Ocean Acoustic Transmission Research Articles

The effect of signal bandwidth on partially saturated broadband ocean acoustic transmission scintillation in a shallow water waveguide

The acoustic field in a shallow water waveguide at small source-receiver separations <3 km are partially saturated. Here, we examine the scintillation statistics of partially saturated broadband ocean acoustic transmissions in the low to mid frequency range collected using a high-resolution towed horizontal coherent hydrophone array system. The standard deviation of intensity fluctuations and scintillation index are quantified as a function of signal bandwidth and source-receiver separation. The amplitude and intensity distributions are compared with corresponding distribution models for the partially saturated field to determine the number of independent statistical fluctuations present in the broadband data.

Model and data comparisons of ocean acoustic intensity statistics in the Philippine Sea 2010 experiment

The statistics of intensity fluctuations in dual-band, low-frequency, ocean acoustic transmissions across 500 km in the Philippine Sea 2010 experiment appear to be at odds with the standard micro-multipath theory predicting them. M-sequence encoded acoustic signals at 200 Hz and 300 Hz were transmitted for 54 h from a ship-suspended multiport source to a distributed vertical line array (DVLA) with 149 working hydrophones covering most of the 5500 m water column. Histograms of the received intensity fluctuations are approximately exponential. Intensity fluctuations at the two frequencies are strongly correlated. Scintillation indices and variances of log intensity are high (&amp;gt;1 and &amp;gt;5.57dB, respectively), suggesting the measurements are not yet in a saturated regime but still significantly scattered. However, for all this, the pulses received at the DVLA are virtually the same width as those recorded next to the source; no pulse spreading or arrival splitting is seen. So, the fading observed in the data does not appear to match this causal mechanism. To explore the connection with micro-multipath theory more fully, we compare the intensity statistics from the Philippine Sea 2010 data with those calculated both from simulated random oceans and from Flatté and Rovner's path integral theory. [Work supported by ONR.]

Intensity statistics for dual-band, 500 km, ocean acoustic transmissions in the Philippine Sea 2010 experiment

The May 2010 component of the North Pacific Acoustic Laboratory's Philippine Sea experiment included low-frequency transmissions over a 500 km path from the APL-UW ship-suspended multi-port source to a distributed vertical line array (DVLA) with 149 working hydrophones spaced over most of the water column. The multi-port source has two acoustic resonances at approximately 200 and 300 Hz. To accommodate those resonances in this experiment, two M-sequence signals at those center frequencies were transmitted simultaneously. Data recorded over a 60 h data window in the experiment show deep fades in both frequency bands, i.e., frequency-dependent variations of 10–20 dB in acoustic arrival intensities on the DVLA. The measurements are processed with Wiener filtering based on source-monitoring receptions of the transmission on a hydrophone 20 m from the source, to compensate for effects of the power amplifier and the transducer. Statistics of the received acoustic intensities are reported for both frequency bands. Correlations in the intensity between these broadly separated frequency bands allow one to explore questions of the role in arrival-splitting (micro-multipathing) in the deep fades observed in this experiment. [Work supported by ONR.]

Probability distribution for energy of saturated broadband ocean acoustic transmission: Results from Gulf of Maine 2006 experiment

The probability distribution of ocean-acoustic broadband signal energy after saturated multipath propagation is derived using coherence theory. The frequency components obtained from Fourier decomposition of a broadband signal are each assumed to be fully saturated with energy spectral density that obey the exponential distribution with 5.6 dB standard deviation and unity scintillation index. When the signal bandwidth and measurement time are larger than the correlation bandwidth and correlation time respectively of its energy spectral density components, the broadband signal energy obtained by integrating the energy spectral density across the signal bandwidth then follows the Gamma distribution with standard deviation smaller than 5.6 dB and scintillation index less than unity. The theory is verified with broadband transmissions in the Gulf of Maine shallow water waveguide in the 300–1200 Hz frequency range. The standard deviations of received broadband signal energies range from 2.7 to 4.6 dB for effective bandwidths up to 42 Hz, while the standard deviations of individual energy spectral density components are roughly 5.6 dB. The energy spectral density correlation bandwidths of the received broadband signals are found to be larger for signals with higher center frequencies and are roughly 10% of each center frequency.

Probability distribution for energy of saturated broadband ocean acoustic transmission: Results from Gulf of Maine 2006 experiment

The probability distribution of ocean-acoustic broadband signal energy after saturated multipath propagation is derived using coherence theory. The frequency components obtained from Fourier decomposition of a broadband signal are each assumed to be fully saturated with energy spectral density that obey the exponential distribution with 5.6 dB standard deviation and unity scintillation index. When the signal bandwidth and measurement time are larger than the correlation bandwidth and correlation time, respectively, of its energy spectral density components, the broadband signal energy obtained by integrating the energy spectral density across the signal bandwidth then follows the Gamma distribution with a standard deviation smaller than 5.6 dB and a scintillation index less than unity. The theory is verified with broadband transmissions in the Gulf of Maine shallow water waveguide in the 300 to 1200 Hz frequency range. The standard deviations of received broadband signal energies range from 2.7 to 4.6 dB for effective bandwidths up to 42 Hz, while the standard deviations of individual energy spectral density components are roughly 5.6 dB. The energy spectral density correlation bandwidths of the received broadband signals are found to be larger for signals with higher center frequencies and are roughly 10% of each center frequency.

