Page 567 badio acoustic ranging 6231 



point of reception in shoal water, or vice versa, the propagation is compHcated. Shoal 

 areas and irregular bottom relief are the causes of regions of sound shadows, thus adding 

 to the difficulties of Radio Acoustic Ranging. 



Where the surface of the water is turbulent, reflections from this boundary will be 

 diffuse and scattered, resulting in a limited range of transmission. The character of 

 the bottom material also has a pronounced effect on the reflection losses at this bound- 

 ary for, if the velocity of sound in the bottom material is nearly equal to the velocity 

 of sound in water, part of the energy of the sound wave will be refracted into the 

 bottom. 



It is apparent that the propagation of sound in a heterogeneous medium is compli- 

 cated by many factors, each of which may be different for each distance measured in 

 subaqueous sound ranging. Some of these factors vary diurnally, seasonally, and 

 regionally. Because of these complications the text of 6231 to 6233 i^ mainly restricted 

 to typical conditions likely to be encountered in the various seasons and localities where 

 subaqueous sound ranging is used by the Coast and Geodetic Survey. 



6231 . Reflection of Sound 



Reflection and refraction are equally important in the propagation of sound through 

 sea water. Except for comparatively short distances, where the transmitted sound is 

 affected by refraction only, propagation is by a combination of refraction and reflec- 

 tions. Refraction aft'ects the length and direction of the sound paths in the medium, 

 while reflection causes an abrupt change of direction at the boundaries of the medium. 



First, consider the case of shoal water with the velocity of sound decreasing from 

 surface to bottom in a fairly regular gradient. Except for very short distances from 

 the sound source, the sound arriving at the point of reception will be by way of a num- 

 ber of reflections from the bottom and surface. The path between each reflection 

 point will be concave downward because of refraction. (See 6232.) The steeper the 

 velocity gradient the greater will be the curvature due to refraction between reflections. 

 For the accuracy required in R.A.R., refraction between points of reflection cannot be 

 ignored. In other words, in computations of distance it cannot be assumed that the 

 path of sound is composed of a number of straight lines between reflection points. And, 

 the greater the refraction, the greater the number of reflections required for sound prop- 

 gation over great distances. These conditions of refraction and reflection in depths less 

 than about 50 fathoms are common in waters along the Atlantic Coast and in the 

 Gulf of Mexico. Wliere the velocity gradient is not uniform the shape of the refracted 

 path must be modified as explained in 6232. This consequently changes the number 

 of reflections a sound wave will undergo in traveling a given distance. 



In general, a sound wave will undergo fewer reflections in traveling a given distance 

 in deep water than in shoal water. This, in part, accounts for the greater distance 

 ranges possible in deep water since less sound energy will be lost because of fewer reflec- 

 tions. Refraction in deep water is mainly due to the influence of hydrostatic pressure, 

 since the change in velocity due to temperature ceases to be appreciable below about 200 

 fathoms. This condition exists in deep water off the Atlantic Coast and in the Gulf 

 of Mexico and is the usual condition found along the Pacific Coast and in Alaska 

 waters. (See fig. 130.) 



The slope of the bottom reflecting surface plays an important part in determining 

 the number of reflections between the source and point of reception. If the source of 

 sound is where the water is deep and the point of reception is in shoal water, assuming a 



