570 



SEISMIC METHODS 



[Chap. 9 



ment. The best procedure is to select a favorable location (see paragraph 

 c below), to drill a hole 25 to 50 feet deep, and to select, from inspection of 

 the well samples, a firm or moist formation for placing the shot. Shots are 

 fired with gradually increased charges and records are taken at 100- to 

 200-foot intervals, beginning as closely to the shot point as possible. 

 These may be utiUzed later for average velocity calculations if desired. 

 The distance most favorable for the desired reflections is then selected. 

 Quality of reception may be improved by suitable adjustment of filters, 

 and the like. From the records and from geologic considerations the 

 observer will decide whether dip shooting or continuous profiling may 

 be necessary. Surface geology data mil determine the weathered-layer 

 procedure. 



(6) Types of shooting. When reflecting beds are consistent and of low 

 dip so that no correlation difficulties are experienced, the normal setup 



with receptor spread to 

 7 ' 2T one side of the shot point 



is used (correlation shoot- 

 ing). Shot distances vary 

 from 200 to 2000 feet to the 

 middle receiver and re- 

 ceiver intervals from 20 to 

 100 feet. If reflecting beds 

 are not consistent or if cor- 

 relation difficulties arise 

 from other sources, con- 

 tinuous profiles are shot. 

 This is a variation of dip 

 shooting. When the spread 

 has been shot in the for- 

 ward direction, the shot 

 point is moved to the loca- 

 tion previously occupied by the last receiver. From this point the first 

 spread is shot in reverse and the next spread in forward direction (Fig. 

 9-90). Since the paths from I to 6 and from II back to I are identical, 

 the travel times are identical and the two records may be tied in. Within 

 the setup shown in the figure, the reflection points are continuous. The 

 "holes" left under the shot points between successive profiles may be 

 avoided by using more overlap. Receivers are evenly spaced between 

 shot points, generally at 100 to 150 feet. 



Dip shooting is applied when there is noticeable dip and when reflecting 

 beds are not consistent, so that presence or absence of structure may be 



Fig. 9-90. Arrangement for continuous profiling. 



' S. J. Pirson, A.I.M.E. Tech. Publ. No. 833, 1937. 



