658 russell. SEDIMENTATION IN NAVAL WARFARE [Ch. 35 



and drum families, also produce noise which may be troublesome, espe- 

 cially when they congregate seasonally in large schools (Loye and 

 Proudfoot, 1946; Johnson, 1948). A great many other biological 

 noises, not all identified with the producing species, have been noted 

 during operations and experiments — the sea is far from silent. Like 

 snapping shrimp, the distribution of many of these organisms is in part 

 dependent on the type of bottom. 



Mining Opekations 



With floating mines, anchored to the bottom, some information about 

 the character of the bottom and of currents in the vicinity is useful to 

 determine whether the mine will remain where it is placed. With 

 ground mines (those lying upon the bottom) , the applications are much 

 more direct. To be effective, a ground mine should remain on the sur- 

 face of the bottom, not sink in, and remain where it is placed. Whether 

 it will do so depends on the type and degree of compaction of the bot- 

 tom material, the amount of scour by waves and currents, and the 

 movement of sand or other sediment. Of secondary importance is the 

 color of the bottom and transparency of the water, as mines may be 

 spotted from planes if they contrast with their background. The foul- 

 ing of mines has also proved to be a problem, and the type and abun- 

 dance of fouling organisms is in part dependent on the character of the 

 bottom. 



Installation of Undekwater Equipment 



In installing underwater equipment for detection or other purposes, 

 a prime consideration is the stability of the installation. Here, again, 

 as with ground mines, the character of the bottom, together with cur- 

 rent and wave conditions, is of major importance. The bottom must 

 be firm enough to bear the weight of the equipment, and stable enough 

 so that the equipment will not be overturned as a result of scour, or 

 covered by shifting sand. With underwater sonar equipment, the 

 acoustic properties of the bottom are also important, and with cable- 

 connected equipment the character of the bottom along the route fol- 

 lowed by the cable may be critical. Ideal conditions occur when the 

 cable is covered by a thin layer of sediment. When the cable is ex- 

 posed, particularly on a rough bottom, catenaries are likely to be 

 formed, and movement of the cable in the earth's magnetic field sets 

 up interfering electrical noise in the equipment. A rocky or coral 

 bottom, particularly in shallow water, is especially bad because motion 

 of the cable on the exposed rocks quickly wears it through and thus 

 involves costly maintenance. The ideal type of bottom for an instal- 



