SWARM95 Experiment Research Articles

The Influence of Random Internal Waves on the Characteristics of a Horizontal Antenna in a Shallow Sea

The effect of random internal waves on the gain and pattern of a horizontal antenna array in a shallow sea has been investigated. An algorithm for calculating the correlation matrix of the point source field at the grating aperture is proposed. The antenna gain is analyzed for different spatial processing methods: the beam forming method, the optimal linear processing method, and the optimal quadratic processing method. The effect of internal waves on the directivity pattern is described by the dispersion of the angular response of the grating. Numerical simulation results are presented for a model summer-type waveguide and an exponential Brunt–Väisälä frequency profile. The empirical internal wave spectrum proposed earlier from the SWARM95 experiment is used. The dependences of the antenna array characteristics on the number of its elements, orientation with respect to the source, radiation frequency, and the acoustic characteristics of the bottom are analyzed.

Intense Internal Waves and Their Manifestation in Interference Patterns of Received Signals on Oceanic Shelf

Data of the SWARM-95 experiment, where intense internal waves on a stationary path led to coupling of acoustic source field modes, have been processed. The spectral-density localization domains, which are due to unperturbed and perturbed fields, are obtained using a double Fourier transform of interference pattern on a hologram. The interference patterns of these fields have been reconstructed by filtering these regions and applying double inverse Fourier transforms to them, which made it possible to reconstruct the unperturbed-waveguide transfer function and the time variability of the medium. The velocity of intense internal waves is estimated based on the recorded perturbed-field hologram.

Temporal sound field fluctuations in the presence of internal solitary waves in shallow water

Temporal variations of intensity fluctuations are presented from the SWARM95 experiment. It is hypothesized that specific features of these fluctuations can be explained by mode coupling due to the presence of an internal soliton moving approximately along the acoustic track. Estimates are presented in conjunction with theoretical consideration of the shallow water waveguide.

Geoacoustic model for the New Jersey Shelf by inverting airgun data

This paper describes geoacoustic inversion of airgun data acquired during the SWARM95 experiment. Hybrid optimization and Bayesian inversion techniques were applied to three airgun data sets recorded by a vertical line array. Optimization results are used to show the consistency of the estimates from all of the shots in terms of histograms and standard deviations of the inverted geoacoustic model parameters. The inversion results from the Bayesian approach are used to show the uncertainties of the estimates in terms of marginal distributions, MAP estimates, and credibility intervals. In the Bayesian inversion, full data error covariance matrices were estimated by ensemble averaging the covariance of the residuals of the measured and modeled data of inversions from many shots. The numbers of shots in the ensemble averages were determined by checking the temporal coherence of the signal. Statistical tests were used to test the validity of the assumptions in the Bayesian approach after incorporating full data error covariance matrices. With these inversion techniques, equivalent geoacoustic models with/without shear wave estimates are extracted for this experimental site. The frequency dependence of the p-wave attenuation, and the correlation between the geoacoustic parameters are obtained from the inversion results. [Work supported by ONR.]

Sound field temporal fluctuations due to mode coupling in shallow water with internal soliton

Sound field temporal fluctuations due to mode coupling, resulting from traveling internal solitons (IS) approximately along the acoustic track is addressed. The spectrum of received signal amplitude fluctuations in the range of very low frequency (∼ F = 0.001–0.01 Hz) is obtained. Maximums in this spectrum, correspond to the times which IS passes the sound field ray cycles. A relationship between F‐spectrum and acoustic frequency is illustrated by showing Frequency‐frequency (F−f) diagram. It is shown that depending on the sound field frequency (f), different mode pairs give the most pronounced contribution to the sound intensity fluctuations. At the same time invariant frequency, F, of maximal fluctuations remains approximately the same for all sound frequency (f) bands. For example in the case of the SWARM95 experiment, F is about 0.0015 Hz for sound frequency, f in the range of 30–150 Hz. Generally, F depends on waveguide parameters and velocity of IS. In this paper, the physical meaning of the invariant frequency and its relationship to the sound frequency is discussed. In addition, the experimental setup for observation of F in future shallow‐water acoustic experiments is proposed. [Work supported by CRDF, PFBR and ONR‐321OA.]