Temporal coherence after multiple forward scattering through random three-dimensional inhomogeneities in an ocean waveguide

An analytical expression is derived for the temporal coherence of an acoustic field after multiple forward scattering through random three-dimensional (3D) inhomogeneities in an ocean waveguide. This expression makes it possible to predict the coherence time scale of field fluctuations in ocean-acoustic measurements from knowledge of the oceanography. It is used to explain the time scale of acoustic field fluctuations observed at megameter ranges in various deep ocean-acoustic transmission experiments. It is shown that this time scale is nonlinearly related to the much longer coherence time scale of deep ocean internal waves through a multiple forward scattering process. It is also shown that 3D scattering effects become pronounced when the acoustic Fresnel width exceeds the cross-range coherence length of the deep ocean internal waves, which lead to frequency and range-dependent power losses in the forward field that may help to explain historic long range measurements.

Modeling broadband ocean acoustic transmissions with time-varying sea surfaces

Solutions to ocean acoustic scattering problems are often formulated in the frequency domain, which implies that the surface is "frozen" in time. This may be reasonable for short duration signals but breaks down if the surface changes appreciably over the transmission time. Frequency domain solutions are also impractical for source-receiver ranges and frequency bands typical for applications such as acoustic communications (e.g. hundreds to thousands of meters, 1-50 kHz band). In addition, a driving factor in the performance of certain acoustic systems is the Doppler spread, which is often introduced from sea-surface movement. The time-varying nature of the sea surface adds complexity and often leads to a statistical description for the variations in received signals. A purely statistical description likely limits the insight that modeling generally provides. In this paper, time-domain modeling approaches to the sea-surface scattering problem are described. As a benchmark for comparison, the Helmholtz integral equation is used for solutions to static, time-harmonic rough surface problems. The integral equation approach is not practical for time-evolving rough surfaces and two alternatives are formulated. The first approach is relatively simple using ray theory. This is followed with a ray-based formulation of the Helmholtz integral equation with a time-domain Kirchhoff approximation.

Internal wave effects on ocean acoustic propagation in shallow water

Complementary models are developed to investigate the effect of internal waves on ocean acoustic transmissions in shallow water. An empirical model, based on temperature variations, and a modified Garrett–Munk spectral model of internal wave generated sound speed variations are in turn inserted into an acoustic transmission model. The Garrett–Munk model is itself an empirical model most often used in deep water; in shallow water a different parametric fit is needed to model the complexity found in the environment. The internal wave effects on the acoustic field and, in particular, the patterns of the field extrema, obtained from the numerical modelling are then compared with the results of 2–3 and 5–7 kHz broadband transmissions collected during a shallow water acoustics experiment off the coast of San Diego. The two modelling approaches are used to gain insight into the effects of internal waves on the motion of dislocations; dislocations are places in the sound field where the amplitude goes to zero and so the phase becomes undetermined. [Work supported by ONR.]

Simulations of an adaptive equalizer applied to high-speed ocean acoustic data transmission'

Computer simulations are carried out to study the feasibility of an adaptive equalizer applied to an hydroacoustic data-transmission channel. The channel is examined with a comprehensive acoustical model to acquire worst-case examples of the ocean acoustic transmission channel. The equalizer performance is investigated by simulations with a computer-generated channel response. Equalizer behavior in a mobile time-variant environment is also studied by use of a simplified, time-discrete multipath channel model. A stochastic gradient lattice equalizer is simulated for a channel which varies due to movement of the transmitter platform. The equalizer was able to track a velocity of up to 0.4 m/s for a favorable transmission geometry, using a transmitter beamwidth of 10 degrees . The results demonstrate the feasibility of coherent modulation schemes for medium-distance ocean acoustic telemetry. It was found that small beamwidths are imperative in maintaining signal coherence and in facilitating adaptive equalization. In particular, narrow-beam transducers will reduce equalizer complexity as well as the frequency spread.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Separating multipaths by global optimization of a multidimensional matched filter

A transmitted signal can arrive at a receiver via several refracted Fermat paths. If the paths are independent in the Fresnel sense, then the received signal can be modeled as the sum of amplitude scaled and time shifted copies of a predetermined replica plus white noise. We present an algorithm that uses the replica to determine the time shifts and amplitudes for each path. It is referred to as an n-dimensional matched filter algorithm by analogy with the well-known matched filter algorithm. The cross correlation between the received signal and the replica oscillates near the center frequency of the transmitted signal. This causes the n-dimensional matched filter output to have many local maxima that are not globally optimal. The time shifts and amplitude scalings for the Fermat paths are determined by maximizing the output of the n-dimensional matched filter. The algorithm is more robust and efficient than others currently available. Simulated realizations of received signals were generated with multipath and noise characteristics similar to an ocean acoustic transmission case. These realizations were then separated into arrival times and corresponding amplitudes by the algorithm. The results of these tests and the general limitations of the algorithm are discussed.