SEISMIC METHODS 709 



the possibility that if correlation is carried too far, small intermediate 

 structural features may be overlooked. 



An illustration of dip shooting together with a vertical projection of 

 the dips into a horizontal plane is given by Figure 433. 



Often the cross section is not used ; instead, tabulations of reflection 

 times and depths for each persistent reflecting stratum are maintained 

 separately. These depths are mapped directly, the data being associated 

 directly with the shot-point. 



Methods of continuous profiling which have already been treated under 

 dip shooting may in part be used with correlation shooting and their 

 discussion will not be repeated at this time. 



Illustrations of Correlation Shooting 



An example of field correlation records is shown. These records are not 

 typical in that they constitute better records than obtained in a normal run, 

 and they are used only to illustrate the method of correlation. 



The set of correlation records shown in Figure 434 was secured in the 

 vicinity of St. John, Kansas, A symmetric split spread of total length 

 equal to 270 feet was used. The low velocity zone correction was 

 determined only for the two nearest seismometer stations and, cor- 

 respondingly, reflection time was picked only on the two center traces. 

 Depths below sea level and correlation grades are given at the bottom 

 of the record and the horizon label at the top. 



It will be noted that not all reflections are picked. This procedure 

 is customary in correlation shooting where only those beds which persist 

 are of value, particularly when a sufficient number of such beds are present. 

 The characteristics of each reflection are carefully studied, particularly the 

 sequence of phases between neighboring reflections. The presence of 

 these phases in succeeding records is the type of correlation evidence sought. 



These records show the possible quality of correlation carried across 

 long distances as the two shot-points were separated by over three miles. 

 The correlating bed labeled A stands isolated and yet can be correlated 

 with certainty due to persistent and distinctive character. This reflection 

 is from the anhydrite layer. Reflections labeled Ti and To represent the 

 Topeka strata, which again reveal excellent persistency in character. 



Treatment of Three-Dimensional Problems 



Equivalent Time Horizons and Equivalent Rays. — The three- 

 dimensional structure may be studied with relative ease by introducing 

 the concept of equivalent time horizons. Consider a reflecting horizon 

 whose slope at all points is continuous and assume that from every point 

 on the horizon a normal ray is drawn to the ground surface. If the velocity 

 varies with depth only and if the extent of folding of the horizon is limited 

 to such a degree that no rays cross, a one to one correspondence is estab- 



