852 



EXPLORATION GEOPHYSICS 



ease of disposition of seismometers, the seismometers and shot-points 

 are required to be on the same line. When the uni-directional spread 

 has shot-points at both ends, it is usually called the "two-hole" spread. 



The symmetric offset 

 _.<3 ^ H^_.-o-_^_a-^__^^. gpj-eadf is used to avoid the 



^°^^'^ long gap between the seismom- 



eters nearest the shot-point. 

 The effect of the offset spread 

 SYMMETRICAL OFFSET IS to displacc reflcction pomts 

 • SPREAD Q^ ^j^g Strata along the cross 



dip, but in general this may 

 be made sufficiently small to 

 — • -o -o o- o be negligible. The right-angle 



UNIDIRECTIONAL j • i • j , , , • 



SPREAD spread is designed to obtain 



line and cross dips simultan- 

 eously. The number of seis- 



— -« o o- • — mometers used in this spread 



SPREAD^ is usually greater than that 



used in the spreads described 

 previously. 

 — o- -o _^_._o_ 0- — por dip shooting, the sym- 



OFFSET UNIDIRECTIONAL . \ . ^ . ./ 



SPREAD metric rather than uni-direc- 



I tional spreads are preferred 



j but sometimes the uni-direc- 



_o. (j, o o 0-- tional spread must be used for 



i "'* spread'''^ ^ variety of reasons : such as, 



j inaccessibility and when, due 



? •shot -POINT to excessive shot-point dis- 



j turbances, the seismometer 



9 seismometer nearest to the shot-point must 



I be located a great distance 



from that point, too far to 



T solve the problem properly by 



Fig. 533.— Types of seismometer spreads. offsetting the Spread laterally. 



(See "open" spreads, p. 853.) 

 In correlation shooting, the uni-directional spread is the most efficient 

 when very short spreads are desired (p. 707). Continuous seismic profiling^: 

 is another frequent application of uni-directional spreads (p. 72)6). (Ve- 

 locity shooting (p. 728) is a special application of this type of spread.) 



Also, in reflection shooting, spreads are sometimes shot in various 

 combinations under certain circumstances. For example, when using 

 equipment with relatively few traces, such as six, it may be advantageous 



t Henry Salvatori, "Apparatus for Seismic Surveying," U. S. Patent 2,117,364, issued Dec. 

 24.. 1936. 



t L. F. Athy and E. V. McCollum, "Method of Making Geophysical Exploration," U. S. Patent 

 2,336,053, Dec. 7, 1943. 



