620 EXPLORATION GEOPHYSICS 



In another modification of this method, the surface wires are laid 

 out as a divided conductor in the form of a rectangle or square as shown 

 in Figure 387. If measurements are to be made along a line midway be- 

 tween the wires, the rheostats Ri and R2 are adjusted to give equal current 

 flow in the two branches of the current. The primary fields from the two 

 surface wires effectively neutrahze each other, and the vector direction and 

 magnitude measured by the detecting coil are chiefly due to the secondary 

 field set up by the current flowing in the subsurface. Obviously, a dis- 

 advantage of the method is the necessity of laying the two surface lines. 

 For general reconnaissance work only one line or "U" may be laid, 

 although interpretation of the vector data is more difficult. This same 

 divided conductor system for energizing the ground has also been em- 

 ployed successfully when magnetometric methods of measurement are 

 used for measuring the magnetic field associated with the flow of direct 

 current. 



METHODS EMPLOYING A VERTICAL COIL ENERGIZING 



SYSTEM 



The most widely employed electromagnetic equipment utilizes a ver- 

 tical coil for energizing the ground. The vertical coil is especially suit- 

 able for energizing conductive zones which are vertical or which have a 

 steep dip, such as veins, etc. This type of equipment permits more rapid 

 field work than the large horizontal loop type. Also, the results may be 

 interpreted more conveniently. The vertical type coils are usually ener- 

 gized with medium to high frequency alternating current because of their 

 relatively small size. 



Energizing Equipment. — A vertical type energizing system of the 

 "high frequency" type is shown in Figure 388. This system operates at 

 frequencies from 30 kc. (30,000 cycles per second) to 50 kc. The cross 

 pieces for supporting the coil are each 10 feet in length, and when set up, 

 the coil dimensions are approximately 7 feet by 7 feet. The coil is 

 mounted on a tripod so that it may be leveled and oriented easily. Bake- 

 lite, or other insulating supports, must be provided for holding the turns 

 of the coil in place. (The number of turns depends on the spacing and 

 frequency.) Formulas for proper design of this coil will be found in 

 text books on radio and high frequency phenomena, t 



The vacuum tube circuit and the batteries for both plate and filament 

 supply are contained in a hard oak case. Shoulder straps and carrying 

 handles are provided for convenient handling of the outfit. To provide 

 the 450 volt plate supply, ten "B" batteries, of medium-duty type (45 

 volts each) are connected in series. No. 6 dry cell batteries are con- 

 nected in series to provide the filament supply. 



t J. H. Morecroft, Principles of Radio Communication (John Wiley and Sons). 

 F. E. Terman, "Radio Engineering" (McGraw-Hill). 



