## 5 Experimental Results  
In order to further verify the feasibility of the proposed method, the research team carried out a sea experiment in the sea near Dalian, China. The sea area is at a depth of 30-40 m, and it has fine sediment. Figure 15 is the configuration of the synthetic echo test. The echo was simulated by two monostatic hydrophones, where the spacing between the two hydrophones was 1.4 m. By rotating the pole that mounted the two hydrophones, the azimuth of the echo was changed from 0 degree to 180 degrees. The receiver and transmitter were mounted on the experimental ship. The receiver and transmitter were 5 m deep in the water. The 20-40 kHz and 40-80 kHz LFM wideband signals were tested using pulse lengths of 1 ms and 3 ms.  
Figure 14: Procedure for discriminating a real underwater target echo from a synthetic echo. Figure 15: Configuration of the synthetic echo test. Figure 16 is the synthesized echo envelope with LFM 20-40 kHz and an azimuth of 45 degrees, and the results after differentiation. Figure 17 is the simulated echo envelope with LFM 40-80 kHz and an azimuth of 45 degrees, and the results after differentiation. From the comparison of Figures 16a and 17a, we can see that the shape and structure of the echo envelope remains almost the same when the carrier frequency changes, and the gradient of the echo envelope differs slightly. For the real target echo, from the comparison of Figures 6a and 7a, the shape and structure of the echo envelope differ notably when the carrier frequency changes, as does the gradient of the echo envelope. The echo in all azimuths was processed, and the echo envelope fluctuation features for the two different frequencies were extracted. Figure 18 is the fluctuation intensity of the real target echo and the synthetic echo of the wideband echo envelope for different frequencies, with a pulse length of 1 ms. Figure 18a is the fluctuation intensity comparison of the real target's wideband echo envelope for different frequencies; Figure 18b is the fluctuation intensity comparison of the synthetic wideband echo envelope for different frequencies. Figure 16: (**a**) Synthetic echo envelope with LFM 20–40 kHz frequency; (**b**) gradient of the synthetic echo envelope from (**a**). Figure 17: (**a**) A synthetic echo envelope with LFM 40–80 kHz frequency; (**b**) gradient of the synthetic echo envelope from (**a**). Figure 19 is the standard deviation of the fluctuation intensity of the real target echo and the synthetic echo of the wideband echo envelope for different frequencies, where the pulse length is 1 ms. Figure 19a compares the standard deviation of the fluctuation intensity of the real target's wideband echo envelope for different frequencies; Figure 19b compares the standard deviation of the fluctuation intensity of the synthetic wideband echo envelope for different frequencies. Figure 20 is the fluctuation intensity of the real target echo and the synthetic echo of the wideband echo envelope for different frequencies, where the pulse length is 3 ms. Figure 20a is the fluctuation intensity comparison of the real target wideband echo envelope for different frequencies; Figure 20b is the fluctuation intensity comparison of the synthetic wideband echo envelope for different frequencies. Figure 19: **(a)** STD comparison of the real target wideband echo envelope with different frequencies, the pulse length is 1 ms; **(b)** STD comparison of the synthetic wideband echo envelope with different frequencies, the pulse length is 1 ms.  
From Figures 18-20, we can see that the fluctuation intensity and its STD of the real target echo changes notably when the carrier frequency changes, but the fluctuation intensity and its STD of the synthetic echo remains almost the same when the carrier frequency changes. The experimental results are consistent with the theoretical analysis and simulation results. For further comparison, Figure 21 is the difference in the fluctuation intensity between the two different carrier frequency echoes.