Broadband Acoustic Signals Research Articles

NEAR-AXIAL INTERFERENCE EFFECTS IN LONG-RANGE OCEAN ACOUSTIC PULSE PROPAGATION

In many long-range ocean propagation experiments the source and receiver are placed close to the depth of the waveguide axis. In this case, rays emerging from the source at sufficiently small angles intersect the sound-channel axis many times and form in its vicinity a large number of caustics with caustics cusps located repeatedly along the axis. In neighborhoods of cusped caustics there exists a very complicated interference pattern. Neighborhoods of interference grow with range and at long ranges they overlap. As a result, a complex interference wave (axial wave), that propagates along the waveguide axis, appears. The goal of this paper is to obtain the representation for the axial wave in the time domain and calculate its magnitude for a realistic model of a three-dimensional range-independent medium. Numerical computations are done for the average profile from the Acoustic Engineering Test (AET) experiment. The pulse center frequency of 75 Hz with 30-Hz bandwidth is used that corresponds to broadband acoustic signals which were transmitted during November 1994 in the eastern North Pacific Ocean as a part of the AET. The propagation range is 3250 km. The sound source is located on the waveguide axis, and the receiver is placed close to the depth of the axis. Through numerical simulation the dependencies of the magnitude of the axial wave on depth of the receiver and propagation range are studied.

Basin‐scale time reversal communications

During November 1994, broadband acoustic signals were transmitted from a 75‐Hz source to a 20‐element, 700‐m vertical array at approximately 3250 km range in the eastern North Pacific Ocean as part of the Acoustic Engineering Test (AET) of the Acoustic Thermometry of Ocean Climate (ATOC) program [Worcester et al., J. Acoust. Soc. Am. 105, 3185‐3201 (1999)]. The AET tomography signal can be treated as a binary‐phase shift‐keying (BPSK) communication signal with an information rate of 37.5 bits/s. With the multipath arrivals spanning 5‐8 s, this data represents an extreme case of intersymbol interference (ISI). The AET array data are processed using time reversal combined with frequent channel updates to accommodate channel variations over the 20‐min long reception, followed by a single channel decision‐feedback equalizer (DFE). The almost error‐free performance using all 20 array elements demonstrates the feasibility of time reversal communications. Further, comparable performance of single receive element communications integrating over multiple transmissions indicates that the ocean provided temporal diversity as well as the spatial diversity provided by the array.

Spatial and temporal sound field fluctuations due to propagating internal waves in shallow water

Fluctuating three dimensional acoustic wave propagation in shallow water presents a challenge due to the anisotropic nature of the environment. Temporal and spatial changes in the water column caused by the passage of internal waves are among the the primary causes of the anisotropy. Three main mechanisms for the acoustic field variations have been determined: 1) adiabatic propagation,. 2) horizontal refraction, and 3) mode coupling. All these mechanisms provide different characteristics of the sound fluctuations, and depend on the angle between the internal wave direction and the source receiver propagation path, as well as frequency and mode number. In a recent multi‐institutional shallow water experiment (SW06) a series of source‐receiver positions were created to examine the effects of 3D environmental fluctuations on the propagation of low to mid frequency (0.05‐3 kHz) broadband acoustic signals while extensive environmental observations were made. These recent observational results confirm the above theoretical hypothesis. Work supported by ONR‐321OA and RFBR.

Human recognition by active and passive acoustic signatures

Recognition of different sensed objects is a problem that often appears in practice. One of the solutions is based on analysis of the signatures of the specific objects. This method was applied for the acoustic detection of walking humans. Human footsteps excite envelopes of broadband acoustic signals in the air due to periodic friction forces between the foot and the ground/floor. The repetition frequency of these envelopes is equal to the footstep rate and usually lies below 3 Hz. High frequencies in these envelopes allow detection and localization of a walker using a narrowband ultrasonic receiver with a high directivity pattern. Consequently, periodic low frequency human motion results in passive ultrasonic detection using this method. This motion has also unique Doppler signatures and is measured using ultrasonic sonar. Common analyses of passive and Doppler signatures allow the extraction of the specific cadence in human motion and recognition of a human while rejecting other moving or stationary ob...

