Chapter 11 



ALLOWANCE FOR THE INFLUENCE OF VERTICAL AND 



INCLINED SURFACES OF SEPARATION WHEN 



INTERPRETING ELECTRIC PROBINGS 



V. I. FOMINA 



When interpreting VEP [VES or vertical electrical sonde in Russian; Editor's 

 remark] curves it is assumed that the surface of separation between layers 

 can, in a first approximation, be considered to be horizontal within the 

 limits of each probe point taken separately*. In practice such conditions 

 are encountered very infrequently, especially when the dimensions of the 

 probe feed lines are great (25-30 km). A structure consisting of two or 

 more tectonic blocks, within each of which the condition of horizontal 

 homogeneity of the medium is observed within certain limits, is often the 

 subject of electro -geophysical prospecting. The boundaries of each component 

 of such a structure (as a result of tectonic disturbances) are zones of vertical 

 or inclined contacts between rocks of different specific resistance. In what 

 follows we shall call these zones "zones of vertical or inclined contacts". 



In this connection it is extremely important to explain the character and 

 magnitude of the distortions introduced into VEP curves by tectonic 

 disturbances in order to be able to estabHsh with greater accuracy the electric 

 cross -section, the depth at which the electrical markerf horizon lies, 

 and the position of the contacts in each component. 



If the distance from the VEP point to the zone of vertical or inclined 

 contact is sufficiently great by comparison with the maximum spacings 

 of AB, the VEP curve will reflect changes in the electric cross-section at the 

 point under examination with sufficient accuracy and may therefore be 

 interpreted by the ordinary method. 



If the distance from the VEP point to the contact zone is small, the observed 

 VEP curve will deviate from the curve for a horizontally homogeneous 

 medium. 



* The use of ordinary comparison curves is permitted if the marker horizon falls within 

 angles that do not exceed 15-20° (sides of the folds). It is assumed that the diurnal surface 

 is also horizontal. 



t An electrically resistant horizon, through which the current does not penetrate 

 easily. [Editor's footnote]. 



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