Chap. 10] ELECTRICAL METHODS 735 



clay (p small), or water (p intermediate). The conductivity of these sub- 

 stances may be altered by adding electrolytes (NaCl or CUSO4). Tanks 

 should be compensated to eliminate wall effects. 



Model experiments are of value in determining the anomalies of ore 

 bodies whose calculation would be too time consuming. The curves re- 

 produced in Fig. 10-65 show that a dipping conductive formation may 

 produce a negative anomaly in resistivity when the electrodes are parallel 

 to the strike; but that it produces a maximum, flanked by two minima, 

 when the electrodes are perpendicular to the strike. These experiments 

 were made in 1929-1930 at the Colorado School of Mines to study the pos- 

 sibility of locating fluorspar deposits associated with faults, rendered con- 

 ductive by circulation of mineralized solutions. They were confirmed later 

 in the field. Similar results were published by Hubbert in 1932, who also 

 supplemented his field data with model tank experiments.** Further 

 experiments were contributed by N.H. Johnson,^^ with a theoretical discus- 

 sion by Howell.^ Other model experiments were made by T. A. Man- 

 hart*' and R. J. Watson.*' 



F. Results Obtained by Resistivity Surveying 



Resistivity methods have been appfied (1) in structural investigations 

 for the purpose of locating potential oil structure, that is, anticlines, salt 

 domes, faults, and the like; (2) in mining (a) in connection with structural 

 investigations, (6) for the location of ore bodies, of brown coal, anthracite, 

 and salt, (c) for the determination of thickness of overburden in pro- 

 posed shaft and tunnel sites and of depth to bedrock in placer projects; (3) 

 in civil engineering (a) in foundation problems, (6) in determining depth 

 to bedrock or rock consistency in proposed dam and tunnel sites, and (c) in 

 highway engineering for the location of construction materials, sand and 

 gravel, and the like; (4) in prospecting for water. 



1. Structural investigations in oil exploration. For comparatively shal- 

 low problems (to 2000 feet or so) resistivity mapping has been found fairly 

 useful in oil exploration. Fig. 10-66 shows a curve of apparent resistivity 

 across an anticline of Devonian limestone flanked by Mississippian shales 

 and sandstone and concealed by glacial formations, boulder clay, and the 

 like. This traverse was run with an electrode separation of 200 feet. Ow- 



« M. K. Hubbert, A.LM.E. Geophys. Pros., 9 (1934). 

 « Ibid., 14. 

 « Ibid., 30. 



4Bo ll)i(l_ 35, 



«Colo.'Sch, Mines Quart., 32(1), 141-168 (Jan., 1937). 

 " A.LM.E. Geophys. Pros., 222-232 (1934). 



