Page 445 echo sounding 5145 



excessively. Attenuation of the echo for this reason is intensified where the transmitted 

 signal is directive, in the form of a narrow beam or cone, as is usual with supersonic- 

 sounding instruments. This limitation is aggravated by the additional fact that, even 

 though the sound may be reflected from the bottom in a normal mamier, the echo may 

 not be detected if the receiving unit, which may also be directional, is at an unfavorable 

 angle for its reception at the instant of its arrival. 



5145. Echoes From Sloping Surfaces 



The depth registered by the echo-sounding instrument is not the true depth under 

 the vessel under all conditions. A hydrographer experienced in echo sounding can fre- 

 quently recognize these unfavorable conditions and sometimes a proper correction can 

 be applied. In sounding over rugged submarine relief, the sound may be reflected 

 back from a multitude of surfaces and a prolonged echo may be received, no part of which 

 is from the bottom directly under the vessel. The sound from a nondirective trans- 

 mitter is reflected first from the nearest reflecting object and this part of the echo is 

 likely to be more intense than any subsequent part, and might be interpreted erroneously 

 as the correct depth. Such echoes are known as side echoes or reflections from slopes. 

 Echo-sounding instruments employing directive signals avoid, to some extent, these 

 confusing side reflections, but still are not entirely free from them. 



As mentioned in 5144, certain discontinuities in the physical characteristics of the 

 water or the accumulation of foreign matter m the water, may cause reflections which 

 may be erroneously interpreted as those from the bottom. This is not a common 

 occurrence but reflections have been received from density interfaces and foreign 

 matter such as kelp, which have been difficult to distinguish from echoes from the 

 bottom. 



From certain types of bottom, the echo is not a single clear-cut sound, but a series 

 of reflections from various points on the bottom, at varying distances from the vessel. 

 Although one of these numerous reflections may be from vertically below the vessel, the 

 selection of the proper echo may be difficult. A graphic record has the great advantage, 

 in such a case, of making possible a careful and lengthy examination to select what 

 appears to be the correct depth. 



Additional discussion of this limitation is included in 563. 



5146. Multiple Echoes 



Multiple echoes are those received subsequent to the first echo, and are due to a 

 multiplicity of reflections back and forth between the bottom and surface. These 

 reflections often register on the dial of a visual indicator, or on the fathogram of a 

 graphic recorder, at multiplies of the true depth. Such echoes are sometimes mistaken 

 for the first echo. As many as 12 echoes of the transmitted sound have been observed 

 in extreme cases. This condition can be controlled to some extent by reducing the 

 amplification of the echo until all the echoes disappear except one, the result of a 

 single reflection. 



5147. Strays 



At times spurious, or false, indications appear on the dial or fathogram of an echo- 

 sounding instrument, which either may be mistaken for the true echo or may be of such 

 a nature as to prevent identification of the true echo. These spurious indications are 

 known as strays. They may be caused by the motion of the vessel tlirough the water, 

 by acoustic or electric noises in the ship, or by electric noises in the echo-sounding equip- 



