Fig. 3 lists the signals when the distance ranges from 1950 to 2050m. From Fig. 2 and Fig. 3, we notice that the multipath structure exists in the propagation signals. The disturbing time errors caused by the depth and distance are about several 0.1 mini-seconds in time. Comparing with the time error, several mini-seconds, which is caused by the fluctuation of the thermocline, it is evident that the multipath structure is robust to the depth and the distance of the receivers. From these physical phenomena comes our idea, a new robust method for the application in shallow water.
2.2 use thermocline to monitor the internal wave and tide
In summer, there exists strong thermocline in the Yellow Sea. Fig. 4 shows that we can use the fluctuation of thermocline to monitor the internal wave and tide. It is easy to see that there are something similar between the internal wave and tide. The difference is that the x-axis represents range and time, respectively. Because of this similarity, we study the method of monitoring tide at first. The final object of our research is the method of monitoring the internal wave in shallow water.
Fig. 4 the fluctuation of thermocline can reflect the internal wave and tide in shallow water.
The fluctuation of the thermocline reflects the internal wave and tide. At the same time, each thermocline corresponds a special multipath structure in the pulsed propagation. From these two hypnoses it is reasonable to develop the method that uses the multipath structure in the pulsed propagation to monitor the internal wave and tide in shallow water.
The program of monitoring the internal wave and tide can simply divided into two parts, the forward problem and the inverse problem. The forward problem studies the acoustic propagation. We have now developed the Beam Displacement and Ray Mode (BDRM) theory for range-independent environment and coupled BDRM for range-dependent environment. Section 3 briefly describes the BDRM theory. [2] gives more detail about BDRM theory. The inversion problem tries to extract the amplitude and arrival time of each pulse by means of Digital Signals Process, and then inverse the internal wave and tide.
