VERTICAL AND INCLINED SURFACES OF SEPARATION 



273 



In the case of a vertical contact covered by a medium of high specific 

 resi?ta.nce the thickness of which is small by comparison with the 

 dimensions of the apparatus it is assumed that the nature of the distortions 

 described above Avill not be changed. 



Complex combinations of vertical and horizontal contacts, which are 

 often hidden, are frequently encountered in practice in geophysical investiga- 

 tions, and the influence of the layer which covers the contact can often not 

 be neglected because of its considerable thickness. In certain of the most 

 simple cases an explanation of the nature of the distortions in the VEP 

 curves can be reduced to solution of a known problein concerning the 

 vertical surface of separation between two separate homogeneous media. 



A M,, N B 

 • *^-^, • 



/>! = I J2. m 



/>2=00 



V V V V 

 ^3=/Jj =00 



(a) 



P-^-p^ =1 "O. m 



l"NB 



■Pt^STa 



V V V V 

 ^3=C0 V 



(d) 





Let us consider the case (Fig. 1, a) in which the cross section on one side 

 of a vertical contact (at the VEP points) is represented by a double -layered 

 medium and on the other side by homogeneous deposits. 



Let us proceed from the hypothesis that a double -layered medium with 

 specific resistivities q-^ and pg can be treated for each VEP cross-section as 

 a homogeneous medium the specific resistivity of which is equal to the 

 apparent resistivity (^i/^)^*. This hypothesis is not open to doubt for the 



* That this suhstitution might be possible was also suggested by D. M. Srebrodol'skii 

 in a thesis in 1936. Documents of the Leningrad State University. 



Applied geophysics 18 



