Low-Frequency Sound Attenuation in the Deep Ocean R. J. Urick Citation: The Journal of the Acoustical Society of America 35, 1413 (1963); doi: 10.1121/1.1918705 View online: https://doi.org/10.1121/1.1918705 View Table of Contents: http://asa.scitation.org/toc/jas/35/9 Published by the Acoustical Society of America Articles you may be interested in Ray and Wave Invariants for SOFAR Channel Propagation The Journal of the Acoustical Society of America 46, 1259 (1969); 10.1121/1.1911850 Low-frequency ambient noise in the deep sound channel—The missing component The Journal of the Acoustical Society of America 69, 1009 (1981); 10.1121/1.385680 Transmission Characteristics of the SOFAR Channel The Journal of the Acoustical Society of America 48, 767 (1970); 10.1121/1.1912201 Sound channel in an exponentially stratified ocean, with application to SOFAR The Journal of the Acoustical Society of America 55, 220 (1974); 10.1121/1.1914492 Deep-Ocean Sound Attenuation in the Sub- and Low-Kilocycle-per-Second Region The Journal of the Acoustical Society of America 38, 648 (1965); 10.1121/1.1909768 Analytic Description of the Low-Frequency Attenuation Coefficient The Journal of the Acoustical Society of America 42, 270 (1967); 10.1121/1.1910566 THE JOURNAL OF THE ACOUSTICAL SOCIETY OF AMERICA VOLUME 35, NUMBER 9 SEPTEMBER 1963 Low-Frequency Sound Attenuation in the Deep Ocean R. J. URICK U.S. Naval OrdnanceLaboratory, White Oak, Silver Spring, Maryland (Received24 May 1963) Signalsreceived from Sofar bombsdropped from an aircraft eastwardfrom Bermuda have beenstudied quantitatively. The energydensity of the signalsreceived at Bermuda, measuredin variousbands between 20 and 1600cps, can be accountedfor by cylindricalspreading beyond a "transitionrange" predicted approxi- mately by simpleconsiderations, plus an attenuationcoefficient in dB per megayardgiven by the linear expression1.5+8.2 f, wheref is the frequencyin kc/sec.At frequenciesbelow 1 kc/sec,this attenuationis far in excessof that which would be producedby absorptionalone. At very low frequenciesthe excessat- tenuation is postulatedto be due to the failure of the Sofar soundchannel to act as an acoustictrap. At frequenciesfrom about 50 to 500 cps, quantitative evidenceis presentedto indicate that the dominant attenuationprocess is scatteringby index-of-refractioninhomogeneities deep in the sea.Other characteristics of Sofar-transmitted signalsare described. INTRODUCTION tention.4.* However, the amplitude, duration, and spectralcharacteristics of Sofar signalshave not been Nsurfaces,the deep the ocean, attenuation farfrom of soundeffects at of frequenciesthe bounding be- studied extensively from a quantitativeviewpoint, even low 1 kc/sec is known to be of the order of only a small though these characteristicsare of dominant interest fraction of a decibelper mile. For example,the formula for long-rangeacoustic communication in the deep sea. This paperdescribes the resultsof a seriesof measure- a=0.033f• (a in dB per kiloyard,f in kc/sec)proposed ments made across the Atlantic Ocean between by Sheehyand Halley,• as a fit to their measurements, wouldindicate a coefficientof only 0.033dB per kiloyard Bermudaand the United Kingdom. The spreadinglaw and attenuation coefficient for this series of shots and at 1 kc/sec, with smaller values at lower frequencies. Measurementsof valuesas low as theserequire propa- the lossproduced by the Mid-Atlantic Ridge are de- gation paths that are measuredin hundredsof miles, scribed,and processesare postulatedto accountfor under conditionswhere reflection and scattering from the excessattenuation over that due to absorptionat the sea surface and sea bed are not involved. This low frequencies. implies,in turn, the useof the Sofarchannel for long- MEASUREMENT PROCEDURE distancepropagation, since the requiredpath lengths and freedom from boundaries cannot otherwise be ob- StandardNavy Sofar signals,containing the equiva- tained.In addition,the useof explosivesis dictatedby lent of 4 pounds of TNT, were dropped on a pre- the needfor high acousticsource levels. assignedtime scheduleby an aircraft as it flew between The existenceof a soundchannel in the deep seawas Bermuda and Great Britain via the Azores during predicted and demonstratedby Ewing and Worzel2 June1962. The locationof the dropsis shownin Fig. 1. about 20 yearsago. Named for its usefor soundfixing The maximumrange, from Bermudato the furthermost and ranging,it has receivedattention as a meansof drop,was 2516 miles. At eachlocation a pair of signals location of downed aviators and, more recently, of set to detonate at depths of 1500 and 3500 ft was missileimpacts? In consequence,the travel times for droppedalong with a sonobuoyfor observingthe Sofarchannel propagation have receivedabundant at- instant of detonation aboard the aircraft. At the Bermudaend, recordingswere made with hydrophones •M. J. Sheehyand R. Halley, J. Acoust.Soc. Am. 29, 464 4 R. A. Frosch,M. Klerer, and L. Tyson,J. Acoust.Soc. Am. (1957). 