In Figure 14-12 are shown the corresponding curves for lateral devices. 

 The point of measurement of the readings is point 0, midway between electrodes 

 M and N. 



In contrast to curves recorded with normal sondes, the lateral curves are 

 markedly dissymmetrical with respect to the center planes of the layers; and 

 their features are considerably more complex. As before, the transitions in 

 the curves corresponding to formation boundaries have been rounded off by 

 the effect of the drill hole. 



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

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

 appears to be displaced downward. The amplitude of the shift is approximately 

 equal to the spacing. 



The lower part of Figure 14-12 corresponds to a resistive layer thinner than 

 the spacing. The bed is indicated by a sharp peak of comparatively low ap- 

 parent resistivity. A slight depression is observed above the layer, followed 

 by a second smaller peak located at a distance below the bottom boundary of 

 the layer equal to the spacing. This secondary peak is called a reflection peak, 

 and the zone of very low apparent resistivity is called the blind zone. The blind 

 zone corresponds to the interval during which the resistive streak is located 

 between the current electrode and the measuring electrodes. 



The lateral is useful for the location of thin, highly resistive streaks, al- 

 though the interpretation may be difficult if several resistive streaks are close 

 together; a lower streak located in the blind zone of an upper resistive streak 

 may be missed; and the reflection peaks may be mistaken for actual resistive 

 streaks in the formation. 



For a resistive layer whose thickness is approximately the same as the 

 spacing (critical thickness), the curve is almost completely flattened. 



Similar generalizations are possible for lateral curves recorded for beds 

 more conductive than the surrounding formations. Whether the layer is thick 

 or thin, the shape of the curve is dissymmetrical; and the anomalies are spread 

 downward, outside of the bottom boundaries. The apparent increase of 

 thickness is roughly equal to AO. 



Resistivity-Departure Curves 



It has already been pointed out that the apparent resistivity, measured by 

 a given device in a drill hole, is dependent on the resistivities of all the different 

 media in the vicinity of the electrodes and may thus differ considerably from 

 the true resistivity of the formation being logged. 



The evaluation of the fluid content of permeable beds from electric log 

 data requires a knowledge as accurate as possible of the true resistivity of the 

 beds. Resistivity departure curves have been computed in order to help derive 



290 



