494 



EXPLORATION GEOPHYSICS 



by Tagg t in Gloucestershire, England. The surface material is limestone 

 overlain by about six inches of loam; the depth of the limestone varies 

 from 50 to 266 feet, and underneath the limestone is either sand or clay. 



? \00 20O 300 40O 



CL » ELECTRODE SEPARATION , f=T. 



Fig. 295. — Experimental values of the apparent resistivity as a func- 

 tion of electrode separation. Solid curve is experimental curve obtained 

 virith_ Megger Earth Tester and broken curve is experimental curve 

 obtained with potentiometer equipment. (Tagg, A.I.M.E. Geophysical 

 Prospecting, 1934.) 



The sites chosen were practically level. The resistivity pi of the surface 

 limestone was obtained from measurements at electrode intervals to 70 

 feet. The averaged value was 6703 ohm-inches. The apparent resistivity 

 Pa is shown plotted as a function of the electrode separation in Figure 295. 



t Tagg, loc. cit. 



