230 DEEP BOREHOLE SURVEYS AND PROBLEMS 



manner not only on the ground but also on its moisture 

 content and the substances dissolved in this moisture. (We 

 shall not submit a table of conductivities and resistivities 

 here because they vary tremendously for the different strata 

 of the earth's crust even in the same rocks. They should 

 be determined experimentally for every place being investi- 

 gated.) With the exception of certain metallic ores which 

 have the property of electronic conductivity (like metals), 

 rocks are capable of transmitting an electric current only 

 by means of the water which they have imbibed.^ Therefore 

 their conductivity is solely electrolytic, and disappears 

 entirely with drying. The solid mineral elements are 

 almost perfect insulators, which the current skirts in 

 following the damp veins. The following approximate 

 laws have been deduced therefrom: 

 The specific resistivity of a rock is 



1. Inversely proportional to the quantity of imbibed 

 water contained in a cubic meter of rock. 



2. Proportional to the resistivity of this water, therefore 

 roughly inversely proportional to the total quantities of 

 salts dissolved per unit volume of the water. 



Thus the resistivity of a rock is in inverse proportion 

 to the total weight of electrolytes dissolved in a cubic meter 

 of the rock. Schlumberger says that these underlying 

 principles are, of course, subject to many modifications, 

 according to conditions; the angle at which sedimentary 

 strata are inclined, for instance, affects the resistance; a 

 rise in temperature reduces the resistance, etc. 



By the accompanying illustration (Fig. 157) we see the 

 measuring apparatus in diagrammatic form. It comprises 

 three insulated cables 1, 2, and 3, suspended in the hole 

 4, and terminating toward the bottom in three electrodes 

 A, M and N immersed in the well water 5. The radii, 

 AM equals r and AN equals r' , are chosen greater than the 

 diameter of the hole. The electrode A serves to send 

 the current into the soil and the electrodes M and N to 

 measure the difference of potential produced by ohmic 



1 BiGNELL, L. G. E., Electric Coring, Oil Gas Jour., p. 33, Feb. 6, 1930. 



