ACOUSTIC THERMOMETRY IN A RANGE-DEPENDENT OCEAN

摘要

Peculiarities of acoustic thermometry over a range-dependent ocean waveguide are considered. On range-dependent transoceanic paths, acoustic mode coupling causes purely predictable fluctuations of the modal travel times and amplitudes which do not correlate directly with changes in the sound speed and modal attenuation over the path. In the case of strong coupling this may distort the results of acoustic thermometry of the ocean as well as acoustic measurements of the ice thickness in the Arctic. It is shown that the mode coupling was the main reason of long-term variations in the modal amplitudes in the signals propagated from a drifting source at ice camp Turpan over a 1 Mm path to ice camp Narwhal during the Transarctic Acoustic Propagation experiment. The correctness of mode filtering on a vertical receive array at Narwhal is analyzed to estimate possible contribution of experimental errors to modal variations. Changes in the waveguide transfer function for individual modes due to mode coupling are analyzed using an approximate analytic solution and numerical modeling for the coupled-mode problem. The modeling results show that variations of the modal phases due to mode coupling are much less significant for measuring changes in the modal travel times, than those of the group delay of the modal pulses in broadband signals.