KUTSCHALE: LOW-FREQUENCY PROPAGATION IN THE ICE-COVERED ARCTIC OCEAN 



from an aircraft with simultaneous measurements of propagation 

 loss from air-dropped SUS charges. 



The purpose of this paper is to present a summary of the im- 

 portant aspects of long-range, low-frequency propagation in 

 deep water of the central Arctic Ocean. The emphasis is on 

 a comparison of field data with theory. Generally good agree- 

 ment is found between experiment and theory employing ray 

 theory, WKB mode approximation, normal-mode theory and the 

 Fast Field Program (FFP) , provided the approximate nature of 

 the former two methods is kept in mind. The normal-mode 

 fojnnulation and the FFP technique are derived from the exact 

 integral solution of the wave equation for sources in a multi- 

 layered, interbedded liquid-solid half space. The FFP, 

 introduced by Marsh and DiNapoli, is a fast, convenient 

 method for computing propagation loss as a function of range 

 and source and detector depth. The FFP integrates directly 

 the exact integral solution of the wave equation employing 

 the Fast Fourier Transform and is particularly well suited 

 for Arctic propagation because of the uniformity of the 

 vertical sound velocity structure. Reflection loss by 

 the rough ice boundaries is easily incorporated in the 

 integral solution. Computed propagation loss is in close 

 agreement with field data. For computations of disper- 

 sion and signal waveforms, normal-mode theory is in good 

 agreement with field data. 



Field data strongly suggest that reliable estimates of 

 propagation loss as a function of range in deep water can be 

 obtained from measurements by aircraft of surface ice rough- 

 ness combined with computer modeling of loss. The FFP seems 

 most useful for frequencies below 100 Hz, while at higher 

 frequencies both ray theory and the FFP yield results in 

 close agreement. 



A characteristic feature of long-range explosive sound propa- 

 gation in deep water of the central Arctic Ocean is that the signals 

 are dispersive; that is, waves of each frequency travel with different 

 phase and group velocities. Dispersion of explosion sounds in shallow 

 water was discovered and extensively investigated by Worzel and Ewing 

 (1948) during World War II. These observations of Worzel and Ewing 

 in shallow water were explained by Pekeris (1948) who had developed 



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