626 ELECTRICAL METHODS [Chap. 10 



components, (c) In the Miiller method the out-of-phase components are 

 made negligible by the use of 60-cycle power, so that the vector amplitude 

 may be measured without reference to its phase by a vacuum tube volt- 

 meter arrangement connected to the pickup coil, (d) The Elhof method 

 measures merely the direction of strike and dip of the ellipse of polarization. 



The next three methods are closely related: (e) Bieler and Watson use 

 two coils in fixed arrangement. One is vertical, the other horizontal; the 

 latter picks up the vertical field component produced by the primary loop, 

 whereas the vertical coil responds to the horizontal out-of-phase com- 

 ponent produced by subsurface current concentrations. (/) Some of the 

 electromagnetic-m^to methods utilize two independent coils connected in 

 series through an amplifier detector. One coil remains fixed in direction 

 on one station while the other, at a second location, is rotated until balance 

 is obtained, the angle of rotation corresponding to the difference in in- 

 tensity and phase angle. Other electromagnetic-ratio methods make use 

 of ratio bridges by which the voltages induced in two coils (usually held 

 horizontally to measure the vertical component) are balanced for phase 

 and amplitude. 



The four remaining procedures are generally referred to as truly in- 

 ductive: (g) In the Sundberg method a large rectangular loop is laid out, 

 and the horizontal component is measured across one cable. Due to the 

 "reflection" of the cable on subsurface conductors, the depth of the latter 

 can be determined from the shape of the anomaly curve. The real and 

 imaginary components of the horizontal field components are measured 

 with a compensator. This method is primarily applicable to structural 

 studies, (h) Another procedure known as the ring induction method uses 

 a circular primary loop of small diameter laid out concentrically with a 

 smaller secondary horizontal coil. The efifect of the primary coil on it is 

 compensated by an auxiliary coil, and the secondary fields are measured 

 according to phase and amplitude. By varying the radius of the ring it 

 is possible to reach different depths and to calculate the resistivities of 

 formations at these depths, (i) In the Mason method the primary field 

 is produced by a vertical loop, and strike and dip of the secondary field are 

 measured with a search coil, (j) The high-frequency Radiore method 

 utilizes a similar arrangement, except that the primary field is produced 

 by a circular loop of comparatively small diameter. The dip of the field 

 resulting from a combination of the primary and secondary components is 

 determined with a search coil. 



With the exception of the Sundberg-inductive, the Miiller, and the ring- 

 induction methods, electromagnetic methods are applied principally in the 

 location of sulfide ore bodies. The methods mentioned as exceptions have 

 been used chiefly for structural and stratigraphic studfes. 



