















j 1 ... - —4MMM ■ 



'*• — * • • ■ - f*' 



v*;": ' .-■•-.••; ^- ■ ;/-;;■.' 



Figure 1 1. Storage scope record at 9 kHz. Pings 0.5 sec per division. 

 Figure shows direct path and three lowest order multipaths. 



and the multipath arising from a single reflection off the surface and bottom. The calcula- 

 tion was done for a combined surface bottom loss of 4.13 dB (assumed independent of angle) 

 at 10 kHz for a slow-scan source near the bottom transmitting to a near-surface receiver. 

 Although these conditions do not apply directly to either experiment, they serve to illustrate 

 the effect of rapidly increasing multipath with horizontal offset. Figure 12 is the result of 

 this calculation and shows the rapid dropoff of the signal-to-noise ratio as the direct path 

 departs from the vertical (i.e., as the horizontal offset increases). In order to maintain a 

 signal-to-multipath level sufficient to yield reasonable picture quality (10 dB, for example) 

 it is necessary to keep the vertical angle less than about 30 deg. Control over horizontal 

 offset was not possible during the SEACLIFF and DOLPHIN experiments and leads us to 

 conclude that multipath contributed to the observed degradation in picture quality. 



19 



