of sperm whale clicks offers additional insight into sperm whale populations. The echo location clicks (usual clicks) of sperm whales can be used to give an estimate of the whale’s length. The acoustic axis of the whale must be aligned with the hydrophone for accurate length estimates to be made. Most previously reported work using acoustics to measure whale lengths has been done with surface hydrophones recording clicks from a diving whale. With that geometry the whale’s acoustic axis will be approximately aligned with the hydrophone. In the case of monitoring a diving whale it is usual that most of the recorded clicks are from a single whale reducing interfering clicks from other whales. When recording clicks with bottom mounted hydrophones no whale/hydrophone orientation can be assumed. The vast majority of clicks recorded are from off axis whales and therefore produce inaccurate length measurements. It is also not unusual for several clicking whales to be recorded at the same time, producing incorrect whale length measurements due to interference between the clicks. Cepstrum Analysis is the favored technique for making acoustic whale length measurements from bottom moored hydrophones. If enough clicks are recorded from the same whale in enough different orientations, the spurious signals in off axis clicks are “averaged out”. It has been observed, however, that multiple whales clicking at the same time produced errors in the results from Cepstrum Analysis. In this paper a new method for selecting on axis sperm whale clicks from which to determine repeatable length estimates will be presented. It uses wavelet decomposition and a unique quality control test to make better whale length estimates.

In this work, a systematic procedure of analyzing marine propeller performance and its underwater acoustics is delineated. The main focus is laid on the numerical prediction of hull pressure fluctuations and far-field acoustics induced by marine propellers via frequency- or time-domain approaches. As the first step, a hydrodynamic boundary element method (BEM) is implemented to analyze the comprehensive marine propeller performance in open water and ship-behind conditions. An elaborate numerical scheme modeling developed tip vortex cavitation is proposed for cavitating propellers. In the frequency domain approach, a potential-based panel method (or pressure-BEM) solving for the diffraction potential on the ship hull is coupled with the hydrodynamic BEM to predict the propeller-induced hull pressure fluctuations. Infinite speed of sound is assumed in the near-field, neglecting the time travel of acoustic waves to reduce the Helmholtz equation to the Laplace equation, which can be solved by a standard BEM solver. Far-field underwater noise is investigated in the time domain using the Ffowcs Williams-Hawkings (FW-H) formulation. The model propeller is mounted on the ship with an infinite plane assumed at the design draft to consider the pressure release or solid wall boundary conditions. Time-dependent surface quantities from the hydrodynamic analysis are taken as the known noise sources to the FW-H equation given in the form proposed by Farassat. The complex hull geometry and its disturbance to the medium are considered by solving the boundary integral equations. A half-space domain is modeled by modifying standard full-space Green’s function to consider the image model beyond the infinite plane. The propeller-induced noise, its reflections from the hull, and their sums as the total noise will be investigated in full-space or half-space domains. Numerical predictions, including the time history of acoustic pressures and noise spectra, under various operating conditions are compared with experimental measurements, observations, and results from finite volume methods to validate the feasibility and accuracy of the present method

Underwater acoustic metamaterials (AMMs) use engineered subwavelength structures to produce novel effective material properties to control water-borne acoustic wave propagation. Transformation Acoustics (TA) is a mathematical construct that is used to specify the material properties that are necessary to achieve exotic manipulation of acoustic fields, and as such, AMMs can be the means through which physical realization of these devices can occur. An acoustic cloak is one possible device that can be designed using TA, but it is one that has garnered the most attention in academic studies due to the multi-faceted complexity of designing, fabricating, and experimentally validating performance. For an object to be indistinguishable from the background medium, an acoustic cloak requires either anisotropic effective dynamic density or anisotropic effective dynamic stiffness, but further requires these properties to be spatially dependent. Devices predicted by TA, such as cloaks, can be expensive and difficult to fabricate due to the geometric complexities of the substructures required to realize the required material properties. In some cases, state-of-the-art additive manufacturing methods are the only means of fabricating such devices. Devices predicted by TA are also difficult to characterize because of their frequency dependent dynamic behavior. Further, since the AMMs of interest in this work operate primarily at low frequencies, freefield methods are not practical. Therefore, this work investigated the characterization of select underwater AMMs predicted by TA using new and existing experimental apparatuses and techniques while evaluating the applicability of different fabrication processes to physically realize testable samples. Three materials were investigated in this work: (i) a layered AMM comprised of aluminum, air-filled, honeycomb panels coupled to one another by compliant rubber rods demonstrating anisotropic effective dynamic density, (ii) a hexagonal elastic lattice comprised of bimode unit cells that exhibits a focusing beam pattern in an underwater acoustic field by exploiting spatially varying density and stiffness, and (iii) an additively manufactured, titanium, diamond-shaped, elastic pentamode lattice that demonstrates anisotropic sound speeds due to anisotropic effective dynamic stiffness. Significant effort was expended on experimental validation of the AMM's acoustic properties, but this work also explored the relationships between homogenization theory, numerical simulation, and fabrication methods, in order achieve more robust understanding of the underwater acoustic AMMs studied here.

