66 



HERSEY 



[CHAP. 4 



where shallow explosions and receivers have proven inadequate because the 

 sediment arrivals are commonly masked. The outstanding requirement is a 

 source that can be lowered to the bottom, there to operate long enough to 

 enable the observing ship to drift away with a deep-suspended receiver while 

 recording the profile. It is not difficult to invent such a system ; possibly the 

 next few years will see one put into successful operation. One untried possibility 

 is sketched in Fig. 17. While the scheme itself is speculative, the component 

 parts either exist or are under construction at the time of writing. 



5. Interpretations 



A. Geophysical 



Continuous-reflection-profiling data provide a means of determining the 

 geometric relations between the reflection record and the structure; from this 

 description of structure and other independent knowledge, geologic inferences 

 can be drawn in the manner usual for such problems. It is not proposed to 

 review the subject of geological interpretation of geophysical data, but a 



ELECTRICAL 

 CROSS- ca 

 FEED ' 

 FROM 

 SOURCE 



10 MULTIPLE REFLECTIONS 

 BETWEEN SURFACE AND BOTTOM 



50 — 



DEPTH 



IN 



FEET 100 

 (C'4800 



FT/ SEC) 



150 



200 



BOTTOM 

 REFLECTION 

 ^ BEDROCK 



=^EFLECTlON 



200 



Fig. 1 8. A recording of multiple -bottom reflections anid a complex reflection from preglacial 

 bedrock in Narragansett Bay, Rhode Island. Note that the bedrock reflection is 

 weakened so as not to record beneath the strongly reflecting portion of the bottom. 

 (Fink and Bunce.) 



number of straightforward observations have been made by now which must 

 be understood to interpret a recording properly ; these will be discussed in the 

 order that they might well be taken up in analyzing a seismic survey. 



A graphic recorder will bring out correlated signal sequences A\'hich have 

 nothing to do with the profiling (except to degrade it). Typical occurrences are 

 power-line cross-feed and rhythmic noise from ship's engines or propellers. 

 These are best identified when the recording is made. It is usually a simple 

 matter to shut off the sound pulse long enough to distinguish such noises. 

 They should, of course, be eliminated or reduced if possible. 



In shallow-water surveying an echo-sounder record often contains echo 

 sequences of several multiple reflections between bottom and surface. Fig. 18 



