306 Lecture 16 
s oi Ye ee Ae [ee 
40 FMS, —————>| 
j}¢—_—_—___—— APPROX. 4 MILES —————————__>| 
Fig. 16.5. Ten-kcps echo-sounder record showing penetration of soft mud 
over bedrock, 
Several kinds of sound source have been designed for this application. The 
"Sonoprobe" developed by the Magnolia Oil Co. produces approximately a half 
wave of 3.8-kcps sound with a peak power of about 10 Mw. This is still rather 
a high frequency for deep penetration, but it has high resolution. Underwater 
sparks have been used with considerable success [6] but always produce a double 
pulse due to the collapse of the gas bubble, and this reduces their effective 
resolution. 
Perhaps the most interesting and promising device isthe Edgerton "Thumper" 
[7]. This is extremely simple in principle, consisting of a flat coil of wire with 
a circular aluminum plate 20 in. in diameter held loosely against it. A pulse of 
current passed through the coil produces a strong magnetic field which induces 
eddy currents in the aluminum plate. The interaction of these eddy currents 
with the magnetic field produces a force which strongly repels the plate from 
the coil, and thus produces an acoustic pulse in the water. The standard unit 
uses a peak current of about 2000 amp from the discharge of a 160- pf capacitor 
charged to about 4 kv and produces a peak pressure of about 250,000 d/em? at 
1 yard. The initial pulse consists of aclean unidirectional pressure pulse lasting 
about 0.5 msec, but unfortunately this was followed in the original design by two 
or three waves of lower frequency caused by various factors. N.I.O. has made 
major modifications to the mounting arrangements of the coil and plate, and it 
seems that this has considerably improved the waveform, but at the time of 
writing the system is still being tested at sea, and the precise waveforms are 
not available. 
N.1I.0. is now using as a receiver a line array of hydrophones towed behind 
the ship so that the array has almost zero sensitivity in the forward direction 
and thus does not hear the noise generated by the ship. The received signal is 
passed through a band-pass filter with a range of 300 cps to 1 kcps, which has 
to be specially designed to have a good impulse response. Using this system 
towed at 7'/, knots, echoes trom strata arriving 0.16 sec after the bottom echo 
have been obtained in water depths of about 900 ft. We understand (private com- 
munication) that considerably better results have been obtained in the United 
