ELECTRICAL METHODS 



495 



A series of values of the apparent resistivity corresponding to several 

 values of the electrode separation is read off the experimental curve. The 

 apparent resistivities are then divided by the surface resistivity pi to de- 

 termine the ratios pa/p\. Because the values of the ratio pa/pi are greater 

 than unity, the reciprocal ratios o-a/ai are also formed, (Table 17.) 



The values of d/a and Q2 corresponding to each of the values of the 

 ratio (Ta/o-i given in Table 17 are now read off Figure 294. The values 

 thus obtained are shown in Table 18. 



The next step in determining the depth consists in plotting a series of 

 curves of Q2 as a function of d, one curve for each value of ojux, — i.e. 



so 



zoo 



too /so 



d - DEPTH ^ rr. 



Fig. 296. — Curves showing the reflection factor Q2 vs. 

 the depth d for various values of aJ<J\. (Tagg, A.I.M.E. 

 Geophysical Prospecting, 1934.) 



one curve for each electrode interval. These are shown in Figure 296. The 

 six curves intersect within the dotted circle, and the center has the coordi- 

 nates d = 142 and Qo = 0.0702 ; that is, the intersection of the curves gives 

 142 feet as the value of the depth and 0.0702 as the value of the reflection 

 factor Q2. 



Also, it is of interest to point out that when Q2 and pi are known, the 

 resistivity p2 of the lower formation can be determined from the relation 



02 = 



_ P2 



PI 



+ 



Pl 



or 



_ Pl(l+Q2) 



''- 1-02 



