BORE-HOLE INVESTIGATIONS 1023 



where polarization is not effective. This point is the cutting end of the bit, 

 where new rock is continually being exposed by the bit. The resistance varia- 

 tions in the circuit, therefore, are predominately those encountered by the bit. 



Numerous high frequency and radio methodsf have been proposed, 

 but such methods have not come into commercial operation for deep bore 

 hole work due chiefly to the enormous and unpredictable changes in capacity 

 and leakage which occur when long lengths of cable are lowered into the 

 hole. As a matter of fact, up to the present, the commercially successful 

 methods utilize direct current or alternating current having a frequency 

 of less than 750 cycles per second. 



The total resistance methods are relatively simple in their practical appli- 

 cation but suffer from one defect: Variations in the earth or mud 

 filling the drill hole in the immediate vicinity of the moving electrode, or 

 electrodes, will sometimes exercise an undue effect on the readings, due to 

 the fact that the total resistance between the electrodes is confined almost 

 entirely to the immediate vicinity of the electrodes. (Compare Fig. 283.) 

 The drilling mud itself is relatively uniform due to the circulation during 

 the drilling process. On the other hand, because the mud is weighted to pro- 

 duce a hydrostatic head in excess of the probable formation pressures, it 

 permeates and impregnates the porous formations.* Obviously, this inva- 

 sion of the mud may influence the effective resistance of the permeable 

 formations markedly. 



The variations produced by changes in the effective diameter of the bore 

 hole constitute another disadvantage of the single electrode method wherein 

 the electrode is not in direct contact with the formation itself. This variation 

 is particularly pronounced when drilling with excessive mud circulation 

 through the relatively soft formations (shales, unconsolidated sediments, 

 etc.) frequently encountered in the Gulf Coast and California areas. When 

 single electrode logs are run, they should be supplemented with caliper 

 logs to show the changes in effective area of the bore hole. 



There are certain advantages, however, which tend to offset the disad- 

 vantages. When the moving electrode is of small size (a few inches in 

 length), the single moving electrode system will give much greater detail 

 and will show the very thin formations better than the multiple moving 

 electrode systems. In addition, the single moving electrode system utilizing 

 a small moving electrode (or the multiple moving electrode system having 

 a short separation between the current and the potential electrode) gives 

 sharper structural breaks corresponding to the tops and bottoms of the 

 resistivity variations than the measuring systems having a greater pene- 

 tration. This greater detail is, of course, superposed on the non-structural 

 variations previously mentioned. 



t T. Zuschlag, "Methods of Investigating the Nature of Subterranean Strata," U. S. Patent 

 1,652,227, issued Dec. 13, 1927. 



H. Lowy, "Methods for Ascertaining the Nature of Subterranean Strata," U. S. Patent 

 1,092,065, issued March 31, 1914. 



* The distance of permeation is primarily a function of the differential pressure, 

 time of contact, viscosity and sealing properties of the mud, and the permeability of the 

 formation. 



