## 4 Analysis of the Experiment Data
### Vertical Structure Change in Channel  
The arrival paths of different path clusters were determined for subsequent characterization of the path clusters. Based on the number of reflections from the sea bottom and the sea surface, the path clusters were divided into the direct path cluster (D), surface path cluster (S), bottom path cluster (B), surface-bottom path cluster (SB), bottom-surface path cluster (BS), and surface-bottom-surface path cluster (SBS). The deep-sea high-frequency sound field can generally be simulated using Bellhop [28], which simulates the channel vertical structure and determines the arrival paths of a multipath of experimental channels, thus providing physical support for the analysis. The measured sound speed profile and the environmental parameters (see Table 1 for a partial list) were input to Bellhop. The absorption loss, density, and sound speed of the sea bottom were set to empirical values. The simulated sound field over a horizontal range of 25 km is shown in Figure 6.  
As shown in Figure 6, an obvious boundary between the direct-arrival and shadow zones appeared. At a range of 14.8 km, it was at a depth of around 3400 m. The contributions of path clusters to the channel depend on their receiving depths. The arrival paths of path clusters of different channels were identified by comparing the experimental and simulation results. Then, the structural characteristics of the channels at different receiving depths were analyzed at 14.8 km. The CIR was simulated using the Fourier frequency domain synthesis algorithm [29]. The transmitted source level was ignored and the actual sea bottom and sea surface parameters were different from those input to the simulation. The simulation results provided only the vertical changes in the channel structure rather than the absolute channel amplitudes. The simulation was intended only to guide the identification of the path clusters of the actual channel. The experimentally and numerically obtained vertical structure changes in the channel in the distance of 14.8 km are compared in Figure 7.  
As shown in Figure 7, the vertical structures of the simulated channels and the measured channels were the same. Five path clusters with different arrival paths (S, B, SB, BS, and SBS, listed in order of arrival) appeared in the shadow zone. The relative delays between the path clusters changed with the receiving depth, but the sequence of the arrival path clusters remained. In the bottom part of the sea, where the direct-arrival zone is, except for the above path clusters, there was also D, and D arrived before S. \begin{table}
\begin{tabular}{c c} \hline \hline
**Environmental Parameters** & **Value** \\ \hline Frequency range (kHz) & 8-12 \\ Transmitter depth (m) & 44.8 \\ Range between transmitter and receiver (km) & 14.8 \\ Receiver depth (m) & 124-3605 \\ Sea depth (m) & 3790 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Parameters used in the simulation. Figure 6: Simulated sound field. Figure 8 shows the details of some clusters. As shown in Figure 8a, the delay between SB and B decreased when the depth increased. This is because the distance difference between B and SB became smaller. Figure 8b shows that, when the receiving depth was greater than 3405 m, D started to arrive, and it arrived before S. As the depth increased, the relative delay between the two path clusters increased and the expansion and absorption losses of D also increased with the depth; hence, the energy of D decreased. Table 2 shows the ratios between the amplitude of different path clusters and the maximum amplitude of all arrived arrival clusters at six different depths. When the depths were shallower than 2405 m, the amplitude ratios of all path clusters except for D, of which there was none at these depths, were greater than the threshold. According to Equation (2), all of the path clusters' energy contributes to the channel. The amplitude ratios of BS and SBS decreased with depth. When depths were greater than 2605 m, the two ratios were smaller than the threshold.