D. E. Weston 61 
sion is underwater, underground, or even in the air. The shock duration is 
similar underwater and underground, so that both spectra show a turning point 
near the reciprocal F, of this shock duration, and a similar high-frequency spec- 
trum. However, the low-frequency spectra are quite different since the average 
underwater spectrum is flat down to about F,, the bubble-pulse frequency. This 
arises because the restoring force underwater isthe hydrostatic pressure, lead- 
ing to a negative pressure phase ofmuch greater duration than the shock. Under- 
ground, the restoring force is the rock rigidity, the positive and negative phases 
are of comparable length, and the f? spectrum law starts just below F,. These 
differences mean that the underwater explosion isa much better seismic source; 
a typical calculation suggests 10% efficiency underwater and 0.05% underground. 
There is experimental evidence for this underwater superiority. 
3.7. USES IN RESEARCH 
A very important aspect which should be included in any account of under- 
water explosions is the use of these explosions in acoustic research. The uses 
are manifold, even excluding seismic investigations of the sea bed, so I will not 
attempt to give a full list. One use is to investigate transmission loss, including 
such things as reflectivity and fluctuation measurements. Another is to measure 
reverberation: volume reverberation including that from biological scatters, 
surface reverberation, bottom reverberation, and reflections from distant topo- 
graphic features. There are several other possible uses, many little explored. 
(Shots were fired in a wide range of conditions - four shots to measure trans- 
mission and three shots near the receiver to measure reverberation.) 
A little more will be said about the investigation of what may be considered 
the central problem in underwater acoustic research, i.e., transmission loss, 
Most investigators apparently use similar techniques, which are not particularly 
novel. Typically, one goes to sea with two ships. One ship is used for sound 
reception, with one or two hydrophones suspended in the sea. The second ship 
opens range while firing charges, possibly using a safety fuse. Alternatively, 
there may be serials at constant range, varying the depth of the charge or the 
hydrophone. Normally, the signal energy levels are measured using analog com- 
puters, but there are possibilities for other processing methods, such as the 
digital computer. For some purposes, peak pressures may be measured. Worth- 
while precautions include monitoring overload in all channels, scheduling some 
close shots as a check on source level and on the whole system, and measuring 
rather than estimating any differences between different charge types. In the 
typical trial, a large quantity of information is collected and a large analysis 
headache is generated. Lastly, note that one should beware of overload; in shot 
work this is of extreme importance and cannot be stressed too often. 
Figures 3.7 and 3.8 show two examples of results achievable with explosion 
sources, both taken with 1-lb charges in water of constant depth. Only a very 
little will be said about the physical meaning of these plots. Figure 3.7 shows a 
composite frequency plot for a range run in the Norwegian Sea, with 58-fathom 
source and 140-fathom receiver. Signal level itself has been plotted, and this is 
a first stage in presentation which is useful when source level may not be 
accurately known. Note that at low frequencies there is hardly any departure 
