A STUDY OF THE INFLUENCE OF THE ENVIRONMENTAL PARAMETERS ON THE TRAFFIC NOISE IN THE SEA

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A STUDY OF THE INFLUENCE OF THE ENVIRONMENTAL PARAMETERS ON THE

TRAFFIC NOISE IN THE SEA

M. Taroudakis

To cite this version:

M. Taroudakis. A STUDY OF THE INFLUENCE OF THE ENVIRONMENTAL PARAMETERS

ON THE TRAFFIC NOISE IN THE SEA. Journal de Physique Colloques, 1990, 51 (C2), pp.C2-

1001-C2-1004. �10.1051/jphyscol:19902234�. �jpa-00230562�

COLLOQUE DE PHYSIQUE

Colloqu© C2, supplement au n°2. Tome 51, Fevrier 1990 C2-1001 ler Congres Frangals d'Acoustlque 1990

A STUDY OF THE INFLUENCE OF THE ENVIRONMENTAL PARAMETERS ON THE TRAFFIC NOISE IN THE SEA

M.I. TAROUDAKIS

Foundation for Research and Technology-Hellas, Institute of Applied and Computational Mathematics, PO. Box 1527, GR-711 10 Iraklion, Crete, Greece

Résume - L' influence des paramètres de l'environnement sur le bruit de circulation des navirs en mer, est étudié en utilisant un model simpli- fié, fondé sur "normal-mode" propagation en fréquence unique. Les résul- tats obtenus indiquent une variation remarquable selon les saisons de 1' anée. Les différences sont plus importantes au fonds costitués des sédiments mous.

Abstract - The influence of the environmental parameters on the traffic noise in the sea is studied with the help of a simplified model based on normal-mode propagation at a single frequency. The results show signifi- cant variation of the traffic noise spectrum level among the various seasons of the year. The differences are more pronounced when the bottom consists of soft sediments.

1 - INTRODUCTION

The presence of noise in the sea is a restrictive factor for the performance of any sonar signal processing system working in this environment. The noise at the input of a hydrophone at some depth in the sea, can be classified into three categories, namely the (a) ambient, (b) self and (c) reverberant noise. The present paper is concerned with the ambient noise and more spe- cifically with the traffic noise, which is a peculiar form of the ambient noise in the sea, owing its origin in the noise radiated by the ships steam- ing in the sea. It dominates the ambient noise spectrum from about 50 to 200 Hz, but is still significant at frequencies as low as 20 Hz and as high as 1000 Hz1. Since low frequency acoustic waves propagate effectively at long ranges, the contribution of distant sources-ships to the traffic noise is very important. Estimation of the traffic noise at a certain area can be done by means of stochastic models2'3 since the parameters related to the ships as sound sources (such as their number, kind, speed, route and posi- tion) are random variables for the specific area of interest. However, a deterministic model for the estimation of the traffic noise can still be applied when there is a good knowledge of an instant distribution of ships at the greater area of the measuring hydrophone. This model is particularly useful for parametric studies of the influence of the various parameters on the traffic noise levels at a certain area. The present study is focused on the environmental parameters and a deterministic model is used to predict seasonal variations of the traffic noise, as well as the influence of the bottom structure on the traffic noise levels in shallow water.

2 - THE MODEL

The general idea of the deterministic model used in the study, is that the total noise level due to a number of ships, can be computed at the input of a hydrophone, by adding the contribution from each individual one, and is based on the assumption that the noise from each ship is uncorrelated to the noise of each other at a specific frequency. All the calculations are re- ferred to spectrum levels in 1 Hz band, and the ships as sources of sound are treated as harmonic point sources at the corresponding frequency. This simplification enables the use of a sound propagation model to compute and extract from the source level of each ship the transmission loss up to the location of the receiver and obtain its receiver level4. Input data of the model are the geoacoustic parameters of the environment, such as speed of sound, density, attenuation coefficients, bathymetry and bottom structure.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19902234

C2-1002 COLLOQUE DE PHYSIQUE

The model used in the present study, is based on a normal-mode representa- tion of the acoustic field in a range independent environment4, which means that the usual simplifications of a horizontal stratified medium are consid- ered. The model is efficiently applied to shallow water environments and low frequencies, which are the frequencies of interest in the study of the traffic noise. Another assumption made in the model is that the sound waves from the various sources bear a random phase distribution.

The source levels of the ships depend on their main particulars (kind, size, speed, SHP etc.), but only empirical data concerning total source levels or source spectrum levels at specific frequencies are available5.

The source code created for the implementation of the method in the comput- er, asks the user to provide the environmental parameters of the area of interest, the location of the ships, given as range and bearing from the receiver and their source spectrum levels for the frequency of interest. The receiver can be either an omnidirectional hydrophone or a horizontal array of hydrophones. In the second case, the program gives the possibility of calculating the horizontal directivity of the traffic noise. For the present study however, only the input levels of the omnidirectional hydrophone are needed.

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It is well known that the sound speed profile in the sea varies from season to season, due mainly to the different temperature structure of the sea. The temperature near the sea surface is higher during summer than during winter, giving rise to higher sound speed values at the surface. In shallow water, the temperature at the surface is generally greater than that in the bottom of the sea. Sometimes, during winter, the sound speed at the surface is lower than that at the bottom. This means that propagation characteristics of the sea vary drastically from season to season. Since traffic noise sources are located at great distances from the receiver, its spectrum levels depend on the propagation characteristics of the environment. It is therefore expected that different noise levels will be observed among the seasons, even when the rest of the input parameters including the specific distribution of ships, remain unchanged. This seasonal variation can be easily studied with the help of the aforementioned model and work carried out within this scope is presented in the next section.