To optimize the use of sound in waters on the continental shelf for naval, commercial, and environmental monitoring applications, the acoustic properties of the ocean bottom must be well understood. The effects of 1) pore water salinity variability on acoustic reflectivity, 2) poroelasticity on geoacoustic inference, and 3) organic carbon on sediment properties were formerly-considered insignificant in sediment acoustics, but due to advancements in other areas of underwater acoustics systems and modeling, have now become significant. Three separate but related studies were conducted to begin to quantify these effects. 1) A high-frequency acoustic reflection experiment was performed on a water-clay interface, while varying the salinity of the water. Results demonstrated significant changes in reflectivity at high incident angles, as well as a transient effect explained by a new coupled salt diffusion/reflection model. Using the model, the effective diffusion coefficient of salt in clay was inferred from the experiment, and reflectivity was then simulated at lower frequencies and longer time-scales. From this modeling effort, at a given time-scale of fluctuation, a characteristic frequency was identified, below which the reflectivity should not be assumed temporally invariant. 2) A model geoacoustic inference procedure was performed on a layered waveguide consisting of water and water-saturated glass beads contained within a glass tube. The resonance frequencies of the system were measured and compared with simulations of the experiment. Within each simulation, various sediment acoustics models were used. The only model that allowed for self-consistency between the inference and an independent set of high-frequency sound speed measurements, was a model that accounted for poroelastic effects. 3) A sediment constituent that has great value to the planet and is ubiquitous in natural marine sediment, organic carbon, has been ignored in sediment acoustics models. To begin to explore this relationship, sediment cores were extracted from a T. testudinum seagrass meadow in the Lower Laguna Madre, Texas, USA. A strong correlation between organic carbon and the primary-wave modulus was identified using a custom-built automated broad-band core and resonance logger and an elemental analyzer. The sediment properties attained from the cores were compared, and a theory explaining the correlations was developed. The acoustic sensitivity to organic carbon in a seagrass meadow has demonstrated promise toward developing an acoustic tool to more rapidly quantify marine organic carbon stores, which is needed in climate science. However, a larger-scale study is required to determine its applicability across a broader range of seagrass meadows and sediment types

The frequency bandwidth of the sound from a light helicopter, such as a Robinson R44, extends from about 13 Hz to 2.5 kHz. As such, the R44 has potential as a low-frequency sound source in underwater acoustics applications. To explore this idea, a series of experiments has been conducted in shallow water off the coast of southern California in which a horizontal line of hydrophones detected the sound of an R44 hovering in an end-fire position relative to the array. Some of the helicopter sound interacted with seabed to excite the head wave in the water column. A theoretical analysis of the sound field in the water column generated by a stationary airborne source leads to an expression for the two-point horizontal coherence function of the head wave, which, apart from frequency, depends only on two parameters: the horizontal separation of the two receivers and the sound speed in the sediment. By matching the zero crossings of the measured and theoretical horizontal coherence functions, the sound speed in the sediment was recovered and found to take a value of 1682.42 ± 16.20 m/s. This is consistent with the sediment type at the experiment site, which is known from a previous survey to be a fine to very- fine sand.

Descriptions of underwater vocalizations produced by aquatically mating phocids are available for many species, but are lacking for the endangered Hawaiian monk seal (Neomonachus schauinslandi). We obtained simultaneous year-round audio and video recordings of a captive adult male Hawaiian monk seal to evaluate underwater vocal repertoire and characterize seasonal trends in vocal behavior. A discriminant function analysis based on 17 acoustic parameters revealed that this seal produced at least six discrete underwater vocalizations. Spontaneous aquatic calls were most commonly produced from September through January, during a period coincident with elevated blood testosterone levels and prior to the annual molt. These seasonal patterns in sound production confirm a protracted reproductive season for this tropical species. While limited to a single individual, this first report of underwater sound production expands our understanding of reproductive behavior in Neomonachus schauinslandi, and establishes a foundation for future research and population monitoring efforts using passive acoustics.

