Ennis, C. C.

English

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USE OF REGIONAL CONSTANT CORRECTION FACTORS FOR REDUCTION OF ECHO SOUNDINGSbyC .C . E N N ISD e p a r t m e n t  o f  T e r r e s t r i a l  M a g n e t i s m , C a r n e g i e  I n s t it u t io n  o f  W a s h in g t o n ,W a s h in g t o n , D. C., U. S. A.(Reproduced from the Report of Proceedings of the Fifth Pan-Pacific Science Congress,Vancouver, June 1933)A t the third meeting of the Pan-Pacific Science Congress held in Tokyo, October 30 to November 11 ,  1926, a paper, entitled “ Correct values of the velocity of sound for echo soundings in the Pacific Ocean” , was presented by Commander N . H . H e c k  and Lieutenant Jerry H . S e r v i c e , both of the United States Coast and Geodetic Survey (1).In that very interesting paper there was discussed a method for simplifying the calculation of echo soundings by using constant correction factors applicable to certain areas in the Pacific Ocean —  in this case for areas along or between designated paral­lels of latitude, the resulting depths having errors not exceeding one per cent.The table of H e c k  and S e r v i c e , applicable only to the Pacific Ocean, is reproduced here in table 1, the tabular values being in percentages and based on an assumed velocity of sound of 4,800 feet (1,463 metres) per second.Table I. — Table for reducing acoustic soundings to correct valuesLatitude6001,0971,2002,1941,8003.2922,4004.389Fathom3,000Metres5.486s3,6006,5844,2007,6814,8008,7785.4009.8756o° N ........................................ 0.0 0.5 1.1 1.9 2.5 3-2 3.9 4-5 5-i55.................................................... 0-3 0.8 1.3 2.0 2.6 3-2 4.0 4.6 5-25°  - 4° ............................................ 0.6 1.0 i -5 2.0 2.6 3.3 4.0 4.6 5-235.................................................... 1-3 1.4 1-7 2.2 2.8 3-4 4.1 4.8 5-330° N. - 40° S ................................ 2.0 1.7 2.0 2.5 3-0 3.6 4.2 4-9 5-445 S ................................................ i -3 1.4 i -7 2.2 2.8 3-4 4.1 4.8 5-350.................................................... 0.6 1.0 i -5 2.0 2.6 3-3 4.0 4.6 5-255.................................................... 0.3 o-5 1.1 1*7 2.3 3-0 3-7 4-3 4-90.0 0.1 0.7 i -5 2.1 2.7 3-5 4.1 4-7Since the presentation and publication of that paper, the Carnegie has traversed a considerable portion of the Pacific within the latitudes 50° N. to 40° S. in her memorable cruise of 1928-9, which was fatefully terminated at Apia, Samoa, in November, 1929.Inasmuch as about 1,200 echo soundings were obtained during this cruise, it was deemed worth while to examine some of the results of that cruise and to ascertain what contribution they might make in answer to the question whether or not it might be feasible and practicable to devise a method for simplifying sounding reductions by the use of factors constant for certain areas, by which a great amount of computational work might be eliminated from the usual methods, and especially in view of the fact that so many soundings are made at stations at which there are no accompanying observations of temperature and salinity of the sea water.As noted by H e c k  and S e r v i c e , "the problem was to find the extent of the areas over which the same velocity may be used for any given depth” . This is indeed the crux of the situation, assuming adequate knowledge of temperature and salinity condi­tions, as well as of sound velocity characteristics.(1) Proc. Third Pan-Pacific Sei. Congr. Tokyo, 1926, Vol. 1, pp. 202-4, 1928-Fig. 1Regions showing nearly uniform sounding velocity correction factors, Pacific Ocean, Carnegie results, 1928-29The arbitrary selection of areas between or along certain parallels of latitude seems open to some criticism for the reason th a t the correction factors vary  w ith longitude as with latitude. Taking as an example the area between the parallels of latitude 50° and 40° N., the regions adjacent to  the American and Asiatic coasts within the said limits have on the average almost identical characteristics of the velocity of sound, both regions apparently differing in this respect from the more central part of the area in question. Similar conditions obtain within other regions of the Pacific.In order, if possible, to  override these objections, as well as to attain  a  higher degree of accuracy in the results, a compilation of all correction factors was made for selected levels a t each Carnegie oceanographic station, which factors were determined in connection with the Carnegie reductions.This compilation