1038 EXPLORATION GEOPHYSICS 



peaks to the left of the chart. The base Hne corresponds in most cases to 

 impervious beds, whereas the peaks are generally opposite permeable strata. 

 The shape and the amplitude of the peaks may be quite different according 

 to the formation, but there is no definite correspondence between the magni- 

 tude of the peaks and the values of permeability. 



The importance of potential measurements is probably equal to that of 

 resistivity measurements in commercial bore hole investigations. In some 

 areas, e.g. in the Gulf Coast area, the potential measurements are of major 

 importance. 



In 1896 P. Bachmetjewf described an experiment which he believed 

 indicated that potentials could be created which were due not to mineral 

 contacts but to the motion of ground waters through subsurface sands and 

 porous materials. These potentials were believed to be due to an effect 

 first discovered and described by C. Quincke^ in 1859. The first practical 

 application of these phenomena to drill-hole exploration appears to have 

 been made by Schlumberger,§ who received a patent for the location of 

 permeable strata traversed by a drill hole. 



The potential existing at the faces of the strata traversed by a drill hole 

 consists, essentially, of two components : one, the potentials of electro- 

 filtration caused by movement of the fluid either into or out of the porous 

 formations traversed by the drill hole ; and two, the electrochemical poten- 

 tials created at the contact of two solutions having different concentrations 

 of dissolved salts. Although the two effects exist simultaneously, they are 

 best described separately. 



Electro filtration Potentials 



In most drill holes, the well is filled with water or mud in sufficient 

 quantities to exert a pressure on the walls distinctly superior to the hydro- 

 static head of the fluid in the rocks. This is particularly true in the numer- 

 ous cases where the muds filling the holes are purposely made relatively 

 heavy to avoid the caving-in of the hole and blowouts of gas or oil. In these 

 cases, the penetration of the circulating mud into the pervious layers 

 generates electrofiltration or electrokinetic potentials. 



The electromotive force produced by the flow of an electrolyte through 

 a pervious dielectric is directly proportional to the differential hydrostatic 

 pressure and the electrical resistivity of the electrolyte and is inversely pro- 

 portional to the viscosity of the electrolyte. It does not depend on the thick- 

 ness of the filtering sheet nor on the radii and number of pores of the 

 pervious medium. The magnitude of the E.M.F. produced by filtration 

 may be expressed by the equation iff 



t See article by W. M. Rust, Jr., "A Historical Review of Electrical Prospecting Methods," 

 Geophysics. Vol. 3 (1938), pp. 1-6. 



t C. Quincke, Annalen der Physik, Series 2, Vol. 107 (1859), pp. 1-47. 



§ C;. Schlumberger,' "Electrical Process for the Geological Investigation of the Porous Strata 

 Traversed by Drill Holes." U. S. Patent 1,913,293, issued June 6, 1933. 



tt C. and M. Schlumberger and E. G. Leonardon, "Electrical Coring: A Method of Determining 

 Bottom-hole Data by Electrical Measurements," A.I.M.E. Geophysical Prospecting, 1934, pp. 237-272. 



