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Low-Frequency Ambient Noise in the Deep Sound Channel -The Missing Component

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Low-Frequency Ambient Noise in the Deep Sound Channel -The Missing Component SACLANTCEN Rep SR-51 SACLANT ASW RESEARCH CENTRE REPORT LOW-FREOUENCY AMBIENT NOISE IN THE DEEP SOUND CHANNEL - THE MISSING COMPONENT RONALD A. WAGSTAFF 15 JULY 1981 NORTH ATLANTIC LA SPEZIA, ITALY TREATY ORGANIZATION This document is unclassified. The information it contains is published subiect to the conditions of the legend printed on the inside cover. Short quotations from it may be made, in other publications if credit is given to the author(s). Except for working copies for research purposes or for use in official NATO publications, reproduction requires the authorization of the Director of SACLANTCEN. This document is releamcd to a NATO Government at the direction of the SACLANTCEN subject to the following conditionr: 1. The reciplenl NATO Government agree8 to use its best endeavours to ensure that the informmtion herein disclosed, whether or not it bears a security classification, 1s not dealt with in any manner (a) contrary to the intent of the provisions of the Charter of the Centre, or (b) prejudicral to the rights of the owner thereof to obtain patent, copyright. or other like statutory protection therefor. 2. If the technical :nformation was originally releared to the Centre by a NATO Government subject to rertrictionm clearly marked on this document the recipient NATO Government agrees to ure Itm beet endeavours to abide by the terms of the restrictbnm RO imposed by the releasing Government. SACLANTCEN REPORT SR-51 NORTH ATLANTIC TREATY ORGANIZATION SACLANT ASW Research Centre Viale San Bartolomeo 400, 1-19026 San Bartolomeo (sP), Italy. national 0187 560940 tel: international + 39 187 560940 LOW-FREQUENCY AMBIENT NOISE IN THE DEEP SOUND CHANNEL -THE MISSING COMPONENT by Ronald A. Wagstaff (Reprinted from J. AcousticaZ Society America 69, 1981: 1009-1014) 15 July 1981 This report has been prepared as part of Project 21. B.W!&+$L B.W!&+$L Direct I # Low-frequency ambient noise in the deep sound A channel-The missing component I R. A. Wagstaff I SACLANT ASW Research Centre, Lu S~ia,I&ly (Received 23 June 1980; qtedfor publication 26 November 1980) There is an important wmponht of the undersea ambient noise which has generally been overlooked or ignored. It is the noise which amvb at a sensor, located in a deep sound channel, by way of ducted sound propagation. It is generated at the surface and becomes channeled m a result of a gradually sloping sound channel axis and through repeated reflections from a sloping bottom. It can be of sufficient level to dominate the local measurement. Examples are given in which neglecting it leads to incorrect interpretation of results and erroneous wnclusions, including agreement between modeling and measurement. PACS numbers: 43.30.Nb, 43.30.Bp winds and shippingg noise source characteristics, pro- Ambient noise in the ocean is a complex phenomenon. cessing technique^,'^*^^ and various forms of noise To begin to understand it, one must have a knowledge models12-l5 to name but a few. In some cases, the in- of acoustic propagation and noise-source distributions vestigations are attempting to account for just? few and characteristics. Further, to understand mea- decibels. It tends to imbue a feeling of confidence, sured noise data requires knowledge of the influence of that the scientific community understands the first- the measurement system on the data. For practical order effects and simply needs to "chip away" on the reasons, past investigations of the noise have concen- second-order effects to achieve a complete understand- trated on some aspects that are considered interesting ing of the ambient-noise field. (In this context, second- or important, while limiting consideration of the others. order effects are those which, if omitted, would affect This being the case, it is not surprising that a mrijor the noise by only a few decibels.) component of the noise can remain unrecognized while There has, unfortunately, been one first-order com- the general appearance is that only relatively minor ef- ponent of the noise which has generally been overlooked fects remain to be investigated. or ignored. It is the noise which arrives at a sensor, located in a deep sound channel, by way of continuously This paper discusses a major component of the noise # refracted (RRR) ray paths. This noise is usually gene- which has either been overlooked or ignored by the bulk rated a considerable distance from the measurement of the scientific community investigating the undersea location. It can be of sufficient level to dominate the ambient noise environment. The component being re- local measurement. There is evidence that some ex- ferred to is the noise from distant sources which ar- perimentalists understood the importance of this com- rives at a sensor, located within a deep sound channel, ponent. 