36 SHOR [chap. 2 



topographic; echoes on the low-frequency traces only are sub-bottom. Officer 

 (1955, 1955a) has tape recorded reflection arrivals and played them back 

 through narrow-band filters for this same ])ur])ose. For such a i)urpose the tape 

 recorder used must have good response in the low-frequency bands. 



Reflection records at vertical incidence can be taken from a single ship with 

 little effort. The first extensive program of this sort was performed by WeibuU 

 (1954) on the Swedish Deep-Sea Expedition, 1947-1948; his method was 

 unique and has not been copied extensively. He distinguished between sub- 

 bottom reflections and topographic reflections by firing several shots at different 

 depths below the surface ; only for the sub-bottom reflection did the time-lag 

 after the bottom reflection remain a constant for shots at different depths. 

 Interference and complication of the record by bubble pulses was eliminated 

 by firing the shots at depths so great that the bubble-pulse interval was much 

 shorter than the bottom-to-sub-bottom interval for which discrimination was 

 desired. Concurrently, the surface reflection was made so late that it normally 

 occurred after all readable sub-bottom reflections were past. This method was 

 performed with pipe-encased charges with a weighted nose. They were fired 

 by means of a special pressure-sensitive detonator. When it was desired to fire 

 charges on bottom a delay disk of sintered copper was placed between the 

 pressure detonator and the sea, delaying build-up of pressure in the detonator. 

 Detection was with a wide variety of relatively insensitive near-surface hydro- 

 ])hones. Recording was done by photographing the trace on an oscilloscope 

 and by means of a wire recorder with subsequent playback. 



Independently and during approximately the same period, Hersey and 

 Ewing (1949) developed a method more akin to that used in the refraction 

 work. Shots of 1 lb of TNT were fired at depths of 100 to 300 ft in the earlier 

 part of the work and at 2 or 3 ft later. A hydrophone at depths varying from 

 150 to 1000 ft fed an audio-amplifier and an octave-band filter. Traces w^ere 

 photographed from an oscilloscope. The principal energy recorded was in the 

 frequencies above 100 c/s. Since that time further development has led to an 

 underway shooting method in which the hydrophone is towed behind the ship 

 and the cable is slacked at the time of each shot. Many thousands of reflection 

 shots have been taken in this manner. On-station reflection shots with charges 

 flred at depths of 150 ft and hydrophones at the same depth were used by 

 Shor (1959) in an area of the east Pacific where a strong reflection occurs 

 late enough to be unaffected by the bubble pulses and surface reflections. 



Development of improved reflection methods is progressing rapidly; major 

 changes have occurred between the writing of this papier and its return in proof. 

 Ewing and Tirey (1961) have developed a system of recording in which the firing 

 of each shot starts the rotation of a recording drum on which are recorded each 

 ])eak and trough of the returning signal as a black dot. The hydrophone and 

 amplifiers used are the same as those of refraction work; the hydrophone is 

 towed behind the ship, and the towing wire is slacked at each shot to allow the 

 hydrophone to sink slowly through the water. With many closely spaced shots, 

 one obtains a record, similar to that of a fathometer, which permits rapid 



