SOUNDING VELOCITY 



By sounding velocity is meant the average velocity 

 of sound over a vertical path from the sea surface to the 

 depth in question. As the sounding velocity is dependent 

 on the actual velocity at intervals along the vertical path 

 and as the actual velocity is a function of the tempera- 

 ture, salinity, and pressure, a knowledge of the vertical 

 distribution of temperature and salinity is necessary be- 

 fore the sounding velocity at any point can be computed. 

 The vertical distribution of temperature and salinity was 

 determined from actual measurements at each oceano- 

 graphic station (that is, about every other day) down to 

 depths which were usually from 2000 to 4000 meters. 

 The deep-water observations indicated that certain of 

 the oceanographic stations had vertical temperature and 

 salinity distributions sufficiently similar to be grouped 

 together. Accordingly, all measured values below 2000 

 meters for a given group of stations were plotted on a 

 single graph which was used for extrapolating the indi- 

 vidual temperature and salinity curves for stations with- 

 in that group. Scaled values of temperature and salinity 

 for the nominal depth intervals down to 2500 meters are 

 given for each oceanographic station in table 2 (see 

 Oceanography I-B). Extrapolated values for depths be- 

 low 2500 meters as determined by groups are shown in 

 table 1. Wherever the vertical distribution curves based 

 on actual measurements extend below 2500 meters, 

 values scaled from these curves have been used instead 

 of the values obtained from group extrapolation. The 

 sounding velocities computed from the conditions found 

 to exist at the oceanographic stations are given in table 

 5 (Oceanography I-B). In this table the values appearing 

 below the heavy line are based on extrapolated tempera- 

 tures or salinities. The sounding velocities given are 

 probably significant to a few tenths of a meter per sec- 

 ond as representing the conditions at the time measure- 

 ments were made, but must not be relied on as repre- 

 senting the conditions at any other time. 



There are seasonal variations in both temperature 

 and salinity in the upper layers. Of these, the variations 

 in temperature have the greater effect on sound velocity. 

 In general, the temperate regions suffer the greatest 

 annual variations in surface temperature, whereas the 

 tropics and polar regions have smaller changes. Sur- 

 face temperatures may vary as much as 10° C in the 

 temperate regions and even more in the vicinity of the 

 boundaries of pronounced streams such as the Japan 

 Current and the Gulf Stream. Little is known regarding 

 subsurface variations in temperature in the open ocean. 

 It seems reasonable, however, to expect that annual 

 variations occur to depths as great as those at which the 

 rapid temperature decrease of the thermocline changes 

 to the gradual temperature decrease at greater depths. 

 Let us assume, then, that significant annual variations 

 in temperature occur down to 500 meters and that the 

 temperature at the surface may be 10° C different from 

 the values measured on the Carnegie . Under such con- 

 ditions the values of sounding velocity given in table 5 

 (Oceanography I-B) would be in error by about 0.2 per 

 cent at a depth of 2500 meters and the error at 4000 

 meters would be about 2 meters per second. 



Vertical sections showing the sounding velocity 

 along the path of the Carnegie have been prepared from 

 the computed values given in table 5 (Oceanography I-B). 



These sections are approximately south-north and west- 

 east, but the abscissas represent great circle distances 

 between oceanographic stations. In order to show the 

 variations, the vertical distances are shown on a scale 

 which magnifies them 1000 times with respect to the 

 horizontal scale. It is believed that sounding velocities 

 shown in these sections, particularly in the Pacific, can 

 be used to reduce future soundings in depths greater 

 than 2500 meters not in the vicinity of pronounced 

 streams with an error of less than one-fifth per cent in 

 the sounding velocity. A horizontal section showing the 

 sounding velocity at a level of 4000 meters (fig. 1) is 

 given for the Pacific. An inspection of this indicates 

 that the sounding velocities represented by the vertical 

 sections can be applied to areas adjacent to the actual 

 sections as follows: sections IV, Vni, X, XI, XII, and 

 XIII apply 200 miles on each side; sections in, V, VI, 

 XV, and XVI apply 100 miles on each side; sections ^/II, 

 IX, and XIV apply 50 miles on each side. 



In the British Admiralty Hydrographic Department 

 Publication No. 282 entitled "Tables of the velocity of 

 sound in pure water and sea-water for use in echo- 

 sounding and sound-ranging" the oceans are divided 

 into twenty-three areas within which echo soundings may 

 be roughly reduced by means of appropriate tables of 

 sounding velocity. The boundaries between these areas 

 were intersected a number of times by the path of the 

 Carnegie and the accompanying vertical sections conse- 

 quently represent additional data on which to base the 

 location of these boundaries. The boundary conditions 

 were assumed to be the means of the sounding velocities 

 given in the British Admiralty tables as applicable, at 

 given depths, to the two adjacent areas. These boundary 

 conditions were then located on the vertical sections, 

 more attention being paid to the deeper layers than to 

 the layers above the minimum. The boundary locations, 

 as indicated by the Carnegie sections, are shown by 

 broken lines superimposed on a chart giving the British 

 Admiralty boundaries. This is shown in figures 2 and 3. 

 The boundary between areas 17 and 3 is shifted some- 

 what to the south. Boundary 6 to 3 could not be very 

 well located and has been omitted. Boundary 3 to 10 is 

 shifted nearly 5° to the north. In the Pacific, boundary 



18 to 20 seems to be south of the south end of Section HI 

 and east of the east end of Section X. Boundary 16 to 

 18, off the South American coast, is also shifted to the 

 south. The eastern tip of boundary 16 to 13 is shifted 

 westward through about 20° of longitude. Boundary 9 to 

 13 is apparently south and west of the Samoan Islands. 

 The southern boundary of area 15 is shifted south and 

 its northern boundary is shifted north. Boundary 13 to 

 16, north of Guam, is shifted south. Boundary 16 to 18, 

 off the Japanese coast, is practically the same, and 

 boundary 18 to 19 in this vicinity is the same. Boundary 



19 to 21, however, is shifted considerably south, being 

 south of the entire Section XVI. In view of this it seems 

 probable that the northward bulge of boundary 18 to 19 

 is not so pronounced. The eastern end of boundary 18 to 

 19 is shifted but slightly to the south. Boundary 16 to 18 

 has an irregularity introduced northeast of the Hawaiian 

 Islands. The boundary between areas 13 and 16, south- 

 east of the Hawaiian Islands, could not be very well lo- 

 cated on Section V. It seems probable that the values of 



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