GEOPHYSICAL METHODS OF INVESTIGATING 229 



is present. Then the ore deposit will become polarized^ 

 and transformed into a secondary element, which discharges 

 itself according to the interruption of the polarizing current. 

 While the period of discharge arises if there is a measurable 

 potential difference on the surface, he has succeeded with 

 the help of polarization phenomena in distinguishing 

 between minerals of metallic conductivity, such as mag- 

 netite, pyrites, lead glance, etc., and groundwater strata. 

 Now the application of direct current also brings a polariza- 

 tion of the searching probes wherein the ground plays 

 the part of an electrolyte, so that in potential line surveys 

 special electrodes or non-polarizable electrodes^ made of zinc 

 plates in concentrated zinc sulphate solution and other 

 chemical types are used. Alternating current of average 

 frequency is now often applied in order to avoid this 

 polarization phenomenon at the probes, and the course of 

 the current in such cases no longer depends only on the 

 ohmic resistance of the separate rock layers but also on the 

 capacitative and inductive alternating-current resistance, 

 which also brings about phase displacement and fluctuations 

 in the current, thus causing variations in the form of the 

 potential lines. However, this latter aspect does not affect 

 electric coring methods, so that we will content ourselves 

 with referring the interested reader elsewhere.^ 



Schlumberger's Method of Investigating Boreholes. — 

 The application of the above method has been extended 

 by the brothers Conrad and Marcel Schlumberger of Paris 

 to investigations on lithological data, dip of strata, faulting, 

 intercalations and other features accounting for the devia- 

 tion of boreholes. They consider the electrical conductivity 

 of the constituents of the earth's crust which fluctuate very 

 widely; that of the badly conducting overwhelming majority 

 of the constituent minerals depending in a very decisive 



^ Schlumberger, C, Ph^nomene ^lectrique produit par les gisements 

 metalliques, Compt. rend., p. 477, 1922. 



^ Bartjs, C, U. S. Geol. Survey Mon., No. 3, pp. 309 et seq. 



^ Ambronn, R.., "Methods of Applied Geophysics," pp. 131 et seq.; 

 Heine, H., Die Einflusse von Induktion und Kapazitat bei geophysikaUschen 

 Potential-liniemessungen, Ztschr. Phys., p. 219, 1926. 



