The multipath structures of pulsed propagation and fluctuations of the thermocline in shallow water
Renhe Zhang (Institute of Acoustics, Chinese Academy of Sciences, Beijing 100080, China),
e-mall: zhrh@public.bta.net.cn
Lei Ma and Ling Xiao (Institute of Acoustics, Chinese Academy of Sciences, Beijing 100080, China),
ABSTRACT
There are strong fluctuations of thermocline and internal wave in summer in the Yellow Sea. Beside the direct measurement, the multipath structures of the pulsed signals can also reflect the variation of thermocline. The data of the 96's and 97's experiments in the Yellow Sea show strong relationship between the thermocline fluctuation and multipath structures in the acoustic propagation. Chirp Signals and M-sequence Signals were used in these experiments. Pulse compression and some other techniques of DSP were also introduced. The variation with the time of the multipath structures was observed. Computed multipath structures from the thermocline fluctuation by Bind Displacement and Ray Mode (BDRM) theory were given out. Strong coincidence between the computed results and experiment exists. This shows that the multipath structures in the acoustic propagation can be used to invert the variation of thermocline and internal wave.
1. INTRODUCTION
The acoustic tomography has been greatly improved in recent years. Using individual ray arrivals resolved in time and angle by a vertical line array, the ocean acoustic tomography can achieved at a horizontal resolution of 4 to 8 km and a vertical resolution of 30 to 80m[1]. Although these are the highest spatial resolutions today, it is still necessary to develop new method for the application in shallow water. For example, the internal wave in shallow water has the spatial resolutions from 0.1 to 10km in horizon and 5 to 20m in vertical. These resolutions are too smaller for today's acoustic tomography method to reach. The purpose of this article is to give out a new method with high resolution and robust property in shallow water.
2. THE IDEA OF THE NOVEL ACOUSTIC TOMOGRAPHY METHOD
2.1 multipath structure is a robust quantity in shallow water
Let's first investigate the pulse propagation in shallow water.
