TABLE OF CONTENTS Bodies of water such as oceans, lakes, and rivers (in short, seas) cover more than two-thirds of the surface of our planet. The climate of the earth is largely conditioned by exchanges of heat and mass between the seas and the atmosphere. Although much of human activity, i.e., shipping, fishing, extraction of natural resources (such as water itself, sedimentary solids, and petroleum), communication, traveling, washing, rejection of waste, warfare, etc., occurs on the surface and within these fluid masses. For humans, seas are, and remain, essentially a hostile, unknown, and unexplored medium (it being understood that the latter includes sedimentary layers located below the seafloor). For this reason ways have been sought of probing the sea at a distance. Doing this by optical means proved unsuccessful, except at rather small distances, because of (fluid) turbidity or (solid) opacity. Other electromagnetic waves are more or less absorbed due to the conductivity of seawater. On the other hand, elastic waves, i.e., longitudinal (acoustic) waves in fluids, or combined longitudinal-transverse waves in solids, propagate well over long distances (i.e., with little attenuation, this being less true in the sedimentary layers) in sea environments and thus constitute excellent vectors for gathering information (including that of a mechanical nature, of great importance in many applications) concerning what lies beneath the surface of the seas.
The first way of probing the sea at a distance is achieved by detecting sounds (generated naturally within the sea or by some artificial source) by means of a...
