5142 HYDROGRAPHIC MANUAL PaGE 444 



5142. Radiation 



The sound leaving the transmitting unit docs not travel in parallel rays, but radi- 

 ates from the source. In sounding, only that part of the sound which is reflected back 

 from the bottom directly under the vessel is of practical importance and the remainder, 

 which travels in other directions, represents a complete loss of energy. This loss might 

 be utilized to produce a stronger echo if concentrated in a direction normal to the bottom. 

 This divergence of the sound represents one of the largest losses of useful energy — 

 because of the spread, at a greater depth a smaller percentage of the total energy 

 reaches the receiving unit, 



5143. Aeration 



Sound energy is also lost through aeration, which is the suspension in water of air 

 in the form of bubbles. The movement of a vessel through the water mixes air with 

 the water along the bottom of the vessel, to an extent that it may even completely 

 envelop the bottom. Sound waves are partly reflected, or dispersed, and partly 

 absorbed by aeration; theoretically when the air in the water represents 10 percent by 

 volume of the water, total reflection of sound will take place. In passing through this 

 aerated lamina, sound suffers a loss which in extreme cases is equivalent to complete 

 attenuation of the sound signal. Moreover, the returning echo will be attenuated 

 equally as much as the outgoing sound. During rough weather aeration may be 

 sufficient to preclude the use of echo sounding. The shape and size of the hull and the 

 speed of a vessel influence the extent of aeration, it being more extensive on small and 

 shallow-draft vessels. Wlien a vessel is going astern the propeller will generally draw 

 sufficient air under the hull to prevent the detection of any echoes. 



5144. Reflections 



Echo soundmg depends on receiving the reflection of sound from the sea bottom. 

 Thus, in addition to other types of attenuation echoes are subject to the luniting factors 

 of the configuration and composition of the bottom. The relief and slope of the bottom 

 may be such that the reflected sound is scattered or dispersed so that little or none of the 

 echo signal is received at the vessel. The composition of the bottom is an, important 

 factor in the intensity of the echo. Nearly 100 percent of the incident sound waves will 

 be reflected from a flat bottom of homogeneous composition, such as sand, rock, or 

 packed mud, while a prolonged echo of weakened intensity may be expected from a 

 bottom of spongy nature, such as unconsolidated mud or silt deposits, or from a bottom 

 on which there is an abundance of marine growth. 



Any discontinuity of the medium between the water surface and the bottom, from 

 which the sound can be reflected, reduces the echo intensity. That energy which is 

 reflected before reaching the bottom as well as that reflected back to the bottom before 

 reaching the surface represents a loss in echo effectiveness. Such factors as changes in 

 water density, turbulence, aeration, and solid matter in suspension in the water may form 

 sufficient discontinuity so that internal reflections take place in the medium. These 

 are generally only minor limitations, but may become important in rivers, and in certain 

 regions where the commingling of waters of different temperatures and densities occurs, 

 and where salt wedges and colloidal suspension exist. 



If the path of the transmitted sound is not perpendicular to the bottom, part or all 

 of the echo may be lost, as the sound will strike the bottom at an angle and be reflected 

 away from the vessel. This may occur when heavy seas cause the vessel to pitch or roll 



