Sound propagation at sea and the Nereus raft (1970s)
The quality of sonar is not only determined by the technical characteristics of its components. With the improvement of these properties, sometimes up to the limit of the physically possible, attention is increasingly focused on the properties of the propagation of sound in (sea)water. Apart from disturbances, sound propagation is also a limitation of the sonar possibilities. Around 1970, for that reason, the first cautious steps were taken in the unknown field of “medium research”. The unknown was not only in the research itself, but also in the methods for obtaining large numbers of measurement values, the computer processing thereof, and the interpretation of the results.
From the onset, it was certain that the work would concentrate on a few specific points that were considered important and had not yet been sufficiently investigated. In particular, consideration was given to the factors that distort the sonar signal in any way. Several circumstances have encouraged the smooth start of this work. For example, the Royal Netherlands Navy supported this work with an assignment and through the provision of facilities. The latter concerns the AFAR (Azores Fixed Acoustic Range) where the first extensive series of measurements were performed. Also, sailing time was made available for the new oceanographic vessel Hr.Ms. Tydeman (A906). In good cooperation, this research vessel was, equipped with several facilities, especially for researching sound propagation in seawater. The smooth start is also largely because the necessary expensive electronic equipment could be purchased. Some parts of it (transducers, hydrophones and electronic circuits) that were not for sale were manufactured in-house by TNO. It is clear that the contacts with foreign laboratories, which had already started such research several years before, were of great importance in this sound propagation research. Thanks to the activity in this new area, a start could be made with a new possibility to predict the reach of a sonar system. The use of special sound paths was also studied.
To overcome the need for two ships to perform sound propagation experiments, a special raft was made. The 800-kilo heavy raft was able, on the open sea and unmanned, to perform the task of one ship. The 4.2 m wide raft could lower three transducers (1-15 kHz) to a set depth and emit underwater sound signals. These signals were then received and processed remotely at the measurement ship. This raft, called NEREUS (Netherlands Experimental Raft-Suspended Electromagnetically controlled Underwater Sound-source), which was unique, was a great saving in needed ship time. For telemetrics, a special version of the VESTA system was developed: VESTEL (VESTa TELemetrics).
A so-called non-linear effect is present in the propagation of sound in water. This effect, relatively small and therefore previously neglected, came to the attention because of the special possibilities. In this context, the laboratory has made a special transducer with which the properties of “non-linear acoustics” are studied. This knowledge allowed for great potential in the design and manufacture of sonar transducers and hydrophones, which are also used by civilian authorities. Considerable experience has also been gained with the design and manufacture of electronic circuits, both for sonar and for measuring and auxiliary equipment. Finally, with propagation studies and experiments, a new area was entered that also requires new activities both theoretically and technically. The previously acquired experience forms an indispensable element in this.