4316 The modelers, however, being among the by way of ducted sound propagation. The discovery major recipients of new knowledge and their models, of this component is not the result of new theory, nor the barometers with which to measure the total accumu- can it be attributed to one individual. The "germ of the lation and utilization of that knowledge, have generally ideaworiginated more than 20 years ago when it waS ignored it. The only exception which comes to mind is observed that the signal level received from a source at Ref. 12, in which it is explicitly treated as a separate a considerable distance could be more than if the source was much closer in range. The application of this prin- component. ciple to establish the existence of noise which travels At frequencies below a few hundred hertz, where throughout the oceans by way of the deep sound channel shipping is the major source, the noise can be domi- had not been made until recently. Its omission in the nated by the noise which travels from afar by way of interpretation of noise measurement results, and the SOFAR channel (deep sound channel) propagation. The lack of documented evidence of its theoretical or ex- noise from nearby and less distant sources may con- perimental investigation, indicates it is not presently tribute only a fraction of a decibel to the total noise recognized nor understood by the scientific community level. This situation could well exist in most areas as a whole. On a limited basis, this paper attempts to of the oceans in the northern hemisphere. Neglecting change this. it can be a serious error, leading to misguided experi- ment design and incorrect interpretation of the noise I. BACKGROUND environment. When one considers how specialized our theoretical There are several reasons why the "SOFAR channel and experimental investigations of the undersea am- noiseu has been overlooked or ignored. Among them bient-noise environment have become, since the clas- are the following: sic paper by Wenz in 1962,' he cannot help being im- pressed. The investigations include the horizontal2 and (1) All areas do not possess a SOFAR channel. vertical directionality of the noi~e,~~~depth dependen~e,~ (2) It is not a significant contribution above a few coherence characteristic^,^ attenuation coefficient^,^ hundred hertz where attenuation is severe. 4 1009 J. Acoust. Soc. Am. 69(4), April 1981 0001 -4966181104 1009-06$00.80 O 1981 Acoustical Society of America 1009 (3) It is ma~kedby high density local shipping. channel and become trapped. Once in the channel, it (4) The most significant noise sources are gene- travels with relatively low loss through cylindrical rally believed to be within a few hundred miles spreading. Northrop et al.," give just such an ex- of the sensor. ample for noise (explosive sources) originated in shal- (5) Consideration of distant noise sources and range low water off Point Arena, California, being recorded dependent acoustic propagation are required. by hydrophones at SOFARdepths near Eniwetok and Mid- (6) Low-frequency vertical directionality measure- way. The maximum levels received correspond to shots ments, which give a direct measure of it, are over the edge of the continental shelf, with levels de- relatively recent. creasing by about 10 dB seaward and about 5 dB shore- ward. The shoreward reduction was attributed to the increased number of reflections necessary to get the II. MECHANISMS sound into the channel. A seaward reduction was attri- There are several mechanisms by which sound can buted to the bottom becoming deeper than channel depth. become trapped within the deep sound channel, includ- For sound generated over the slope, they state ". the ing diffraction and scattering of sound originating near first bottom reflection is from the continental slope, the surface and the source being physically located where the effect of the steeper bottom slope (3i0)be- within the channel. The contributions from these comes important in channeling BR/SR ("bottom-re- mechanisms, however, can probably be considered flected/surface-reflectedv) rays into RRR ("continuously second- or third-order effectswhen compared to the refracted") rays. For example, for a surface shot in total amount of noise Which is trapped in the channel. 300 m of water a ray that is initially the 0" ray steepens to 11" before it strikes the bottom at a depth of 475 m. There are at least two first-order effects which, to- and the ray becomes continuously refracted. Steeper gether can account for the levels that are measured. rays become RRR after one or more bottom reflections The first is the axis of the deep sound channel approach- ing the surface with increasing latitude where large on the lower continental slope. " An enhancement in concentrations of shipping exist. This happens in the level (less propagation loss) is gained as a result of the RRR propagation having a more favorable geometric North Pacific and the North Atlantic. Figure 2 of Ref.
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