744 



ELECTRICAL METHODS 



[Chap. 10 



moist layers of different capillarity occur above one another. It is seen 

 that three-layer curves encountered in water prospecting require a careful 

 analysis of near-surface stratigraphic conditions and that the same resis- 

 tivity curve may indicate totally djfferent conditions. 



(a) 



^•-^^' ■/ /\/' ■/ / / 



Fig. 10-76. Various resistivity indications on conductive and nonconductive 



ground waters. 



G. Electrical Logging 



Electrical logging is the application of resistivity mapping with fixed 

 electrode separation to the location of formation boundaries in uncased 

 wells. Methods and equipment, theory and results are discussed in 

 Chapter 11. 



VI. POTENTIAL-DROP-RATIO METHODS 



A. General 



Compared with resistivity methods, the advantages of potential-drop- 

 ratio procedures are that sharper indications are obtained on vertical 

 formation boundaries, and that in favorable cases of horizontally stratified 

 ground the indications are more directly related to depth. The P.D.R. 



