ELECTRICAL METHODS 



619 



METHODS WHEREIN THE EFFECTS OF THE PRIMARY 

 FIELD ARE MINIMIZED 



There are many methods by which the field from the surface energizing 

 system may be minimized at the detecting coil. Theoretically, this can be 

 accomplished with a conductive energizing system utilizing two grounded 

 electrodes. For this work, a direction-finding coil is used, and its axis of 

 rotation is on the extension of the imaginary line connecting the two current 

 or energizing electrodes. When in this position very little of the primary 

 field from the surface wire is picked up by the coil. On the other hand, the 

 position of the coil allows it to pick up the secondary field from the sub- 

 surface conductive zone. 



The terminals of the detector coil are connected to a detector and a two 

 stage audio-frequency amplifier for the higher frequencies, or a three stage 

 amplifier for the lower frequencies. The out-put of the amplifier is con- 

 nected to a vacuum tube voltmeter. By keeping the amplification constant 

 relatively simple field work will often show the presence of a shallow con- 

 ductor. Under such conditions, it will be found that the maximum signal 



Zo», of . 



WMMMWMV'O'-O 



4 



Fig. 387. — Conductive method of energizing the subsur- 

 face. £i and £2, electrodes; R^ and R2., resistors for con- 

 trolling current; G, generator; 7i and I^, line ammeters. 



will be obtained when the detecting coil is held in a vertical position, and is 

 located vertically over the subsurface conductor. The method, however, is 

 not generally recommended due to the difficulties encountered by phase-shift, 

 and the unknown distribution of the primary current between electrodes. 

 Another method,t applicable to the higher frequencies only and hence 

 of limited use, consists in setting up a primary high frequency electro- 

 magnetic field polarized about a linear axis. The direction of the sec- 

 ondary field produced by the induced current in a conductive body is 

 determined at points on the linear axis. The direction-finding apparatus 

 is similar to that described and shown in Figure Z7^. The energizing 

 apparatus may be of the simple Hartley oscillator type. The method 

 works fairly well at depths not exceeding 25 to 150 feet in dry desert 

 countries where long vein conductors are encountered. 



t J- J. Jakosky, "Method and Apparatus for Locating Conductive Bodies," U. S. Patent 

 1,792,910, issued Feb. 17, 1931. 



