Gallagher 



Table 3 



MAXIMUM DIFFERENCES IN REFLECTION COEFFICIENT VALUES 

 CORES NO. 1 AND NO, 2-HAP 



Incldenf Direct ^ Bottom Loss Difference 



Angle Measurement Direct Minus Sutton 



Core No. 1 35° 0.17 (15.0 db) 0.45 (7,0 db) +8.0db 



48° 0.20(14,0db) 0.54 (5.4 db) +8.6 db 



58° 0. 14(17. Odb) 0.51 (5.7 db) +11.3db 



Core No. 2 14° 0.06 (24.5 db) 0.39(8.1 db) +16.4 db 



79° 0.09 (21 .Odb) 0.54 (5.4 db) +15.6 db 



consistently true over oil angles. The impedance values of the layers, the 

 thicknesses of the layers, and the angles and speeds at which the sound rays are 

 propagating through the layers apparently combine to form constructive and 

 destructive interferences and thereby regulate the amount of acoustic energy 

 returned to the interface. 



The maximum differences in the values of these reflection coefficients, and 

 of bottom loss, derived from the measured and Sutton data, are shown in Table 3. 

 It is interesting to note that whereas the magnitude of the sound velocity differ- 

 ences between the measured and Sutton data in core no. 2 is somewhat less than 

 that in core no. 1, larger differences in bottom loss are seen in core no. 2. 

 These large differences may occur aperiodically over various angles, depending 

 upon the layering effects of the model. It should be noted that the differences 

 are smaller at other angles and at some angles there is a zero difference. It 

 should also be noted that since the bottom loss differences are logarithmic 

 ratios, a change in one unit of reflection coefficient at the lower range will 

 produce a different corresponding change in db than will a change in one unit 

 of reflection coefficient at the upper range of the scale. 



Table 4 indicates the differences in bottom loss between the measured and 

 Sutton derived data In cores 1 and 2 and the data obtained from field acoustic 

 tests at a frequency of 3.7 kHz. Considering the suspected high measured 

 velocity values for core no. 1, It Is interesting to note that the field data 

 report generally lower bottom losses for most incident angles. The exceptions 

 are at 42° and at angles approaching normal Incidence. In a general sense, it 

 would be expected that even higher sound velocities would be required to pro- 

 duce higher impedance values in order to obtain the lower bottom losses reported 

 by the field test data. However, In noting the previous remarks. It can be seen 

 that this will not necessarily follow. Generally, the 3.7 kHz field test data 

 exhibit lower bottom losses than those for both cases for core no. 2. 



95 



