SECT. 4] SOUND IN THE SEA 487 



Sofar channel from Vema, research ship of the Lamont Geological Observatory 

 of Columbia University, and Diamantina of the Australian Navy at 33° 13'S, 

 113° 43'E, approximately the antipodes of Bermuda, were detected appreciably 

 above the background noise of geophones mounted on the bottom of the slope 

 of Bermuda. (Mr. Carl Hartdegen of the Columbia University Geophysical 

 Field Station at Bermuda directed the receiving observations ; shots were fired 

 by Dr. John Nafe.) 



H. Propagation in the Sea-Bed 



Sound propagation in the sea-bed is the concern of exploration seismology as 

 well as sound. It will be treated more extensively in Volume 3. 



I. Scattering of Sound in the Sea 



The term scattering is applied to reflection of waves by an object. If a beam 

 of plane waves of sound in water impinges on a sphere of material whose 

 impedance differs from that of water, energy is reflected or scattered uniformly 

 in all directions. The term is usually taken to include not only the energy 

 deflected from the direction of the beam but also the forward scatter or signal, 

 and the back scatter or echo. The signal which would have been carried by that 

 part of the beam intercepted by the object is thus reduced to the forward 

 scatter, and, if there are numerous obstacles, the signal can be considerably 

 attenuated. 



The body of the sea does not normally contain scatterers capable of seriously 

 reducing the intensity of beams of sound. Bubbles, fish, vegetation, dis- 

 continuities in temperature, turbulence, all have some effect, but except in 

 special circumstances it is true to say that scatter by the body of the sea is 

 negligible compared with that by the surface and the bottom. 



Although sea waves are often very large compared with the wavelength of 

 sound used for submarine signalling, back scatter from the surface is not 

 considerable because the sound beams are nearly horizontal at the surface. 

 Moreover, as they are in general bent downwards by temperature gradients, 

 scatter by the surface occurs only at close range and possibly at a limited number 

 of other regions. The result is that in the deep ocean, even in rough weather, 

 long range sonar apparatus is remarkably free from disturbance by back 

 scatter, and the echo from a distant object is not masked by "surface re- 

 verberation". 



On the other hand the bottom of the sea is a powerful scatterer, especially 

 when the sound beam reaches it at an angle and when the scale of the irregu- 

 larities is large. Scattering by the bottom does not, however, normally impair 

 reception of sound from a distant source because the signals, especially when 

 they are short, reach the detector before the scattered energy. But things are 

 very different in echo detection by sonar apparatus, which may receive re- 

 verberation from the bottom and the surface before, during and after reception 

 of the echo. Reverberation comes from an area of the bottom or surface pro- 

 portional to the range R, so the ratio of echo intensity to reverberation intensity 



