52 HERSEY [chap. 4 



(c) The Seismic Profiler: a high-tension spark discharged under water 

 (Knott and Hersey, 1956). Recently the boomer, an eddy-current-actuated 

 sound-pulse generator, was developed by Harold Edgerton primarily for use 

 with the seismic profiler but also for other applications in acoustical oceano- 

 graphy. The design lends itself to inexpensive alteration of the radi- 

 ated pressure-pulse shape and duration. The name seismic jxrofiler has also 

 been given to the method emjiloying exjjlosives by Ewing and Tirey (1{)61). 

 Possibly this is a suitable generic name for this kind of seismic instrument 

 system, whether the source is high explosives or an electro-mechanical trans- 

 ducer. 



(d) The Snb-botfom Depth Recorder: similar to the seismic profiler. A spark 

 source is used for high frequencies and fine resolution ; a repeating gas explosion 

 (propane-oxygen) is used for lower frequencies and deeper penetration (Beck- 

 mann et al., 1959). 



The Field Research Laboratory, Socony Mobil Oil Corporation, has developed 

 a gas exploder in the form of a vertical shock tube with one end under water. 

 Starting with spark ignition near the top a shock wave develops as gases (pro- 

 pane, oxygen) burn. The shock wave strikes the water surface at the bottom, 

 generating the sound pulse. The pressure-time wave-form (see Fig. 3) contains 

 both short and long duration pulses {in litt., manuscript in preparation). 



A . Sound Sources 



Fig. 3 contains the pressure-time curves available for the above sound 

 sources. They were all taken with broad-band pressure gauges at distances 

 from 6 to 60 ft from the source. The directional properties of these pulse 

 generators (excepting Padberg's) have not as far as we know been investigated 

 in any detail. All electrically powered sources employ a large condenser for 

 energy storage and various methods of triggering its discharge. Stored energy 

 varies from 4 to over 25,000 joules in models that have been extensively used. 

 Acoustical energy is from 0.5 to 10% of the electrical energy. [For a detailed 

 discussion of power supplies and transducers see Hersey et al. (1962) and 

 Caulfield (1962).] 



B. Receiving Hydrophones 



Piezoelectric and magnetostriction hydrophones have been used more or 

 less indiscriminately by various investigators, who have developed habits 

 depending on preference, convenience and estimates of performance in towing. 

 Of the published accounts of investigations at sea the Sonoprobe system 

 employs a receiver fixed by a rigid mounting to the rail of the shi]), whereas 

 both the seismic profiler and the sub-bottom depth recorder employ single 

 hydrophones towed well astern. Mostly, hydrophones are chosen for broad- 

 band, flat response (at least 30 to 10,000 c/s) and for best towing. Since noise, 

 whether ambient, self noise, or flow noise, is almost always more intense at low 



