Si-Zhao Bi

Acoustic signals can travel thousands of kilometers in seawater, and the characteristics of long-range sound propagation are different from short range propagation. This paper is based on a long-range underwater acoustic experimental data obtained from the western Pacific Ocean, where the farthest propagation distance is nearly 2000 km. The ocean environment information and vertical line array information are carefully processed. We analyze the attenuation of long-distance acoustic propagation in seawater and multi-path arrival structure characteristics under the complete acoustic channel environment of the ocean. In terms of the attenuation law of long-distance propagation energy, with the increase of propagation distance, the effect of seawater absorption on the attenuation of sound energy becomes prominent, and the selection of absorption coefficient is very important for the accurate prediction of sound field energy. Absorption in seawater of low frequency signals is small, and the transmission loss of acoustic signal with 100 Hz center frequency increases only by about 6 dB when the propagation distance increases from 1000 km to 2000 km. In terms of multi-path arrival structure characteristics of deep-sea acoustic channel for long-distance sound propagation, the thermocline sound velocity profile in the experimental sea area has a higher sound speed, making the number of eigenrays reaching the receiving point more and the multi-path arrival structure more complex. The arrival structure formed by sea surface reflected eigenrays is at the earlier position of the overall arrival structure and has relatively strong energy. Owing to the influence of subtropical water over the northwest Pacific Ocean on the sound speed profile, the time of some eigenrays arriving at the receiving point is earlier, and the length of multi-way arrival structure on the time axis is prolonged.

Since the earth is approximately a sphere, the sound speed equivalent surface on the range-depth plane is not a parallel plane, but a concentric sphere at a long distance. Therefore, for a long-range sound propagation, the effect of the curvature of the earth cannot be ignored. In this paper, conformal mapping is used to propose a method of earth curvature correction without changing the existing sound field calculation model, and has the characteristics of good portability and simple calculation. The simulation results in a typical environment show that because of the earth curvature, the location of the convergence zone moves toward the sound source, and its movement can reach 10 km at 1000 km in distance. Before and after the earth curvature correction, the transmission loss difference can reach up to 15 dB at a particular location. Through the analysis of the simulation results in four typical sound speed profiles in Northwest Pacific Ocean, it is found that the movement of the convergence zone and the distance change are approximately linear, and the movement of the convergence zone increases by about 1km for every increase of the propagation distance by 100 km. For deep-sea channel propagation, the earth curvature will cause the arrival structure to move forward as a whole on the time axis, and the degree of the forward motion will gradually increase with the distance increasing. At 1000 km, the amplitude of the forward motion can reach 136 ms. In addition, the earth curvature will also cause the depth and time extension of the arrival structure. For the received time-domain waveform at a certain depth, there is a big difference between before and after the earth curvature correction. The modified results from different earth approximation models show that the accuracy of the calculation can be satisfied by approximating the earth as a standard sphere.