SECT. 4] SOUND IN THE SEA 495 



movements of the ships introduce an unwanted variable ; moreover, the in- 

 fluence of the depths of the transmitter and receiver on propagation is one of 

 the most important aspects of this study, and the lowering of stabilized sonar 

 transducers a few hundred feet below the surface introduces complications. The 

 solution is obvious if submarines are available. 



Recently directional transducers have been abandoned in many propagation 

 research programs. Explosives, pulse generators or single frequency oscillators, 

 small compared with the wave-length, are emjDloyed in their stead. Likewise 

 omni-directional receivers are preferred. Such choices eliminate the need for 

 the precise alignments discussed above, location being effected by cross bearings. 



G. Echo-Sounding and Ranging 



The mapping of the bottom of the sea, naturally an important concern of 

 oceanographers, is made comparatively easy by an adaptation of sonar ap- 

 paratus. A directional transducer is made to face vertically downward ; it is 

 caused to radiate short pulses of sound on a regular schedule so arranged that 

 between pulses it can serve as the receiver for the bottom echo. The receiver 

 feeds a graphic recorder which automatically correlates the echo of each pulse 

 with its neighbours, thus presenting a profile of water depth beneath the ob- 

 serving ship. The graphic recorder which was introduced by Marti (1922) for 

 echo-sounding shortly after World War I is generally useful in the sound- 

 ranging problems of oceanography. It has been applied widely to echo ranging, 

 and also to a variety of special problems such as seismic research (see Volume 3), 

 bottom photography (Edgerton and Cousteau, 1959) and coring (Hersey, 1959). 

 Simple sound-ranging techniques have sufficed for the latter two problems and 

 others like them for which fine control of instruments near the bottom is 

 wanted. All reduce to one problem of measuring the height of an instrument off 

 bottom. The first successful solution was by Edgerton and Cousteau (1959) for 

 positioning a camera to photograjDh the bottom in deep water. In their early 

 experiments a sound pinger was attached to the camera. When it was the right 

 distance from the bottom the sound from the ping and its bottom echo made a 

 characteristic cadence to their ears. The same information, presented on an 

 oscilloscope, provided an accurate measure of height off bottom ; obviously the 

 same signal fed to a graphic recorder provides a permanent record. Less 

 obviously, if the sound pinger is made to actuate on a precise schedule com- 

 mensurate with the graphic recorder's sweep time, then the depth can be read 

 under otherwise impossibly noisy water conditions. 



Sound ranging, which is well known from the work of the U.S. Coast and 

 Geodetic Survey in the 1930's, fell into disuse for a time (except in seismic and 

 acoustical research) following the introduction of various radio navigation 

 schemes such as Shoran and the like. Recently several scientists have turned 

 back to it as the only practical means of locating deep-submerged instruments. 

 Swallow's (1955) deep current observations depend on measuring the bearing 

 of a pinger attached to a drifting float at the observing ship, depending on 



