378 



Subsurface Geologic Methods 



inches and eighteen inches for the normal curve and between five and six 

 feet for the long normal curve and is indicated on the Schlumberger log 

 heading as "AM." 



Lateral Curves — The spacing is considered as the distance between the 

 potential-measuring electrode, M, and a point midway between the two 

 current electrodes, AB. This spacing is always large as compared with the 

 distance, AB, and is indicated on the Schlumberger log heading by "AO" 

 (fig. 162). 



Depth of Investigation — -The depth of investigation is an indefinite 

 matter since, for both the normal and lateral arrangement, it varies with 

 many factors. It can be stated, however, that in general the greater the 

 AM or AO spacing the greater the depth of investigation. 



Characteristics of Resistivity Curves 



A resistivity log has primarily a twofold purpose: one, to locate and 

 determine the boundaries of all resistive formations; the other, to deter- 

 mine the fluid content, both qualitatively and quantitatively, of permeable 



<2) 



AB - Current. Electrodes 

 MN - Potential Electrodes 



AM small compared to AB 



a 



(AM on log heading) 



AB - Current Electrodes 

 MN - Potential Electrodes 

 AB small compared to AM 



T 



(AO on log heading) 



__J_ 



Figure 161. Schematic diagram showing 

 arrangement of electrodes for record- 

 ing normal resistivity curves. 



Figure 162. Schematic diagram of elec- 

 trode arrangement for recording lat- 

 eral curves. 



