1050 EXPLORATION GEOPHYSICS 



Various modifications of the potential method have been proposed. For instance, 

 Hummel t proposes to measure the potentials at null-point conditions without the use 

 of a manually-controlled or automatically-controlled potentiometer. In Hummel's 

 method, two electrodes are usually lowered into the bore hole ; one is a current elec- 

 trode and the other a potential electrode. Sufficient current is passed into the well 

 through the current electrode to balance out exactly the natural ground potentials 

 measured between the potential electrode in the well and the other potential elec- 

 trode at the surface. A record is made of the variation in current necessary to create 

 the zero potential condition. 



The eflfects of extraneous earth potentials may also be eliminated or minimized 

 by use of an auxiliary potential circuit. In this arrangement, the extraneous earth 

 potentials are fed into the measuring circuit in such a manner that they oppose or 

 neutralize the same variations in the measuring circuit.* 



Karcher § has developed a means for measuring polarization during drilling. 

 The polarization in question is a back E.M.F. which opposes the voltage causing the 

 current flow and persists after the voltage is removed. The method utilizes a bit 

 which is insulated from the remainder of the drill stem and connected electrically 

 to a recording potentiometer at the surface of the ground. It is claimed that because 

 the electrical properties of the earth formations are affected by their porosities, the 

 phenomena of polarization are also affected by the porosities ; consequently, a variable 

 which depends on the polarization is diagnostic with respect to the amount of the 

 porosity of the earth formation in question. In operation, the current is allowed to 

 flow for some definite period, for example, one minute, during which time the re- 

 sistance to ground is recorded.ft The drilling bit is then disconnected from the current 

 supply and connected to the potential measuring device. The instantaneous reading 

 of the potential is, therefore, the diagnostic variable which depends on the amount of 

 polarization produced by the original current applied to the measuring circuit. 



It is contemplated that the above operation would be repeated as the drilling 

 proceeds, and means are provided for performing this operation automatically. 



A means of conducting electricity through the drill stem to the bit has been 

 developed by Hawthorne.|t It should be noted that if the measurements are made 

 while drilling is in progress these potentials are superimposed on the rapidly vary- 

 ing potentials generated by the bit. Measurement of the latter potentials when 

 the entire bit and drill stem are in contact with the mud or earth has been described 

 previously. 



Complementary Application of Resistivity and S.P. Data 



In the preceding description of the resistivity and the S.P. curves, 

 interpretation vv^as treated separately. In practice, however, the two curves 

 supply data which is complementary for most areas or geological provinces. 

 The interpreter therefore must consider both curves jointly. The following 

 examples illustrate such general interpretation problems, as applied to the 

 cases of soft formation (mostly sands and shales of low or moderate 

 resistivity) and hard formations (mostly limestone of high resistivity). 



t J. N. Hummel, "Process for Inspecting the Ground," U. S. Patent 2,084,143, issued June 

 15, 1937. 



t J. J. Jakosky, "Method and Apparatus for Electrical Exploration of the Subsurface," U. S. 

 Patent 2,162,086. issued June 13, 1939. 



§ J. C. Karcher. "Method and Apparatus for Determining the Porosity of Rock Formations," 

 U. S. Patent 2,085,664, issued June 29, 1937. 



tt T. C. Karcher. "Method and Apparatus for Exploring Bore Holes," U. S. Patent 1,927,664, 

 issued Sept. 10, 1933. 



tt David G. Hawthorne, "Apparatus for Subsurface Surveying," U. S. Patent 2,096,359, issued 

 Oct. 19, 1937. 



