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Latitude

Figure 3.18 Vertical section of water temperatures (°C) over the TexasLouisiana continental shelf in the Gulf of Mexico in February (Etter and Cochrane, 1975).

Latitude

Figure 3.18 Vertical section of water temperatures (°C) over the TexasLouisiana continental shelf in the Gulf of Mexico in February (Etter and Cochrane, 1975).

devoid of acoustic energy since it is insonified by diffraction and by sound scattered from the sea surface. The rate of sound leakage out of the surface duct can be quantified in terms of an empirical leakage coefficient (aL), which expresses the attenuation (dBkm-1) of sound trapped within the duct. This leakage coefficient varies with the surface roughness (wave height), duct thickness (SLD or MLD), sound-speed gradient below the layer and acoustic frequency (Urick, 1983: 153-4).

Over short distances, or even at a fixed location over time, the thickness of the mixed layer may vary because of internal waves. These waves propagate along the density discontinuity at the base of the mixed layer (Figure 3.21). Internal waves complicate the propagation of underwater sound by causing variations in TL over short distances. Fluctuations of echoes between a source and receiver separated by the MLD or SLD are attributable in part to the existence of internal waves.

Urick (1982: chapter 6) reviewed the available open-literature algorithms for estimating TL in the surface duct. In particular, an analysis of the

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