VERTICAL AND INCLINED SURFACES OF SEPARATION 



291 



curves along the profile allows us to determine the true position of the 

 asymptotic branch of VEP curve No. 464 as depicted in Fig. 14. The first 

 asymptotic rise is, in all probability, due to the influence of the eastern 

 contact which increases the value of Qf^ The further lowering in the values 

 of Qi^ with increase in the separations of AB is due to the influence of the 

 western contact. 



458 



o 460-(o) A2p-(b) ^4p-(c) 

 1(d) E^(e) ^B(f) vvv(g) 



Fig. 15. a — ^VEP points; b — drill holes in existence at the moment when the materials 

 were being studied; c — drillings made after the materials had been studied; d — Upper 

 Cretaceous deposits C^25 ^ — lower Cretaceous deposits C^^; / — Paleozoic deposits P^; 

 g — top of the reflecting horizon of infinitely high resistivity from electro-prospecting data. 



We obtain confirmation of this hypothesis by examining VEP point 

 No. 462, which comes next on the profile, since the sharp increase in the 

 ordinate of the minimum which is to be observed on the curve points to 

 the siting of the point close to the edge of the eastern contact. The distance d 

 for this VEP point is close to the depth H. On the other hand the abscissa 

 of the maximum decrease in Qf^, which denotes the western contact, is 

 decreased in passage between points 464 and 462 by comparison with 

 point 464. Its right-hand asymptotic branch should be corrected in con- 

 formity with this, as is shown in Fig. 14. 



The analysis of distortions in VEP curves thus carried out enables us to 

 determine the position of the non -horizontal contact and the right-hand 

 branch of the VEP curve with greater reliabihty and from this, the value 

 of S. Possible values of S obtained from VEP curves are given in the lower 

 portion of the drawing. The most reliable values established by the analysis 

 of the distorted curves are joined by a continuous line. 



Fig. 14 gives an example of the comparison of VEP curves sited on both 



d 

 sides of the contact for approximately equal values of — . This type of 



H 



juxtaposition of the curves enables one to determine the true path of the 



asymptotic branch as a hne running at an angle of 45° from the point of 



19* 



