Chap. lOJ 



ELECTRICAL METHODS 



779 



tensities and phases are determined in reference to the loop current by 

 compensator arrangements. The out-of-phase component (generally due 

 to subsurface conductors) may be obtained in reference to the in-phase 

 field (usually due to the loop) by the Bieler-Watson method. Fields in 

 successive intervals may be determined relatively by measuring the ratio 

 of the field vectors and their phase difference. Finally, the in-phase com- 

 ponents on successive stations may be compared (field ratiometer). 



With a reception coil of round or square shape the orientation of the 

 plane of polarization of the ellipse and its projections can be determined. 

 When elliptical polarization is small, this is equivalent to a measurement 

 of the direction of the resultant horizontal field and the inclination of the 

 field. The reception coil must be capable of rotation about a vertical and 

 a horizontal axis. Transit bases are frequently used for this purpose. 



/* 



Z. 



0.2 ae 1 ^ 

 (a) 



12 3 4 5 6 7 e 



(b) 



Fig. 10-100. Fields of circular loops: (a) inside of loop, (6) outside of loop 

 (adapted from Miiller). 



The standards are changed to a semicircular support in which the reception 

 frame can be rotated about a horizontal axis. A small vertical circle is 

 usually provided so that the tilt angle of the frame in the minimum position 

 may be determined. The horizontal azimuth on the horizontal circle is 

 read in respect to markers on the base line or in respect to magnetic north. 

 The Swedish investigators have used a long staff with a horizontal cross- 

 bar, about which a square frame may be rotated. The crossbar is pro- 

 vided with peepsights and a clinometer so that both horizontal azimuth 

 and tilt angle of the frame can be determined. Coils are generally wound 

 on wooden or aluminum frames. They have a diameter of 40 to 50 centi- 

 meters, from 500 to 1000 turns of wire, a D.C. resistance from 75 to 150 

 ohms, and an inductance of from 4 to 5 henry. The amplifiers are usually 

 transformer coupled; they have two to three stages; and carry headphones 



