696 EXPLORATION GEOPHYSICS 



section that this method of showing the geologic structure may he more 

 misleading than informative. 



Average Dip and Strike. — In many areas of gently dipping beds, the 

 reflections are so numerous that they cannot be shown as dips and strikes 

 on a horizontal plan map without great confusion, due to their superposi- 

 tion. Also, in such regions very small errors in the component move-out 

 times cause large errors in the resolved direction of the dip. 



To improve this situation, the remaining seven columns of the com- 

 puting form are designed to facilitate the computing of average values 

 for certain vertical zones within which the reflecting strata have proven 

 to be very nearly parallel. These zones may be chosen between certain 

 depths or between geologic marker beds, usually after enough data has 

 been obtained to reveal at least a qualitative picture of the geologic structure. 



The values of sin a and of sin j3 in each group are averaged separately, 

 different weights being given according to the quality of the reflections. 

 As a rough basis for weighting, a grade A reflection is given a weight of 4, 

 grade B a weight of 3, grade C a weight of 2 and grade D a weight of 1. 

 Averaged components obtained in this manner are then resolved in the 

 usual way and computed as individual reflections, with the difference that 

 the depths are not computed, but averaged. The horizontal distances are 

 then computed for an average depth in each zone. 



These averaged values may now be plotted on a plan map with increased 

 accuracy and decreased confusion. It is advantageous to plot separate 

 zones on individual maps for greater simplicity of interpretation. These 

 maps are a valuable aid in contouring theoretical surfaces (phantoms) 

 within the averaged zones. 



End Shots 



In choosing a computing procedure it is natural to build a routine 

 around a specific type of operation. In the case of the method just 

 described, the normal set-up is the split or symmetrical instrument spread 

 with reference to the shot-point. This is believed to be the best type of 

 set-up under most conditions, but not infrequently in the course of field 

 work situations are met which require the use of end shots. An end shot 

 as referred to here is an instrument set-up in which the shot-point is 

 placed outside the spread of seismometers, usually in the projection of the 

 line of seismometers and not more than a few hundred feet from the 

 nearest seismometer. 



In practical work no computing system can be inflexibly geared to a 

 single type of set-up, because inevitably abnormal situations arise and 

 must be solved. Therefore the theory of interpretation developed should 

 be capable of very wide adaptability so that almost any conceivable situation 

 may be freed of perplexing difficulties. It is not possible to discuss in 

 advance all of the variety of abnormal set-ups which may be encountered, 