A parametric study of interactions between acoustic signals reflected by the seafloor

This paper deals with the study of broadband acoustic signals reflected by the seafloor and recorded on a vertical array in shallow water areas. Only the first bottom reflected path is considered here. In previous studies (see e.g., Guillon and Holland, JASA, 122, 2974), two different aspects of the "coherence" of these signals were examined (the maximum of the cross-correlation coefficients and the phase of the cross-spectra) and it was shown that these parameters are sensitive to the geoacoustic nature of the seafloor. These results show that the interactions between signals can be used efficiently for improving geoacoustic inversion schemes or for transmission loss predictions. Before going to such applications, a detailed parametric study is needed to investigate the sensitivity of the interactions measurement to various parameters (range, geoacoustic structure,...). This is done using a numerical model, based on the spherical wave decomposition of plane waves, that simulates the signals reflected by the seafloor. The parametric study lead shows the relative importance of the various parameters and gives indications for the possibility of inverting these parameters.

Frequency dependence and intensity fluctuations due to shallow water internal waves

A theory and experimental results for sound propagation through an anisotropic shallow water environment are presented to examine the frequency dependence of the scintillation index in the presence of internal waves. The theory of horizontal rays and vertical modes is used to establish the azimutal and frequency behavior of the sound intensity fluctuations, specifically for shallow water broadband acoustic signals propagating through internal waves. This theory is then used to examine the frequency dependent, anisotropic acoustic field measured during the SWARM'95 experiment. The frequency dependent modal scintillation index is described for the frequency range of 30-200 Hz on the New Jersey continental shelf.

Spatial and temporal fluctuations of the sound field during shallow water 2006 (SW06) experiment

During a multi-institutional shallow water experiment in the summer of 2006 (SW06) a series of source-receiver positions were designated to examine the effects of 3D environmental fluctuations on the propagation of low to mid frequency (0.05–3 kHz) broadband acoustic signals. During each scheduled transmission a ship equipped with an acoustic source and a vertical receiver array was positioned at a fixed coordinate determined by the real-time measured position of an internal wave (IW) regime. The passing IW packet in each one of these events was measured by additional research vessels to assess the oceanographic conditions. At the source-receiver ship, the passing IW was observed and its speed and wavelength was measured from surface features using ship’s radar within 3-NM radius from the leading edge of the IW on approach. Each event was photographed at 30-s intervals. In addition, thermistor array data were collected simultaneously. A total of 57 passing IW events were recorded during 21 days through which broadband acoustic signals were transmitted at different source-receiver angles with respect to the IW front. This paper presents preliminary data processing of the observed data. [Work supported by ONR-321OA and RFBR.]

Calibration of source level, expected transmitted power, and reverberation for long‐range sonar in fluctuating range‐dependent ocean waveguides

The ocean waveguide is temporally and spatially varying. An acoustic signal transmitted through the ocean or scattered from objects will get randomized. Estimating parameters of sonar, the environment, and scatterers requires a statistical approach that incorporates medium uncertainties into the signal analysis. Here we discuss source level, transmission loss (TL), and reverberation model calibration using data acquired with a long‐range sonar on the New Jersey STRATAFORM. Broadband acoustic data measured by a desensitized hydrophone in a towed horizontal receiving array at numerous locations from a vertical source array are analyzed. The match‐filtered data are compared to the expected TL output from a parabolic equation model that accounts for bathymetric variations. A maximum likelihood estimator is implemented to provide a global inversion of the data for source level, attenuation due to scattering, and match‐filter degradation in the multi‐modal ocean waveguide. An estimate is also provided of the coherence bandwidth for broadband acoustic signal transmission in this environment. A challenge in calibrating bottom reverberation models with sonar data lies in separating the scattered intensity from moving objects, such as fish groups, and distinguishing them from the statistically stationary background reverberation. An approach is presented for this purpose.

