Chap. 10] ELECTRICAL METHODS 751 



medium of poor conductivity. Another procedure is to consider a vein 

 as a sheet of negUgible thickness, that is, equivalent to a plane of ideal 

 reflecting characteristics. If the plane is a good conductor, its effect is 

 equivalent to producing an image of opposite sign, while an insulating 

 sheet, assumed to be so thin that charges do not collect on it, produces 

 an image of a source of like sign. In this manner both resistivity and 

 P.D.R. distributions can be calculated. ^^ 



4. Ore bodies. The method by which anomalies of ore bodies are 

 calculated depends entirely on their shape and geometric disposition. 

 For ores in horizontal or nearly horizontal stratification (certain types of 

 lead- zinc and lead-silver ores) the treatment is the same as for horizontal 

 beds. For vein-type deposits in vertical positions the P.D.R. 's may be 

 calculated by assuming two adjacent contacts or a thm conductive or in- 

 sulating sheet. This procedure is applicable when the depth of overburden 

 is small; if such is not the case, the problem can be treated as a two-layer 

 case with a vertical vein in the lower medium that reaches up to the 

 interface. Dipping ore bodies may be considered as two contacts of equal 

 dip or as thin sheets, as the case may be. Lenticular bodies of limited 

 dimensions may be assumed to be equivalent to polarized doublets. The 

 more complicated cases are best treated by model experiments. 



5. Surface geologic features. Owing to the great sensitivity of the 

 P.D.R. method to horizontal resistivity variations, considerable anomalies 

 will be produced by slight changes in the composition and structure of 

 surface and near-surface formations. In regard to vertical contacts, the 

 theory holds only if beds are infinite in vertical direction; for horizontal 

 formation boundaries, the theory assumes that they remain the same in a 

 horizontal direction. The field technique described below is intended to 

 eliminate surface geologic features which are characterized by a limited 

 extent in either horizontal or vertical direction, or both. 



In Fig. 10-80, A' , C, and B' are the three potential stakes and E the 

 next power electrode. The arrangement is moved over a conductive body 

 of limited depth extent, with an electrode separation equal to the width 

 of the body. Assume as unit of length one-half of the electrode separation 

 and let the depth extent of the conductive body vary from one to eight 

 units. The corresponding ratio curves become more un symmetrical with 

 increasing depth extent. Therefore, to detect bodies of great depth extent 

 and to eliminate near-surface interference, it is but necessary to take a 

 reverse profile. Upon averaging the two curves, one obtains curves with 

 amplitudes proportional to depth extent, and anomaly peaks over the 

 center of the body."" 



«i Howell, op. cit., 34-37 (1932). 



"« F. Kihlstedt, A.LM.E. Geophys. Pros., 62-74 (1934). 