This dissertation studies five models that make up a cross section of the geoacoustic models that have been used to study sandy sediments: a simple fluid model, the effective density fluid model (EDFM) of Williams, the viscous grain-shearing (VGS[lambda]) model of Buckingham, the Biot-Stoll model, and the corrected and reparameterized extended Biot (CREB) model of Chotiros. The first objective is to use numerical experiments and model/data comparisons to determine the usefulness and assess the physical validity of these five models. The second objective is to ascertain the current state of knowledge of sandy sediments and describe what truths can be learned from model/data comparisons. To complete these objectives, the models' predictions of geoacoustic quantities such as wave speeds, attenuations, and bottom loss are compared with published measurements and to each other through Bayesian inference and computational studies. It is determined that while each model has its uses, no one model fully captures the wave physics of sandy sediments.

When sound is projected into the ocean, the backscattered signal may provide information about the object(s) from which the sound has scattered. When the backscattered sound comes from an aggregation of strong scatterers, such as a school of fish at their swim bladder resonance frequency, a phenomenon known as Coherent Backscatter Enhancement (CBE) may occur and this phenomenon could aid in discriminating fish schools from other scatterers in the ocean water column. When CBE occurs, the addition of the in-phase path pairs enhances the scattered field intensity by as much as a factor of two in the direction opposite to that of the incident wave. This thesis describes the results from simulating aggregations of randomly placed scatterers using the Foldy (1945) equations. The simulations are verified and validated by checking conservation of acoustic energy and by replicating results from near-field optics and acoustics CBE experiments. Simulations of far field CBE from aggregations of scatterers are presented and compared with far-field backscattering from ideal spherical scatterers. Parametric scalings for the height of the CBE peak are proposed for both near- and far-field geometries to relate the extent of multiple scattering effects to aggregation properties. Possible means are presented for distinguishing when backscattered returns come from a school of fish or from a single large scatterer.

This dissertation presents work in which computational modeling is used to advance two areas of interest in modern acoustics. First, we develop computational models to study the poorly understood physics underlying Contrast-Enhanced Ultrasound (CEUS) bioeffects and Diagnostic Ultrasound (DUS)-induced lung hemorrhage. A better understanding of these problems is critical to the development of science-based safety guidelines. Because inertial cavitation is thought to cause CEUS bioeffects, we model a spherical bubble in viscoelastic soft tissue, driven by experimentally-measured ultrasound waveforms (1.5 − 7.5 MHz), with known bioeffects thresholds. Bioeffects thresholds were compared with calculated cavitation metrics. Experimentally-determined bioeffects thresholds were found greater than accepted thresholds for inertial cavitation in water, and that the ratio of maximum radius to equilibrium radius correlated strongly with bioeffects thresholds as a function of frequency. Separately, we investigate DUS-induced lung hemorrhage and model an acoustic wave in soft tissue (water) traveling toward an alveolus (air) with a perturbed surface. First, a trapezoidal wave with ultrasonically relevant properties (5.0−12.5 MPa pressures amplitudes, 1.3−6.0 μs wave durations), drives the interface. We showed that acoustic waves may be capable of depositing sufficient baroclinic vorticity to drive significant interface perturbation growth, long after the wave passes. Interface perturbation growth scaled with the circulation density (i.e., the circulation per unit length of the interface) and exhibited power-law behavior at late time. To approximate ultrasound-induced stresses and strains on alveoli, we subjected interfaces of varying initial perturbation amplitude to 1.5 MHz ultrasound pulses with peak amplitudes from 1 − 5 MPa and observed interfacial viscous stress amplitudes up to approximately 60 Pa and interfacial strains up to 38%. While the viscous stresses appeared beneath capillary failure thresholds, the calculated strain was sometimes greater than expected failure thresholds for alveolar epithelium. Since this work considers only a single pulse, and the vorticity driving the deformation is expected to accumulate over multiple pulses, we conclude that vorticity-induced deformation is worthy of further investigation. Finally, we develop area statistics, a computationally efficient method for estimating Probability Density Functions (PDFs) of acoustic Transmission Loss (TL) in uncertain ocean environments. Such PDFs of TL are useful for real-time Naval applications but impractically expensive to obtain via traditional computational methods. This work describes how to approximate the PDF of TL at a point of interest using TL statistics from the field surrounding the point of interest. The idea is that local TL variations due to environmental fluctuations are represented by spatial variations in TL in single baseline TL field calculation. The baseline calculation used the most probable values for uncertain parameters describing the sound speed profile, bathymetry, and geoacoustic bottom-layer properties. Area statistics-generated PDFs were compared with 2000-sample Monte Carlo-generated PDFs via the L1 error, in ten ocean environments with varying properties and uncertainties, for source frequencies of 100, 200, and 300 Hz and depths of 91, 137, 183m. The area statistics-generated PDFs had an L1 error < 0.5 in 91% of the 11,000+ test locations (depths from 20 m - 4.5 km, ranges from 70 km). Area statistics PDFs were generated in milliseconds, and required one-millionth the computational effort required for Monte Carlo PDFs and thus suitable for real-time applications.

