AVERAGE BOTTOM REVERBERATION INTENSITIES 



321 



since at the lower frequencies the corrections for at- 

 tenuation are not as large. 



In Table 6, cobbles and boulders have been 

 grouped under rock; and fine sand, foraminiferal 

 sand, and medium sand have all been grouped under 

 sand. The last column in Table 6 gives the results of 

 averaging a similar grouping of the 0-degree values 

 of Table 2, including coarse sand under sand; the 

 designations sand-mud and mud-and-sand of refer- 

 ences 2 and 3 have been replaced by the customary 

 SAND-AND-MUD. If all the entries of Table 6 

 are averaged with equal weight, we obtain the over- 

 all averages in Table 7. 



Table 7. Overall mean values of backward scatter- 

 ing coefficient at 10-degree grazing angle. 



Bottom type 



10 log m" 



ROCK 



MUD 



SAND-AND-MUD 



SAND 



-20 + 5 

 -27 + 5 

 -28 ± 5 

 -32 ± 5 



The values of Table 7 do not differ significantly 

 from other estimates of the mean bottom scattering 

 coefficients, also based on the data of references 2 

 and 3. In reference 7 it is estimated that the quartile 

 deviations of the mean bottom scattering coefficients 

 are about 5 db for each bottom type. In view of the 

 crudeness of a classification system which includes 

 all bottom types in only four categories, a quartile 

 deviation of this magnitude is not surprising. Thus 

 this estimate of the deviation from the mean has 

 been included in Table 7. It must be remembered 

 that the values of 10 log m" in Table 7 are true 

 values; that is, they were determined by subtracting 

 6 db from the values of m" inferred from comparison 

 of equation (54) of Chapter 12 with the measured 

 reverberation levels. The expected reverberation 

 levels with horizontal beams will therefore be 6 db 

 greater than the levels that would be predicted by 

 the use of equation (54) and the values of 10 log m" 

 in Table 7. 



15.4 AVERAGE BOTTOM REVERBERATION 

 INTENSITIES WITH HORIZONTAL 

 TRANSDUCERS 



Bottom reverberation levels are a function of range 

 and water depth and in addition depend on refraction 

 conditions, transducer orientation, and bottom type. 



For most practical echo-ranging purposes, however, 

 the transducer is oriented so that the transducer axis 

 is horizontal, parallel to the ocean surface. Under 

 these circumstances, over level bottoms, the data 

 which have been presented in this chapter can be used 

 to make some prediction of average bottom rever- 

 beration levels. The results of reference 2, discussed 

 in Section 15.3.1, show that under most conditions 

 the transmission anomaly due to refraction is negli- 

 gible a;t ranges up to and including the range of the 

 reverberation peak. The results of references 1, 2, and 

 3, described in Section 15.2, all show that in water 

 other than isothermal or nearly isothermal the range 

 of the reverberation peak tends to be about 6 times 

 the depth between the projector and the bottom, and 

 that this peak corresponds approximately to the 

 range at which the 5- to 6-degree ray from the pro- 

 jector strikes the bottom. The data of references 2 

 and 3, which were discussed in Sections 15.3.1 and 

 15.3.3, show that the angle at which this ray strikes 

 the bottom usually is about 10 degrees in nonisother- 

 mal water. Thus, knowledge of the average value of 

 m"at a grazing angle of lOdegrees, coupled with equa- 

 tion (54) of Chapter 12, enables prediction of the aver- 

 age height and range of the reverberation peak over 

 different bottom types in any water depth. It is of 

 course necessary to know the value of A in equation 

 (54). However a value of A equal to 4 db per kyd is 

 probably a good approximation, and for the short 

 ranges at which the reverberation peaks are usually 

 observed, deviations in practice from 4 db per kyd 

 should not be very significant, except possibly when 

 the water is quite deep. 



Once the height and range of the bottom rever- 

 beration peak are determined, the most significant 

 quantity for echo ranging is the rate at which the 

 reverberation decays as a function of range past the 

 peak. At ranges less than the peak the reverberation 

 usually decreases with decreasing range; such ex- 

 amples as Figure 7 and other similar figures in refer- 

 ences 2 and 3 show that the bottom reverberation can 

 hardly increase with decreasing range as rapidly as 

 the expected echo level.* At some ranges less than 

 the principal reverberation peak (where the main 

 beam strikes the bottom) reverberation from side 

 lobes can be very high. However, it seems on the 

 whole that bottom reverberation is likely to be most 

 troublesome at ranges past reverberation peak. 



At ranges past the reverberation peak in noniso- 

 thermal water, the results of reference 1 indicate that 

 on the average the reverberation falls off at about the 



