GEOLOGY 83 



These methods have been used by commercial companies for 

 the location of mineral deposits and petroleum-bearing rocks. 

 Government departments will be primarily interested in their use 

 for the detection of underground water-supplies. For this purpose 

 the electrical method is most suitable. 



The electrical or 'resistivity' method depends on the principle 

 that strata saturated will conduct electricity better than dry strata. 

 Therefore, by applying an electric field to the ground and measur- 

 ing the potential at various points, the degree of resistivity of 

 underlying rocks can be measured and the depth and extent of 

 water can be estimated. Depths from two hundred and fifty to 

 five hundred feet are usually given for the effectiveness of the 

 electrical method for locating minerals. These figures do not apply 

 in the case of water, however, for under favourable conditions it 

 is claimed that the depths and extent of water-bearing formations 

 can be determined down to one thousand feet or more. The 

 Australian work has shown that under suitable conditions the 

 presence of saline water can be detected; this would have great 

 practical value in obviating the risk of sinking wells only to find 

 that the resulting water is useless for either stock or irrigation. It 

 must be stressed that only trained geologists can produce satisfac- 

 tory results, as the interpretation of any geophysical observations 

 involves a full understanding of the rocks. 



It seems desirable that this method of locating supplies should 

 have extended trials with as little delay as possible. The necessary 

 instruments can now be purchased for ;£^200 or even less, so experi- 

 ments need not prove too expensive. Interest has already been 

 aroused in parts of Africa and some work fully worthy of considera- 

 tion has been carried out. In South Africa, the Geological Survey 

 of the Union is pursuing investigations into the value of various 

 geophysical methods of prospecting for minerals and for under- 

 ground water, the work being done by trained geologists. In 

 addition, there are a number of experts of varying qualification, 

 who work on contract for mining companies, for the purpose of 

 discovering valuable mineral resources. 



In Southern Rhodesia Mr. Shaw (1934), working on under- 

 ground water, has made investigations which were checked by 

 bore-holes. The drillings confirmed the indications from resistivity 



