## 6 Thematic Taxonomy of UWSNs
### Underwater Acoustic Communications
#### 6.2.1 Sound Velocity  
Acoustic waveforms in water are dependent on the sound velocity and the environment. According to [52], several key factors affect the sound velocity in water: temperature, salinity, and hydrostatic pressure through empirical experimentation. Figure 7 shows how these factors influence the velocity of sound in the ocean; key points of these factors are discussed below. 1. Temperature. The sound velocity and water temperature are closely related to each other: the velocity will be higher with an increase in water temperature. When approaching the surface of the water, the temperature increases as well as the sound velocity. 2. Salinity. The second factor that affects the velocity of sound in water is the salinity ratio. However, the salinity factor has a smaller effect on the velocity of sound compared to the temperature. Different concentrations of dissolved salts in pure water affect sound velocity. The level of ocean salinity is typically 35 p.s.u; this value varies depending on the characteristics of the water, and the effect of rock, soil, and atmosphere. Another factor regarding salinity levels is that they vary according to the depth of water. 3. Hydrostatic Pressure. The hydrostatic pressure factor has also effect on the velocity of sound in the water. Hydrostatic pressure increases the velocity of sound with depth [53]. The increase in depth is directly proportional to the increase in hydrostatic pressure. Figure 6: Relationship of SDN, IIoT, IoUT and Industry 4.0. Sound Velocity Profile  
The ocean comprises two main regions based on ocean depth. Each level of depth produces different sound velocity variations, known as sound velocity profiles. 1. Ocean Depth below 200 m. The surface layer (0-100 m) is subject to change of environment, wind, and temperature. The wind circulation can mix up this layer and convert wind power to isothermal (mixed layer). The sound velocity is reduced dramatically if the wind speed is higher than 7 m/s due to the domination of bubbles found at a distance greater than 10 m below the surface of the water. In the seasonal thermocline region (100-200 m), the temperature changes seasonally. The temperature decreases according to the depth of the water. Consequently, in the winter season, the thermocline is weak since the surface of the water is continuously cool. 2. Ocean Depth above 200 m. At depths of 200-100 m, there is a region with minimal sound speed known as the main thermocline. At this depth, the water temperature begins to increase. In the deepest zone, known as the deep isothermal layer, the temperature characteristics depend on the density of water and water salinity. Nevertheless, the impact of hydrostatic pressure on the sound velocity is significantly higher compared to temperature and salinity. The sound rays interchange through the medium at a fixed rate, even with the variable speed of sound. The categorization of ray bending is either qualitative or quantitative. In qualitative ray bending, the sound speed will increase according to the depth in parallel with the increasing number of bubble populations. The population of bubbles decreases with increasing paths at the sea surface. When the acoustic energy concentrates within a layer at the sea surface, a reflection will occur. It does not spread in all directions, because it causes the speed of sound to be minimal where the course of the wavefronts propels toward the water depth. This velocity profile is known as the SOFAR (Sound Fixing and Ranging) channel. Conversely, in quantitative ray bending, a sound ray moves horizontally through the points, and the sound velocity increases linearly, in parallel with the depth. Long Range Propagation  
The reduction of sound in signal-to-noise amplitude due to long-range propagation, especially in the SOFAR channel, is influenced by geometry, attenuation, and thermometry. According to  
Figure 7: Sound Velocity Factors.