VERTICAL AND INCLINED SURFACES OF SEPARATION 



281 



a medium with resistivity {Qi^h)L^ Q2 (Fig- l, candd). A similar case may 

 be encountered when working over an antichnal fold, the core of which is 

 composed of rocks of high resistivity while the overlaying strata of anticlinal 

 crest and limbs are of reduced resistivity. 



Use is also made in this case of approximately constructed curves and the 

 error in construction for the case ^2 < (Qi, i^l ^^ preliminarily estimated 

 by comparison of the approximated curve with an HVC reference graph. 



Fig. 7. o— approximately constructed || VEP curve; 6— approximately constructed X VEP 

 curve; c — H line; d — position of the non-horizontal contacts; e — VEP curve for a homo- 

 geneous horizontal medium. 



Let us examine the special case of the HVC reference graph when q-^ = 99^2 

 (Fig. 1, d). This case may be identified with the case which we adduce in 

 Fig. 1, c, if one allows in the latter case that the thickness of the layer with 

 a resistivity q-^ at the VEP point is small in comparison with the dimensions 

 of the arrangement. 



A cross-section consisting of a two-layered medium disposed on both 

 sides of the vertical contact is taken for the HVC reference graph (Fig. 1, e). 



It is necessary, when constructing approximated curves, to determine the 



