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



5 and the solid circles correspond to shot points occupied by the ice 

 breaker "Northwind." The signals were recorded aboard T-3. A signal 

 along Path 6 will show the dispersion of waves of the first two normal 

 modes. Profiles 1 and 2 correspond to flight lines for air-dropped 

 SUS charges recorded on T-3 in 1968. Peak signal intensities for 

 these profiles will be shown later, but let me point out here that 

 the signal strengths as a function of range are considerably weaker 

 along Profile 2 than along Profile 1 because of a corresponding 

 difference of ice roughness. 



Figure 5 shows typical signals detected by hydrophones at 

 depth and geophones on the ice surface. The signals often begin 

 with a series of pulse-like waves corresponding to the deep penetrating 

 RSR sounds. These waves may have the largest amplitudes. If shallow 

 water intervenes, these waves are weak or absent. Sound spectrograms 

 show that these waves consist of a superposition of a number of normal 

 modes. Following these pulse-like waves, a train of nearly sinusoidal 

 waves arrives in which frequency clearly increases with time. This 

 dispersion is characteristic of Arctic SOFAR propagation, and it is 

 useful for identifying weak signals by ear from high-speed playbacks 

 of tape recordings: the signals give a tone of increasing frequency. 



Figure 6 shows the nearly sinusoidal waves corresponding to 

 the first two normal modes at a range of about 650 nautical miles from 

 the shot . 



Figure 7 shows a sound spectrogram of waves corresponding to 

 the first two normal modes. The signal was detected on the ice by 

 a geophone as part of a special infrasonic listening system capable 

 of responding to waves over the frequency band from 0.1 to 30 Hz. 

 This spectrogram illustrates clearly the spectrum of waves in the 

 extremely low- frequency band from about 2 to 30 Hz. The reverberation 



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