ELECTRICAL METHODS 609 



The time phase angle between the primary and secondary fields is therefore 

 approximately equal to 180°, and the direction of the resultant field is 

 substantially constant or fixed. Thus, if a direction-finding loop were 

 revolved about an axis parallel to this fixed direction, no signal would be 

 heard in the headphones, because the field would always be parallel to the 

 plane of the coil. 



Usually, electrical prospecting methods which utilize inductive pick-up 

 include appropriate electrical equipment either for evaluating the elliptical 

 polarization properties of the magnetic field (e.g., double coil method) 

 or for minimizing the effects of the elliptical polarization at the detector. 

 However, some methods (e.g., Conklin horizontal loop method) involve 

 balancing the effects of two coils without regard to the elliptical polari- 

 zation properties of the resultant magnetic field. The latter methods will 

 be discussed first. 



A study of the propagation of electromagnetic waves in the earth 

 (inductive energizing) was made by Haycock, Madsen, and Hurst f in 

 the form of a careful summary of the relevant experiments performed by 

 numerous workers up to the present (1949). A resume of the advantages 

 and disadvantages of the various frequencies is included in that study. 



The use of radio waves wherein the transmission of radio stations was 

 utilized was investigated by Silverman and Sheffet.^ Penetration is dis- 

 cussed and attenuation graphs of various frequencies are included. 



HORIZONTAL LOOP METHODS 



Conklin§ was probably the first investigator to energize the ground 

 successfully by passing alternating current through a large, horizontal, 

 insulated loop or coil laid on the surface of the ground. 



Considerable work has been done by subsequent investigators both in 

 the United States and abroad,tt using modified equipment. 



Magnetic Field Due to a Square Coil. — When current is passed 

 through a large coil lying in a horizontal plane, the magnetic field produced 

 within the coil by this current will be vertical; i.e., perpendicular to the 

 plane of the coil. The magnitude of the magnetic field may be evaluated 

 as follows: Referring to Figure 381, which represents a square coil 

 carrying a current /, it is desired to compute the magnetic field at a point 

 situated at a distance x from the side AB and a distance y from the side 

 BC. The field due to the side AB is given by Equation 2. That is, 



Hab = — (sin /?2 — sin /?i) 



t O. C. Haycock, E. C. Madsen, and S. R. Hurst, "Propagation of Electromagnetic Waves in 

 Earth," Geophysics, Vol. XIV, No. 2, April, 1949, pp. 162-171. 



t D. Silverman and D. Sheffet, "Note on the Transmission of Radio Waves Through the 

 Earth," Geophysics, Vol. VII, No. 4, Oct., 1942, pp. 406-413. 



§ H. R. Conklin, "Method and Apparatus for Determining Subterraneous Conductors," U. S. 

 Patent 1,241,197, issued Sept. 25, 1917. . , ^ 



tt See, for example, Hans Lundberg, "Recent Results in Electrical Prospecting for Ore, 

 A.I.M.E. Geophysical Prospecting, 1929, p. 87. 



