Decoherence effects in 3D fluctuating environments: Numerical and experimental study (original) (raw)
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Our study focuses on the subject of acoustic wave propagation through spatially fluctuating ocean. The fluctuations are here linear internal waves (LIW) and we developed an experimental protocol in water tank in order to reproduce the effects of LIW on ultrasound propagation. The present paper gathers the results obtained in terms of coherence function (second-order moment) for various configurations. Typical regimes of the ΛΦ plane developed by Flatté were explored, resulting into coherence function becoming narrower as the saturation increases. We also relate the coherence function to an array gain degradation parameter, δAG, which accounts for how the system performance will be mitigated in a given configuration. δAG was calculated for various sizes of vertical linear array (VLA) and showed an important dependence on the VLA's length. Typically, in any case (scaled experiment, computer simulations and simplified theory), we note that the longer the VLA, the greater the corresponding δAG. Moreover, as the saturation induced by medium fluctuations increases, δAG increases as well. This highlights the need for corrective signal processing techniques when large VLAs are used in a fluctuating environment. Signal processing techniques from various domains (e.g. adaptive optics, radio) are also studied.
IEEE Journal of Oceanic Engineering, 1999
This paper presents results of combined consideration of sound coherence and array signal processing in longrange deep-water environments. Theoretical evaluation of the acoustic signal mutual coherence function (MCF) of space for a given sound speed profile and particular scattering mechanism is provided. The predictions of the MCF are employed as input data to investigate the coherence-induced effects on the horizontal and vertical array gains associated with linear and quadratic beamformers with emphasis on the optimal ones. A method of the radiation transport equation is developed to calculate the MCF of multimode signal under the assumption that internal waves or surface wind waves are the main source of long-range acoustic fluctuations in a deep-water channel. Basic formulations of the array weight vectors and small-signal deflection are then exploited to examine optimal linear and quadratic processors in comparison with plane-wave beamformers. For the vertical arrays, a particular attention is paid also to evaluation of the ambient modal noise factor. The numerical simulations are carried out for range-independent environments from the NorthWest Pacific for the sound frequency of 250 Hz and distances up to 1000 km. It was shown distinctly that both signal coherence degradation and modal noise affect large-array gain, and these effects are dependent substantially on the processing technique used. Rough surface sound scattering was established to cause the most significant effects.
Array measurements of long-range, low-frequency signal propagation in the seabed
The Journal of the Acoustical Society of America, 1979
Conclusion. Louis C. Maples (Naval Underwater Systems Center, New London, CT 06320) "A Different Point of View on the Role of Attenuation as a Component of Total Propagation Loss in Underwater Acoustics," a paper given at the 96th Meeting of the Acoustical Society, and its sequel, Part II, presented at the 97th Meeting, presented a mathematical model for propagation on the axis of a sound channel which proposed simple random-phase addition of multipath signals as the mechanism determining spreading loss, rather than the usually accepted cylindrical spreading. Interpretation of experimental attenuation data using the new expression results in lower coefficient values that those of Thorp, Browning et al., based on the same data. Earlier studies involved eye-fitting to published plots, and leastsquares fits to calculated propagation data produced by Weinberg's generic model. The present paper compares "old and new" attenuation model analyses of actual raw data, as planned for Part II, but made possible only in July 1979, with obtaining of the original records. [Work supported by the Naval Underwater Systems Center.]
Sonar Processing Performances in Random Environments
promitheas.iacm.forth.gr
We present a numerical model for stochastic propagation of acoustic waves in fluctuating marine environment. This model predicts the de-coherence effects of the propagated acoustic signals (space decorrelation, and time-distortions of transmitted waveform) which degrade the performances of sonar processing and induce further limitations of the detection capabilities of sonar systems. We investigate the effects of random sound speed fluctuations on the performance of linear horizontal arrays from numerical runs of the stochastic propagation model. As an illustration, we present quantified results for the performance degradation of conventional beamforming for spatial processing, associated to a towed linear array, in presence of realistic random fluctuations of the medium: waves in a deep ocean environment. We observe trends in the behavior of the degradation of the array gain which are summarized within an approximate closed analytical formula. This formula will be helpful for quickly correcting the Array Gain term in the sonar equation and for evaluating in a more realistic way the detection range in a fluctuating oceanic environment.
Mid-frequency measurements of array signal and noise characteristics
IEEE Journal of Oceanic Engineering, 1997
Experiments using seismic-type arrays with lengths in terms of wavelengths λ, from 20λ at 50 Hz to 143λ at 340 Hz have been conducted in the Mediterranean Sea and Northwest Atlantic Basin to ranges of 750 km. Signal-gain cumulative distribution functions (CDFs) were experimentally determined as a function of acoustic aperture and integration time. We found that for an array
IEEE Journal of Oceanic Engineering, 2004
The temporal variability of the spatial coherence of an acoustic signal received on a bottomed horizontal array has been calculated for 276-Hz narrow-band signals. A conventional plane wave beamformer was applied to the received signals. The temporal variability of the array's omnipower, beam power, and array gain are related to variability in the sound-speed field. The spectral characteristics of array omnipower are nonstationary and changed as the spectral characteristics of the temperature field varied. The array omnipower and beam-power variability tracked each other in time and varied by as much as 15 dB over time intervals as short as 7 min. Array gain varied up to 5 dB and usually tracked the omnipower variability. A contiguous 24-h section of data is discussed in detail. This data section is from a time period during which the high-frequency fluid dynamic perturbation of the sound-speed field was of smaller amplitude than other sections of the 16-d data set. Consequently, this section of data sets an upper bound for the realizable array gain. The temporal variability of array gain and spatial coherence at times appears to be correlated with environmental perturbation of the sound-speed field, but are also correlated with changes in the signal-to-noise ratio. The data was acquired during the Office of Naval Research's South China Sea Asian Seas International Acoustics Experiment. The 465-m 32-channel horizontal array was placed on the bottom in 120 m of water at the South China Sea shelf break. The acoustic source was moored in 114 m of water 19 km from the receiving array.
IEEE Journal of Oceanic Engineering, 2001
An experiment was carried out over a nine day period from August 18, 1996, to August 27, 1996, to examine acoustic wave propagation in random media at frequencies applicable to synthetic aperture sonar (SAS). The objective was to test experimentally the hypothesized imaging effects of variations in the sound speed along two different acoustic paths as put forth by Henyey et al. The focus of this paper is on describing the experiment and carrying out an initial analysis of the data in the context of the effect of ocean internal waves on imaging resolution. The oceanographic instrumentation and environmental measurements that form the primary reference for understanding the acoustic results were fielded by Oregon State University. The oceanography is summarized to the extent needed to discuss important aspects relative to the acoustics experiment. In the acoustics experiment transmissions at 6, 20, 75, and 129 kHz between sources and receiver arrays were carried out. Source to receiver separation was about 815 m. All sources and receivers were mounted on bottom-deployed towers and were at least 9 m off the seafloor. The analysis concentrates on the 75-kHz data acquired during one day of the experiment. The time span examined is sufficient to examine a diurnal tidal cycle of the oceanographic conditions. The results indicate the IW phase perturbations would have a significant effect on imaging for even the most benign conditions of the experiment if no autofocusing scheme is used. Also, though autofocusing should be useful in recovering the focus for these conditions, there are conditions (e.g., for the path that has a turning point at the thermocline and during times when solibores are present) where more sophisticated compensation schemes would be needed.