460 EXPLORATION GEOPHYSICS 



the pairs of contacts AB and BC and also to determine the difference in 

 phase angle between them. 



When in complete balance, i.e., no sound in headphones, 



R2 sin tan-i -7^ 

 y 2 -^1 



f^i ~ r. • 1 ^'' 



Ki sm tan~^ -r^- 



X2 



where Ri and R2 are the resistances and Xi and X2 are the capacitative 

 reactances of the two arms of the bridge ; also, 



62 — 61 = tan~^ r^ tan~^ -^ 



where 62 — 61 is the difference in phase angle between V2 and Fi. This 

 difference is positive when V2 leads Vi and vice versa. 



In applying this method, it is essential that the earth at contact B be 

 intermediate in potential with respect to contacts A and C. Observations 

 are usually made along straight line traverses. From data obtained with 

 this bridge both potential and phase variations can be plotted. 



A.C. Method Utilising Three or More Current Electrodes. — In this method, sev- 

 eral current electrodes are used and the strengths of the currents in each circuit are 

 made to vary in a predetermined manner, f When three power electrodes are used, the 

 total current h, which enters the earth through a common power electrode, will be 

 distributed in the subsurface so that currents of magnitude h and h, respectively, reach 

 the other two power electrodes. The ratio of the currents h and h is varied by varying 

 the effective impedance of each branch circuit. 



The characteristic and important advantage claimed for the method is that the depth 

 of maximum current density in the ground depends on the ratio 7^. Hence, because the 



I2 h 



potential difference is a function of the ratio y- and because the ratio -y—can be varied 



so as to penetrate to any preassigned depth, the measured potential difference 

 theoretically can be made to correspond to a current penetration to any desired depth. 



Method Employing Commutated Current.^ — Serious limitations of 

 the D.C. methods outlined above are the errors introduced by undesired 

 earth potentials and the necessity of employing non-polarizing electrodes 

 which are difficult to handle, except under favorable terrain conditions. 

 Marked disadvantages of the audio-frequency A.C. methods are their poor 

 penetrating power and the indefiniteness of the minima detected with 



t H. M. Evjen, "Electrical Method of Geophysical Exploration," U. S. Patent 2,169,685; issued 

 Aug. 15. 1939. U.S. Patent 2,172.557; issued Sept. 12. 1939. 



t H. M. Evjen, "Apparatus for Making Geophysical Explorations," U. S. Patents No. 2,314,873, 

 Mar. 30, 1943, and 2,342,626, Feb. 29, 1944. 



D. S. Muzzey, Jr. and R. D. W. Miller, "Apparatus for Electrical Exploration," U. S. Patent 

 No. 2,363,987, Nov. 28, 1944. 



D. H. Clewell, "Geological Exploration System," U. S. Patent No. 2,454,911, Nov. 30, 1948. 



