## 4 Analysis of the Experiment Data
### RMS Delay Spread  
RMS delay spread assesses the complexity of the channel structure and characterizes the dispersion of propagation delay. The instant and average RMS delay spreads were calculated with Equations (4) and (6), respectively. Figure 11a,b present the instant RMS delay spread temporal variations at depths shallower and greater than 500 m, respectively, and Figure 11c shows the average RMS delay spread as a function of the receiving depth. The average RMS delay spread fluctuated greatly at depths shallower than 500 m, and the instant RMS delay spreads at depths of 144, 246, and 328 m varied widely over time. Figure 12 shows representative three-dimensional amplitude variations of the CIR at depths of 144 and 246 m.  
The CIRs at both depths contained S, B, SB, BS, and SBS. At a depth of 144 m (Figure 12a), the energy of the channel was almost entirely contributed by B, SB, BS, and SBS. The energy contribution of the channel mainly came from the last four path clusters, with a relatively small delay before 116 s. The approximate instant of RMS delay spread at 144 m ranged from 0.2 to 0.3 s. After 116 s, the energy of the S was increased by fluctuations of the sea surface, which changed the grazing  
SBS with short relative delay, and the instant RMS delay spread increased. The average RMS delay spread at 144 m was 0.32 s.  
At depths shallower than 500 m, the maximum value of the average RMS delay spread was 0.51 s at 246 m. As shown in Figure 12b, S highly contributed to the energy of the CIR at most sampling times and was separated by a large relative delay from the other path clusters. Consequently, the average RMS delay spread was high at 246 m. When the energy contribution of S (with a higher relative time delay than the other four path clusters) was lowered by sea surface fluctuations, the average RMS delay spread was low; otherwise, increased. The average RMS delay spread decreased when the receiving depth increased to depths below 500 m. Figure 13 shows representative three-dimensional plots of the CIR amplitudes at depths of 805 and 905 m.  
The arrival paths of the path clusters at both depths were the same as those at depths shallower than 500 m. The path clusters were S, B, SB, BS, and SBS. However, most of the energy of the channel at 805 m was contributed by B and SB, which presented small relative delays. BS and SBS provided low-energy contributions. When the receiving depth increased to 1505 m, S (with a larger relative delay than the other path clusters) gradually gained energy and contributed more strongly to the CIR. Moreover, the relative delay difference between the last two path clusters and the two middle path clusters increased. As BS and SBS also contributed to the energy of CIR, the average RMS delay spread increased with increasing receiving depth. As the energies of B and SB increased, their contributions became more concentrated in the delay domain below 1505 m. As the energy contributions of S, BS, and SBS to the channel were small, the RMS delay spread decreased with increasing energies of B and SB. As shown in Figure 14, when the depth reached 1905 m, the energies of both clusters decreased from those at 1805 m. The decreased energy contributions of these two path clusters increased the average RMS delay spread at this depth. Below 1905 m, the average RMS delay spread fluctuated the same way as the fluctuations at depths near the sea surface. As the receiving depth increased, the expansion and absorption losses of B decreased and the energy contributions of other path clusters (except for SB) related to the sea surface decreased; accordingly, the average RMS delay spread decreased between 1905 and 2605 m.  
In the 2605-3305 m range of receiving depths, the energy of the channels was sourced from S, B, and SB. The energy contribution of S to the channel was also small at these depths. As most of the energy was provided by B and SB, the RMS delay spreads at these depths were very small (\(\sim\)0.1 s).