502 EXPLORATION GEOPHYSICS 



metrical inhomogeneities is due to F. H. Brown.t Brown's apparatus 

 consisted essentially of a differentially wound receiver and three electrodes. 

 The electrodes were mounted in a straight line with the "central" electrode 

 midway between the two exploring electrodes. One winding of the receiver 

 was connected to the central electrode and one of the exploring electrodes, 

 and the other winding was connected to the central electrode and the other 

 exploring electrode. This method operated on the principle of causing 

 current to traverse different depths of the earth forming one circuit, and 

 at the same time causing current to flow through the other earth circuit. 

 The relative strengths of the two currents could be compared audibly by 

 means of the differential coil in the receiver. 



Schlumberger J utilized equipotential studies around the power elec- 

 trode to determine inhomogeneities. A non-symmetrical distribution of 

 subsurface current flow was indicated by a distortion of the equipotential 

 lines. 



Lee § has proposed a method wherein the ground is "partitioned," prefer- 

 ably into symmetrical parts, with the station center at the midpoint as in 

 the Brown arrangement. Measurements are made on each side of the mid- 

 point for the purpose of comparing one side with the other. The compari- 

 son may be made of the computed resistivity values or any function of 

 these values, such as measured potentials, currents, or resistances. This 

 arrangement measures the resultant effect produced by local zones of 

 anomalous conductivity and dip of structure. 



The potential ratio methods generally utilize a central electrode as a 

 reference point. Zuschlagtt illustrates and describes one method for com- 

 paring electrical potentials about the midpoint of the electrode configura- 

 tion. Methods of this type indicate a non-symmetry of position, but they 

 do not isolate the near-surface effect or locate the region of its occurrence 

 along the traverse line of measurement. 



Experimental results show that the potential electrodes are subject to 

 a far greater disturbance as a result of near-surface effects than the power, 

 or energizing, electrodes. Hence, the near-surface effects can be minimized 

 by employing an electrode system wherein the two potential electrodes 

 remain in a fixed position, while the two power electrodes move outward. 

 This condition is illustrated in Figure 313f. 



An accentuation of near-surface effects occurs when the electrode move- 

 ment is such that the power electrodes remain stationary and the poten- 

 tial electrodes move. It is this accentuation of near-surface effects which 

 so greatly handicaps the potential ratio methods and other methods wherein 

 only the potential electrodes are moved. 



t F. H. Brown, "Electrical Apparatus for Determining tlie Location of Metallic Ores," U. S. 

 Patent 817,749, issued April 17, 1906. 



t C. Schlumberger, "Process for Determining the Nature of the Subsoil by the Aid of Elec- 

 tricity," U. S. Patent 1,163,468, issued Dec. 7, 1915. 



§ F. W. Lee, "Method of Conducting Geological Survey," U. S. Patent 1,951,760, issued 

 Mar. 80. 1934. 



tt T. Zuschlag, "Electrical Prospecting," U. S. Patent 1,951,387, issued Mar. 20, 1934. 



