BORE-HOLE INVESTIGATIONS 



1035 



electrodes are above or below the current electrodes. Along with this dissymmetry it is 

 observed from the figure that for the lateral the positions of the minima are shifted 

 quite noticeably downward with respect to the exact depths of the conductive layers, 

 and for the normal a slight upward shift of the minima is also discernible. This often 

 results in the maximum of one curve matching in depth the minimum of the other 

 curve, a feature which can make logs recorded in actual practice seem very confusing 

 to those not acquainted with the behavior of the devices used. 



Fig. 640. — Normal and lateral curves opposite thick, highly 

 resistive formations. 



It may be further observed that the depressions read on the normal curve opposite 

 the conductive layers are very smooth and considerably broader than the thickness of 

 the beds, and the boundaries of the limestone formations are poorly marked so that 

 accurate determinations are practically impossible. 



If a two-electrode normal is used, or if a normal device is employed in which the 

 upper electrode is far enough from the other two so that it is always located outside 

 of the highly resistive bed, the curve will tend to be symmetrical, but the formation 



