CHRISTIAN: THE ACOUSTIC OUTPUT OF EXPLOSIVE CHARGES 



Since I have gone to some lengths to emphasize the need for more 

 and better data before we can solve source level problems, let me also 

 mention a case where we will have to rely on processing techniques, 

 even if we obtain perfect data. This is the shallow burst, where it 

 is not possible to record the total output wave of the charge separate 

 from the rarefaction wave that is reflected back into the water from 

 the surface. For example, the first bubble period of the very popular 

 Mk-61 SUS fired at 60 feet is about 120 milliseconds, and the entire 

 train of explosion pulses that comprise the charge output lasts for 

 several hundred milliseconds. There is no point in the water at 

 which this pressure wave can be recorded faithfully, because the 

 longest time interval between the direct and surface reflected waves 

 that one can find is 24 milliseconds. This maximum interval occurs 

 directly below the charge, as shown in Figure 12. The parabolic 

 curves of Figure 12 are isopleths of the time of arrival of the re- 

 flected wave; values decrease rapidly as the gage location approaches 

 the surface. (The cognomen "surface cut-off time, t " of Figure 12 

 is the explosions research community's jargon for the time separation 

 between the shock front and the surface-reflected wave. ) Whether one 

 unscrambles the two signals by deconvolution in the frequency domain, 

 as suggested by Hovem (1970) and by Hanna and Parkins (1974) , or by 

 extrapolating time domain functions, as done by Gaspin and Shuler 

 (1971) , some sort of special processing must be applied to obtain 

 source levels. Figure 13 illustrates the degree of spectrum dis- 

 tortion introduced by the reflected pulse if it arrives well beyond 

 the direct wave (top pair of curves) , hard on the heels of the direct 

 wave (center pair) , or in the midst of the direct wave (bottom pair) . 



I have now completed my long list of reasons why it will always 

 be difficult — if, indeed, possible — to predict explosion source 

 levels in 1/3-octave bands to within 1 dB at low frequencies. Many 



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