1030 



EXPLORATION GEOPHYSICS 



The lower part of the figure shows the case of a resistive layer thinner than the 

 spacing. This case is characterized by a depression opposite the layer and two symmet- 

 rical small peaks, c and d, on each side of the depression. This feature illustrates the 

 main disadvantage of the normal device ; beds thinner than the spacing, no matter how 

 resistive they may be, appear on the logs as being conductive. 



12 3 4 5 fig 



fin\- 1 



AM<3d 



Fig. 636. — Resistivity curves for normal sonde. 



The Lateral Sonde. — In Figure 637 are shown the corresponding curves for 

 lateral devices. The point of measurement of the readings is O, midway between elec- 

 trodes M and A''. 



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. 



As the sonde is lowered through the thicker layer indicated at the top of Figure 

 637, the sequence of events may be described as follows. When electrode A'^ enters the 



