Page 443 echo sounding 5137 



5137. Other Instruments 



In 1939 a Hughes Veslekari deep-water graphic-recording instrument (527), of 

 British manufacture, was installed on the ship Oceanographer and a year later a simi- 

 lar instrument was added to the sounding equipment of the ship Explorer. Also, in 

 1939 performance tests were made on a Hughes portable graphic-recording instrument 

 MS 12 D (528) . The value of such an instrument for use in hydrographic surveys made 

 from launches and small boats was at once apparent. Specifications were prepared for 

 a portable graphic-recording instrument particularly adapted, to meet the requirements 

 of the Coast and Geodetic Survey. The 808 Fathometer (523), built by the Submarine 

 Signal Company, resulted from these specifications and was first put into service in 

 1940. In actual field use during that year this instrument gave excellent results 

 although minor faults were disclosed which were subsequently corrected during the 

 winter 1940-41. Although primarily intended for use on launches as a semiportable 

 instrument, a few permanent installations have been made on larger vessels. 



514. General Limitations of Echo Sounding 



Certain general limitations are encountered in echo sounding that tend to limit 

 the useful range of the instruments and infiuence the accuracy of the results. These 

 limitations may be divided into two broad classes; those due to the properties of the 

 medium through which the sound passes, and those embodied in the echo-sounding 

 equipment. 



The transmission of the acoustic waves used in echo sounding is dependent on 

 certam properties of the medium, or on the result of certain external influences on 

 this medium, and on the reflecting surface. For echo soundings the medium is the water 

 through which the sound passes and the reflecting surface is the sea bottom. Ideally, 

 for echo sounding, they should possess the following characteristics: constant physical 

 characteristics throughout the entire depth of water, resulting in a constant velocity 

 from surface to bottom; zero attenuation of sound; and 100 percent reflection from a 

 bottom parallel to the water's surface. In practice these ideal conditions never exist; 

 nevertheless echo sounding under the existing conditions gives results satisfactory for 

 most purposes. Some of the conditions that influence echo sounding are (1) salinity of 

 the water, (2) temperature of the w^ter, (3) type of bottom, (4) aeration, (5) absorption, 

 and (6) turbulence. Some of these factors are interrelated. 



Many of the factors that limit the use and accuracy of echo-sounding instruments 

 are inherent and basic and therefore can be controlled only to a limited degree. There 

 are others, however, which are controllable to a greater extent and, unquestionably, 

 ways will be found to control them in increasing degree as the science of echo sounding 

 progresses. 



5141. Absorption 



As the sound passes through the water some of its intensity is lost and hence, under 

 certain conditions, the echo may be so weak that it cannot be detected. Sound, like 

 other forms of energy, suffers a loss in intensity due to friction, which is referred to as 

 the loss due to the viscosity of the medium. This loss of sound energy is by con- 

 version to heat and is sometimes referred to as absorption. Absorption of this nature 

 is important only when high supersonic frequencies are employed, and may be ignored 

 at sonic frequencies. 



