SECT. 4] 



SOUND IN THE SEA 



485 



scattering, no sound reaches beyond the grazing ray ; thus detection by echo 

 fails for an object beyond that ray. 



However, unless the frequency is so high that the energy is all absorbed 

 before the sound reaches the bottom, each ray is reflected with some loss, or no 

 loss, according to the angle of incidence and the nature of the sea-bed. It then 

 proceeds upwards along a path which is the mirror image, about a perpendicular 

 to the sea-bed, of its downcoming path, as in Fig. 2, where the sea-bed is 

 horizontal. Thus the grazing ray reaches the surface again horizontally, whereas 

 the ray which had been emitted in a horizontal direction becomes horizontal 

 again at the depth of the source, and the process goes on repeating itself as long 

 as geometric spread, scattering and absorption do not reduce the intensity to 

 a trivial value. If the emitted beam of sound is narrow, i.e. if the angle between 

 the extreme rays is small at any rate in the vertical, there are at depths of the 

 order of that of the source a number of regions in which the intensity is negli- 

 gible, alternating with others in whiah it is appreciable. Even if the beam is 



VELOCITY 



RANGE 



SURFACE 



ZJ. 



Fig. 3. Sound rays from directional source when velocity increases with depth. 



wide or even if the source emits fairly uniformly in all directions, as for instance 

 an explosion does, the rays reaching the bottom at small angles of incidence, 

 like 3 and 4 in Fig. 2, are reflected with some reduction and may not carry 

 sufficient energy to fill the "shadows", so that the intensity near the surface 

 may even then go through maxima and minima instead of decreasing uni- 

 formly with range. 



A second simple case can occur when the sea has been well mixed by a 

 warm, moist wind, and the weather turns fine, calm and cold. The temperature 

 near the surface is then less than it is lower down, and the rays are bent up- 

 wards (Fig. 3). After reflection they travel in a path concave upwards and are 

 reflected again and again, thus hugging the surface and never reaching the 

 bottom if the original beam is emitted in a horizontal or nearly horizontal 

 direction. There is little loss and the sound can carry many miles although an 

 object a few fathoms below the surface, even if it is at close range, may never 

 receive any sound and may, therefore, fail to be detected by the echo method. 

 Even a purely isothermal layer, formed by mixing and extending from the 

 surface to a few fathoms below it, produces a similar though less marked effect, 

 since velocity is then controlled only by pressure and, therefore, increases 

 slowly with depth according to (8). 



