3.3 SURFACE-BOTTOM MULTIPATH 



The presence of a 9-kHz pinger aboard the Box L during the DOLPHIN experiment 

 allowed measurement of the normal-incidence bottom reflection coefficient. This in turn 

 allows the calculation of the relative magnitude of the direct-path slow-scan signal to higher 

 order multipaths that reflect off the surface and bottom. This calculation will be presented 

 in this section and will show that the direct-to-multipath ratio can degrade rapidly as the 

 direct path departs from vertical. 



During the period when the 9-kHz pinging was in operation, the geometry was as in 

 fig. 10a. Pings were transmitted from the 200-ft-deep baffled conical transducer and received 

 by another conical transducer near the surface and recorded. Figure 10a shows the direct 

 path, while figs. 10b, 10c, and lOd show multipaths with single, double, and triple bottom 

 reflections. 



(a) PINGER GEOMETRY 

 SHOWING DIRECT PATH 



) / ) I ) 7 V 1 1 1 1 / 1 // 



(bl FIRST MULTIPATH 



!c> SECOND MULTIPATH (d) THIRD MULTIPATH 



Figure 10. 9-kHz pinger geometry. 



A segment of tape containing in excess of 20 pings was played back onto a storage 

 scope. The horizontal sweep was triggered on the direct-path arrival. Figure 1 1 is the 

 resulting storage-scope record. The three pulses, from left to right are the returns due to one, 

 two, and three bottom reflections. The spacing between the center pulse and pulses on either 

 side corresponds to twice the 2620-ft water depth. After subtracting the background, we find 

 the amplitudes of the three pings to be in the ratio 4.38:1.13:0.348. The decibel difference 

 between the first and second is 1 1.77 dB and 10.22 dB between the second and third. 



The difference in amplitude between the first-second and second-third pulses is due 

 to a surface reflection loss, a round trip from surface to bottom to surface, and a bottom 

 reflection loss. Using the known depth and an absorption coefficient of 0.8 dB/kiloyard, 

 we calculate a combined surface-bottom loss of 2.97 dB from the difference between the first 

 and second pings and 5.28 dB between the second and third pings, yielding an average of 

 4.13 dB for the combined normal-incidence bottom and surface reflection losses. Consider- 

 ing the glassy seas experienced during the DOLPHIN experiment, this figure agrees well with 

 coastal type bottom loss measurements (Ref. 6). 



By means of the transducer directivity response and the measured bottom loss, the 

 transmission loss for the direct path or any multipath can be computed for the SUBSAT 

 geometry. In particular, we will calculate the signal-to-noise ratio between the direct path 



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