PORTER: SOFAR PROPAGATION OF WIDE-BAND SIGNALS TO LONG RANGES 



along several different paths. In contrast, suirveillance systems, 

 and the propagation models designed to analyze them, are based on 

 the reception of narrow-band components. We need to know more 

 about the interrelationships of these very different signals after 

 they have traveled through the ocean. 



The acoustic field at a long range from a localized source is 

 a strong function of the time dependence of the projected pulse. For 

 example, the propagation loss estimated from shot data does not reveal 

 the semi -periodic variation with range that is observed with single 

 tone or CW sources. As most calculations of transmission loss are 

 based on the transmission of a single harmonic, we should not be 

 surprised when such models fail to accurately predict sound levels 

 from shots. Clearly, predictions of signal level should be based on 

 models that include the spectral characteristics of the source and 

 receiver as well as the channel propagation. 



In this paper, we illustrate some of the observed phenomena found 

 in wide-band propagation, discuss a propagation model, and present 

 the results of some simple loss calculations. We have investigated 

 wide-band propagation in the Mediterranean Sea and have found clearly 

 delineated mode and ray arrival structures. Dispersed high-order 

 modes have been observed at frequencies as high as 300 Hz and at 

 ranges of 600 km. Spectrum analysis of shot records has revealed as 

 many as five identifiable group-velocity profiles (modal dispersion 

 curves) that correlate with propagation modes as high as 70. It is 

 typical of these records that the highest frequencies arrive last. 

 These group velocity profiles can be identified because only a few 

 high-order modes are received. This quite unexpected result is a 

 consequence of interference between the upward and downward traveling 

 waves that constitute each mode. 



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