10 EXPLORATION GEOPHYSICS 



devices in the Quincy Mine in Michigan in 1893, attempting to locate 

 copper-bearing lodes. This was followed by the work of Osborn, who 

 experimented with methods of mapping electric equipotential lines on the 

 Mesabi iron range and in the Lake Superior copper areas. 



At the beginning of the twentieth century, F. H. Brown carried out 

 investigations and secured patents on a method wherein the resistance 

 between two grounded points was measured. Several patents on modifica- 

 tions of this method were secured by Brown and McClatchey about 1900. 

 Resistance measurements, as proposed by Brown, had a marked disadvan- 

 tage : the ground in the immediate neighborhood of the electrodes exerts a 

 large and undefinable effect on the measurements. In 1912 and 1913 

 C. Schlumberger proposed the direct current equipotential line method and 

 Bergstrom the alternating current equipotential methods. The first signifi- 

 cant proposal regarding the use of electromagnetic methods was made in 

 1913 by Schilowsky, who investigated the electromagnetic efl^ects produced 

 by subsurface anomalies when the ground was energized by alternating 

 current. 



Schlumberger's studies from 1912 to 1914 in the Calvados Silurian 

 basin (France) probably constitute the first tectonic studies by geophysical 

 methods using artificial fields of force. Additional studies of the French 

 Sain-Bel and Serbian Bor ore deposits, carried out during the same period, 

 showed that the qualitative concepts of locating ore deposits by their elec- 

 trical conductivity had been transformed into an effective technique. Wen- 

 ner's work about 1916 did much to simplify the calculation of resistivity 

 data obtained by passing a current between two electrodes and measuring 

 the potential between two auxiliary electrodes. 



The concept of "apparent resistivity," which greatly simplified interpre- 

 tation, was introduced in 1922 and led to the possibility of systematic 

 geophysical studies over large areas. The first large scale petroleum survey 

 was carried out by C. and M. Schlumberger in 1923, and led to the proving 

 of the existence and the mapping of the Aricesti dome in the Rumanian 

 plain. 



Prior to the twentieth century, only a few isolated instances of suc- 

 cessful application of geoelectrical methods for the discovery of com- 

 mercially valuable ore bodies were recorded. With advancement in the 

 art and greater utilization of metals, more attention was directed toward 

 the discovery of ore bodies. The more obvious mineralized areas were 

 approaching depletion, which furnished an incentive for the development 

 of new means of detecting the presence of ore in imknown areas. The 

 application of geophysical principles to mining problems reached its maxi- 

 mum during and immediately following World War I. Electrical methods 

 were developed concurrently in France, Germany and Sweden, as well as 

 in America, where men engaged in this work soon far surpassed previous 

 accomplishments by perfecting highly specialized methods and equipment. 

 Geophysical methods applied to mining have not resulted in any spectacular 



