ELECTRICAL METHODS 545 



A photographic view of tlie instrument is shown in Figure 333. The panel con- 

 trols are marked with the same designations used in the wiring diagram. Operation 

 of the instrument is simple and rapid : (a) The desired current range is selected by 

 means of the shunt switch ^'2;* (b) the potentiometer 7?5 is adjusted to neutralize the 

 potentials existing in the potential electrode circuit; (c) the energizing circuit is 

 closed, and the potentiometer Rx is adjusted to give a null reading or balance on the 

 galvanometer G. The reading of the calibrated potentiometer Rx for null reading of 

 G is the desired E/I ratio. 



The instrument may be used with direct, commutated, or alternating current. In 

 the latter two cases, an alternating current galvanometer, preferably of the rectifier 

 type, is employed. 



Potential Gradient Methods. — The theory of these methods is 

 analogous to that of the gradient methods in gravimetric and magnetic ex- 

 plorations. Successful utilization of the gradient of the electrical potential 

 would allow the field data to be presented in such a manner that those 

 conductors which are better than the retjional average could be plotted as 

 positive gradients while conductors which are lower than the regional 

 average could be plotted as negative gradients. (The contact between for- 

 mations of dififerent electrical conductivities would produce an inflection 

 or zero gradient in the gradient profiles.) Furthermore, if it could be as- 

 sumed that there is no distortion of the equipotential bowls surrounding 

 an energizing electrode, the depth to a formation boundary would be indi- 

 cated as the distance of the point of zero gradient from the energizing 

 electrode. In practice it is found that this distance is a fraction of the 

 potential bowl radius. This fraction is unfortunately not a constant but 

 varies with different resistivities of the two layers. 



In actual field application, these methods are subject to the same limita- 

 tions as the resistivity methods, and the simple interpretation of the field 

 data is applicable only in very limited cases. 



Ratiometer Methods 



In the ratiometer methods, potential drop ratio measurements are made with the 

 aid of a modified bridge circuit.t A potential drop ratio compensator t developed by 

 Th. Zuschlag and called a "Racom" is illustrated schematically in Figure 334. 



A source of power is connected to the current electrodes 5" and S', the electrode 

 S' being located at such a distance that only the potential distribution due to electrode 5" 

 need be considered. The bridge circuit makes contact with the ground at three 

 points A, B, and C. The two ratio arms AD and DC contain variable known resist- 

 ances Ri and R2 and the fixed contact resistances Ra and Ro of the electrodes A 



* The current range used depends on the resistivity and the contact conditions in 

 the area where the measurements are being conducted, 

 t C. Schlumberger, U. S. Patent 1,163,468. 

 H. Lundberg, "Potential Method for Elektrisk Malmletning," Jernkontorets Ann. 1919; see 

 also A.I.M.E. Geophysical Prospecting, p. 50, 1932. 



J. G. Konigsberger, Zeit fiir Geophysik, 1930, Vol. 6. 



A. Broughton Edge, British Patent Application 19120/30; see also Broughton Edge and 

 Laby, Geophysical Prospecting, pp. 50-56 (Cambr. Univ. Press, 1931). 



t H. Lundberg and Th. Zuschlag, "A New Development in Electrical Prospecting," A.I.M.E. 

 Geophysical Prospecting, 1932, pp. 47-62. 



