1032 



EXPLORATION GEOPHYSICS 



is the presence of a zone of low resistivity below 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 ^one. 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, although 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 

 reflection peaks may be mistaken for actual resistive streaks in the formation. 



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

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



SPONTANEOUS - POTENTIAL 

 Millivolt* 



RESISTIVITY RESISTIVITY 



ohm* m'/Hi elwit m'/m 

 Jp tSfi l}I_ 



Fig. 638. — S.P., normal and lateral curves in low resistivity formations. 



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. 



Examples of Resistivity Curves. — Figures 638 and 639 offer further illus- 

 trations of the characteristics of normal and lateral curves, in the case of beds of 

 moderate or low resistivity, which can be considered as soft formations. The solid 

 curve in the middle track is a short normal of 16" spacing. The dotted curve is a lateral 

 of 18' 8" spacing. The curve in the right-hand track is a long normal of 64" spacing. 

 The S.P. curve is reproduced in the left-hand track. In Figure 638 the shaded curves 

 represent recordings of the curves on reduced sensitivity (200 ohm-meters full scale) 

 when the recording of the regular sensitivity (20 ohm-meters full scale) goes oflf-scale. 

 The depth scales and sensitivity scales are not the same on the two figures. 



Layer 1 of Figure 638 is a good example of the behavior of the various curves in 