Numerical study of the influence of roughness on coherence of signals reflected by the seafloor

The context of this work is the study of broadband acoustic signals reflected by the seafloor and recorded on a 15‐hydrophone vertical array. Two different aspects of the coherence of these signals were examined: in the temporal domain with the maximum of the cross‐correlation coefficients and in the frequency domain with the phase of the cross‐spectra. In previous work, it was shown that these coherence parameters are sensitive to the geoacoustic nature of the seafloor which may open the door for a new method of probing the seabed properties on both large scales (i.e., deterministic) and small scales (i.e., statistical). The aspect examined in this present work is the influence of the seafloor roughness on the coherence. To do so, the time series are simulated by a three‐step process: 1) computation of the plane‐wave reflection coefficient, 2) computation of the spherical wave reflection coefficient by a numerical integration of the Sommerfeld integral, and 3) Inverse Fourier Transform. Using these simulations, with the measured data as a guide, the potential for detecting and extracting roughness estimates from the coherence in complex multilayered seabed environments is explored.

Effects of offshore mesoscale eddies and fronts on inshore shallow water acoustic propagation

Shallow water shelf areas inside of western boundary currents have two distinctly different ocean acoustic environments determined by the type of front that separates deep offshore from shallow inshore. Prograde fronts allow for the ducting of offshore internal waves up onto the shelf as the main source the sound speed fluctuations over the internal wave band. Retrograde fronts block the propagation of offshore internal waves setting up stability conditions that allow for propagation of locally generated, large amplitude non‐linear solitary waves as the major source of sound speed variability. Here, acoustic propagation is examined for both environments with data from two similar fixed system propagation experiments, one for the prograde environment off the coast of south Florida near the site off the Acoustic Observatory, and the second for the retrograde front environment of the Mid‐Atlantic Bight the SWO6 site. Offshore mesoscale features of fronts and eddies are shown to determine mean sound speed profiles and the energy of the internal wave fields. In turn, intensity fluctuations and temporal coherencies of broadband acoustic signals over several octaves are observed to vary with variations of the sound speed. For some locations, observations of mesoscale features alone can predict sonar performance.

Fiber optic in-line distributed sensor for detection and localization of the pipeline leaks

When a gas or liquid pipeline leaks, it will generate broadband acoustic signal. This acoustic pressure will induce an optical phase signal of the optical fiber fixed on the surface of pipes. In this paper, we propose a hybrid configuration of Mach-Zehnder and Sagnac interferometer as sensing frame. In this interferometer, there are two light paths that have the same optical length but travel different sequence paths. Because the propagation lights of the two light paths pass through the leaking point at different times, the resulting phase signals differ respectively. After interference, we demodulate the sensing phase signal by a broadband phase generated carrier (PGC) circuit and then the leaking point can be acquired from null frequency of the output spectrum. This system has many advantages, such as its in-line configuration, all fiber structural design and polarization-insensitive for sensing fiber. This system also has very wide dynamic range which can be greater than 76dB with the minimum detectable phase signal about 3.3×10−4 (rad/Hz).

Propagation of impulsive broadband signals in a coastal ocean setting during the 1996 Shelfbreak Primer experiment

The 1996 Shelfbreak PRIMER experiment provided an opportunity to study the impulse response of a coastal ocean waveguide using broadband acoustic signals from explosive sources. Acoustic transmission paths in the vicinity of the shelfbreak included various orientations ranging from up‐slope to along‐slope, including interactions with a shelfbreak front. Acoustic receptions were acquired with a 16‐element vertical line array (VLA). In addition, thermistor string data acquired at the receiving array suggest that a group of high‐amplitude, internal waves traversed the test site during this portion of the experiment. Modal decomposition of broadband signal arrivals was performed to investigate variation of the impulse response during source deployments. Variations in the received spectra of individual normal modes were compared with numerical predictions for impulsive broadband signals propagating in range‐dependent environments constructed using oceanographic data available for the test site. The potential for mode coupling and three‐dimensional effects to shape the modal and spectral distribution of received energy is being investigated. Numerical and experimental results will be discussed. [Work supported by Office of Naval Research.]

