516 EXPLORATION GEOPHYSICS 



respectively, and the thickness of each layer is set equal to Xd where d 

 denotes the actual thickness of the layer and A its coefificient of anisotropy. 

 Evidently from a physical viewpoint these consequences affect the inter- 

 pretation of resistivity data in that the values of the apparent resistivity 

 must be modified to take into account the anisotropy of the formations 

 being investigated. 



FIELD PROCEDURE AND EQUIPMENT 



Electrode Configurations for Measuring Resistivities. — Various 

 electrode spacings and configurations may be employed in resistivity 

 measurements.f As a general rule, the configurations employed are 

 chosen because of their symmetrical arrangement which allows simpli- 

 fication of formulas. The literature, with special reference to many pro- 

 fessional-type papers, is replete with various configurations which reput- 

 edly are superior to all other configurations. There is no theoretical basis 

 for judging one configuration superior to another. In practice, however, 

 it has been found that certain configurations, used in conjunction with 

 the proper field technique, may permit better evaluations of certain factors 

 that introduce errors. Briefly stated, these undesirable factors are intro- 

 duced chiefly by: (1) near-surface effects, (2) variations in depth of cur- 

 rent penetration, (3) lateral changes in geology. 



The electrode configuration employed should preferably be such that 

 the maximum flow of current will take place in the depth range desired 

 in the measurement. Also, the potential electrodes should be so positioned 

 with respect to the current electrodes that they receive the maximum 

 influence from the subsurface flow of current. 



Obviously, an electrode arrangement wherein the current electrodes 

 are very close together will be very ineffectual for deep subsurface inves- 

 tigations, because the percentage of current penetrating to the desired 

 depth range will be small and its variations will not be measurable at the 

 surface. In practice, the separation between the current electrodes usually 

 should be from three to six times the desired depth of measurement. 

 The optimum value for any given electrode configuration must be deter- 

 mined experimentally for any given area. 



The location of the potential electrodes is important, because if the 

 potential electrodes are positioned too near the current electrodes, they 

 will be predominately influenced by shallow effects. As the distance 

 between the potential and the current electrodes is increased, the deeper 

 zones have a greater effect on the measurement, but at the same time 

 the observed potentials become smaller and the effects of near-surface 

 variations become proportionally greater. Usually, the best "detectability" 



t H. M. Evjen, "Electrical Method of Geophysical Exploration," U. S. Patent No. 2,172,557, 

 Sept. 12, 1939. 



J. J. Jakosky, "Method and Apparatus for Electrical Exploration of Subsurfaces," U. S. Pat- 

 ent No. 2,174,343, Sept. 26, 1939; "Electrical Exploration of Subsurfaces," U. S. Patent No. 

 2,256,742, Sept. 23, 1941. 



R. G. Piety, "Method of Surface Prospecting," U. S. Patent No. 2,390,270, Dec. 4, 1945. 



