VERTICAL AND INCLINED SURFACES OF SEPARATION 293 



of the dipole MN across the contact, the position of the dipole AB being 

 considered as constant. 



In analysing the distortions of DQP curves we make use of a vertical 

 cross-section ^^, which is constructed in the following manner; the abscissa 

 of the values of ^^ taken from the plus and minus DQP curves are plotted 

 on an arithmetical scale on lines orientated vertically from the centres 

 of the corresponding MN dipoles. 



When the same vertical and horizontal scales are used, the values of Qj^ 

 of DQP curves are distributed along lines at an angle of 45° to the line of 

 the profile. 



If we make separate use of the plus and minus branches of the DQP 

 curves "\ve obtain vertical cross -sections from which we can obtain a qualitative 

 idea of the character of the variations in the values of Qj^ along the profile 

 in both the vertical and the horizontal directions. 



If there is a non -horizontal boundary along the profile, we shall not observe 

 a zone of increase or decrease in the values of ^^^ [relative to [q^ ^)^ of the 

 horizontally layered medium] in constructing the vertical cross -section 

 in this manner, at the site of the non -horizontal contact. In this case the 

 line Avhich connects maximum deviations of Qj^ wth the same sign will coincide 

 approximately with the position of the surface of the non -horizontal boundary. 

 Reciprocal signs of the deviations of Qj^ will be observed at the site of the 

 non -horizontal contact in vertical cross -sections constructed from the plus 

 and minus branches of DQP curves. 



Vertical cross-sections for the plus and minus branches of DQP curves 

 are given in Fig. 16 for the region in which an electric prospecting party of 

 the Spetsneftgeofizika Office (Penza province) carried out investigations. 



A region of decrease in Qj^ is to be seen quite clearly in the eastern part 

 of the upper profile for the DQP branches. The characteristic DQP curve 

 No. +27 given in Fig. 17 shows an area in which Qj^ is decreased. 



The abscissae of the maximum decreases in Qj^ are plotted over the vertical 

 cross-sections as in VEP studies*. 



In that part of the profile which is being examined there are two further 

 sections with anomalous deviations of p^, initially increasing and subsequently 

 decreasing, and consequently giving rise to subsidiary maxima and minima. 

 The lines which connect the abscissae of these points form vertical zones 



* It is not always essential to depict the abscissae of the deviations of q^^ from {o■^ j^j^ 

 when analysing distortions of DQP, since in the majority of cases the anomalous region of 

 deviation on the cross -section manifests itself with sufficient clarity and is always sited in 

 the immediate vicinity of the vertical contact owing to the principle upon which the profile 

 is constructed. 



