SECT. 1] REFRACTION AND REFLECTION TECHNIQUES AND PROCEDITRE 35 



6. Reflection Methods 



Two major types of reflection work have been done in the deep sea : variable- 

 angle reflection shooting and vertical reflection work. The former has been done 

 generally in conjunction with refraction lines, although a few special reflection 

 profiles have been performed, primarilj^ by Officer (1955, 1955a). The latter has 

 been of two types, continuous iDrofiling (described by Hersey, Chapter -i) 

 and point recording. Reflection shooting in the deep sea has attained neither 

 the degree of sophistication aj^parent in shallow-water oil exploration work 

 nor the general use of the deep-sea refraction work, for reasons that will 

 become apparent below. 



The problems of reflection work are primarily detection and classification. 

 Detection of sub-bottom reflections requires first an adequate signal source to 

 overcome the background. This is easily obtained with small explosive charges 

 using a quiet hydrophone from a motionless ship ; it becomes more difficult 

 when a moving ship and/or non-explosive source is used. The second require- 

 ment is a method by which reflections from below the sea floor may be identified 

 amid the signals created by reflections from various points on an irregular sea 

 floor. This second requirement impinges on the problem of classification. The 

 sub-bottom reflection must be distinguished from the bottom reflection, and 

 one sub -bottom reflection must be distinguished from another where inter- 

 ference exists. The earliest method of detection of seismic reflections on land 

 depended solely on amphtude ; every large new arrival was identified as a 

 reflection. In a few extremely favorable areas such methods gave usable 

 results (see Weatherby, 1949). Similarly, at sea such methods can be used 

 occasionally. The land work has shifted over to identification on the basis of 

 correlation of arrival time between closely spaced recordings, both at various 

 angles from a single shot and between nearby shots at vertical incidence ; the 

 trend in the A\ork at sea is the same. 



In any deep-sea refraction line, reflections from the sea floor are recorded 

 and reflections from below the bottom may be present. Under favorable 

 circumstances many useful data can be obtained by firing a closely spaced 

 series of small charges at ranges short of that at wliich the first refracted arrival 

 can be detected. Two methods of firing are used ; to avoid complexity of the 

 arrival caused by the bubble pulses and the surface reflections, charges are 

 frequently fired at depths so shallow that the bubble blows out to the surface 

 before any bubble pulses can occur. To provide low-frequency energy for deep 

 penetration into the bottom, charges must be fired at greater depth. Frequently 

 charges at the surface and at depth are alternated (see Hersey et al., 1959). 

 From variable-angle reflection profiles such as these, the depth of the reflecting 

 layer and the velocity above the reflector can be obtained. Recording is nor- 

 mally by the same method as the refraction profile, with three frequency 

 bands recorded on a photographic oscillograph. Distinction of the sub-bottom 

 arrival from topographic echoes is done primarily by frequency content ; 

 echoes appearing on the high-frequency as well as the low-frequency traces are 



