Passive acoustic monitoring (PAM) is one of the most effective and widely used methods for marine mammal detection and population density estimation. In mo

The results of experimental studies of underwater acoustic communication at frequencies of 500–1000 Hz using vertical chains of hydrophones (linear arrays) are presented. The experiments were carried out in the coastal area of Lake Baikal covered with continuous ice. It is demonstrated that the use of vertical arrays in shallow water areas with ice cover can effectively suppress natural background noise when such receiving modules cover the entire water layer over depth. This allows to reduce by an order of magnitude the bit error rate in comparison with reception by a single hydrophone.

Cross-correlation functions of ambient and shipping noise were measured by near-bottom hydrophones in 100 meter-deep water in the Straits of Florida. This paper is based on cross-correlations in the 20-100 Hz frequency band between two hydrophones 5 km apart. The propagation conditions between the hydrophones were nearly range-independent. The noise cross-correlation functions provides an approximation to the deterministic Green's functions, which describes sound propagation in opposite directions between the hydrophones. Using a background sound-speed profile, 44 eigenrays were found and relevant perturbation matrices were calculated for propagation with and against the flow. Modelled Green's function has 10 peaks, which have been matched to the peaks of the cross-correlation function.Sound speed and flow velocity profiles in water have been retrieved by minimizing the mismatch between positions of the modelled and measured peaks. Acoustic frequencies being low, errors in ray-theoretical modeling limit the accuracy of the sound speed profile reconstruction. However, due to the reciprocity principle, the current velocity profile inversion remains accurate and agrees with independent measurements. The feasibility of retrieving geoacoustic parameters of the seafloor in addition to the water column parameters will be discussed.

—In experiments on a stationary acoustic track under a solid ice cover, estimates of possible sound signal propagation time variations at distances of ∼4 km with a period of more than 100 s were obtained. The experiments were carried out on Lake Baikal in the spring period, when the vertical profile of the sound speed has two sections characteristic of freshwater areas: an upper layer with a near constant sound speed and a lower layer with linear growth of sound speed. Under these conditions, the variations of the propagation time did not exceed ~10^–4 s. Numerical modeling showed that the variations of propagation times due to the variability of the medium are minimal for the case when the sound source and receiver are located in the upper layer. It is demonstrated that in this case it is acceptable to take the sound speed in the upper quasi-homogeneous layer as the effective value of the sound speed, which determines the propagation time. The obtained results allowed us to formulate recommendations on under-ice acoustic positioning of autonomous underwater vehicles.

In experiments on a stationary acoustic track under a solid ice cover, estimates of possible sound signal propagation time variations at distances of ∼4 km with a period of more than 100 s were obtained. The experiments were carried out on Lake Baikal in the spring period, when the vertical profile of the sound speed has two sections characteristic of freshwater areas: an upper layer with a near constant sound speed and a lower layer with linear growth of sound speed. Under these conditions, the variations of the propagation time did not exceed ~10–4 s. Numerical modeling showed that the variations of propagation times due to the variability of the medium are minimal for the case when the sound source and receiver are located in the upper layer. It is demonstrated that in this case it is acceptable to take the sound speed in the upper quasi-homogeneous layer as the effective value of the sound speed, which determines the propagation time. The obtained results allowed us to formulate recommendations on under-ice acoustic positioning of autonomous underwater vehicles.

The article presents the results of a full-scale experiment aimed at assessing the temporal variability of the impulse response of a hydroacoustic channel and efficiency of coherent underwater acoustic communications using bottom-mounted transmitters and receivers at frequencies of ~10 kHz on the Black Sea shelf in autumn. Three prominent maxima of variable amplitude were observed in the impulse response structure throughout the experiment (~36 h). The range of variability of the root-mean-square decoding error was ~11 dB; the bit error ratio varied from 0 to 0.10. A strong relationship was found between the values of decoding errors and the amplitude of the maximum arrival in the structure of the reference impulse response, corresponding to a group of rays with one reflection off the surface, as well as the coefficient of variation of high-frequency fluctuations of the amplitude of this arrival in the instantaneous estimate of the impulse response. Numerical modeling was used to confirm the hypothesis that in autumn conditions, characterized by the absence of a pronounced seasonal thermocline, the main hydrophysical cause of the variability of the amplitude of the main arrival, and, as a consequence, the effectiveness of underwater acoustic communications, consisted in an insignificant (fractions of a degree) change of temperature in the upper layer of sea water.