272 V. I. FoMiNA 



It is knoAHi that the nature and degree of the observed distortions 

 depend on: 



. d 



(1) the size of the ratio — , where d is the distance from the contact to 



H 



the VES point; H is the depth at which the reflecting horizon hes; 



(2) the correlation — between the specific resistances of the media on 



both sides of the contact; 



(3) the angle of inclination of the contact surface. 



CV-2S and CV-3S reference graphs, which are mentioned in some instruc- 

 tion manuals of electro -geophysical prospecting, had been calculated to 

 evaluate the influence of the vertical contact as long ago as 1938 (^^ ^» ^\ 



An examination of these reference graphs shows that the vertical contact 

 of two media (given that the electrodes MN do not pass through it) brings 

 about specific alterations in the values of q^^. Thus, if the spacings of AB 

 are orientated perpendicular to the vertical contact, the VEP curve at spacings 



„f ^ close to . WU show a sha, deflee.on of .Ke v.ue of ,, fro. ,„ 



dependent on the ratio — (increase for Q^^ Qx-, decrease for q^ <. Q^f. 

 Qx 



. . AB 



The beginning of these deflections is observed with spacings of an 



d ' . . ^ AB ' AB ^ 

 order of — . They attain their maximum value at == a; at > d 



2 ^ 2 2 



AB 

 the deflections are gradually decreasing and at > lOd they are practically 



imperceptible (CV-3S reference graph). 



In the case of spacings of AB being orientated parallel to the vertical 



contact, the VEP curve will, as is known, show an even deflection beginning 



,AB 

 with separations of ^ 0.8 — 1.0 c? (increase for Q2> Qi or decrease for 



Q2 < Qi) in the value of Qf^ from Qi, to an asymptotic value determined 

 by the formula 



^\ _ 2g2 



Qi AB Q1 + Q2 ^^^ 



^2 AB 



For — =00, the asymptotic value Q/^ = 2^^ ; for < d distortions 



Qi 2 



in the curve are insignificant (CV-2S reference graph). 



