## 3 Sea Experiment in the East Sea
### Data Analysis  
Figure 6 shows an example of the reverberation data. The time origin indicates the time at which the sound wave begins to emanate from the source. The horizontal axis indicates time and the vertical axis indicates the sensor number for each line array. We choose two different results using the same CW with a 3 kHz center frequency and 1 s duration in a similar bathymetry (See Figure 2). The upper figures show the reverberation acquired on the first day at a source depth of 127 m and a receiver array depth of 117 m (ping 107), and the reverberation of the lower figures are measured at a source depth of 36 m and a receiver array depth of 72 m (ping 669). As shown in Figure 6, the reverberation time of the lower figure is shorter.  
This result is related to wave propagation physics in deep water. We simulate a ray tracing for the above two source depths and plot it in Figure 7. The bathymetry is set to be flat and the sound speed profile of Figure 2b is used. In Figure 7, the field of the bottom-reflected rays is more spread as the source depth is deeper. This is the reason for the longer reverberation time in the upper figures of Figure 6. In addition, we observed some striation patterns for the sensor number (array length) due to the ship speed.  
To analyze the dependency of the array depth on the reverberation in more detail, we observe the beam time signal measured in \"trajectory B\" of Figure 2. Along this trajectory, a total of 88 LFM pings with a 3 kHz center frequency, a 1 kHz bandwidth, and 1 s duration are launched while varying the source and receiver depths. Figure 8 shows the beam time series from the starboard direction for the ping numbers. The reverberation time for the ping number varies accordingly as the source and receiver depths are varied.  
Figure 6: Two examples of the measured CW reverberation intensity for the time and sensors of each line array in the triplet line array. The upper figures are the results of ping 107 (\(z_{s}\) = 127 m and \(z_{r}\) = 117 m) and the lower figures are those of ping 669 (\(z_{s}\) = 36 m and \(z_{r}\) = 72 m).  
In Figure 9, we compare two beam time series measured at a continental slope at a depth of 500 m (ping 107) and at a deep ocean floor with a depth of 1000 m (ping 324), respectively. The source and array depths are similar in both cases. Ping 107 is a CW with a 3 kHz center frequency and 1 s duration and ping 324 is LFM with the same center frequency and duration, and a 1 kHz bandwidth. For the early region of the beam time series, time signals with a large amplitude are clipped. At a glance, the dependency of reverberation time for the relative beam angle is different in two figures. Since the duration of the source pulse is the same at both pings, it is presumed that this tendency is mainly due to the varying bottom bathymetry. The continental slope region has a range and azimuth-dependent bathymetry; however, the deep ocean floor region is deeper and relatively range-independent.  
Figure 10 shows an example of spectrograms of the CW with a 3 kHz center frequency and 0.1 and 1 s durations. These are the spectrograms of the sensor data that were generated by ping 120 and ping 121. The horizontal axis indicates the frequency shift. Interestingly, a broadening of the tonal energy can clearly be observed. Such a broadening comes from the Doppler effect of the moving source and receiver and the fixed scatterers. Recently, [PERSON] and [PERSON] [24] have reported a similar phenomenon through the observation of a reverberation spectrum of TREX 13 data and modeled it in the Pekeris waveguide using a mapping technique.  
Figure 8: (a) Beam time series for the time and ping number from starboard direction. (b) Source and receiver depth corresponding to the ping number.  
Figure 7: Ray propagation according to source depth. (a) \(z_{g}\) = 127 m, (b) \(z_{r}\) = 36 m.