| Popis: |
Adiabatic mode theory is used to calculate the path in the horizontal plane of vertical acoustic normal-modes (at frequencies of 25 and 50 Hz) that have propagated through an idealized cold-core eddy of radius 125 km, inside which the sound-speed along the SOFAR axis varies by 15 m/s. The eddy centre is at a fixed distance (625 km) from the source, and receiver positions beyond the eddy on a line at 5° to the source-centre axis are considered. The arrival directions of the normal-modes, in conjunction with their relative amplitudes as functions of depth, are used to obtain the apparent transverse displacement of the source for various transducer depths. External to the eddy, the depth of the SOFAR axis is 1250 m. The transverse displacement is found to be a slowly varying function of range beyond the eddy and appears to approach a finite value as the range becomes infinite. For source and receivers at the SOFAR axis, for example, the dominant mode (at any frequency) is Mode 1, and at both 25 and 50 Hz the transverse displacement of Mode 1 is 9 km at ranges of 1000 km and beyond. For the eddy model considered, the degree of horizontal refraction of normal-modes at a given frequency decreases as the mode number is increased. The transverse displacement therefore decreases as the depth of the transducers is moved away from the SOFAR axis. As transducer depth decreases to 300 m for example, Mode 16 becomes dominant at 25 Hz, and the transverse displacement at 1000 km range is only 4.3 km, while at 50 Hz the dominant mode is 27 and the corresponding displacement is 4.7 km. Thus the transverse displacement at a given depth is also a slowly varying function of frequency. |
