274 



V. I. FOMINA 



asymptotic portion of the VEP curve, since for sufficiently large spacings 

 over a double-layered medium 



The approximate construction of VEP curves under these conditions is 

 achieved by graphic summation of the ordinates of two theoretical probe 

 curves, the first of which corresponds to a horizontally layered medium, 

 and the second to cases with a single vertical surface of separation for which 

 CV-2S or CV-3S reference graphs have been calculated. 



A curve is selected from a CV-2S or CV-3S reference graph which de- 

 pends on the direction of the probe spacings and in which the modulus- 



h 



and takes account of various values for 



ju = — is equal to the ratio 



Qi iQik,)L 



the required spacings of AB. 



This approach to the construction of the curves follows from a considera- 

 tion of known formulas used in electric profiling for calculating ^^ for the 

 case of the vertical contact of two separately homogeneous media: 



(a) If the electrodes are placed perpendicularly to the contact then 



Qk 



{i + A(i._o[. 



^^^'+j^'-'-''A]4^T:r 



/2 (4>x+L)^-P_ 



]■ 



(2) 



when all the electrodes are on one side of the contact in a medium with specific 

 resistivity q-^, and 



k 



1 



L^~l^ 



{4^x + LY-l^_ 



(3) 



if the electrode B is in a medium with specific resistivity ^2? and the electrodes 

 AM and N are in a medium with specific resistivity q-^. 

 (b) If the electrodes are placed parallel to the contact then 



Qk= QiV- + k 



y{y+i) 

 I 



l/4c^2 + j2 j/4^2+(^+/)2_ 



(4) 



If the expression in braces is denoted by q^ and if we take the above hypo- 

 thesis into consideration, the value of Qj^ for each of the formulas given 

 above can be expressed as the product 



Qk=i.Quk}LQk- (^) 



(?i k)L ^^'^ ^ double -layered medium is expressed in its turn by the known 

 formula 



