616 



EXPLORATION GEOPHYSICS 



Double Coil Method 



The double coil phase measuring system employs a large horizontal 

 loop for energizing the earth. The Bieler- Watson detecting methodf 

 consists essentially of two coils permanently fixed at right angles to each 

 other and connected in opposition through an amplifier and telephones. 

 The larger or vertical coil is wound with a fixed number of turns of 

 fine wire and is connected in parallel with a condenser of suitable value 

 for tuning the coil to the frequency of the alternator. The smaller or 

 horizontal coil is tuned by the same condenser and is connected to a 

 compound multipoint switch in such manner that its number of turns 

 may be varied. The amplifier and headphones are connected in series 

 with one of the coils. 



To determine the resultant or total magnetic field at any station, the 

 larger coil is mounted in an approximately vertical plane and oriented in 

 the desired direction. The apparatus is then rocked backwards and for- 

 wards slowly and the number of turns in the horizontal coil is varied by 



/ior/zonfa/ Co// 





PAoftes 



Fig. 384. — Double coil method for determining components of the 

 elliptical electromagnetic field. 



the multipoint switch until the null point or minimum signal is obtained 

 when the double frame is vertical. The number of turns in the horizontal 

 coil necessary to obtain this balance is noted. 



If the axes of the coils are parallel to the axes of the ellipse, only one 

 such reading is necessary, as will be evident from the following 

 considerations : 



Consider for simplicity that the horizontal coil is perpendicular to the 

 major axis of the ellipse and the vertical coil is perpendicular to its minor 

 axis. The induced E.M.F. of the horizontal coil (Figure 384) is altered 

 by varying the number of turns until it balances the induced E.M.F. in 



t E. S. Bieler and H. G. Watson, "Apparatus for Use in Discovering Ore Bodies," U. S. 

 Patent 1.794,666, issued Dec. 12, 1927. 



