Chap. 10] 



ELECTRICAL METHODS 



743 



4. Location of water. Resistivity and related surface-potential measure- 

 ments are probably the most promising geophysical methods for the 

 location of water. ' The problem is not simple and requires a careful study 

 of the stratigraphic situation since the occurrence of groundwater is quite 

 variable and the conductivity of water itself may vary. The manifold 

 conditions applying in electrical water prospecting have been discussed in 

 detail by Heiland^^ and Tattam.*^ 



Experience has shown that in many cases the water itself, though pot- 

 able, is conductive and occurs at the bottom of a dry layer, which in 

 turn is covered by a surface medium of Intel-mediate conductivity (see Fig. 







/ / 



' / / / / 



Fig. 10-75. Location of gravel lenses by resistivity mapping (after Wilcox). 



10-76, diagram [a]). Conversely, the bottom layer may have a high resis- 

 tivity (crystalline bedrock, and the like) and the water may occur as an in- 

 termediate layer of good conductivity above it (b) . The typical three-layer 

 curves so obtained may degenerate into the extreme curve ([a], e) when the 

 top layer is of very good conductivity, and into (c), e when the top layer has 

 poor conductivity. Water subject to rapid circulation in beds of large 

 pore volume and permeability often acts as a nonconductor compared with 

 other beds (curve [cj) . Curve (d) is less frequently observed and occurs when 



" Amer. Geophys. Union Trans. (Hydrology), 574-588 (1937). 



6*C. M. Tattam, Colo. Sch. Mines Quart., 32(1), 118-138 (Jan., 1937). 



