AEROGRAPHER'S MATE 3 & 2 



distance increases from the source. Long ranges 

 are possible when this type of structure prevails, 



2. Rays curved downward. A negative tem- 

 perature gradient (temperature decreasing with 

 increasing depth) produces a negative velocity 

 structure, which is a common occurrence near 

 the sea surface. The sound rays transmitted 

 from a source near the surface are bent sharply 

 downward resulting in extremely short ranges 

 when there is a rapid decrease of temperature. 

 Beyond this range, a shadow zone occurs at 

 the surface in which, theoretically, sound 

 intensity is negligible. The sharpness of the 

 temperature gradient determines the spread of 

 the sound beam. For example, if the decrease 

 in temperature to about 30 feet totals a degree 

 or more, most of the sound beam would miss 

 a shallow target located at a range of 1,000 

 yards. 



3. Rays curved upward. A positive tempera- 

 ture gradient causes a sound velocity increase 

 with increasing depth, and sound rays are 

 refracted upward. Longer ranges are attained 

 with this temperature structure than with a 

 negative gradient because the rays are refracted 

 upward from greater depths. Unless the sea 

 surface is very rough, most of the rays are 

 repeatedly reflected at the surface to longer 

 ranges. 



4. Split-beam pattern. A combination of 

 isothermal water overlying water with a negative 

 gradient produces a layer effect. The sound 

 rays from a near-surface source split at the 

 point where the temperature begins to decrease 

 (layer depth). Part of the ray is refracted back 

 toward the surface, whereas the other part is 

 refracted strongly downward. At the point where 

 the rays split, a shadow zone is formed into 

 which very little sound energy penetrates and 

 in which a target may escape detection. 



5. Sound channel. A negative gradient in 

 the surface layer overlying an isothermal or 

 positive gradient produces a sound channel. 

 This channel is rare and transitory in the 

 surface layer of the open sea since the thermal 

 conditions causing it are unstable. However, 

 it is quite common at depths where it is termed 

 the "deep, or permanent," sound channel. 



What usually occurs in practice is a combi- 

 nation of the various situations (e.g., positive 

 over negative, negative over isovelocity gradi- 

 ents, etc.). However, by keeping in mind that 



at any point the ray will be bending toward 

 minimum velocity gradients, even the more 

 complex situations are simplified. 



Theoretically, the essential difference between 

 shallow and deep water sound transmission is 

 the interference effects produced by multiple 

 reflected transmission paths. Because of these 

 effects, underwater sound problems are divided 

 into two principal categories: 



1. Shallow water (water less than 100 fathoms; 

 i.e., water over the Continental Shelf). 



2. Deep water (water with a depth greater 

 than 1,000 fathoms). 



The area between 100 fathoms and 1,000 

 fathoms (i.e., the continental slope) is only 

 a small portion of the oceanic area. 



Shallow Water Transmission 



The interference effects in shallow water 

 transmission are dependent on several environ- 

 mental factors, the most important being 

 (1) water depth, (2) physical characteristics 

 of the sea surface and bottom, and (3) sound 

 velocity structure. 



If we consider the present operational depths 

 of conventional submarines and the sound 

 frequencies in use, then shallow water can be 

 defined as water over the Continental Shelf 

 where depths seldom exceed 100 fathoms. Water 

 depth determines to some extent the types of 

 transmission paths that occur and the range and 

 angle of incidence at which sound rays strike 

 the bottom. 



Shallow water bottom composition and its 

 associated degree of roughness control, to a 

 large extent, the reflective capabilities of the 

 bottom, and thus the attenuation of sound. Also, 

 these factors govern the degree of reverberation 

 that contributes to the masking of target echoes. 

 In contrast to deep-sea sediments, which are 

 mostly mud or ooze, the sediments on the 

 Continental Shelf are of diverse types which 

 vary considerably in their composition. Areas 

 of mud, mud-sand, sand, gravel, rock, or coral 

 are not uncommon over ohelf regions. 



Because of the bottom complexity of sedi- 

 ment distribution, let us consider three aspects 



382 



