220 



TRANSMISSION OF EXPLOSIVE SOUND IN THE SEA 



and partially transmitted at the boundaries between 

 the strata, and a complicated sequence of multiple 

 reflections may take place. Finally, the pulse may be 

 transmitted horizontally through the bottom, the 

 disturbance of the bottom at each point being accom- 

 panied by a corresponding disturbance in the water. 

 In this phenomenon the impact of the explosive wave 

 on the bottom below the charge sets the bottom into 

 vibration, and this vibration is propagated radially 

 outward like a surface wave on the water, or, to use 

 a more accurate analogy, like a surface-bound earth- 

 quake wave, its frequency and velocity being in- 

 fluenced, however, by the water overlying the 

 bottom. 



The three following subsections deal in turn with 

 the simpler and the more complex aspects of these 

 phenomena. Section 9.4.1 treats ordinary reflections, 

 using the concept of sound rays, and discusses arrival 

 time data for certain parts of the "earthquake wave," 

 since these data can also be interpreted in terms of 

 rays. Sections 9.4.2 and 9.4.3 discuss the detailed 

 form of the pulses transmitted by the "earthquake 

 wave," which can be understood only by abandoning 

 the ray concept and treating water and bottom as a 

 single dynamical system. 



In the theoretical portions of all these sections it 

 will be assumed for simplicity that the bottom is 

 smooth and horizontally stratified; and an effort will 

 be made to interpret the experimental material in 

 terms of this idealization. It must be remembered, 

 however, that there may often be small-scale ir- 

 regularities in the bottom which will scatter the ex- 

 plosive pulse, and large-scale departures from hori- 

 zontal stratification, which will complicate the trans- 

 mission phenomena. 



9.4.1 Reflection Coefficients and 

 Times of Arrival 



When a pulse of sound strikes a plane boundary 

 between two media of different acoustic properties, 

 the reflected pulse has a lower amplitude than the 

 incident pulse and in general a different phase. A 

 theoretical derivation of the amplitude and phase 

 relations to be expected at the boundary between 

 two ideal fluid media has been given in Section 2.6.2. 

 Actual ocean bottoms may differ in their properties 

 from the ideal media considered there, however. To 

 describe completely the reflecting properties of a 

 given bottom, one should specify the amplitude re- 

 duction and phase shift for all frequencies and all 



angles of incidence. An equivalent description, which 

 could be related to this by the methods of Fourier 

 analysis (see Section 9.2.4) would be provided by 

 recording the exact form of the reflected pressure- 

 time curve for an explosive pulse for all angles of inci- 

 dence. So far, however, no pressure-time curves have 

 been recorded for explosive pulses reflected from the 

 bottom. The only quantitative data on bottom re- 

 flections which are available are those obtained at 

 WHOI" in connection with the long-range propaga- 

 tion studies discussed in Section 9.3.2. These data 

 will now be described. 



As mentioned in Section 9.3.2, the series of experi- 

 ments for which analyses of bottom reflections were 

 carried out was made using a shallow hydrophone at 

 80 ft and a deep hydrophone at 1,600 ft, with shots 

 of 3^-lb TNT fired at depths of the order of 50 ft at 

 ranges up to 30 miles. Two recording channels with 

 different frequency responses were used for the shal- 

 low hydrophone, and five for the deep hydrophone. 

 The reflection coefficients of the bottom were deter- 

 mined for each of these channels by making plots 

 similar to that in Figure 22 for the pulses undergoing 

 respectively one, two, and three bottom reflections 

 and then measuring the vertical displacements be- 

 tween the lines corresponding to different numbers of 

 reflections. The values obtained are given in Table 7. 



Table 7. Reflection coefficients for the bottom in the 

 region near Latitude 26°46' N, Longitude 76°25' W. 



This method of analysis, while probably the best that 

 can be applied to the data available, is rather crude 

 in that the angle of incidence of the rays on the 

 bottom changes with range and also with the order of 

 the reflection; if, as is often the case, the reflection 

 coefficient varies strongly with angle of incidence, 

 only a vague average over a range of angles will be 

 obtained. 



The values given in Table 7 suggest a decrease of 

 reflection coefficient with increasing frequency, an 

 effect which would not occur at a plane boundary 

 between two ideal acoustic media. Unfortunately 



