SONIC DEPTH WORK 



During the summer of 1927 while the Carnegie was 

 being overhauled prior to the beginning of her seventh 

 cruise, sonic depth-finding equipment loaned by the 

 United States Navy Department was installed. This 

 equipment was of a type well suited for deep-sea sound- 

 ing and consequently fitted the needs of the Carnegie . A 

 Fessenden type of oscillator having a 30-inch steel dia- 

 phragm was located in the keel below the after part of 

 the engine room. This oscillator, which was the source 

 of sound of 540-cycle frequency, was actuated electro- 

 magnetically, being supplied with alternating current of 

 540-cycle frequency at 180 volts and direct current at 

 115 volts. A 5-kilowatt remote-controlled motor gener- 

 ator set for the alternating-current supply was located 

 in the toolroom just off the engine room on the port side; 

 the control panel was located in the engine room near 

 the forward end. Six Navy hydrophones, any three of 

 which could be used at one time, were located along the 

 port garboard strake below the chartroom. 



The depth finder proper was located in the control 

 room (a deckhouse on the port side of the forward end of 

 the quarter-deck). The depth finder acted as the clock 

 for measuring the time required for the sound to travel 

 from the surface to the bottom and return. It consisted 

 of a tuning fork-controlled rotary converter which drove 

 a large bakelite disc at constant speed. Riding on, and 

 driven by this bakelite disc, was a smaller accurately 

 machined brass disc mounted on a splined shaft carry- 

 ing a series of commutators which made and broke the 

 electrical circuit of a relay; this, in turn, operated the 

 oscillator, thus sending out signals at periodic intervals. 

 By means of a calibrated screw the radius at which the 

 brass disc rode on the balcelite disc, and consequently 

 the time interval between signals, was continuously var- 

 iable between limits set by the dimensions of the bake- 

 lite disc. The outgoing signals and the returned echoes 

 were audible in the telephone receivers, and in taking a 

 sounding the position of the brass disc on the bakelite 

 disc was adjusted until the outgoing signals occurred 

 simultaneously with the returned echoes of the immedi- 

 ately preceding signals. Under this condition, the time 

 required for a signal to travel to bottom and return was 

 the same as the time interval between two successive 

 signals. A dial operated by the calibrated screw indi- 

 cated, in effect, the latter time interval. 



A table, based on an arbitrarily selected sound 

 velocity of 1450 meters per second, was made for con- 

 verting dial readings into approximate depths, due 

 consideration being given the horizontal distance be- 

 tween oscillator and hydrophones. As the velocity of 

 sound in sea water is a variable depending chiefly on 

 temperature, salinity, and pressure, the approximate 

 depth was then multiplied by the suitable correction 

 factor selected from a table applicable to the area in 

 which the sounding was taken. To the value thus ob- 

 tained, a further correction for draft was applied. 



As originally installed, the outgoing signal was 

 brought to the receivers from the secondary of an air- 

 core transformer, the primary of which was in the al- 

 ternating-current circuit of the oscillator. Thus there 

 would be heard, first the electrically conducted impulse 

 of the outgoing signal, then the outgoing signal as a di- 

 rect sound wave picked up by the hydrophones, andfinally 



the reflected sound wave as picked up by the hydrophones. 

 As the first two arrived but a short time apart, it re- 

 sulted in a blurred sound of considerable intensity, which 

 had to be matched in time of arrival with a fainter sound 

 of shorter duration. Later on the arrangement was 

 changed and the air-core transformer eliminated, so 

 that the outgoing signal was registered only as the direct 

 sound wave picked up by the hydrophones. This resulted 

 in a sharper outgoing signal in the receivers, and a con- 

 sequently greater ease and accuracy in getting a balance. 

 After this change in arrangement, a further constant cor- 

 rection of half the distance between oscillator and hydro- 

 phones was added. 



The correction factors applicable to a certain local- 

 ity were grouped into a table of ratios of the average 

 velocity of sound down to the applicable depth, to the 

 basic velocity of 1450 meters per second. These were 

 based on 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-sound- 

 ing and sound -ranging." The variation in pressure at a 

 given depth, due to the variation in gravity with latitude, 

 was considered to be small enough to be disregarded. 

 The range in temperature normally encountered is from 

 -2° to +30° C, whereas the salinity range is within 31.00 

 to 38.00 parts per thousand. Correction factors were 

 computed for salinities of 31.00 and 38.00 parts per 

 thousand and all even degrees of temperature from -2° 

 to +30° C, using tables 2 and 3 of the British Admiralty 

 publication cited above. From these factors a set of 

 straight-line curves was drawn, one curve for each de- 

 gree. Although the isothermal variation of velocity with 

 salinity is not linear, it was sufficiently so for this pur- 

 pose. 



Curves based on the data in table 1 give the correc- 

 tion factor to the basic velocity at any salinity and tem- 

 perature at atmospheric pressure. The amounts to be 

 added to the values derived from table 1 because of 

 pressure effect, as taken from table 4 of the British Ad- 

 miralty publication, are shown in table 2. 



A set of correction factors was prepared every two 

 days from actually measured temperatures and salinities 

 in the following manner. Vertical distribution curves of 

 temperature and salinity were plotted, and from these 

 curves were scaled the values at the nominal depths (in 

 meters) of 0, 25, 50, 75, 100, 200, 300, 400, 500, 1000, 

 1500, 2000, etc. The temperature and salinity measure- 

 ments usually extended to depths of from 2000 to 4000 

 meters. The vertical distribution curves were extrapo- 

 lated to depths ordinarily about GOO meters greater than 

 the deepest soundings obtained in the area in question. 

 The extrapolations were made with the help of composite 

 curves based on measurements made in areas where the 

 deep water was homogeneous. These group extrapola- 

 tions are discussed in the section on sounding velocity. 

 From the velocity correction curves, values of correc- 

 tions were obtained for th" conditions of temperature 

 and salinity prevailing at the nominal depths. To these 

 were added the corrections, due to pressure, corre- 

 sponding to the appropriate depth and temperature, and 

 taken from table 2. The sum of these two corrections 

 was entered in a column headed "velocity corrections" 

 opposite the proper depth. The procedure for getting the 



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