In this work, an eigenray finding method for simulating refractive multipath phenomena in an unbounded underwater environment is presented. To simplify the problem, the environment consists of a horizontally-homogeneous two-layer medium in which isogradient sound speed profiles are used in each layer. Eigenrays are found through an iterative Newton-Raphson method on the Snell invariant, and the travel time is found from the computed Snell invariant. A derivation for the maximum number of ray paths that need to be considered to account for all refractive multipaths is included. Given a point source/point target pair, the method will output a launch angle and travel time for the eigenray connecting the source and target. The time accuracy of the eigenray finding method is confirmed using a forward ray-trace. From the travel time of eigenrays, time series data is formed for each receive element. Refractive multipath phenomena are shown using the time series data of a target near the layer boundary. An FFT beamformer is used to beamform the simulated time series data which reproduces an image of the acoustic targets, which shows a time resolution sufficient to support the coherent addition of signals in beamforming.

Three-dimensional acoustic propagation in shallow water waveguides is studied using the longitudinally invariant finite element method. This technique is appropriate for environments with lateral variations that occur in only one dimension. In this method, a transform is applied to the three-dimensional Helmholtz equation to remove the range-independent dimension. The finite element method is employed to solve the transformed Helmholtz equation for each out-of-plane wavenumber. Finally, the inverse transform is used to transform the pressure field back to three-dimensional spatial coordinates. Due to the oscillatory nature of the inverse transform, two integration techniques are developed. The first is a Riemann sum combined with a wavenumber sampling method that efficiently captures the essential components of the integrand. The other is a modified adaptive Clenshaw-Curtis quadrature. Three-dimensional transmission loss is computed for a Pekeris waveguide, underwater wedge, and Gaussian canyon. For each waveguide, the two integration schemes are compared in terms of accuracy and efficiency.

In the ocean, the acoustic signal from a remote source recorded by an underwater hydrophone array is commonly distorted by multipath propagation. Blind deconvolution is the task of determining the source signal and the impulse response from array-recorded sounds when the source signal and the environment’s impulse response are both unknown. Synthetic time reversal (STR) is a passive blind deconvolution technique that accomplishes two remote sensing tasks. 1) It can be used to estimate the original source signal and the source-to-array impulse responses, and 2) it can be used to localize the remote source when some information is available about the acoustic environment. The performance of STR for both tasks is considered in this thesis. For the first task, simulations and underwater experiments (CAPEx09) have shown STR to be successful for 1.5-4 kHz broadcast signal. Here STR is successful when the signal-to-noise ratio is high enough, and the receiving array has sufficient aperture and element density so that conventional delay-and-sum beamforming can be used to distinguish ray-path-arrival directions. Also, an unconventional beamforming technique (frequency-difference beamforming) that manufactures frequency differences from the recorded signals has been developed. It allows STR to be successful with sparse array measurements where conventional beamforming fails. Broadband simulations and experimental data from the focused acoustic field experiment (FAF06) have been used to determine the performance of STR when combined with frequency-difference beamforming. For the source localization task, the STR-estimated impulse responses may be combined with ray-based back-propagation simulations and the environmental characteristics at the array into a computationally efficient scheme that localizes the remote sound source. These localization results from STR are less ambiguous than those obtained from conventional matched field processing in the same bandwidth. However, when the frequency of the recorded signals is sufficiently low and close to modal cutoff frequencies, STR-based source localization may fail because of dispersion in the environment. For such cases, an extension of mode-based STR has been developed for sound source ranging with a vertical array in a dispersive underwater sound channel using bowhead whale calls recorded with a 12-element vertical array (Arctic 2010).

