SEISMIC METHODS 



739 



different beds of approximately similar characteristics are in contact. 

 Hence, even when the plane of faulting is very sharp, it will rarely act 

 as a good reflector of wave energy over an appreciable section. 



A correlation showing a vertical displacement is the best kind of 

 evidence when supported, of course, by general dip information in the 

 region, but such evidence is not often found. Sometimes reflections indi- 

 cating the fault drag are detected. 



Correlations do, however, supply important negative evidence, par- 

 ticularly when the records have been obtained by the continuous profiling 

 method. If correlations for several reflections cease roughly on a ver- 

 tical or sloping line, as depicted on a cross section, and continue again at 

 a certain distance beyond this line, strong evidence of a fault zone in that 

 interval is obtained, the dip of the zone roughly indicating the hade in 

 the plane of the cross section. This method, though strikingly effective 

 on paper, cannot always be depended upon because of its negative 

 character ; also, correlations must be reliable and persist for several strata, 

 and the zone of fracture must not be too extensive.* 



Mapping of Structure by Angular 



Divergence or Interval 



Change Method 



This method is based on the premise 

 that a change of interval between two 

 or more reflections recorded over an 

 area is an indication of structure in 

 that the geological section is normally 

 thinner over the crest of a structure 

 than in the surrounding area, t 



The principles utilized in the method 

 are best explained by referring to Fig- 

 ures 450, 451, and 452. Figure 450 

 shows a schematic geological cross sec- 

 tion through an anticlinal structure and 

 the ray paths of rays which originate 

 at a shot-point and are reflected from 

 the surfaces of layers 3 and 5 to the seis- 

 mometers Si and So.** The low velocity or aerated layer 1 is variable in thickness and 

 in physical characteristics. Layer 2 consists of consolidated rock having elastic proper- 

 ties which are different from those of layer 1. Layer 3 is rock having elastic properties 

 which are different from those of layer 2. Layer 4 has the same general character as 



Fig. 450. — Diagrammatic cross section showing 

 rays reflected from various reflecting horizons 

 located above an anticlinal structure. (After Mc- 

 Collum. U. S. Patent 2,118,441.) 



* The methods of refraction shooting and continuous electrical profiling often offer 

 a more convenient method of locating shallow faults. 



t E. V. McCollum, "Method of Making Geological Explorations," U. S. Patent 2,118,441. 

 Issued May 24. 19.38. 



E. V. McCollum and L. F. Athy, "Geophysical Method of Determining Geological Struc- 

 tures." U. S. Patent 2,118,442. Issued May 24, 1938. 



E. V. McCollum and G. C. McGhee, "Method of Making Dip Determinations of Geological 

 Strata," U. S. Patent 2,001,429. Issued May 14, 1935. 



** In this analysis it will be assumed that ray paths can be approximated with suffi- 

 cient accuracy by straight lines, an assumption justified only for relatively gentle dips. 