Vertical structure and arrival times of the axial wave based upon conductivity, temperature, and depth data acquired during the Long‐range Ocean Acoustic Propagation Experiment

In the LOAPEX experiment at some distances from VLA the sound source was deployed to depths quite close to the sound‐channel axis. This took place at stations T1000, T500, T250, and T50 where, in particular, the source was deployed to 800 m and the depth of the sound‐channel axis at these ranges is about 680 m. The interference of the wave fields corresponding to near‐axial rays leads to formation of a complex interference wave (‘‘the axial wave’’) that propagates along the axis being a component of the axial crescendo. The talk will present the results on simulation of the axial wave as a function of depth of the receiver and the calculation of its arrival time at frequencies 56.25–93.75 Hz. In the time domain the pulse propagation modeling is carried out for the broadband acoustic signals with the pulse center frequency of 75‐ and 37.5‐Hz bandwidth. The obtained results are compared with those for the lowest mode in the adiabatic approximation. Calculations are made for the range‐dependent ocean model based upon LOAPEX CTD data and use the integral representation of the axial wave in a local coordinate system introduced in the vicinity of the range‐variable sound‐channel axis. [Work supported by ONR.]

Factors affecting broadband acoustic emission measurements of a heterogeneous reaction

Broadband acoustic emission signals were obtained by attaching a piezoelectric transducer, sensitive up to 750 kHz, to the external wall of a 1 L jacketed glass reactor. Measurements were acquired of itaconic acid particles mixing in toluene; the total area of the acoustic emission signal from 55-500 kHz increased when the particle concentration, particle size or stir rate were increased. Signals at frequencies above 200 kHz were less sensitive to changes in particle size than those at lower frequencies. From calculation of the area of the signal in the range 55-200 kHz as a percentage of the signal area over the range 55-500 kHz, for mixtures of different size ranges of itaconic acid, it was possible to obtain an estimate of the mean particle size of a mixture. The heterogeneous esterification reaction of itaconic acid and 1-butanol was monitored non-invasively. A decrease in the overall acoustic signal area between 60 and 500 kHz was observed as the reaction progressed. Particle size and concentration information were contained in the amplitude of the acoustic emission signal, while the emission frequency yielded information on changes in the mean particle size.

Effects of sea surface on underwater communication

During High Frequency Experiment in 2003 (HFX-03) various ocean parameters such as the surface wave spectrum, salinity, and temperature profiles were recorded simultaneously with the broadband acoustic signals. Including broadband probe chirps as well as several other communication sequences. Received data are collected and analyzed from different receiver arrays. The statistics of arrival rays and the performances of communication system are studied in two periods of calm and rough sea conditions. Surface reflected rays show variable coherence during different sea surface conditions. This has large influence on the structure of the received signals. Strong dependence between sea surface condition and the performance of communication system is observed. Two types of receiver structures are used and their performances are compared: (1) single channel receiver with decision feedback equalizer, (2) multichannel receiver with signal combining scheme. Similar trends of dependence are found on both receivers. [Work supported by ONR-321OA.]

Shallow Water Temporal Coherence of Broadband Acoustic Signals in the Presence of Internal Waves

Temporal coherence is closely related to single path phase rate. The single path phase rates in a shallow water environment were obtained by applying two different multipath models [Dyson, Munk, and Zetler, J. Acoust. Soc. Am., vol. 59, pp. 1121-1133, 1976 and Mikhalevsky, J. Acoust. Soc. Am., vol. 66, pp. 751-762, 1979] on simulated data with internal waves. The estimated phase rates were consistent between the two models. In contrast, previous application of the same models on deep water data showed a factor of 8 difference in their phase rates. The phase rates derived from the models become independent of range after a uniform phase distribution is reached in the (-/spl pi/,/spl pi/) region. The shallow water coherence time varies considerably with depth at a given range. The depth dependence is explained with a correlation function constructed with mode depth functions.

