VERTICAL AND INCLINED SURFACES OF SEPARATION 277 



purposes and the curves obtained can be used to explain the nature of distor- 

 tions in probings arising from, the influence of non-horizontal boundaries. 



The approximate construction of curves distorted by the influence of 

 a horizontal boundary can be extended to the more complex case shown 

 in Fig. 1, b, when the medium at the VEP point is a triple -layered cross- 

 section. 



Fig. 3 shows curves obtained for this case: 



^0 = 9^1' ^^0 = 1^1- 



In this case gjf has already been determined on an SN-20 triple -layered 

 comparison curve for each separation taken; the value of S has been approx- 

 imately determined with the same degree of error as in the double -layered 

 case cited above. The ordinate of the minimum point is increased under 

 the influence of the vertical contact. Its abscissa remains practically cons- 

 taut. In this case the variation in the co-ordinates of the minimum point 



d . 

 and the value of S in relation to the distance — • is the same as would obtain 



H 



if instead of sudden increase in the resistivity at the contact there were to 

 be a gradual increase in the resistivity of the conducting horizon. 



In connection with this, it is obvious that a fictitious rise of the marker 

 horizon may be obtained in interpreting VEP curves when fixing the para- 

 meter which corresponds to the conducting horizon above the marker 

 horizon. For VEP curves parallel to the contact, the position of this contact 

 may be defined either from the beginning of the deviation of Qj^ from q^^ j^ 

 or from the alteration in the type of curve as the VEP point approaches 

 the contact (transition from a double -layered curve to a triple -layered curve 

 of type A), while for a triple -layered cross-section it may be defined from 

 the increase in the ordinates of the minimum points. It is obvious that these 

 criteria may not be of use in all cases as the electric cross -section at the 

 VEP point is not sufficiently studied in the majority of cases. A more accu- 

 rate determination of the position of the vertical contact may be made 

 from VEP curves in which the spacings are perpendicular to the contact. 



By using the approximate method described above for constructing curves 

 for VEP in which the spacings are orientated perpendicular to the surface 

 of the contact, let us make a preliminary comparison of certain of the curves 

 obtained with the curves of the HVC reference graphs. The comparison 

 shows that the calculated curves coincide with the approximated curves 

 for a double -layered cross -section. 



Approximated curves for a triple -layered cross -section of type H, are 

 shown in Fig. 4 and for a double -layered cross-section in Fig. 5. A value 