Another physical parameter that has a strong influence on the propagation characteristics in shallow water, is the bottom. Since the acoustic energy encounters the bottom many times, its propagation characteristics are very much influenced by its structure. A quantitative indication of this influ- ence is also presented in the next section.

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The model is applied to a shallow water environment. The sound speed struc- ture in the water is simplified in the form of a linear profile and realis- tic data are used. The sound speed in the surface is taken 1500 m/sec for January, 1508 m/sec for April, 1530 m/sec for July, 1513 m/sec for October.

The sound speed at the bottom is always 1510 m/sec. The water density is 1025 ~ ~ / m ~ . Various types of bottoms are also used ranging from very soft material (clay) at the upper layers, to harder sedimentary materials such as coarse sand. The parameters of the various bottoms used in the analysis, appear in Table I.

Table I : Bottom geoacoustic parameters of the study.

Density ( ~ ~ / m ~ ) Sound Speed (m/sec)

Clay 1420 1530

Silt-Clay-Sand 1450 1550

Sandy Silt 1600 1600

Silty Sand 1830 1680

Coarse Sand 2000 1850

Attenuation coefficient: 0.09 dB/m Khz.

The study is performed for different distributions of same kind of shlps at constant shipping density (32 ships at the intercept range of 100 Km) and the results obtained show similar trends although the calculated traffic noise levels vary according to the specif.ic distribution of ships. Empirical data are used for attributing source levels to the various ships. In every case, a 6 dB/octave decrease of the source level is considered for perform- ing the analysis at various frequencies.

The calculated data are sorted in two main groups. The first group consists of the results obtained by changing the sound speed profile in a shallow water of specific bottom characteristics. These results are further sorted according to the bottom type they are referred to, and they are used to pre- dict the seasonal variations of the traffic noise levels. The second group consists of the results obtained by keeping the sound speed profile constant and changing the bottom type. The results are again sorted according to the season they are referred to and they are used to predict the influence of the bottom characteristics on the traffic noise. The results obtained by the study can be summarised as following.

There is a seasonal variation of the traffic noise levels which depend on the subbottom characteristics. The variation is greater when the bottom consists of soft material, such as silty sediments, than in the case of harder sediments. Generally, good propagation characteristics during winter, lead to higher traffic noise levels in that season. Figure 1 presents the traffic noise spectrum levels at the frequency of 100 Hz for a specific traffic scenario, when the sediment consists of mixed silty-clayey and sandy material. There is a 10 dB difference among summer and winter levels, with winter levels being higher, and this is true for all the receiver depths.

This variation has been found to be a typical one for that type of sedi- ments. Figure 2 presents the spectrum levels at the same frequency and traffic scenario, but with the bottom consisting of silty material. There is only a slight variation among the seasons.

Soft material at the bottom absorbs greater amount of acoustic energy than do harder sediment. This means that traffic noise levels are expected to be lower when the bottom consists of soft material. This was verified by the model, and variations of more than 30 dB among the various types of bottoms during summer were calculated. Figure 3 presents the spectrum levels at the input of a 50 m depth receiver for two water depths, during summer and win- ter, with respect to the sediment type. An increase of the spectrum level is calculated when going from soft sediments to harder ones during both summer and winter. The variations are greater during summer, and this can be easily explained by the fact that propagation conditions during summer guide the acoustic energy towards the bottom, and thus the bottom parameters become then much more important. The higher levels during winter can also be seen in this diagram.

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By using a simplified model for the estimation of the traffic noise in shal- low water, the seasonal and bottom effects on the traffic noise spectrum levels can be studied. Application of the model to a shallow water environ- ment with typical seasonal variations of the sound speed profile, and real- istic shipping situation, show strong variations of the traffic noise levels among the seasons in environments with soft bottoms, while slight variations are calculated in harder bottoms. Moreover, traffic noise levels in environ- ments with hard bottoms are higher than in environments with soft bottom sediments. The variations in this case are more important during summer than during winter.

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1. Wenz, G.M., J.Acoust.Soc.Am., 34, 1936-1956 (1962).

2. Dyer, I., J.Acoust.Soc.Am., 53, 564-570 (1973).

3. Laval, R. and Drezet, J.M., "An Analytic Method to Predict the Statisti- cal Characteristics of Distant Shipping Noiset1 in Adaptive Methods in Under- water Acoustics edited by H.G.Urban, pp.37-48, D.Reide1 Publ. Comp.

Dordrecht, 1985.

4. Taroudakis M.I. and Nikolaidis N.X., Tech. Chron.- B, 9, 47-72 (1989).

5. Urick, R.J., Principles of Underwater Sound, p 299, McGraw Hill, N.Y.,1975

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WATER DEPTH: 2 0 0 rn FREQUENCY: 1 0 0 Hz SEDIMENT Silt-Clay-Sand

5 0 . 0 0

! 1

JAN APR JUL OCT

Season

Fig. 1 Seasonal variations of the traffic noise spectrum level at 100 Hz in 200 m depth sea environment with soft bottom sediments.

Receiver

WATER DEPTH: 2 0 0 rn FREQUENCY: 1 0 0 Hz SEDIMENT: Silt

5 0 . 0 0

!

JAN APR JUL OCT

Season

Fig. 2 Seasonal variations of the traffic noise spectrum level at 100 Hz in 200 m depth sea environment with medium density bottom sediments.

//A' - - - 2 0 0 rn water depth

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Summer 1

/

/ WATER DEPTH: 200/100 m I FREQUENCY: 1 0 0 Hz

RECEIVER DEPTH: 5 0 rn

40.00 ! I

CI Si-CI-Sa Si Si-Sa Sa

Sediment Type

Fig 3. Variations of the traffic noise levels at 100 Hz in shallow water with respect t o the sediment type.