492 



EXPLORATION GEOPHYSICS 



TABLE 16. 



COMPUTED VALUES OF pjp, CORRESPONDING TO VARIOUS 

 ASSUMED VALUES OF Q,t 



When Q2 is positive (0 < Q2 < 1 ) then 5 > and therefore 



-^>1 or 0<-^<l. 

 pi ~ o"l 



When Q2 is negative (Qo _< and < | Q2I ^ 1 ) then 



5<OandO < l^"! < M and 



therefore 



0< ^'*- < 1. 

 ~ pi ~ 



FamiHes of curves showing the relationship between o-a/o-i and d/a when 

 Q2 is positive and the relationship between pa/p\ and d/a when Q2 is nega- 

 tive, for a given set of conditions, are shown in Figure 294. 



In the practical case the apparent resistivity, pa, as a function of the 

 electrode separation, a, is known from measurements and application of 

 Equation 18, while the surface resistivity, pi, is obtained from similar meas- 

 urements with small electrode separations. The procedure then in deter- 

 mining depth is almost the reverse of that for determining the sample-^ 



curves as described above. That is, the -^ vs. d/a curves for various Q2's 

 were determined by assuming values for Q2 and d/a. In the practical case, 



t Tagg, A.I.M.E. Geophysical Prospecting, Vol. 110, 1934. 



