Seven-Tenths Read Free

below the surface. The very earliest sonars were simple echo rangers. The sinking of the Titanic in 1912 had made finding some sort of defence against icebergs an urgent matter, but it was actually World War I – and specifically the development of submarine warfare – which encouraged sonar technology and in so doing gave the biggest boost to modern oceanography. * One offshoot of military research was an efficient echo sounder for cable laying.
    In 1854 Lieutenant Matthew Fontaine Maury of the US Depot of Charts and Instruments had published a profile of the Atlantic seabed, his Bathymetrical Map of the North Atlantic Basin with Contour Lines Drawn in at 1,000, 2,000, 3,000 and 4,000 Fathoms. This was based on only a couple of hundred deep soundings achieved with a weighted line, and its imposing title suggested a survey rather more thorough than it actually was. Nevertheless, it interested a good many people, among them a rich industrialist, Cyrus W. Field. Field had long dreamed of linking America with Europe by telegraphy. The seabed between Newfoundland and Ireland was, according to Maury’s survey, mostly plateau. Suddenly, the romantic notion of voices emerging from the deep took on real commercial possibility. In 1856 the Atlantic Telegraph Company was founded and two years later it laid the first transatlantic cable. Unfortunately, Maury’s ‘plateau’ turned out to include over 1,000 miles of fracture zone: the spine, in fact, of the Earth’s largest mountain range, the mid-Atlantic Ridge. This runs for 7,000 miles with peaks rising two and a half miles above the seabed. Within three months the cable had broken. Under the continued pressure of the Company’s commercial determination, though, Maury’s team were inspired to develop better sounding devices and bathygraphy. The result was a permanent telegraph link, finally laid in 1866. Yet even the best sounding techniques using plummets and lines had great limitations (see Chapter 6). At least another half-century was to elapse before the novel alternative of sonar was devised.
    In 1922 a historic moment in cartography and oceanography occurred when the USS Stewart used a Navy Sonic Depth Finder to draw a continuous profile across the bed of the Atlantic, 68 years after Maury had published his map. But whereas Maury’s had involved large amounts of guesswork, the Stewart ’s involved none, aside from certain problems of interpretation. Otherwise the soundings were fast, accurate and simple to take. The principle of sonar is straightforward. A pulse of sound is bounced off the sea floor and received by the vessel which sent it. Having made small allowances for such things as the speed of the vessel, it is a matter of the simplest arithmetic: divide the time elapsed by two and multiply the result by 4,800, which gives the depth in feet, 4,800 feet per second being the mean speed of sound in water. (The layman is astonished only that, with a principle as thoroughly understood and long turned to practical use, it should have taken so long to develop radar, that apparently magical World War II conceptual breakthrough. Since then, with a good deal of technical wizardry but zero intellectual daring, radio pulses and even laser beams have been bounced off the Moon, ranging it to a matter of a few centimetres.)
    There are, however, certain difficulties to this kind of bathymetric depth sounding. One can be intuited from the phrase ‘mean speed of sound in water’, since in practice the speed of sound in seawater varies. It travels faster, for instance, if the temperature, pressure or salinity increase. This is to leave aside for the moment all the difficulties in interpreting the returning signals, which may be severely affected by shoals of fish and other powerful sources of scattering. A further drawback is that a series of pulses focused vertically downwards beneath a ship’s keel will give only a series of depths. In order to build up a contour map of how