in Table 6 at standard oceanographic station depths assuming the above 

 pressure-depth relationship. Again, if it is desired to use in situ 

 pressures instead of depth to make the simultaneous correction, the 

 pressure vadue is presented at each depth. Some interpolation may be 

 required in these tables; however, for depths less than 3,000 meters, 

 the difference between tables for identical temperature and salinity- 

 values is usually less than 0.2 m/ sec. Table 7 is presented for ease 

 of conversion of meters/ second to feet/ second if desired. 



The assumption that the sound speed at a particular depth is a 

 weak function of the temperature and salinity structure above that 

 depth is supported by NEL^. In addition, direct comparisons were made 

 at the Hydrographic Office between the speeds presented here and 

 those computed by using the actual pressures. Sound speeds were 

 computed at standard depths, in one case using computed in situ 

 pressures and in the other using computed pressures resulting fronn 

 an assumed water column at O'C. temperature and 35%o salinity. 

 In the Gulf of Alaska and the Philippine Trench, this method indicated 

 differences of less than 0.1 m/sec or less above 4,000 meters depth. 

 In the Philippine Trench at 8,000 meters, the difference was less 

 than 0.2 m/sec. These computations indicated that the change in 

 pressure attributed to differences in temperature and salinity from 

 O'C. and 35%o have only a minor effect on the sound speed at any depth. 



B. SOUND SPEED NOMOGRAM 



The sound speed nomogram and structure form was drawn from the 

 values of sound speed given in the tables. As in the tables, the pressure- 

 depth relationship is based on a O'C. and 35%o water column. The main 

 body of the nomogram presents sound speed as a function of tempera- 

 ture and depth with salinity held at 35%o. Values are plotted to the 

 nearest tenth of a meter per second and can probably be read to the 

 nearest 0.5 m/sec. An inset is provided to correct for salinities other 

 than 35%o; for salinities less than 35%o the correction is negative, for 

 salinities greater than 35%o the correction is positive. For the normal 

 range of salinity, temperature, and depth for oceardc and nearshore 

 waters the salinity correction is accurate to the nearest 0.1 m/sec. 

 For anomalous oceanographic conditions but within the range of 

 variables covered by the tables, the error may increase to 0.3 to 

 0.4 m/sec; i.e., for fresh water at 2,000 meters depth or 32%o (or 38%o) 

 salinity at 11,000 meters. 



