Chap. 10] ELECTRICAL METHODS 763 



tions reached by the action of the electrodes. It is claimed that the depth 

 penetration is the same as the electrode separation. Since the ability of 

 formations to furnish polarization e.m.f.'s is, closely related to their elec- 

 trolytic content and hence to their resistivity, there is some question as 

 to whether an altogether different physical property is recorded in these 

 measurements. Although the power supply to the ground is small 

 (1-2 watts) the question arises further how much polarization e.m.f. is 

 produced in the nonpolarizable electrodes. 



VIII. ELECTROMAGNETIC METHODS 



Electromagnetic methods constitute one of the largest and most diversi- 

 fied groups of electrical prospecting. They differ from the potential 

 methods in that the electromagnetic field and not the surface potential of 

 the ground currents is measured. Electromagnetic methods are divided 

 into two groups. In the first, energy is supplied to the ground by contact; 

 in the second group, energy is supplied inductively, that is, by insulated 

 loops. The first group is sometimes called electromagnetic, and the second 

 inductive. 



A frequency of 500 cycles is most commonly used in electromagnetic 

 methods. When a simpler technique is desired, when phase shifts are to 

 be kept to negligible values, where a substantial depth has to be reached 

 (as in Schlumberger's method of electromagnetic dip determination or in 

 Koenigsberger's ring induction method), low frequencies of the order of 

 25-60 cycles are preferred. More than one operating frequency may be 

 required when highly conductive layers near the surface are to be pene- 

 trated. Electromagnetic methods using frequencies of the order of tens 

 of kilocycles are referred to as "high frequency" methods. Such fre- 

 quencies are likely to energize noncommercial conductors and to produce 

 excessive terrain effects. A band between 300 and 900 cycles is a prac- 

 tical compromise. Lower frequencies would make energy transmission 

 too inefficient and would eliminate the telephone as a practical null de- 

 tector. Higher frequencies lack depth penetration and, produce too much 

 interference. 



Transmission units are: long cables, connected to a generator and 

 grounded at both ends ; rectangular or square loops ; or circular coils. They 

 are fed by generators driven by gasoline engines, storage battery operated 

 buzzers, commercial lighting plants, or vacuum-tube oscillators. 



Receiving devices are of widely diversified construction, depending on 

 quantities measured. Their two fundamental constituents are a reception 

 frame with several hundred turns of wire and an amplifier. Use is made 

 of null methods wherever possible, with telephones as null indicators. 