The propagation of acoustic waves through water-saturated granular sediments has been widely studied, yet existing propagation models can not adequately predict the speed and attenuation of sound across the range of frequencies of interest in underwater acoustics, especially in loosely packed sediments that have been recently disturbed by storms or wave action. Advances in modeling are currently dependent on experimental validation of various components of existing models. To begin to address these deficiencies, three well-controlled laboratory experiments were performed in gravity-settled glass beads and reconstituted sand sediments. Sound speed and attenuation measurements in the 0.5 kHz to 10 kHz range are scarce in the literature, so a resonator method was used to investigate a reconstituted sand sediment in this range. The literature contains laboratory and in situ measurements of sound speed and attenuation at higher frequencies, but existing models can not predict both the speed of sound and attenuation simultaneously in some sediments. A time-of-flight technique was used to determine the speed of sound and attenuation in monodisperse water-saturated glass beads, binary glass bead mixtures, and reconstituted sediment samples in the frequency range 200 kHz to 900 kHz to investigate the effect of sediment inhomogeneity. The effect of porosity, independent of changes in other sediment physical properties, has not been demonstrated in the experimental literature. Therefore, a fluidized bed technique was used to independently vary the porosity of monodisperse glass bead samples from 0.37 to 0.43 and a Fourier phase technique was used to determine the speed and attenuation of sound. Collecting these results together, measured sound speeds showed positive dispersion below 50 kHz while negative dispersion was observed above 200 kHz for some samples. Attenuation measurements showed an approximately f⁰̇⁵ dependence in the low frequency regime and an approximately f³̇⁵ dependence for large-grained samples in the high frequency regime. The laboratory experiments presented in this work demonstrate that both sound speed and attenuation in idealized loosely packed water-saturated sediments can not be simultaneously predicted by existing models within the uncertainties of the model input parameters, but the independent effect of porosity on sound speed can be predicted.

Underwater acoustics has recently emerged as a viable tool for assessing ecosystem health and exploring the estuarine soundscape. Recent acoustic surveys have mapped distributions of both seagrass meadows and kelp forests, and scientists are currently developing remote sensing capabilities to improve ecological assessments of these communities. Furthermore, researchers are beginning to focus on the propagation and ecological significance of bioacoustic signals within estuarine landscapes. The research presented here includes a thorough examination of the interaction of acoustic energy and macrophyte tissue as it pertains to habitat assessment and ecosystem function. Modeling experiments investigated the interaction of acoustic energy and submerged macrophyte tissue. Both seagrasses and kelp exhibited a similar acoustic response by increasing the acoustic compressibility of a seawater medium. The increase in acoustic compressibility was driven by free-gas volumes contained within the macrophyte tissue. Interestingly, the tissue served to limit the acoustic compressibility of the gas volume below the magnitude predicted by effective medium models. Separate inquiries of high-frequency sound propagation and the seagrass canopy revealed a significant temporal component to acoustic transmission. Specifically, sound transmission throughout a seagrass canopy was altered by the formation of free gas bubbles and the pressurization of aerenchyma channels, which was mediated by photosynthesis. The photosynthetic controls on sound propagation were species-specific, and patterns of acoustic transmission provided a reasonable proxy for gross primary production in Syringodium filiforme plants. Finally, the interaction of sound energy and submerged macrophytes appears to have important ecological implications. This research suggests that seagrass meadows scatter high-frequency sound energy and provide an acoustic refuge to fish from marine mammal predators. This refuge is highly seasonal, specific to different seagrass species and dependent on the abundance of above-ground biomass. Seagrasses also may influence the transmission of low-frequency sounds used by soniferous fish. Propagation characteristics of low-frequency sounds are highly dependent on frequency and result in differential transmission distances among individual fish species. It is clear from this body of work that submerged macrophytes are an important feature of the underwater soundscape. Future research should continue to exploit this feature for remote sensing purposes and examine its ecological significance.

