G. L. D
W. A. Kuperman
Marine Phys. Lab., Scripps Inst. of Oceanogr., San Diego, CA 92152-6400
C. W. Clark
D. K. Mellinger
Cornell Univ., Ithaca, NY 14850
Beamformed time series of fin whale calls from the North Atlantic recently have been obtained. Spectrograms of these recordings have an unusual appearance---three to four downsweeping clicks occur in rapid succession, almost as if the whale was stuttering. These arrivals can be explained in terms of the differences in group velocity between the lowest-order modes that propagate in the shallow water (about 350 m) waveguide. By matching predicted spectrograms calculated from a shallow water waveguide model with actual spectrograms, estimates of both the range of the whale and the compressional wave velocity in the ocean bottom are obtained. The approach makes various assumptions, including that the whale emits a signal with a downsweep rate of 25 Hz per second. However, known deviations from the assumptions can be incorporated into the modeling. The results have several implications such as that whale signals may provide loud, broadband, tomographic sources of opportunity for determining oceanographic/geophysical properties, and that some differences in whale call recordings from different populations of the same species may be caused by propagation effects. Might whales take advantage of propagation-induced distortions in order to learn something about the ocean medium? [Work partially supported by ONR.]