33, 1804 (1961). 2 M. Ewing and J. L. Worzel, Geol. Soc.Am. Mem. 27 (1948). 5 G. M. Bryan, M. Truchan,and J. Ewing,J. Acoust.Soc. Am. 3H. H. Baker, Bell Lab. Record39, No. 6, 195 (June 1961). 35, 273 (1963). 1413 1414 R. J. URICK Fro. 1. Location of Sofar bomb drops. at variousdepths; however, only the data from a As a result,Sofar signals received at a greatdistance hydrophoneat 3900ft, in water14 000 ft deep,are havea typicalenvelope that showsa gradualbuildup describedin thispaper. The taperecordings were later and reaches a sudden climax at the instant that the analyzedin theconventional way by playingback into axiallytraveling energy arrives. This abruptending is bandpassfilters, squaring, and integrating to obtainthe the characteristicthat makes them useful for triangu- energydensity of the receivedsignals in the various lation purposes. filter bands. Alongthe path of the dropsshown in Fig. 1, the transmission of sound in the Sofar channel is inter- SOFAR CHANNEL AND SIGNAL ENVELOPES ruptedby the Mid-AtlanticRidge, which forms an The Sofar soundchannel is an internal acousticduct acousticbarrier for deeppropagation across the Atlantic havingits axis, or depth of minimum velocity, at depths Ocean.Bathymetric charts ø shownumerous places in rangingfrom about 5000 ft in thetropics to nearthe thevicinity of the acousticpath where the crestsof the surfacein highlatitudes. Its existencedepends upon Ridgepierce the depthof the Sofaraxis and thuscut the fact that the velocityof soundincreases with both off near-axialrays carrying the principalportion of the transmittedenergy. The attenuatingeffect of the Ridge temperatureand pressure; at shallow depths, where the will be evident in the results to follow; in spite of it, temperaturedecreases with depth, the temperature effect is dominant and the velocity decreases;at great signalswere received 5 to 10 dB abovenoise at the depthswhere the sea is nearlyisothermal, the pressure maximum range (Drop 29), althoughthe dropsjust effectcauses the velocityto increaseagain. Figure 2, beyondthe Ridge(Drops 20, 21) appearto havebeen taken from the now-classicpaper on the subjectby cut off by the shadowingeffect of the crestsand were not received at Bermuda. Ewingand Worzel, •'shows a typicalvelocity profile in The sound-velocitystructure along the line of drops midlatit.udes•together with a ray diagramfor a source on theaais. Ray pathsnear the axis carry most of the 6 ChartsBC 0307N, BC 0308N, publishedby the U.S. Navy .energy•.n,.• .a.r,e •oc[ated with the longest travel time. OceanographicOffice. SOUND ATTENUATION IN THE DEEP OCEAN 1415 DO0 FIG. 2. Ray diagramof the Sofar soundchannel. [Reproduced from Ewing and Worzel.2-] was estimated from velocity data provided by the been done by Brekhovskikh,7 that for both sourceand National OceanographicData Center. Figure 3 is a receiver on the axis of a sound channel the difference in velocity contour cross section drawn from N.O.D.C. travel times along the axial ray and along the ray hydrographicdata for the month of Juneat stationsin leavingthe sourceat an angle00 is tOo2/6,where t is the the vicinity of the drop path. The feature most worthy axial travel time. The straightline in Fig. 5 showsthis of note is the decreasingvelocity at shallow depths calculatedduration for 00-12.2 ø for the ray of maxi- toward the right, reflectingthe lower surfacetempera- mum inclination propagating without reflection at tures toward the northeast along the drop path. How- either the seasurface or seabed. The agreementis sur- ever, the depth of the Sofar axis is approximatelycon- prising, consideringthat the Sofar channelhas by no stant, although at the northeasterlyend its depth is means the linear gradient on either side of its axis re- made uncertainby the occurrenceof nearly isovelocity quired for this approximation.The absenceof a clear conditions. beginningto the receivedsignal is perhapsdue to the Figure 4 showssignal envelopes in the band 50-150 contributionby rays which leave the sourcebeyond cpsfor a shotat 3500 ft as receivedby the hydrophone 12.2ø and which sufferrepeated reflection at either or at 3900 ft at Bermuda. The vertical scale shows the both seaboundaries. Beyond the Ridgeat abouta range band level in decibelsrelative to the intensity of a plane of 1800 miles, the durations based on the above cri- wave of 1 dyn/cm2 rms pressure.The envelopesexhibit terionare muchshorter because the early portion of the the expectedlengthening and weakening with increasing signalsare buried in noise,and becausethe near-axial