HANNA: DESIGN OF TRANSMISSION LOSS EXPERIMENTS 



If one could determine the impulse response of the ocean between 

 any two points, then, in principle, one can predict what will happen 

 to any signal which propagates between these two points. As a 

 practical matter, however, one can only aspire to measure a band- 

 limited version of this impulse response. If it is furthermore 

 realized that the modelers are interested in not just an experimental 

 determination of the impulse response, but in its interpretation 

 through physical properties of the environment, it becomes clear 

 that measurement planning must consider: 1) the expected properties 

 of the impulse response, and 2) the limitations imposed by signal 

 sources and processors upon the measurement of this impulse response. 

 In the course of this paper, both these topics will be considered. 



THE IMPULSE RESPONSE 



For the purpose of illustrating some properties of the impulse 

 response, it will be assumed that the medium is not dispersive 

 (that is, the medium simply attenuates the amplitude equally at all 

 frequencies and introduces at most a phase reversal upon reflection 

 from the ocean surface). Consider, then, the hypothetical, idealized 

 impulse response of Figure 1 which consists of four arrivals time- 

 delayed according to the history in the upper right-hand corner. 

 There are two pairs of arrivals separated by a time At. The total 

 history is assumed to correspond to the four arrivals of a single 

 order for some source-receiver geometry. For the sake of excimple 

 the two time differences and amplitudes were selected as shown and 

 the spectrum of the resulting impulse response displayed in the 

 figure. 



By way of interpretation, the 40 Hz periodicity corresponds to 

 the time delay At while the 2 Hz periodicity corresponds to the time 

 delay AT. For a more complicated arrival structure there will be a 



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