The effect of internal waves on low-frequency underwater acoustic communications

Internal waves are abundant in coastal waters and are known to have a significant impact on acoustic signal propagation. The effect includes significant signal fading to loss of signal coherence depending on the acoustic signal frequencies. This paper presents the analysis results using binary phase-shifted keying signals for underwater acoustic communications (ACOMMS) during the SWARM95 experiment. As reported in the open literature, the acoustic data were strongly affected by the presence of internal waves. For example, at 200–400 Hz, the signal characteristics changes drastically at the scale of ∼2–6 min. The most noticeable changes include the depth where the signal intensity peaks, the multipath arrival (time) pattern, and the signal amplitude and phase at a fixed depth. At these frequencies, the ACOMMS packet length for a reasonable amount of data will be long (due to the limited bandwidth) and comparable to the signal coherence time. The bit error rate of a packet varies depending on the temporal coherence within a packet (intrapacket). We report the temporal coherence measurements as a function of packet transmission time and relate that to the performance of the channel equalizer. [Work supported by the U.S. ONR.]

Evidence of three-dimensional waveguide propagation in SWARM 95 data

During a period of the SWARM95 experiment, strong nonlinear internal waves passed across two tracks that had airgun pulses propagating along them. Environmental data for this period indicate that the angles between the tracks and the internal wave-fronts, which were roughly planar, were very different—one angle being close to zero, and the other approximately 42 deg. Two-dimensional PE simulations for these waveguides show dramatically different results for depth-averaged, pulse-integrated energy variations. Specifically, the observed levels can be reproduced for the waveguide with the large incidence angle, but not for the one with the small incidence angle. For the latter case, data show significant variations in pulse shapes and in the integrated energy (≊5 dB), while simulations show very small changes in both of these characteristics. Results from several recent computational and theoretical studies suggest that the cause may be three-dimensional effects from horizontal refraction and modal interference due to the nonlinear internal waves. The adiabatic mode parabolic equation [Collins, J. Acoust. Soc. Am. 94 (1993)] is used to quantify the three-dimensional influence of the internal waves on the integrated energy variations. The results demonstrate experimental evidence of three-dimensional effects from strong nonlinear internal waves. [Work supported by ONR.]

Matched-field inversion of seabed geoacoustic properties complemented by chirp sonar surveys

Matched-field inversion of range-dependent seabed parameters is studied using broadband (525–725 Hz) acoustic data collected on the New Jersey Shelf during the SWARM95 experiment. An efficient global optimization scheme is used to minimize an objective function defined by the Bartlett processor output in both beam and mode space. With the input from chirp sonar bathymetry and sediment thickness data, a broadband acoustic field is calculated by a coupled normal mode model and range-dependent geoacoustic properties are inverted. An independent chirp sonar inversion of acoustic impedance profiles was also performed along the same track by using chirp sonar data. The inversion results indicate good agreement between two inversion methods and show their effectiveness if they are used as complementary to each other. The effect of a strongly range-dependent water column on the inversion performance and possible broadband inversion using single hydrophone is also studied. [Work supported by ONR.]

Matched-field processing in shallow water in the presence of internal waves

The performance of matched-field processing algorithms in shallow water, in the presence of soliton packets, has been investigated through analysis of data from the SWARM95 experiment. In particular, acoustic data collected with a vertical array and a moving up-sweep source have been analyzed, in the frequency range 275–350 Hz. The source was placed below the surface mixed layer and its distance from the array varied between 2.5 and 27 km. Source localization using different matched-field algorithms has been performed, in order to assess the robustness of these processors in the presence of environmental mismatch and their degradation as a function of range. The primary result of the analysis is that these processors failed to localize the source accurately, even at the shortest range (2.5 km). Although broadband processing and inclusion of corrections for array tilt improved the results, the errors in source range (of the order of 1 km at short ranges and 5 to 10 km at long ranges) and depth remain significant. Alternative processing via mode filtering and subsequent matched-mode processing yielded more accurate results, particularly in cases were mode coupling was not an issue of concern.

Effect of transverse solitons on acoustic transmission

Effects of solitary internal waves (solitons) on acoustic signals propagating transversely to the solitons were studied by transmitting 60- to 200-Hz LFM pulses along a 15-km path in shallow ocean water during periods of high and low internal wave activity. The data were collected as part of the SWARM95 experiment on the New Jersey continental shelf. Acoustic normal-mode analysis reveals amplitude fluctuations in the received acoustic modes in the presence of a series of transverse internal waves. Contemporaneous measurements without transverse solitons lack such fluctuations. The acoustic environment and internal wave field were modeled using data from thermistors, CTDs, a high-frequency acoustic flow visualization system, and ship-borne radar images. Theoretical studies [J. C. Preisig and T. F. Duda, IEEE J. Ocean Eng. 22(2), 256–269 (1997)] predict that 2D range- and depth-dependent models are insufficient to describe the mode amplitudes for acoustic scattering by highly transverse waves. In order to test this prediction, the data are compared to 2D and 3D parabolic equation solutions in data-driven simulated environments. [Work supported by ONR.]

