Sea Technology

JUN 2017

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Page 27 of 72 June 2017 / st 27 Sound Speed Profiles A sound speed profile describes the behavior of an acoustic signal in deepwater, which varies throughout the water channel. We analyzed the influence of the sound speed profile on the normal mode wave form and sound field characteristics, as well as determined the relationship of the normal mode wave form to the sound speed profile in situ and the average sound speed profile. The sound speed profile is averaged through the water column. Integrity of Specifications Currently, inversion methods for sound speed profiles are undertaken in a few ways. Ocean acoustic tomography can be used to infer the ocean environment by acquired acoustic signals, but complexities such as internal waves, eddies, uneven seabed and other factors cause difficulty in this method. To acquire the sound field data, the sound field signal received at a certain time is compared with the copying field at a corresponding time. This method does not need much prior knowledge about the specific ocean envi- ronment, but it does involve many copying parameters, and the calculation is time consuming. Normal Mode Theory If the sound speed profile is known, the time domain signal wave form of the sound field can be obtained when a wide-band pulse signal is emitted. Normal modes can be separated effectively by warping transformation, which aims to transform each dispersion mode into a single modal signal with a characteristic frequency. Data on one order of normal mode can be acquired after the warped signal is separated into several orders of normal mode and filtered in the frequency domain. It is also possible to recover the original signal in the time domain of each order mode after inverse warping transformation. Wave Number In physics, the acoustic wave equation governs the prop- agation of acoustic waves through a material medium. It is a second order partial differential equation that describes the evolution of acoustic pressure or particle velocity as a function of position and time. A simplified form of the equa- tion describes acoustic waves in only one spatial dimen- sion, while a more general form describes waves in three dimensions. To track wave number evolution in more range-depen- dent environments, it is necessary to decrease the length of the sliding window to improve spatial resolution. However, this is done at the expense of wave number resolution. This trade-off is the uncertainty principle. Application of nonlin- ear spectral estimation techniques such as auto regression makes it possible to shorten the window length while retain- ing wave number. Depth At 500- to 800-m depth, the amplitude of the signal var- ies vigorously, which indicates that the area where we tested our model is a thermocline. The definitions of these layers are based on temperature and pressure. Because frequency and wavelength are inversely propor- tional characteristics of sound waves, low-frequency signals produce long sound wavelengths. These long-wavelength The first two order modes of the sound signals received in four sound speed profiles have the most energy.

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