HANNA: DESIGN OF TRANSMISSION LOSS EXPERIMENTS 



spectrum of the impulse response discussed earlier, with an appro- 

 priate choice of receiver depth (about 800 feet in this case) , which 

 would be indistinguishable from that caused by the bubble pulse of 

 the source at 60 feet. Similarly, referring now to Figure 3, the 

 source at 800 feet has a bubble pulse frequency of approximately 

 50 Hz (the surface reflected path has been gated out in the time 

 domain) and, again, a particular receiver depth could induce a com- 

 parable periodicity in the spectrum of the impulse response. 



The first point to be made, then, regarding measurement of the 

 impulse response of the ocean is the nature of the limitation in- 

 duced by our attempt to produce a source with a flat spectrum. The 

 rule of thumb which follows from this point is that a source should 

 be chosen (or tailored) such that its distinct spectral features are 

 very different from those features of interest in the spectrum of 

 the impulse response. 



A second potential difficulty may arise in the choice of a signal 

 processing scheme. If it is desired to measure the spectrum of the 

 impulse response in detail over a wide band, then a natural choice of 

 processing is digitization of the data and FFT spectrum analysis. 

 Even though this processing permits very narrowband analysis, some 

 frequency average of the spectrum will be desirable for at least one 

 of two reasons: 1) it may be necessary to average over spectral 

 variations of the source which are not strictly repeatable from event 

 to event (such as the 6 Hz variation in Figure 2) , and 2) it may be 

 necessary to average over certain fine structure of the spectrum of 

 the impulse response itself which is known (or expected) to change 

 rapidly from measurement to measurement. These factors are further 

 explored by Hanna and Parkins (1974) . This frequency average can be 

 selected only with knowledge of the detailed structure of both the 

 source and impulse response spectra. 



515 



