HANNA: DESIGN OF TRANSMISSION LOSS EXPERIMENTS 



the transmission loss, the inferred reflectivity is plotted assuming 

 that the measured loss is either the 35 or 100 Hz filtered result 

 while the computed loss is the rms sum. There are two important 

 points to be made here: 1) the inaccurate transmission loss model 

 (viz. , the rms sum) induces spurious character into the inferred 

 reflectivity which is not only frequency dependent, but source- 

 receiver geometry dependent; 2) the inferred reflectivity loss is 

 consistently negative for angles less than 6 to 7 degrees. 



Water-refracted Paths 



Up to this point is has been assumed that the four bottom- 

 interacting paths can be resolved from all other paths in the problem. 

 This is not always the case for ranges corresponding to low grazing 

 angles. To demonstrate this fact, consider first the ray plot of 

 Figure 14 where are shown the rays from a source at 800 feet which 

 arrive in the range-depth window from 25 to 35 nm and to 300 meters. 

 Those paths which reflect from the surface are distinguished according 

 to whether they interact with the bottom or belong to the RSR family; 

 also shown are the RR rays. Consider the rays which intersect the 

 receiver depth at 300 feet: the last bottom-interacting path arrives 

 at a range of 31 nm, yet even the ray-trace shows non-bottom- 

 interacting paths arriving in the overlapping range from 29.5 to 31 

 nm. In reality, however, the refracting paths make their influence 

 felt before 29.5 nm in the form of the shadow zone field of the RR 

 caustic. The relative travel time between the refracted field and 

 the bottom interacting field is sufficiently small so as to not be 

 resolved by 1/3-octave processing at low frequencies. Thus, attempts 

 to measure the bottom-interacting field at these ranges may be 

 thwarted by the additional influence of the refracting field. 



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