738 EXPLORATION GEOPHYSICS 



of records placed side by side in proper sequence may be viewed simply 

 as one record and the corresponding actual ray paths may be treated 

 by equivalent ray paths. Reflections can be picked not only over the 

 traces constituting a single record but over those traces of other records 

 for which the reflection seems reliable. 



The continuous profiling method outlined above is sometimes modified 

 in such a way that symmetrical spreads are involved — an arrangement 

 which requires double the number of shot-points for the same spread 

 length. Here the entire length from A to C (Figure 448) is covered 

 by the available set of seismometers and only half the seismometers are 

 transferred for successive spread changes.* Each shot-point then need 

 be used but once. This method of overlapping seismometers is gaining 

 favor because the dip is more readily computed for symmetrical spreads 

 than for uni-directional spreads. 



Continuous profiling by symmetrical spreads as described in the last 

 paragraph is sometimes modified to achieve a compromise between 

 quality of correlation and speed of progress in mapping a prospect. Instead 

 of a complete overlap, shot-points are separated by a distance greater 

 than half the spread length and the seismometer stations nearest a shot- 

 point are used only in conjunction with that shot-point. The common 

 tie-in between records is lost but the gap on the reflecting bed between 

 successive spreads may be kept sufficiently short that correlation is reliable. 



Fault Mapping t 



The most common method of investigating faults by seismic pros- 

 pecting is a negative one in that fault areas are generally first detected 

 on a shot-point line when an area of poor reflections is encountered. If 

 parallel lines also reveal similar gaps on the cross sections and if these 

 gaps can be aligned, the fault may be delineated. An examination must 

 be made for assurance that it is not an anomalous cause such as unfavor- 

 able surface conditions which is responsible for the poor results. 



Reflections from a fault plane are helpful when available. Usually, 

 however, reflections are not recorded from a fault plane for reasons 

 which will be evident from the following considerations. Wave energy 

 is reflected from a fault plane only at points where two beds of different 

 elastic constants or density come into sharp contact. Generally, however, 

 the displacement of the beds at the fault is relatively small, and there 

 are many portions of the fault plane along which either the same beds or 



* Referring to Figure 448, the first shot-point would be B, the second C, and so 

 on. The disposition of the 10 seismometers supplying the traces for the first record 

 would be symmetrical about B and would cover the distance between A and C. An 

 odd number of seismometers and traces, however, is best adapted for this spread in 

 order that one seismometer appear opposite the shot-point. 



t Henry Salvatori, "Mapping Faults by the Reflection Method," Geophysics, Vol. 2, No. 4, 

 October, 1937. pp. 342-356. 



M. C. Kelsey, "Studies in Fault Detection with the Reflection Seismograph," Geophysics, Vol. 

 XIV, No. 1, pp. 21-28, January, 1949. 



A. Wolf, "Seismic Method of Locating Faults," U. S. Patent No. 2,449,921, September, 1948. 



