BORE-HOLE INVESTIGATIONS 



1031 



layer, a slight depression is recorded on the curve. The apparent resistivity then 

 increases gradually until upper electrode A has entered the layer, at which time the 

 slope of the curve becomes sharper. A maximum is reached just above the lower 

 boundary of the bed. Below this maximum the decrease of apparent resistivity is rapid, 

 and a value slightly greater than the resistivity of the adjacent formations is attained 



Fig. 637. — Resistivity curves for lateral sonde. 



at a distance below the bottom boundary equal to the spacing. This corresponds to the 

 moment when the upper electrode of the device passes the bottom of the layer. 



It is observed, for the case of a bed thicker than the spacing, that the upper 

 boundary of the bed is not well defined on the lateral curve and, as a whole, the bed 

 appears as being displaced downwards. The amplitude of the shift is approximately 

 equal to the spacing. 



The lower part of Figure 637 corresponds to the case of a resistive layer thinner 

 than the spacing. The bed is indicated by a sharp peak of comparatively low apparent 

 resistivity. A slight depression is observed above the layer, but a more striking feature 



