Introduction to passive underwater detection

Since the Second World War, passive sonars have played a key role in underwater warfare, as they enable targets to be detected while remaining discreet and energy-efficient. At the same time, the constraints on the use of active sonars are increasingly stringent: protection of wildlife, coordination of transmissions during joint operations with other navies. Although the budgets allocated to passive underwater detection (DSM) have decreased since the end of the Cold War, this sector of activity has retained a certain dynamism, as the threats are certainly fewer in number but increasingly silent, and therefore more difficult to detect.

This article covers the basics of passive DSM, a field that touches on a wide range of disciplines, from hydrodynamics to signal processing and, of course, acoustics. The aim of this article is not to turn the reader into a seasoned sound engineer, but rather to provide him or her with the basic elements needed to access the many voluminous books on passive DSM. The presentation of DSM basics follows the path of the signal radiated by noisemakers (ships, freighters, submarines, underwater fauna, offshore platforms, etc.). This source signal then propagates through the water, where it is received by antennas, processed and presented to the operator. We have chosen this order because it corresponds to the usual way of writing the terms in the passive sonar equation. We do not deal here with more downstream functions such as tracking, identification/classification or localization by passive telemetry or trajectography. We have therefore limited ourselves to the level of what is known as primary detection, i.e. the binary decision as to whether or not a (single) source is present in a given direction.

To give the reader an idea of the practical use of passive sonars, we begin by describing three real-life examples of passive detection at sea. The first relates to the sonar of a World War II battleship, the Prinz Eugen. The second dates from the Cold War and concerns a submarine network monitoring the passage between Greenland, Iceland and the north of the United Kingdom. The third is more recent, and concerns the detection of distress beacons in the black boxes of aircraft lost on the ocean floor. For all three situations, detection ranges are available in the open literature, which, in this highly secretive field, is relatively rare. The question is whether these ranges are realistic, and if so, under what conditions. At the end of the article, you can read how to solve these examples of the sonar equation.

We'll start by recalling what an acoustic wave is, and the advantages that this physical quantity has over others (optical, magnetic, electrical, etc.). We then introduce the passive sonar equation. Each term...