FITZGERALD: CONVERGENCE ZONE DEPENDENCE ON FREQUENCY 



(111.1 Hz). Receiver depth is 1100 m. This Figure contains plots 



of P P +1 and interference wave length 

 mm 



„ \ / C + C 



_ 2tt 1 versus average I _ m m+i 



mr m+1 ~ K - K , J phase velocity | m, m+1 



m m+1 ' ^ 1 ' 



for the two source frequencies. 



The plots of P P , indicate that the principal modes of 

 m m + 1 



propagation are the RSR and highest phase velocity SOFAR modes, an 

 expected feature for these source depths . The lower plot shows that 

 the interference wavelengths for adjacent modes follow the same 

 trends except for phase velocities slightly less than the speed of 

 sound at the surface. The 111.1 Hz modes, which have a turning point 

 just below the ocean surface, rapidly decay above that turning point 

 and do not sense the surface. The 13.89 Hz modes, however, decay 

 more slowly, sense the surface, and are affected by it. 



Looking again at the low- frequency excitation, it appears that 

 the excited low-frequency modes can be divided into two groups. The 

 group with the lower phase velocities has an average interference 

 wavelength of about 62 km. The other group has an average inter- 

 ference wavelength of about 65 km. The high-frequency modes can 

 be divided into several groups based on the excitation plot but all 

 groups have an interference wavelength of about 65 km. 



Figure 4 shows the calculated transmission loss curves for the 

 two frequencies. As anticipated, the high-frequency curve is composed 

 of sharply defined convergence zones having an average spacing of 

 65 km. At the low frequency, the first convergence zone is composed 



of two peaks having different interference wavelengths. As range 



th 

 increases, these two peaks gradually move apart. By about the 20 



672 