was then classified according to  the values of the factors with a view to establishing regions having similar characteristics within themselves. W hile such a selection must necessarily be of a more or less arbitrary nature, the variations from region to region are sufficiently marked to warrant the regional selections.In this manner 14 regions were classified and designated I to X IV , as shown in Fig. 1.The correction factors used in the Carnegie reductions were based on the velocity of sound of 1,450 metres per second. This value was arbitrarily selected (1) sim ply for the reason th at “ it is an approximate round number, is somewhere near the true velo­city, and in nearly all cases gave additive values” .The factors represent the ratios, average velocity of sound in metres per second down to the applicable depth divided b y  the basic velocity, 1,450 metres per second. These average velocities were determined for each Carnegie oceanographic station from(1) Soule, F .M .  and Ennis, C .C . Trans. Am . Geophys. Union, n th  Annual Meeting, pp. 264-74, 1930.the British Adm iralty velocity tables (i), being corrected for the observed temperatures and salinities and foi pressure b y  means of tables 2 and 3 of the publication referred to. In this paper the correction factors are expressed in percentages for the sake of sim­plicity, thus 2.35 signifies factor 1.0235.As previously stated, the Carnegie factors were classified and arranged in groups of stations where the values were in close agreement at corresponding levels. Group means were formed and the resulting values were considered as applicable to the regions embracing the groups of stations, 14 in number, and designated “ Region I ” , etc., as shown on the map (Fig. 1).Table 2 gives the mean values of the Carnegie correction factors for the respective regions at every 500-metre interval of depth from 1,000 metres down to the greatest depths. The table also gives the oceanographic stations included within each region. Depths less than 1,000 metres were not considered because of the more or less unstable conditions as compared with those at greater depths. Furthermore, about 95 per cent of the depths determined from the Carnegie observations exceeded 2,000 metres.Fig. 2Correction factors, echo soundings, Regions I  to X IV , Pacific'Ocean, Carnegie results, 1928-9.(1) Admiralty Hydrogr. Dep. Pub. No. 282, London, 1927.Tabls 2.—Regional mean values correction factors in per cent, determined for adnic depth seductions basis sound velocity 1,450 metres per second, Carnegie results, 1928-9.3 o.s-sc c— 5O O)>2X 22 gm usS T -> 2 25  S ’? S SU5r t> lO« 3sag>°9o OO o O 00o 00 lO ic 3 o oo OH coIO1*.o o lOQOH oci ci ci ci ci ci coCOcoco ^ »o&.E a>Û 5OCOOiNWrtNNOOiNNiOOOrt1*-«*-<0©©©©©©o o o o o o o o o o o o o o o o^COO^COîONO^HN'tiOOOOOO^Ot^ OOO^ COCOOOi-HCOÇDOC^ kOt^e c C 'ic ^ M C ^ e o c c c o c O T ^ T ^ T ^ '^ L O iO L OM H r H H  0 . 0  O  CO CO CO ^  ^cocoocoi^ o ^ h oooHcoiooooeooO  O  O  O  O  O  CO CO COe0oooc0C0C0C0C0^ <^^ <^i0fc0»0O O O O O C O C O  co ^ oC O C O O O e O C O C O C O C O * * ^ ^ i O * O i O k O00co1C0000 O)00cocOt*»00o colOt*»o coci ci ci ci cocococo3 s s s o 8 o ocoa00oHsci o o cococococo** •*#*O O O O C O C O C O C O C OO O O O p O O O N O O < C O i f li0*HCi©»-*OTf,t'»O»-4Tj<ç0COCO OCO CO CO CO CO CO **^'«»*H f O ^ O i O O O O O OO O iO O O O O O ^ C O O iOcooocicococococo**NCïCîNNCOCOWCO^^cooooococoC^OTifiOhOi«ci ci ci ci ci ci CO CO CO8888888888888888  o w o io o io o ^ o io o io ô io o io  r - ' r *  ç i  p i CO CO T f T f tC  ln  çp 5P t "  r -  00  00: -sValeurs moyennes des facteurs de correction régionaux du Carnegie exprimées en pour cent et réduites de la vitesse de base de 1.450 mètres par seconde à une vitesse de base de 4.800 feet par. I t  will be noted th a t the tabular values for Regions I, X II, X III  and X IV  vary  considerably from those for the remaining regions, and th at those for Regions X II  and X III  are in very close agreement with each other. The ten regions, II  to  X I, exhibit characteristics sufficiently similar to justify  their being grouped into one series. Over the route traversed by the Carnegie, this series comprises the area approxim ately between latitudes 35° N. and 30° S. The means of the factors for this series and the maximum departures of the factors for any region from the means are given in the last tw o columns of the table. I t  is significant th at these departures are of the order of only one tenth of one per cent a t 3,000 metres, decreasing to about zero at the greater depths.For the