4. Loss due to propagation in the medium (TL )

4.1 Ocean environment

The Earth is aptly named: liquid water represents only a small part of the globe's volume (0.002%). However, it covers more than 70% of the Earth's surface. The hydrosphere is a very thin layer (thickness: 0.06% of the globe's radius) whose simplest model is that of salt water, stratified in temperature . There is a surface layer between 50 and 200 m, with a temperature close to that of the surface (which is highly variable). The deep layer, above 200 m, where the temperature varies very little around 3.5°, with some exceptions: estuaries, near ice, volcanoes, etc....