1096 Subsurface Geologic Methods 



In areas such as central Kansas, where favorable structures are 

 formed over and along buried topography, it is more reasonable to 

 assume that the velocity zones are horizontal near the surface and that 

 they dip as a function of depth, since it has been frequently noted that 

 geologic dips are more severe with increasing depth because of differential 

 compaction. With this assumption we may use the modified straight-path 

 computation method suggested by Stulken which determines depth, dip, 

 and displacement by use of certain average velocities. ^^ 



The assumption that iso-velocily surfaces are all horizontal and that 

 the reflecting horizons transgress the iso-velocity surfaces would seem to 

 be extremely limited in application. Such a condition might possibly 

 arise immediately beneath an angular unconformity or for the special 

 cases in which reflections are obtained from a fault plane or from the 

 flanks of buried topography (where differential compaction is unimport- 

 ant) . However, because of the ease of mathematical computation, it is a 

 favorite assumption of seismologists. Various curved-path methods are 

 described on the following pages. 



The most reliable velocity information is obtained from well surveys 

 in which a detector is lowered by a well-logging cable into the well and 

 direct measurements are made of the travel time of the seismic waves. 

 Several well-shooting associations have been formed to encourage the 

 dissemination of such velocity information. Swan ^^ has indexed the many 

 wells shot for velocities in this country and abroad. 



Results of well shooting are presented as time versus depth, interval- 

 velocity, and average-velocity curves. From such curves the mathematical 

 relationship governing the increase of velocity with depth may be de- 

 termined. The use of simple mathematical relationships between the 

 velocity and time or depth gives a very convenient means of extrapolating 

 data to depths greater than that of the actual velocity measurements. Ex- 

 tending velocity information is frequently required, as data are often not 

 available at depths from which reflections can be consistently obtained. 

 Handley has shown that seismic reflections may correlate with either the 

 resistivity or the self-potential curves of electric-well surveys.^'^ However, 

 he states that in many areas a closer correlation is obtained with the self- 

 potential curve. Such a correlation is certainly to be expected since the 

 self-potential or porosity curve is indicative of the degree of compaction 

 and, thus, of the acoustic impedance of the material. We should expect 

 to obtain reflections at points in the stratigraphic section where the 

 acoustic impedance changes rapidly. 



In most areas it is found that the assumption of a linear increase 

 of velocity with depth is justifiable for a limited range of depth. Several 



^^ Stulken, E. J., Effects of Ray Curvature Upon Seismic Interpretation: Geophysics, vol. 10, no. 4, 

 pp. 472-486, Oct. 1945. 



°6 Swan, B. G., Index of Wells Shot for Velocity: Geophysics, vol. 9, no. 4, p. 540, Oct. 1944; 

 vol. 11, no. 4, p. 538, Oct. 1946; vol. 14, no. 1, p. 58, Jan. 1949. 



"' Handley, E. J., Can Geophysical Reflections be Correlated with Geological Horizons? : Oil and 

 Gas Journal, voL 47, no. 44, pp. 84-87, Mar. 1949. 



