Test and Analysis of Compression Wave Sound Velocity Measurement in Ice by Direct Method

Abstract

The sound velocity in ice is of great significance to the acoustic propagation characteristics of ocean channels under ice cover and the acoustic characteristics of underwater targets. Freshwater ice has a uniform crystal structure under normal conditions. The freezing process of sea ice is more complicated compared to fresh water ice. The sea ice contains a lot of bubbles, brine, solid particles and other impurities. Due to the large difference in the structure and composition of ice in different states, it is very different from the common sound transmission media in the past. Therefore, in order to accurately describe the acoustic propagation characteristics of the ocean channel under the ice cover, it is necessary to measure the longitudinal sound velocity in the ice accurately. A direct method for measuring the speed of longitudinal waves in ice is studied in this paper. The propagation time of high-frequency pulses in the ice is used in this research to determine the speed of longitudinal waves. For the same piece of ice, the method of averaging multiple measurements was used to reduce the measurement error. Six tests were performed on typical ice with different structural characteristics. The six tests were the fresh water in Songhua Lake area of Jilin Province in 2019, the high mud content sea ice in the coastal area of Bohai Bay in 2020, the low mud content sea ice in Liaodong Bay area in 2021, the artificial quick-frozen sea ice and fresh water ice at -20°C, and the summer Arctic sea ice obtained in the eleventh Arctic scientific survey in 2020. The sound velocity of longitudinal waves in ice was measured at 12kHz, 25kHz, 50kHz, 75kHz, and 100kHz. The acoustic properties of ice are determined by its own state. The measurement results have certain fluctuations, because the test results were closely related to the sample test environment and test process. The measurement results of longitudinal wave sound velocity were described by the measurement mean value and standard deviation in order to scientifically express the longitudinal wave sound velocity in ice. The test results showed that the longitudinal sound velocity in high mud content sea ice was about 3360±162m/s, 3270±78m/s in low mud content sea ice, 3769±54m/s in artificial sea ice, 3772±57m/s in lake ice, 3729±23m/s in artificial fresh water ice, and 3893±86m/s in Arctic ice. Under the same measured temperature, the compression wave sound velocities in most ice media are stable at 3750 m/s. The research methods and results in this paper provide references for related research on the acoustic properties of ice media, the acoustic propagation characteristics under ice cover and target acoustic characteristics in ice area.