A transponder for underwater acoustic data communications is prototyped. The mobile transponder emits a data sequence whenever it detects a ping from a base station. The data sequence includes GPS coordinates and UTC time sent over a conservative and brief 12 kbps turbo-coded BPSK link, and a 6 kB JPEG image sent over an ambitious 67 kbps turbo-coded 16-QAM link. The range of the transponder from the base station can also be accurately derived. Several challenges exist in decoding the underwater signals at the base station receiver, including Doppler distortion and multipath. While experimental results show that the ranges for decoding the 16-QAM signals with a single hydrophone are limited to less than 25 m, the BPSK signals prove to be much more robust, decoding at ranges of up to 625 m. Experiments with delays and transducer tether length indicate methods for improving reliability in the presence of reverberation and thermocline. This transponder uses mostly off-the-shelf parts and is anticipated to be improved when paired with advanced sonar array devices.

This thesis describes the development and testing of the Combustive Sound Source (CSS), which is a broadband underwater sound source. The CSS is being developed as a clean, safe, and cost effective replacement to underwater explosive charges, which exhibit an inherent danger to marine life and researchers using the charges. The basic operation of the CSS is as follows. A combustible mixture of gas is held below the surface of the water in a combustion chamber and ignited with an electric spark. A combustion wave propagates through the mixture and converts the fuel and oxidizer into a bubble of combustion products, which expands due to an increase in temperature, and then ultimately collapses to a smaller volume than before ignition, producing a high intensity, low frequency acoustic signal. The thesis begins by discussing the background, history, and purpose of developing the CSS. It continues by describing the current apparatus and the essential components and convenient features added to the latest mechanical design. The general operation is discussed along with a description of an experiment conducted to determine the acoustic output and robustness of the current CSS. The results of this experiment are presented in terms of the effect of volume, ignition depth, oxidizing gas, combustion chamber size, and repeatability of acoustic signatures. Discussion of apparatus robustness is presented to suggest improvements for future CSS designs.

This manuscript describes a methodology to combine environmental models, acoustic signal predictions, statistical detection models and operations research to form a framework for calculating and communicating performance. This methodology has been applied to undersea target detection systems and has come to be known as Performance Surface modeling. The term Performance Surface refers to a geo-spatial representation of the predicted performance of one or more sensors constrained by all-source forecasts for a geophysical area of operations. Recent improvements in ocean, atmospheric and underwater acoustic models, along with advances in parallel computing provide an opportunity to forecast the effects of a complex and dynamic acoustic environment on undersea target detection system performance. This manuscript describes a new process that calculates performance in a straight-forward "sonar-equation" manner utilizing spatially complex and temporally dynamic environmental models. This performance model is constructed by joining environmental acoustic signal predictions with a detection model to form a probabilistic prediction which is then combined with probabilities of target location to produce conditional, joint and marginal probabilities. These joint and marginal probabilities become the scalar estimates of system performance. This manuscript contains two invited articles recently accepted for publication. The first article describes the Performance Surface model development with sections on current applications and future extensions to a more stochastic model. The second article is written from the operational perspective of a Naval commanding officer with co-authors from the active force. Performance Surface tools have been demonstrated at the Naval Oceanographic Office (NAVOCEANO) and the Naval Oceanographic Anti-Submarine Warfare (ASW) Center (NOAC) in support of recent naval exercises. The model also has recently been a major representation for the "performance" layer of the Naval Meteorological and Oceanographic Command (NAVMETOCCOM) in its Battlespace on Demand strategy for supporting the Fleet with oceanographic products.

