ELECTRICAL METHODS 463 



A map which shows surface topography and areal geology is necessary 

 for final interpretation of the electrical data. This map should be drawn 

 to the same scale as the equipotential contour map.* 



The interpretation is largely empirical in character and must be based 

 on previous experience with the method and knowledge of the local 

 geology. Usually, it is relatively simple to make a qualitative interpre- 

 tation of the results by outlining the indicated conductive zones and deter- 

 mining by inspection which of them are best defined and are in best accord 

 with the geologic possibilities. In general, potential data alone will not 

 permit determinations of the depth of the conductive body, its strike, 

 length, or width. Usually, shallow deposits give narrow and pronounced 

 potential peaks, while the deeper lying bodies produce broader and less 

 pronounced peaks. The steepness and the width of the peak are thus 

 oftentimes an index as to the depth of the body. 



Various modifications of the general field technique may be employed 

 to suit special conditions. If an ore body is partly accessible, one of the 

 energizing electrodes may be connected to the ore body, thereby making 

 the potential of the surface of its conductive portion the same as that of 

 the energizing electrode. The shape of the ore body may then be inferred 

 from the equipotentials observed at the surface of the earth. Another 

 modification takes advantage of the anisotropy of sedimentary rocks. The 

 dip and strike of formations may be indicated from the increased con- 

 ductivity in the direction of bedding planes, t 



Model Experiments. — The interpretative techniques of the equi- 

 potential point and line methods may be facilitated by experiments with 

 small scale models or test tanks. The tanks usually are filled with layers 

 of different conductive materials, such as moist sand, clay, etc. ; or they are 

 filled with a weak electrolytic solution (such as soluble salt added to fresh 

 water) in which are immersed model ore bodies made of a conductive 

 material. The model may conveniently be made by forming a block of 

 wood into the desired shape and covering it with a layer of 20 to 24 gauge 

 sheet copper, with joints soldered. 



Laboratory model experiments are of value because they show the 

 type of anomalies which would be obtained under the simple conditions 

 selected for the experimental work. However, because actual geological 

 conditions are usually quite complex, the application of model results 

 to the solution of field problems is valid only to the extent that the 

 conditions pertaining to the laboratory work exist in the field. 



* Generally the potential irregularities produced by topographic features are eval- 

 uated empirically, although in certain cases these anomalies can be computed by 

 making a sufficient number of simplifying assumptions. 



t E. G. Leonardon and S. F. Kelly, "Some Applications of Potential Methods to Structural 

 Studies," A.I.M.E. Geophysical Prospecting. 1929, pp. 180 to 186. 



C. and M. Schlumberger and E. G. Leonardon, "Electrical Measurements in Anisotropic 

 Media," A.I.M.E. Geophysical Prospecting, 1934, pp. 159-181. 



