730 



ELECTRICAL METHODS 



[Chap. 10 



spacings from 5 to 20 feet, thus obtaining the resistivity (pi). (2) Plot pi to 

 scale on the resistivity* axis and draw a line from pi to meet tangentially 

 the first part of the curve. (3) Read resistivities at six or eight points 

 on the curve just drawn and apply Tagg's method to that part of the curve. 



Table 72 



Table 73 



a p< 



5 106 



10 120 



15 140 



20 154 



25 165 



30 166 



40 158 



50 120 



60 87 



70 65 



80 49 



90 35 



100 26 



110 18 



120 12 



This procedure jdelds p2 , and hi , as well as fci = 



P2 



Pi 



(4) Estimate 



P2 + Pl 



the depth to the third layer by applying the approximation hi -\- h2 = 

 f d, where d is the distance to the inflection point (between the two points 

 of maximum curvature). (5) Calculate the apparent resistivity pi of the 

 two upper layers (pi and pi) in parallel from {hi + /i2)/pi = h/pi + /i2/p2 • 

 (6) Apply Tagg's method to the bottom part of the curve between the 

 lowest two points of maximum curvature. A more accurate value is then 



