1034 EXPLORATION GEOPHYSICS 



As for the lateral, the thickness of each bed and the distances between beds are 

 so small in comparison with the spacing that the curve recorded approximates that 

 which would be read opposite a single homogeneous layer with a thickness equal to the 

 total thickness of the group and a resistivity equal to an average of the resistivities 

 of the dififerent beds. 



Figure 639 is related to thin resistive beds located in conductive formations. In 

 the upper part a single thin resistive bed is shown, which is indicated on the lateral 

 by a characteristic peak and blind zone, on the long normal by a marked depression, 

 and on the short normal by a very slight depression. The thickness of the layer is 

 evidently slightly less than the short normal spacing. 



The resistive layer of the middle part of Figure 639 is slightly thicker, as indicated 

 by the fact that the short normal shows a peak. It is still thinner than the long normal, 

 however. 



The lower part of Figure 639 shows how the presence of a sequence of thin resis- 

 tive streaks at short distances from one another may complicate the interpretation. 

 The depression of the lateral opposite permeable layer b may be because it falls inside 

 the blind zone of resistive streak a, or it might also be because the thickness of bed b 

 is approximately equal to the lateral spacing. Lower, both normal curves show a series 

 of depressions and kicks which could not be easily interpreted without the help of the 

 lateral curve. The lateral shows definite peaks such as c, e, f, g, h. Peak / is obviously 

 a reflection peak corresponding to resistive streak g, since they are separated by a 

 distance AO. Peak i which is shown on the short normal, and which corresponds to a 

 depression on the long normal, does not appear on the lateral since it is inside the blind 

 zone of resistive streak g. 



The Logging of Hard Formations 



Features of Normal and Lateral Curves in the Logging of Hard Formations. — 



Figure 640 signifies, by means of a qualitatively established example, some typical 

 indications given by normal and lateral devices opposite a thick, highly-resistive forma- 

 tion, such as limestone, containing porous or more conductive zones. 



To understand the behavior of these curves it should be remembered that opposite 

 a highly-resistive formation the current from electrode A is almost entirely confined 

 to the bore hole ; part of it flows up the hole and part down the hole, dividing in inverse 

 proportion to the resistance of the two paths. The resistance of each path is determined 

 mostly by the resistance of the mud column in the hole between the current electrode 

 and the nearest conductive bed in that direction ; once the current reaches the conduc- 

 tive bed it has there, depending on the thickness and the conductivity of the bed, a 

 relatively low resistance path away from the hole. 



This circumstance can be used to explain why the lateral device, which has elec- 

 trodes M and A'' below the current electrode, records a curve which is lopsided in a 

 downward direction. The deeper the device is into the resistive bed the greater is the 

 proportion of the current flowing downward in the hole from electrode A, and there- 

 fore the greater is the potential drop in the mud measured between electrodes M and A''. 

 The apparent resistivity corresponding to this potential drop will thus be largely 

 dependent on the size of the hole, the resistivity of the mud, and the position of the 

 device with respect to the boundaries of the highly resistive zones. 



The unsymmetrical appearance of the normal curve is explained in a similar 

 manner for the three-electrode arrangement illustrated, in which electrodes M and A'" 

 are above the current electrode. As soon as all three electrodes are inside the hard 

 formation, the upper electrode is no longer near zero potential, as is basically assumed 

 for the normal, and the arrangement actually measures the potential drop due to the 

 current flowing in the mud between M and A'^. 



It is thus seen that three-electrode devices give unsymmetrical curves in hard 

 formations, and the direction of the lopsidedness depends on whether the measuring 