purpose of comparison with the results of H e c k  and S e r v i c e ,  the values in table 2, based on the velocity of sound of 1,450 metres per second, have been reduced to the basic velocity of 4,800 feet (1,463 metres) per second b y applying the ratio 1450/1463 =  0.991. These reduced values, given in table 3, m ay be of interest to  institutions, such as those of the United States, viz. N avy, Coast Guard, and Coast and Geodetic Survey, which use the assumed velocity of 4,800 feet per second.In order to show the consistency and uniform ity of the correction factors, the values in table 3 have been plotted, the results being shown in Figure 2. The two curves at the bottom of the graph afford a  comparison of the Carnegie results for latitudes 350 N. to 30° S . with those of H e c k  and S e r v i c e  for latitudes 30° N. to 40° S . It  is inte­resting to note the striking sim ilarity of these two curves, although the Carnegie values are about 0.1 to  0.2 per cent, greater than those of H e c k  and S e r v i c e .T a b l e  4 .— Comparison depths determined from constant correction factors with those computed for each station in Carnegie reductions.Sound.sta.Near oc. sta. RegionApprox.aepthmetresCorr. fact, per cent. Diff. metres Prob.errori«metresSta. Reg. Area® Reg. Area0334 35 VI 3,671 3 .30 3 .30 3 .29 0 0 ± 5 0360 38 VI 5,437 2 .93 2 .83 2 .8 4 2 0 ± 2 5366c 44 VI 4,210 3 .5 0 3 .5 6 3 .5 3 3 1389« 50 II 2,800 2 .96 2 .90 2 .9 5 2 0409« 55 II 2,685 2 .7 8 2.86 2.91 2 3429« 60 I 3,880 3 .12 3 .18 . . . . 2452« 65 I 3,490 3.03 3 .01 1473« 70 III 4,785 . 3 .8 0 3.81 3Ì83 0 i506 75 III 3,776 3 .30 3 .3 0 3 .33 0 1559 80 IV 3,477 3 .2 6 3 .2 4 3 .20 1 2622 85 v 3,760 3 .43 3 .43 3 .33 0 4637 85 v 1,014 3 .49 3 .45 3 .12 0 . 4707 90 v 4,589 3 .8 2 3 .81 3 .7 4 0 4770 95 v 4,290 3 .6 7 3 .6 8 3 .5 8 0 4 ± 2 5819 100 VII 5,754 4 .3 8 4 .41 4 .3 8 2 0 ± 3 0887 105 IX. 5,534 4 .3 2 4 .3 0 4 .26 1 3934* 109 X I 8,169 5 .86 5 .85 5 .86 1 0943 110 X I 3,066 3.11 3 .06 3.03 2 2980 115 X III 5,410 3 .9 8 3 .93 31023 120 X IV 5,754 3 .96 4 .0 0 31072 125 X IV 4,546 3 .26 3 .23 1 ± 3 01131 130 X II 3,208 2 .73 2.71 11179 135 VIII 4,648 3 .65 3 .70 3Ì76 2 *5 ± 2 51225 140 VIII 1,398 2 .9 8 2 .70 2 .8 9 4 1 ±101227 140 VIII 4,770 3 .84 3 .7 7 3 .8 3 3 01231 141 VIII ‘¿,557 2 .93 2 .79 2.88 4 11258 143 X 5,800 4 .28 4 .33 4 .4 0 3 7 ± 3 01280 145 X 5,492 4 .10 4 .14 4 .23 2 71332 150 VIII 4,131 3.43 3 .44 3 .5 0 0 31385 155 VII 4,966 3.95 3 .96 3 .94 0 01442 159 V 1,868 3 .00 2 .99 2 .8 0 0 4 ±101463 160 V 1,179 3 .37 3 .32 3.01 1 31470 160 V 2,608 3 .06 3 .04 2 .90 I 4 ± 1 5, (a) Mean of factors for Regions II-X I -(lat. 35® N.-30° S.), designated “area* to differentiate from “region” .(b) Probable error assigned to final depths in Carnegie reductions.(c) Depths by “shotgun” method: id) Fleming Deep.In order to  test out the results obtained b y the use of constant factors as compared with those determined for each oceanographic station in the Carnegie reductions, table 4 was prepared. Comparisons were made a t or near every fifth oceanographic stationalong the whole cruise in the Pacific. In some cases, in order to include some of the smaller depths, selections were more frequent.The depths determined in the Carnegie reductions being accepted as the true depths, comparison was made with them, both on the basis of the regional factors and of those for the area, latitude 350 N . to  30° S., as given in table 2. The columns headed "d if­ferences”  in table 4 show the differences between the "tru e”  depths and those determi­ned b y  the use of constant factors. I t  will be seen th at those differences vary  from zero to about one-tenth per cent. Differences of this order seem almost negligible when we consider the several sources of errors, instrumental and observational, in the deter­mination of echo soundings. The probable errors estimated for the Carnegie depths range from about 0.5 to 1.5 per cent.It  is hoped that the seeming possibilities indicated herein, necessarily limited in scope, m ay serve to emphasize and stress the need of more thorough and comprehensive surveys, not only for a fuller knowledge of the characteristics of the "w aters that cover the deep”  and of their great systems of circulation, but also of the configuration of the ocean bottoms.0  0  0

Use of Regional Constant Correction Factors for Reduction of Echo Soundings

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Use of Regional Constant Correction Factors for Reduction of Echo Soundings

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