Q 12 15

1900 2000 2100

Figure 3.21 Fluctuations in the depth of the mixed layer caused by the passage of internal waves. Water temperature is measured in °F. The MLD is defined by the 58°F isotherm. (LaFond, 1962; The Sea, Vol. 1, 731-51; reprinted by permission of John Wiley & Sons, Inc., all rights reserved.)

extensive data set collected during the AMOS program (Marsh and Schulkin, 1955) has been used to characterize TL in the surface duct in the frequency range 2-8 kHz. Graphical results derived from Condron etal. (1955) are summarized in Figures 3.22 and 3.23. These figures demonstrate the importance of source-receiver geometry (relative depths) in surface duct propagation, particularly in those cases where the source and receiver are situated on opposite sides of the layer depth (referred to as cross-layer geometries).

3.7.3 Low-frequency cutoff

At very low frequencies, sound ceases to be trapped in the surface duct. The maximum wavelength for duct transmission may be derived from the theory of radio propagation in ground-based radio ducts to be (Kerr, 1951: 20)

3 Jo where n(z) is the index of refraction at any depth z in the duct and n(H) the index of refraction at the base of the duct. Using values of sound speed and sound-speed gradient appropriate for sound propagation in the mixed layer, Equation (3.9) reduces to

for the maximum wavelength (Amax) in meters trapped in a mixed-layer duct of depth H in meters. For example, a mixed layer 30 m thick would trap a maximum wavelength of 1.4 m, corresponding to a frequency of

1900 2000 2100

Figure 3.21 Fluctuations in the depth of the mixed layer caused by the passage of internal waves. Water temperature is measured in °F. The MLD is defined by the 58°F isotherm. (LaFond, 1962; The Sea, Vol. 1, 731-51; reprinted by permission of John Wiley & Sons, Inc., all rights reserved.)

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