SEISMIC METHODS 7Q7 



all interfaces giving reflections are parallel or coincide with bedding or 

 apparent layering of formations. 



Mineralogical replacements and lenticular masses within igneous rocks 

 have been found to give seismic reflections. It may follow that similar 

 conditions in sedimentary formations may also give reflections. 



Figure 432, a seismic record section in southern Oklahoma, is a demon- 

 stration of reflection correlation whereby a possible stratigraphic oil trap is 

 indicated. 



Reflections which correlate with each other from station to station, but 

 do not seem to correspond with any known geologic marker, often are in 

 agreement with the resistivity and/or self-potential correlation on electric 

 well logs.f 



Requirements for dip shooting are more severe. A 500-foot spread 

 length is generally much too short. The actual value of the spread length 

 to be used in any area depends on the values observed for AT in that 

 area, because the value of AT must be appreciably greater than the mag- 

 nitude of probable error.** (See also the tabulation of AT versus spread 

 length for a typical case, page 673.) 



In correlation shooting, the identification of the same phase of a reflec- 

 tion is of paramount importance; in dip shooting, the AT values corres- 

 ponding to the reflections under investigation are of paramount importance. 

 As far as type of spread is concerned, in correlation shooting the uni- 

 directional spread is favored, while in dip shooting more use is made of 

 split spreads and continuous profiling. 



The dip spread is applicable for correlation shooting, even though 

 short correlation spreads are inapplicable for dip shooting. That this 

 condition is fortunate may be inferred from previous considerations ; 

 that is, generally where correlation shooting is used, the dip is so small 

 that accurate dip data could not be obtained without prohibitively long 

 spreads. However, in regions where dip shooting is utilized, correlations 

 quite frequently appear and are plotted. If dip data contradict correlation 

 data, whether dip or correlation or both are to be used rests on the fol- 

 lowing consideration : Because the reflection time interval for dip deter- 

 mination is obtained on a single record, it is inherently more accurate 

 than reflection time intervals which are recorded on separate records 

 at different shot-points. However, when correlations are carried over 

 shot-points separated by large distances, the difference of reflection 

 time is increased, and this permits increased accuracy. The balance of 

 advantage must also be governed by the reliability of correlations and 



t Personal communication from E. J. Handley, geophysicist, Century Geophysical Corporation. 

 ** The accuracy of the travel-time values is of the order of 0.001 sec. at its best 

 and therefore values of AT of larger magnitude than this are required. Important 

 inherent deterrents to an accuracy in the AT value greater than 0.001 sec. are: 

 general irregularities in the subsurface such as lenticular structure or lateral strati- 

 graphic variation ; erratic characteristics of the reflecting horizon ; inaccuracies in 

 the correction for the low velocity layer ; etc. 