Pulse propagation through continental-shelf internal solitary waves

A newly developed pseudospectral numerical model is used to study pulse propagation through continental-shelf internal waves. A time-domain mode coupling analysis is performed on plane-wave pulses propagating through a simple internal solitary wave (ISW) described by downward undulation of a thermocline having the shape of hyperbolic secant. Initial acoustic fields made up of eight-cycle Hanning-weighted sine waves with 224- and 400-Hz center frequencies (vertically shaded by desired acoustic mode shapes) are used. Two-dimensional and three-dimensional numerical results were compared to investigate the validity of two-dimensional techniques in the case of nonparallel acoustic-ISW alignment. Similar to the results of Preisig and Duda [IEEE J. Oceanic Eng. 22(2) (1997)], it has been observed that the horizontal refraction and azimuthal scattering seem to be minor for up to 65-deg oblique incidence angle. Results of two-dimensional propagation and mode-coupling analysis for the ISW packets measured during the SWARM95 experiment are also discussed. [Work supported by ONR.]

Analysis of internal wave and bottom effects on the acoustic field generated by a towed source during the SWARM95 experiment

Thirty-second duration 250- to 375-Hz LFM pulses were transmitted from a towed acoustic transducer to study the effects of internal waves and the sea bottom on an acoustic signal propagating along a 30-km transmission path. The data were taken on the New Jersey continental shelf as a part of the multi-institutional SWARM95 experiment. Environmental data were collected by using CTDs, thermistor strings, a high-frequency acoustic flow visualization system, and a chirp sonar system. Geoacoustic properties of the bottom were inverted from the chirp sonar data with a 1.5-m vertical resolution and 40-m penetration. A deterministic sound speed field in the water column was constructed by imposing the high-frequency visualization measurements on both CTD and thermistor string data. Environmental data indicated that two distinct geological provinces and two major internal wave packets were crossed during the 30-km towed transmission. Using a full-wave broadband PE model, vertical structures of sound field were calculated for comparison to data obtained by a 32-element vertical line receiver array. Preliminary model/data comparisons indicate the dominance of water column effects over the bottom effects. [Work supported by ONR.]

Modeling of the out-of-plane pulse propagation in the SWARM95 experiment

The acoustical data collected in the shallow water acoustics in a random medium (SWARM95) experiment on the continental shelf off New Jersey in 1995 have shown that the temporal variations of the time series of the airgun signals (center frequency 75 Hz and bandwidth 50 Hz) are both range and azimuthal dependent. A full-wave broadband PE model is used to simulate such shallow water pulse propagation at different horizontal bearings. The time-dependent shallow water internal wave fluctuations constructed with a broad spectrum of excitation and the solitonlike thermocline displacements based on the in situ observations are included in the numerical simulation as well as the bathmetry. The vertical wavefronts and the 2-h time histories of the repeated transmissions from airgun positions above and below the thermocline are calculated and displayed. The simulated data and the experimental data of the receiver arrays at different locations with an approximately 10-km cross-range distance are compared. The results suggest that the azimuthal dependence of the received signals correlate to both the internal wave fields and the bathmetric variations.

Acoustic travel time and intensity fluctuations measured in the SWARM95 experiment

During the summer of 1995, a multidisciplinary, multilaboratory experiment entitled ‘‘SWARM’’ (shallow-water acoustics in a random medium) was conducted on the continental shelf off New Jersey. The purpose of this experiment was to examine the effects of internal waves, both linear and nonlinear, upon acoustic transmissions in the 10 to 1000-Hz range. Towards this goal, numerous oceanographic sensors were deployed along a fixed across shelf track, together with a fixed acoustic transmission range. Shipborne acoustic sources also allowed out of plane (along shelf) studies. Results will be presented of recent data analyses showing the nature of the measured acoustic travel time and intensity fluctuations, and how they correlate to environmental forcing, particularly by internal waves and the shelfbreak front. [Work supported by ONR.]