VERTICAL AND INCLINED SURFACES OF SEPARATION 289 



the abscissae the greater is the angle of inchnation of the contact to the 

 diurnal surface (when the strata overlaying the contact are of equal 

 thickness) . 



In the given case the western limb of the anticline dips at a greater angle 

 than the eastern. At the same time the latter obviously exhibits a step struc- 

 ture. The vertical cross-section of the resistivities constructed from the VEP 

 curves gives a qualitative confirmation of the above hypothesis (Fig. 13). 

 Here the region of increased values corresponds to the anticlinal upfold. 

 On the western and eastern flanks of the anticline the character of the be- 

 haviour of the isolines varies. One might consider that the vertical electric 

 cross-section gave a sufficiently good qualitative indication (by the transi- 

 tion from low to high resistivity) of the position of non -horizontally incHned 

 strata and that there was no necessity for detailed analysis of the abscissae 

 of the maximum deviations of Q/^ from {q^ y.)^. 



Determination of the position of the inchned contact by means of the ver- 

 tical cross -section is, however, not always sufficiently precise. In addition 

 the value and direction of the anomalous variation in Q/^ can be evaluated 

 as a result of analysis of the abscissae of the maximum deviations of Q)^. 



In practice the points of maximum deviation in ^^, the abscissae of which 

 are depicted above the profile, do not ahvays correspond to a sufficiently 

 clearly defined anomalous break in the isolines. In VEP No. 466 (Fig. 14), 

 for example, an area of anomalous increase in Q[^ is to be noted. Its abscissa 

 is equal to 5 km, which corresponds to the distance from VEP No. 466 

 to the eastern non-horizontal contact (Fig. 13). There is, however, no ano- 

 malous behaviour of the ohm isolines to be observed in the vertical cross- 

 section at VEP point 466 and the position of the right-hand asymptotic 

 branch of VEP No. 466 can therefore be taken to be true. The position of 

 the right-hand asymptotic branch of VEP curve No. 466 should also be 

 determined from the analysis of the abscissae of the maximum deviations 

 of Qf^, as is shown in Fig. 14. 



At VEP point No. 460, situated at the peak of an anticline (Fig. 13), 

 the anomalous lowering of ^^ is denoted by the course of the isohnes for 

 16Qm. and above. This lowering can be treated as an alteration in the electric 

 cross-section at the area of VEP No. 460 (inclusion of an additional conduct- 

 ing horizon). This lowering is, however, conditioned by the influence of the 

 western and eastern non -horizontal boundaries (as may be seen from the 

 disposition of the abscissae of the maximum deviations of Qf^ over the profile). 



The corrections indicated in Fig. 14 must be introduced into the VEP 

 curve on this basis. 



In VEP No. 464 (Fig. 14), the point of maximum decrease in ^^ is given 



Applied geophysics 19 



