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The continuously upward-refracting propagation conditions that generally prevail in Arctic waters (as demonstrated by the ray trace in Figure 3.28) cause repeated interactions with the ice canopy and tend to create a low-pass filter, favoring the propagation of low-frequency (<300 Hz) signals. The situation is further complicated when the geometry of the sea floor and the acoustic wavelength combine to yield shallow-water conditions with the attendant increase in bottom interaction opportunities.

Measurements of TL in the Arctic are limited since access to this region has historically been restricted to the spring season when conditions are favorable for manned camps. Consequently, there is a relatively poor understanding of the seasonal variability based on historical data. This situation has gradually improved with the introduction of autonomous sensors that can be deployed through the ice from aircraft throughout the year.

One of the principal characteristics of acoustic propagation measurements under an ice cover is the rapid increase in TL with range at frequencies above about 30 Hz (the low-pass filter effect). The loss mechanism has been attributed primarily to scattering at the ice-water interface. Other possible mechanisms include dissipative processes in the ice canopy, conversion of waterborne energy into energy traveling in and confined to the ice canopy and increased absorption in the water column.

Buck (1968) summarized available Arctic TL data (Figure 3.29). These curves give the average measured TL in the Arctic Ocean at a number of frequencies. The standard deviation of the various data points from the smooth curve is stated for each frequency. The dashed line shows spherical spreading (TL = 20log1o r, where r is the range measured in meters). It is

Range in nautical miles 1 10 100 1,000 10,000

Range in nautical miles 1 10 100 1,000 10,000

Range in kilometers

Figure 3.29 Average curves of Arctic TL versus range based on measured data. Note that range is plotted on a log scale (Buck, 1968).

Range in kilometers

Figure 3.29 Average curves of Arctic TL versus range based on measured data. Note that range is plotted on a log scale (Buck, 1968).

evident that sound propagation in the Arctic degrades rapidly with increasing frequency, particularly above about 30 Hz. At short-to-moderate ranges, ducting improves propagation relative to spherical spreading. At long ranges, repeated encounters with the under-ice surface degrade propagation.

3.13 Coherence

Coherence is defined as a measure of the phase and amplitude relationships between sets of acoustic waves. In the ocean, which is characterized by both temporal and spatial variations, the effects of the medium on small-amplitude wave propagation can be described in terms of coherence time, coherence bandwidth, spatial coherence and angular coherence. As described by Ziomek (1985: chapter 7), for example, this information can be obtained from the generalized coherence (or autocorrelation) function.

Temporal coherence (fluctuations) refers to changes in a received signal (relative to a steady signal) over a period of time. Spatial coherence refers to the changes in the signals received at different locations in the ocean at a given time. Urick (1982: chapters 12 and 13) discussed additional aspects of temporal and spatial coherence.

Signal fluctuations can be caused by a number of physical processes including:

1 source or receiver motion;

2 oceanic fine-scale features;

3 sea-surface motion (waves and tides);

4 scatterers;

5 internal waves and internal tides; and

6 ocean currents and eddies.

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