Clark and Yarnall 



of these phenomena covering a relatively broad frequency range. These 

 signatures, however, should be interpreted with the understanding that 

 the received signal provides a measure of disturbances in the sound 

 speed stratification of the medium, and not a direct measure of dis- 

 turbances in the density stratification. It should be expected that 

 directivity factors are involved; e.g., the acoustic response is a 

 function of the angle between the acoustic path and the direction of 

 propagation of the internal wave. Also, the visual technique employ- 

 ed for this initial study of the signatures has limitations which are 

 difficult to define. Even within the limitations that all of these 

 factors impose on the analysis the acoustic probe appears to provide a 

 measure of spectral distribution and time of occurrence of short 

 period internal waves which may be impractical to obtain by other 

 means. 



TIMLLY RELATED SIGNATURES 



Both wind driven surface waves and short period internal 

 waves are spatially limited in their effect upon the medium. The 

 amplitude of a surface wave, whose wavelength is small compared to 

 water depth, decreases rapidly with depth in a manner that is well 

 understood. Similarly, the locus of maximum disturbance for stability 

 oscillations will be a horizontal plane at some level within the medium. 

 On the scale of tidal phenomena, however, "long wave" behavior is to 

 be expected, and substantially all of the water column can be involved 

 in the motion. Under these conditions we move into a realm of large- 

 scale, spatially coherent changes in the medium for which we might 

 expect qualitative correspondence with previously discussed features 

 of the propagation model. 



We do, in fact, find that the most outstanding features of 

 the propagation model are demonstrated in the data. The spectral 

 signatures of the tidal scale processes are clearly evident in signal 

 phase, and they display the fundamental tidal frequencies. On the 

 other hand, signal amplitude, which provides spectral signatures of 

 surface waves and short period internal waves at the fundamental fre- 

 quencies of these phenomena, is a complex interference pattern on the 

 time scale of tidal events and is difficult to interpret directly. 

 These features are evident in Figure 19, which illustrates a tidally 

 related signature. Note particularly in Figure 19 that signal ampli- 

 tude, R(t) , forms an almost syinmetrical pattern mirrored from left to 

 right across the valley of the phase excursion at 1400 hours. Mirror 

 symmetry across both peaks and valleys is demonstrated by the prop^ 

 agation model (Fig. 8) although through a much broader range of phase 

 excursion. 



Figure 19 calls for additional discussion; it has been 

 included primarily to be illustrative of visually transient features 

 of the data which have been attributed to non- linear tidal harmonics 

 (shallow water tides) . The spectral lines corresponding to these 

 effects do not show up in the tidally related phase fluctuation 



325 