Analysis and modeling of broadband airgun data influenced by nonlinear internal waves

To investigate acoustic effects of nonlinear internal waves, the two southwest tracks of the SWARM 95 experiment are considered. An airgun source produced broadband acoustic signals while a packet of large nonlinear internal waves passed between the source and two vertical linear arrays. The broadband data and its frequency range (10-180 Hz) distinguish this study from previous work. Models are developed for the internal wave environment, the geoacoustic parameters, and the airgun source signature. Parabolic equation simulations demonstrate that observed variations in intensity and wavelet time-frequency plots can be attributed to nonlinear internal waves. Empirical tests are provided of the internal wave-acoustic resonance condition that is the apparent theoretical mechanism responsible for the variations. Peaks of the effective internal wave spectrum are shown to coincide with differences in dominant acoustic wavenumbers comprising the airgun signal. The robustness of these relationships is investigated by simulations for a variety of geoacoustic and nonlinear internal wave model parameters.

Modal analysis of broadband acoustic signals propagating in the top layer of the ground

Experimental measurements of the frequency dependence of sound propagation losses in the top layers of the ground were performed in the frequency range between 80 and 420 Hz. These measurements revealed that the distribution of energy on the frequency-range plane has a regular structure. The features of this structure were modeled in the modal approach for a gradient model of the ground. It was shown that average sound speed in the ground and the approximate depth of the layers may be estimated through analysis of the experimentally measured frequency-range distribution of energy. The technique explores the waveguide properties of the top layer of the ground. According to the mode theory, two asymptotes coming from the point (0,0) should exist on the wave number-frequency plane. From the slope of these asymptotes, the minimum and maximal sound speeds in the waveguide were found to be 100 and 530 m/s. The approximate depth of the layers was estimated. Comparisons of obtained sound speeds and depth of layers with independently estimated values show satisfactory agreement. [Work supported by ONR Grant N00014-02-1-0878.]

Studies of waveguide propagation effects on frequency properties of sperm whale communication signals

The Littoral Acoustic Demonstration Center (LADC) conducted a series of passive acoustic experiments for studying ambient noise environment and marine mammal (namely, sperm whales) acoustic phonations in the Northern Gulf of Mexico by long-term autonomous recordings of broadband acoustic signals on a single bottom-moored hydrophone. It was previously reported that extensive spectrogram analysis of sperm whale phonations revealed persistent null pattern in animal spectral signatures that could be associated with either waveguide propagation effects or with a uniqueness of an animal phonation apparatus. The previous modeling addressed the propagation effects on echolocation signals of deep-water foraging animals. It was demonstrated that the null patterns in the analyzed spectrograms could not be explained by the propagation effects for an animal echolocating at foraging depths. This paper presents the results of modeling waveguide propagation effects on spectral properties of sperm whale near-surface communication signals, codas, that could be associated with distinctive sequences of clicks repeatedly present in the LADC recordings. The possibility of the individualization of sperm whale phonations based on the frequency properties of click recordings on a single hydrophone is discussed. [Research supported by ONR.]

Spatial coherence of acoustic signals measured during the 1998–1999 North Pacific Acoustic Laboratory (NPAL) experiment

Little experimental data have been reported in the literature about spatial coherence of low-frequency sound waves propagating over megameter ranges in the ocean. In 1998–1999, the North Pacific Acoustic Laboratory (NPAL) carried out a comprehensive low-frequency long range sound propagation experiment with the use of the receiving billboard acoustic array. By signal-processing of the NPAL data, we calculated spatial coherence of the broadband acoustic signals (with 75 Hz carrier) after their propagation of about four thousand km in North Pacific. The results obtained are presented in this paper. First, the horizontal coherence of the NPAL signals is discussed. The results show that the horizontal coherence radius varies in the range 500–1000 m. This value is in a good agreement with predictions of a 3D modal theory of low-frequency long-range sound propagation in a fluctuating ocean. Secondly, the vertical coherence of the NPAL signals is presented. After averaging over several days of the experiment, the vertical coherence reveals non-monotonic dependence on the distance between hydrophones. Finally, the horizontal-vertical coherence is studied. This coherence rapidly degrades with the increase of vertical separation between hydrophones. [Work supported by ONR.]