ELECTRICAL METHODS 



613 



Fig. 383. — -Diagram of connections for absorption method. 

 (After Carlson and Hanson, U. S. Patent 1,325,554.) 



ill the more intense field. Instead of tilting one coil, a variable poten- 

 tiometer or a tap switch may be used to vary its output and the relative 

 field strengths may be calculated from the relative settings of the poten- 

 tiometer or the number of turns in the coil. 



Other noteworthy references on both empirical and theoretical aspects 

 of electromagnetic prospecting are given below. f 



Absorption Method. — A modification of the electromagnetic 

 method previously described employs a balance system X which is essen- 

 tially a dififerential trans- 

 former in which the induct- 

 ance of the loop or exploring 

 coil is measured by an im- 

 pedance bridge or differential 

 transformer. (Figure 383.) 

 When a current of constant 

 frequency is supplied by the 

 alternator and proper balance 

 obtained, a zero null or mini- 

 mum signal will be heard in 

 the headphones. If, now, the 

 coil is moved over an area hav- 

 ing a greater or smaller con- 

 ductivity than that correspond- 

 ing to the initial balance, the inductance of the coil will be changed 

 resulting in an unbalanced condition, and a signal will be heard in the 

 phone. 



This type of equipment, or a modification thereof, is oftentimes useful 

 for the location of buried metal objects within a few feet of the surface. 

 By making the coil from 1 to 5 feet in diameter and taking parti- 

 cular care to balance out capacity and other undesirable feed-back cur- 

 rents, a simple outfit can be made for the location of pipe lines, "buried 

 treasures," etc., which occur within a few inches or a foot or two from 

 the surface. In operation, the exploring coil is carried approximately 6 

 inches from the ground and the observer slowly walks over the area to 



t A. S. Eve, "Absorption of Electromagnetic Induction and Radiation by Rocks," A.I.M.E. 

 Tech. Pub. 316 (1930). . ,„ 



A. S. Eve, D. A. Keys, and F. W. Lee, "The Penetration of Rock by Electromagnetic Waves 

 at Audio Frequencies," Proc. I.R.E. 17, 2072 (Nov., 1929). 



J. W. Joyce, "Electromagnetic Absorption by Rocks," U. S. Bur. of Mines Tech. Paper 497 

 (1931). „ „ , „ ^ 



C. B. Feldman, "The Optical Behavior of the Ground for Short Radio Waves, Proc. I.R.E. 

 18 (June, 1933), 764. ^ „ ^ ^ , ^ 



R. L. Smith-Rose, "Electrical Measurements in Soil with Alternating Currents, Jour, of Inst, 

 of Elect. Engrs. (London), August, 1934, 221-237. ^ 



R. L. Smith-Rose and J. S. McPetrie, "The Attenuation of Ultra-Short Radio Waves Due to 

 the Resistance of the Earth," Proc. Phvs. Soc. 43 (London), Sept., 1931, 592-620. 



F. E. Terman, "Radio Engineer's Handbook," 708-709 (McGraw-Hill, New York, 1943). 



C. A. Heiland, "Geophysical Exploration," pp. 310-314, 657-667 (Prentice-Hall, New York, 

 1940). 



L. Kerwin, "Use of Broadcast Band in Geologic Mapping," Jour, of Applied Physics, 18 

 (April, 1947), 407. . . ,. t- 



t W. L. Carlson and E. C. Hanson, "Means for Locating Ore Bodies by Audio-Frequency 

 Currents," U. S. Patent 1,325,554, issued Dec. 23, 1919. 



