BORE-HOLE INVESTIGATIONS 



1033 



a thick, resistive formation. Both normal sondes give fairly symmetrical curves. The 

 lateral curve corresponding to layer 1 is low in the top part of the bed for an interval 

 corresponding to the spacing. The curve then sharply increases and tends to level off 

 at a high value of apparent resistivity. The maximum is reached at a very short 

 distance above the lower boundary of the layer, and the curve approximately regains 

 the shale line at a distance below the base of the layer equal to the spacing. 



SPONTANEOUS -POTENTIAL 

 millivolts 



RESISTIVITY 

 ohms m*/m 



RESISTIVITY 

 ohms nr/n\ 



Fig. 639. — S.P., normal and lateral curves for thin resistive beds. 



In the upper part of Figure 638 the S.P. log shows four main negative deflections 

 (2, 3, 4 and 5), corresponding to four permeable layers, with some minor depressions 

 {a, b and c) in the crest of the deflections indicating thin shale streaks inside these 

 permeable layers. 



The average thickness of each permeable bed is greater than the short normal 

 spacing, and this curve makes a fair distinction between the layers, although the thin 

 streaks a, b and c are poorly marked. For the long normal, however, the thickness of 

 the permeable layers is of the same order of magnitude as the spacing of the sonde. 

 The resulting curve is rounded and no distinction between individual beds is possible 

 (except for layer 4 which is thicker than the others). 



