BORE-HOLE INVESTIGATIONS 



1049 



RECORDING 

 GALVANOMETER 



CABLE 



/ / / / / y , / ^ / J 



' y / / / / /_/ ' ' '_' iJ i_' ^ 



are taken to overcome them. A constant difference of potential may normally appear 

 between these two electrodes in the absence of any S.P. current. This constant differ- 

 ence of potential is not recorded on the S.P. log ; it is counterbalanced by means of a 

 potentiometric circuit not shown in the 

 figure. 



Accordingly, the potential of electrode 

 M is actually measured on the S.P. log 

 with reference to an arbitrary fixed poten- 

 tial. However, the variations of the 

 potential, i.e., the deflections on the S.P. 

 log, do not depend on this fixed potential. 

 It is these variations which are a measure 

 of the potential differences created in the 

 mud by the S.P. current and which make 

 it possible to characterize the formations. 

 Under normal conditions, the excursions 

 toward the negative characterize perme- 

 able beds, whereas excursion toward the 

 positive characterize impervious beds. 



The electrodes employed for commer- 

 cial potential or porosity measurements 

 are preferably made from a material hav- 

 ing a low electrochemical contact poten- 

 tial with the mud. In practice lead is the 

 material commonly employed. Theoreti- 

 cally, all electrodes lowered into bore holes 

 should be of the non-polarizing type to 

 minimize errors caused by electromotive 

 forces arising from contacts between a 

 metal and the water of the bore holes ; 

 practically, however, the error introduced 

 by use of lead electrodes is negligible be- 

 cause of the homogeneity of the bore hole 

 mud with which they are in contact. 



An alternative method which has been 

 proposed by Schlumberger,t but not em- 

 ployed commercially to a large extent, places two small or "point" electrodes within 

 the bore hole, the electrodes being a fixed distance apart. The difference in potential 

 between the two electrodes within the bore hole is measured and recorded at the 

 surface of the ground. The readings obtained when employing two electrodes within 

 the bore hole give the gradient of the potential, and it is necessary to convert this 

 gradient to a potential difference along the bore hole. In areas where bad ground 

 currents prevail, this method has the advantage over the method described above 

 that it is much less affected by ground currents, due to the short distance between the 

 electrodes. However, interpretation of the gradient curve is almost impossible due 

 to tlie many thin strata encountered in the usual bore hole. 



Another modification employs an extended electrode, with a small or "point" elec- 

 trode spaced about 50 feet below it. Such a system is practically unaffected by extra- 

 neous ground currents and has the advantages of the two "point" electrode system. In 

 practice, the metallic braid over the cable, if of the shielded type, will serve as the 

 extended electrode. 



Fig. 647. — Schematic arrangement for measur- 

 ing potentials in a bore hole. 



t C. Schlumberger, "Electrical Process for the Geological Investigation of a Porous Strata 

 Traversed by Drill Holes," U. S. Patent 1,913,293. 



