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1 2 5 10 20 50 100 200 500 1,000 10,000 100,000 500,000

Frequency (Hz)

Figure 6.2 Average deep-sea ambient-noise spectra. (Urick, 1983; Principles of Underwater Sound, 3rd edn; reproduced with permission of McGraw-Hill Publishing Company.)

using noise levels (specified in units of spectrum level, dB re 1 |xPa), this summation process is easily accomplished using the "power summation" operator, denoted by the symbol 0, and defined as:

i=i where Li is the level of the ith noise source (dB) and n the number of contributing noise sources. This operation effectively converts the noise levels (Li) to units of intensity, sums the intensities, and then converts the sum back to units of decibels.

For example, in Figure 6.2 at a frequency of 100 Hz under conditions of moderate shipping and sea state 6, the individual noise levels are about 69 and 71 dB, respectively. In effect, however, the noise level obtained using Equation (6.1) is:

or 2 dB higher than the level due to surface weather alone.

6.2.1 Seismo-acoustic noise

The term seismo-acoustics is used broadly in reference to low-frequency noise signals originating in Earth's interior and the oceans. In the frequency range below 3Hz, Orcutt (1988) defined three specific frequency bands distinguished by the physics of the noise sources:

1 Microseism band (80 mHz-3 Hz) contains high-level microseism noise resulting from nonlinear wave-wave interactions.

2 Noise-notch band (20-80 mHz) contains noise controlled by currents and turbulence in the boundary layer near the sea floor.

3 Ultralow-frequency (ULF) band (<20mHz) contains noise resulting from surface gravity waves.

6.2.2 Shipping noise

Shipping noise can exhibit both spatial and temporal variabilities. The spatial variability is largely governed by the distribution of shipping routes in the oceans. Temporal variability can be introduced, for example, by the seasonal activities of fishing fleets.

The noise generated by coastal shipping and by high-latitude shipping can contribute to the nose field in the deep sound channel in tropical and subtropical ocean areas. Specifically, coastal shipping nose is introduced into the deep sound channel through the process of downslope conversion. High-latitude shipping noise is introduced through the latitudinal dependence of the depth of the sound channel axis. These mechanisms are explained below.

Wagstaff (1981) used ray-theoretical considerations to illustrate the mechanisms involved in the downslope conversion process. The following hypothetical arrangement was assumed (Figure 6.3): (1) the continental shelf extends from the coastline to the shelf break (approximately 11.4 km from shore), with an inclination angle of about 1° from the horizontal; (2) the continental slope extends from the shelf break seaward to a depth in excess of 1,000 m, with an inclination of about 5°; and (3) the deep sound channel axis is located at a depth of 1,000 m. Then, in an 11-km band extending seaward from the shelf break, downward-directed radiated noise from surface ships can enter the sound channel by direct reflection off the sea floor.

Kibblewhite et al. (1976) demonstrated the importance of the latitudinal dependence of the sound channel axis depth in introducing high-latitude shipping noise into the deep sound channel. In Figure 6.4, the sound-speed structure in the north Pacific Ocean is related to the local water masses. Also shown are the critical depth and the bathymetry. Figure 6.4 vividly illustrates the shoaling of the sound channel axis as it approaches the Arctic region. Thus, low-frequency noise from shipping sources near latitude 50°N will be refracted into the (relatively) shallow sound channel and then propagate with little attenuation to lower latitudes.

Tappert et al. (2002) used the UMPE propagation model to simulate a range-dependent phenomenon that occurs over a sloping bottom when a source is located on the sea floor in shallow water with

-h—Distributed shipping—H

-h—Distributed shipping—H

Sound channel axis

Figure 6.3 Illustration of the conversion of coastal shipping noise, represented by high-angle rays, to noise in the deep sound channel, represented by horizontal rays (adapted from Wagstaff, 1981).

Sound channel axis

Figure 6.3 Illustration of the conversion of coastal shipping noise, represented by high-angle rays, to noise in the deep sound channel, represented by horizontal rays (adapted from Wagstaff, 1981).

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