Chap. 10] 



ELECTRICAL METHODS 



809 



Considerable ground was covered with high-frequency methods about 

 ten years ago in Canada and elsewhere, and numerous conductive zones 

 were located. Some proved to be commercial ore bodies; but others, and 

 probably the majority, did not. Because of this experience the trend in 

 inductive methods has been consistently toward lower frequencies. On 

 the other hand, the I.G.E.S. has reported fair success with high frequencies. 

 An example is shown in Fig. 10-125. This survey was made in the Zeehan 

 copper-nickel field where the ore occurs near the peridotite and pyroxenite 

 portions of basic dikes intruded into Cambro-Ordovician slates. The ore 

 is a mixture of pyrrhotite, pentlandite, and chalcopyrite. The more 

 uniform indications were obtained in the southern part of the area (lines 

 45, 46, and 49). The three remaining profiles are located in the north- 

 eastern part of the area where the trend of the dike is northeast instead 

 of north. The effect of differences in the depth (and width) of the ore 

 bodies is clearly recognized. Indications in the northeastern portion of 

 the area were verified by trenching. 



IX. RADIO METHODS 



A. General 



"Radio" methods of electrical prospecting make use of frequencies 

 ordinarily employed in wireless communication. They range from several 

 hundred kilocycles to several megacycles (corresponding to a wave length 

 of several kilometers to several hundred meters). In regard to their 

 mineral-locating possibilities, radio methods 

 have been fertile soil for the imagination 

 of laymen, radio amateurs, and even phys- 

 icists! While theoretically the conditions 

 for locating ore bodies or oil by radio waves 

 appear to be very simple, there are, in prac- 

 tice, innumerable interferences from topog- 

 raphy and near-surface variations in mois- 

 ture and mineralization . Further, the depth 

 penetration is very limited in the type of 

 ground occurring in temperate climates. 

 Where indications from deep ore bodies or 

 oil deposits have been obtained, it has not 

 been definitely proved that the indications 

 did not come from shallow petrographic or 

 structural variations genetically related to 

 the subsurface mineralization. 



In the application of radio methods to 



/<? 



fff* W' 10* to' fO* w^ 

 Freqatncy, ci/cles ptr second 



Fig. 10-126. Attenuation in 

 sheet 124 feet thick, for vari- 

 ous resistivities and dielectric 

 constants (after Joyce). 



