304 Lecture 16 
ably spaced pulses which are superimposed on the echoes as shown in Fig. 16.2. 
The counters are reset by the transmission. Assuming a velocity of sound of 
800 fathoms/sec, and with a helix rate of 1 rps, the chart width represents 400 
fathoms and the position of the echo can be readily determined to the nearest 
fathom. Part of a chart produced froma precision echo-sounder survey is shown 
in Fig. 16.3 and demonstrates the remarkable consistency with which the depth 
can be measured. 
Among other things, the use of precision echo-sounders has enabled geolo- 
gists to understand much more about the sedimentation processes in the sea. 
Laughton [2], for example, has surveyed a channel between two abyssal plains, 
formed by the passage of turbidity currents. An old-fashioned echo-sounder 
could have given the precision to show the feeder channels and the general slope 
of the plain only if its timing were checked every few minutes. 
An echo-sounder using a transducer mounted on the hull of a ship usually 
fails even in moderately rough seas due to "blanketing" of the transducer by air 
bubbles dragged under the shipas she pitches. This has been overcome by mount - 
ing the transducer in a streamlined body towed on a short warp (typically 40 ft) 
from a boom alongside the ship. This arrangement will give satisfactory results 
in all weathers. 
16.2.2. Multiple-Beam and Scanning Echo-Sounders 
To quote Professor Brackett Hersey, the conventional echo-sounder (even 
the precision echo-sounder!) is a blunt instrument. With a typical beam angle 
of 30°, it "illuminates" a large area of the sea floor and misses all the fine 
detail. To obtain narrower beam angles involves using large transducers and 
stabilizing them against the roll ofthe ship; but even if such a system is achieved, 
a large number of runs is required to survey an area in detail. Thought is being 
devoted to these problems and various systems have been tried, but none is 
really operational yet. 
Howson and Dunn [3], following suggestions by D.G. Tucker [4], have experi- 
mented with multiple-beam echo-sounders in which the beams are formed by an 
interferometer using two strip transducers, and also with high-speed electronic 
beam-scanning techniques. These enable a comparatively wide strip of sea floor 
to be surveyed on a single run. In the multiple-beam system, identification of 
the beams is a problem, since the records from all of them appear on the same 
chart; M. J. Tucker [5] has suggested that this can be overcome to some extent 
by tilting the transducer so that the pattern is unsymmetrical and port and star- 
board beams are distinguishable by their intensities. Figure 16.4 shows an ex- 
ample of a record taken with such asystem. The beams were very narrow in the 
fore-and-aft direction (1.3°) and separated by about 15° athwartships. The trans- 
ducer was tilted to the angle shown so that the central (most intense) beam 
pointed to starboard. This record shows that the bed of the small canyon crossed 
tilted downwards 2.5° from the horizontal in a direction of 110°T. 
The present difficulty with the electronic sector-scanning technique is that 
the sea-bed profile athwartships is presented on a CRO, and no means of making 
a permanent record has been devised except photography of the traces, which 
would not be satisfactory in practical use. The basic difficulty is, of course, that 
three dimensions have to be recorded. 
