833 HYDROGRAPHIC MANUAL PaGE 782 



833. Velocity of Sound Records 



For liydrographic surveys utilizing echo sounding or R.A.R., a knowledge is 

 required of the velocity of sound in the water surveyed. The values to be used are 

 based on water temperature and salinity observations made in the area, or on special 

 tests, and these form a part of the record of almost every hydrographic survey. 



8331 . Temperature and Salinity Observations 



The observations of w^ater temperatures and salinities are recorded on Form 717, 

 Record of Temperatures, Salinities, and Theoretical Velocities (see 6341), and the tem- 

 perature and salinity graphs are plotted on Form jB-1528-5, Graph of Temperatures and 

 Salinities (see 6342). These forms, or the temperature curves derived from ihe bathy- 

 thermograph slides (473) if that instrument is used, shall be arranged in chi'onological 

 order and bound in a separate folder for transmittal to the Washington Office. The 

 contents shall be appropriately identified on the outside of the folder. A WTitten 

 report is not required. 



The temperature and salinity curves should be inked and a typed copy shouk^ be 

 made of the record of observations, unless they were recorded in black ink originally, so 

 that photostat copies can be made for interested persons. 



8332. Velocity Corrections 



Velocity corrections (561) are determined for limited areas in which, and periods 

 of time during which, the physical characteristics of the water are relatively stable, so 

 that mean values will suffice for use in correcting echo soundings. For gach area and 

 period of time, mean regional temperature and salinity curves must be drawn, from 

 wdiich the curve of velocity corrections (5614) may be derived by either the numerical 

 or the grapliic method- (see 5613 and 5615). These curves and computations should 

 be clearly identified as to applicable area and time, assembled in sets in chronological 

 order, and bound together in a folder for transmittal to the Washington Office. The 

 originals wdll suffice — neither typed copies of the computations nor smooth copies of the 

 curves are required. 



The method of computing velocity corrections has been standardized and a written 

 report is not required unless, for some reason, the hydrographer finds it necessary to 

 use a method not described in this Manual. In such case a short report explaining the 

 method and giving the reason for using it should accompany the curves and the com- 

 putations. 



The records of simultaneous comparisons (see 5522) should also be included in this 

 folder, together with any tests of other sounding apparatus applicable to the equipment 

 used in the survey. 



8333, Horizontal Velocity for R.A.R. 



An abstract of the results of the tests to determine the apparent horizontal velocity 

 of sound should be transmitted to the Washington Office with the records of an R.A.R. 

 survey. The abstract should be a tabulation of the elapsed times measured and the 

 corresponding horizontal distances, measured or computed, from which the apparent 

 velocities w^ere derived. The date and time of each test shall begiven. The method by 

 w^hich each horizontal distance was determined shall be indicated. 



