563 HYDROGRAPHIC MANUAL PaGE 540 



563.. Errors Caused by Bottom Slope 



The transmitting and receiving units of echo-sounding instruments using sonic 

 frequencies are generally nondirectional (see 5152A), and those which utilize supersonic 

 frequencies are usually made more or less directional. When a sound impulse emanates 

 from a sonic transmitting unit, the sound is propagated in all directions in the medium 

 and the first echo that is registered on the indicator of the echo-sounding instrument 

 will have traveled to the bottom and back by the shortest possible route — in other 

 words, it will have been reflected from the bottom surface nearest to the vessel that 

 offers a normal reflecting surface. Wliere the sea bottom below the vessel is sloping, 

 the shoalest registered echo sounding, theoretically, will not have been reflected from 

 vertically below the vessel, but from some point on the slope which is some little distance 

 away horizontally. 



Echo-sounding errors caused by the slope of the sea bottom present an exceedingly 

 complex problem because of the wide range of conditions encountered in actual practice. 

 Such an error depends on (a) the slope of the bottom with respect to the horizontal; 

 (b) the depth of the water; (c) the shape of the bottom; (d) the reflecting characteristics 

 of the bottom; (e) the dimension of the cone of sound transmitted if the transmitting 

 unit is directive; (/) the frequency of sound transmitted; (g) the intensity of the trans- 

 mitted signal; and (h) the sensitivity adjustment of the instrument. 



Wliere the slope that the bottom makes with the horizontal is known, the true 

 depth at the place of sounding can be theoretically determined by applying a correction 

 to an echo depth obtained on a sonic instrument in one of a number of ways all of which, 

 however, employ the basic relation 



COS 6 



in which h is the vertical depth under the vessel, e is the echo depth registered on the 

 echo-sounding instrument, and 6 is the angular slope the bottom makes with the 

 horizontal. 



The determination of the angular slope of the bottom is by no means a simple 

 matter. It is sometimes difficult to realize when examming a line of echo soundings, 

 and particularly the profile so distinctly shown on a fathogram, that it is not a true line 

 profile, or section. It is actually a composite profile made up of echoes reflected from 

 many points of the bottom, some to one side and some ahead or astern, within the 

 effective cone through which the sound from the oscillator is radiated as the vessel 

 moves over the water surface. The diameter of the area on the sea bottom, covered 

 by this cone, depends on the angular spread of the transmitted sound and the depth of 

 the water. The width of the band effectively sounded along the track of the vessel 

 over a flat horizontal bottom is given by 



'?^=2d^'tan 6 



in which w is the diameter of the cone at the sea bottom, d is the depth of water, and d 

 is the angle between the edge of the cone and the vertical. This is on the assumption 

 that echoes are received from all points of the area within the cone, probably an infre- 

 quent occurrence as only a fraction of the reflected energy is received. 



To make accurate corrections for the slope of the bottom would, therefore, require 

 detailed knowledge of the submarine topogi-aphy. For many types of bottom, fairly 



