ELECTRICAL METHODS 



511 



magnitude of which is based on experience under similar geological and 

 electrical conditions. In these cases, the results should be considered as 

 qualitative until proper control is available. 



Small Scale Experiments. — In small scale experiments designed to 

 determine the effects of ore bodies, conductors (model conductors) of 

 various shapes are immersed in tanks partially filled with water of appro- 

 priate conductivity. The model ore bodies may be made by covering any 

 convenient material with copper foil. The tank should be relatively large 

 and should have dimensions about ten times the maximum dimension of 

 the model object contained in the tank. 



Fig. 308. — Type anomalies produced by model formations of various dips. Resistivity values 

 obtained by moving a fixed Wenner configuration of electrodes normal to the strike. 



Extensive experimental work has been done with small scale models, t 

 A few general types of anomalies characteristic of model formations 

 are shown in Figures 308 and 309 taken from tests by Clyde H. Wilson. 



Representative results of tank tests to determine the zone of influence 

 in a Wenner electrode configuration are shown in Figure 309. In these 

 tests a glass partition, oriented at right angles to the line of electrodes, 

 was moved in a direction along the line of electrodes.* Curve No. 1 was 

 obtained for the higher partition, and curve No. 2 for the lower partition. 

 The results show that the zone of influence is associated with the dis- 

 torted potential bowl surrounding each of the energizing electrodes. 

 The peaks of the curves shift farther away from the power electrodes 

 at the greater depth. The results indicate that a distorted potential bowl 

 exists around each source and sink (the power electrodes) and the actual 

 potential as measured is the resultant of the effects in the vicinity of each. 



This phenomenon explains the limited resolving power of the Wenner 



t M. King Hubbert, "Theory of Scale Models as Applied to the Study of Geologic Structures," 

 Bull. Geological Society of America, 1937, Vol. 48, pp. 1459-1520. 



M. King Hubbert, A.I.M.E. Geophysical Prospecting, 1934. 



C. A. Heiland, Colorado School of Mines Bulletin, 1929-1930. 



J. H. Swartz, "Resistivity Measurements upon Artificial Beds," U. S. Bureau of Mines, 

 Information Circular 6445, Feb. 1931. 



* It will be seen of course that movement of the partition will produce the same 

 result as would be obtained if the partition were to remain stationary and the elec- 

 trodes moved in a fixed configuration. For experimental tests, movement of the parti- 

 tion is more convenient and far more rapid. 



