**5. Ambient noise in shallow water**

Shallow water ambient noise is highly random due to the waveguide nature of the environment and bottom reflection [12]. There are two definitions of shallow water: hypsometric and acoustic. The hypsometric definition is based on the fact that most continents have continental shelves bordered by the 200 m bathymetric contour, beyond which the bottom generally falls off rapidly into deep water. Therefore, shallow water is often taken to mean continental shelf waters shallower than 200 m. Using this definition, shallow water represents about 7.5 percent of the total ocean area.

Acoustically, shallow water conditions exist whenever the propagation is characterized by numerous encounters with both the sea surface and the seafloor. By this definition, some hypsometrically shallow water areas are acoustically deep. Alternatively, the deep ocean may be considered shallow when low-frequency and long-range propagation conditions are achieved through repeated interactions with the sea surface and the seafloor.

Shallow water regions are distinguished from deep water regions by the relatively greater role played in shallow water by the reflecting and scattering boundaries. Also differences from one shallow water region to another are primarily driven by differences in the structure and composition of the seafloor. Apart from water depth, the seafloor is perhaps the most important part of the marine environment that distinguishes shallow water propagation from deep water propagation. The most common shallow water bottom sediments are sand, silt, and mud, with compressional sound speeds greater than that of the overlying water.

In shallow water, in the absence of local shipping and biological noise, wind noise dominates the noise of distant shipping over the entire frequency range [1]. The reason for this is that the deep favorable propagation paths traveled by distant shipping noise in deep water are absent in shallow water.

Shallow water is known for its variability. Waters close to shore and in busy harbors are dynamic locations, where rapid noise changes take place. Even though shallow water has the variability as a notable feature due to variable background of biological and shipping activities, at high frequencies and even at high wind speeds, this level is significantly constant from location to location at the same wind speed. Although noise field characterization is complex in shallow waters, it has to be carried out because of its greater applications in naval operations.

Ambient noise is more in shallow water as the noise is pinned between the seafloor and the surface of the ocean. In shallow water (depth of 5–200 m), acoustic systems like sonar, echo sounder, and sub-bottom profiler suffer a huge loss due to ambient noise [13]. Wind speed in shallow water may vary from 0.5 m/s to 300 m/s; hence it causes disturbance to surface water, and these disturbances thereby propagate towards the seabed. Due to these factors the SNR value reduces. This poses a greater challenge to developing a method or algorithm to improve the SNR value. Hence, to improve the effectiveness of underwater acoustic instruments, the measurement and characterization of ambient noise is more significant.