Advances in ocean modeling (Barron et al., 2006) have improved such that ocean forecasts and even ensembles (e.g., Coelho et al., 2009) representing ocean uncertainty are becoming more widely available. This facilitates nowcasts (current time ocean fields / analyses) and forecasts (predicted ocean fields) of acoustic propagation conditions in the ocean which can greatly improve the planning of acoustic experiments. Modeling of acoustic transmission loss (TL) provides information about how the environment impacts acoustic performance for various systems and system configurations of interest. It is, however, very time consuming to compute acoustic propagation to and from many potential source and receiver locations for multiple locations on an area-wide grid for multiple analysis / forecast times, ensembles and scenarios of interest. Currently, to make such wide area predictions, an area is gridded and acoustic predictions for multiple directions (or radials) at each grid point for a single time period or ensemble, are computed to estimate performance on the grid. This grid generally does not consider the environment and can neglect important environmental acoustic features or can overcompute in areas of environmental acoustic isotropy. This effort develops two methods to pre-examine the area and time frame in terms of the environmental acoustics in order to prescribe an environmentally optimized computational grid that takes advantage of environmental-acoustic similarities and differences to characterize an area, time frame and ensemble with fewer acoustic model predictions and thus less computation time. Such improvement allows for a more thorough characterization of the time frame and area of interest. The first method is based on critical factors in the environment that typically indicate acoustic response, and the second method is based on a more robust full waveguide mode-based description of the environment. Results are shown for the critical factors method and show that this proves to be a viable solution for most cases studied. Limitations are at areas of high loss, which may not be of concern for exercise planning. The mode-based method is developed for range independent environments and shows significant promise for future development.

In June 2003, the Littoral Acoustic Demonstration Center conducted an acoustic characterization experiment for a standard seismic exploration array. Two moorings with Environmental Acoustic Recording Systems (EARS) were deployed in the northern part of the Gulf of Mexico to measure ambient noise and collect shot information. A 21-element seismic airgun array was towed along five parallel linear tracks with horizontal closest approach points to the EARS buoy position of 63, 500, 1000, 2000, and 5000 m. Calibrated acoustic pressure measurements collected during the experiment were analyzed to obtain zero-to-peak sound pressures, sound exposure levels, and pressure levels in 1/3-octave frequency bands. In addition, the experimental data were modeled by using a modified underwater acoustic propagation model to fill in missing data measurements. The resulting modeling procedure showed good agreement between measured and modeled data in absolute pressure amplitudes and frequency interference patterns for frequencies up to 1000 Hz. The analysis is important for investigating the potential impact on marine mammals and fish and predicting the exposure levels for newly planned seismic surveys in other geographic areas. Based on results of the experiment conducted and data analysis performed, a new experimental design was proposed to maximize the amount of collected data using the available equipment while minimizing the time needed for the source ship. The design used three patches, one with 3º angular spacing between the lines at a reference depth. Embedded is a smaller patch with 1º spacing and within that a still smaller patch with one half degree spacing. This arrangement gives a reasonably uniform distribution of shots versus solid angle with a large variety of emission and azimuthal angles for different ranges. Due to the uncertainty of positioning systems, the angular space is divided into solid angle bins. Simulations predicted more than 200 shots per bin for emission angles greater than 13 degrees. Statistical analysis of collected data will be performed on the proposed bin basis. An experiment based on the proposed design was conducted in Fall 2007. The data measurements collected during the experiment are currently being analyzed and will be reported in the near future.

Acoustical interactions with ocean sediments effect a wide range of sonar applications in littoral environments. An important factor in understanding the acoustical behavior of the ocean bottom is how the sediment density changes with depth. Although there are existing techniques for obtaining information about sediment gradients, these methods are unable to provide direct measurements of the sediment density gradient without significantly disrupting the test site and requiring significant diver support for installation and implementation. The proposed X-Ray Attenuation Measurement (XRAM) device aims to improve upon these existing techniques with the goal of being a portable diver operated device that can perform direct in situ measurements of sediment density gradients without significant disruption of the ocean bottom. To accomplish this, the XRAM utilizes the attenuation of x-rays passing through the sediment to measure the density as a function of depth, and is arranged in a compact, portable design that can be deployed and operated by a single diver. The layout and basic design of the XRAM device is discussed, and a physical model of its operation is developed. Results of experimental testing on homogeneous liquid samples and liquid/solid mixtures to evaluate the effectiveness of the XRAM device in measuring density gradients are presented. Based on the analysis of these results, recommendations of improved performance for future development are given.

The average temperature of the ocean can be determined by measuring the traveltimes of acoustic signals from a source to a receiver. In the temperate deep ocean, a narrow acoustic pulse transmitted from a source results in a reception at long ranges consisting of many (possibly overlapping) arrivals. One of the mathematical structures used to describe and interpret acoustic propagation in the ocean is normal mode theory. The identification of individual normal mode arrivals in a reception is difficult because modal arrivals are spread in time by geometric dispersion causing them to overlap and interfere with each other. Current signal processing methods aimed at identifying individual normal mode arrivals require the use of vertical arrays of receivers which are rare because they are very expensive to build and deploy. A new signal processing method using phase-only filters to compensate for the geometric dispersion of normal mode arrivals is presented. This compensation increases the peak signal to noise ratio of the individual modal arrivals while simultaneously compressing them in time, helping to isolate them and their arrival times from overlapping neighbors. The properties of the phase-only filters and their ability to help isolate and identify modal arrivals is investigated through the processing of computer simulated receptions. By processing a reception with a bank of phase-only filters characterized by different amounts of dispersion compensation, a plot dubbed the Dispersion Diagnostic (DD) Display is generated. The use of phase-only filters does not require vertical arrays of receivers because modal phase is constant across depth. DD Displays generated for a reception from a receiver at a single depth show compressed modes which are isolated from their neighbors and for which traveltimes can be determined. Thus, the dispersion processing opens up the use of horizontal arrays or single hydrophones in mode identification, broadening the capabilities of ocean acoustic researchers. It is shown that the use of DD Displays in conjunction with vertical arrays helps to improve the accuracy of the traveltime estimates for low order modes, as well as extending the number of modes the array can effectively isolate. The optimum placement of acoustic sources when dispersion processing is the goal is examined as well as the dispersion properties of sound speed profiles of varying shape. The results of processing experimental receptions made during the ATOC Alternate Source Test in July 1996 are presented which show less dispersion than expected, possibly indicating propagation characterized by significant mode coupling.

The design and analysis of a low-frequency, resonant, tonpilz type, rare earth iron underwater sound transducer using rods of Tb.27Dy.73Fe1.95 are described. An equivalent circuit is presented which predicts the performance of the transducer in the presence of eddy currents and demagnetization. The effect of eddy currents on the impedance of a rod is discussed. The predicted performance of a laminated transducer is compared with that of an unlaminated transducer. The performance of the rare earth iron transducer is compared with the performance of the same transducer with identical ceramic active elements. Data on transmitting current response, transmitting voltage response, free field voltage sensitivity, coupling coefficient, efficiency, and linearity are presented. Advantages, disadvantages, and possible uses of rare earth iron material are discussed.

This report proposes a novel technique for measuring of acoustic fields in water. A Laser Hydrophone is proposed taking advantage of the properties of Total Internal Reflection. A theoretical analysis of the idea is presented followed by a prediction of the operating characteristics of an actual system. Actual data were taken with the proposed system and it is compared to the predicted.

In recent years the United States Navy has concentrated most of its Anti-Submarine Warfare (ASW) research and development efforts toward passive sonar. Its ability to locate enemy targets without being detected gives the passive sonar system a supreme strategic advantage over its active counterpart. One aspect of passive sonar signal processing is the time delay estimation of an underwater acoustic wavefront. From this estimation the location and velocity of the radiating source (target) can then be determined. This report compares two popular methods of estimating time delay utilizing computer simulations of each: the cross correlator and the beamformer.

This Research Report presents some of the equations of underwater acoustics that relate to the signal excess noise received by a transducer. The basic structural equation is developed, as are defining equations for each term in that equation. An analysis is performed utilizing typical values to ascertain if the elements of the structural equation can be simplified. Results delineate that several terms of that equation can be neglected while maintaining a relative high degree of accuracy.

Although an extensive body of data on the speed of sound in the ocean is available from hydrographic casts, considerably more precise measurements can be made of explosion travel times over long paths through the deep ocean sound (so far) channel. A novel method is presented for analytically combining the two types of data to produce a functional description of the spatial variation of sofar speed. This method is based on the fact that the integral, over a path, of a series representation of the reciprocal of speed yields a series representation of the travel time over that path, the same set of coefficients entering linearly into both series. Both types of observations may then be combined in the same matrix equation for estimating the coefficients. An example is computed for the Pacific Ocean in the form of a spherical harmonic function of degree 6. Hydrographic data consisted of values averaged for 4013 one-degree squares. Approximately 400 temporally independent travel-time measurements, over paths ranging in length from 17° to 110° , were used. The paths were concentrated in the northeast Pacific. The estimated variance of a single point observation of sofar speed was 1.56 (meter/sec)2 while the variance of a single observation of harmonic-mean sofar speed was 0.016 (meter/sec)2. These values were used, where appropriate, for weighting the data during least-squares estimation of the coefficients of the spherical harmonic function.