ELECTRICAL METHODS 443 



CLASSIFICATION OF METHODS 



The electrical methods are more diversified than any of the other geophysical 

 methods, and a rigid classification is difficult. The following classification is one 

 which corresponds best to field practice. 



(1) Conductive Methods 



These include all methods in which both the energizing and the measuring 

 electrodes make direct contact with the ground. The energizing current may 

 be direct or alternating. Two types may be enumerated: 



(a) Methods Applicable Under Steady Slate Conditions. 



These include the self-potential method, the direct current and low fre- 

 quency alternating current equipotential line methods, and the various 

 modifications of the resistivity methods. The characteristic feature of 

 these methods is that the diagnostic variable, resistivity, for example, is 

 determined under conditions such that steady state relationships exist, 

 and the typical alternating current phenomena (real and imaginary com- 

 ponents, phase differences, etc.) need not be considered in the interpre- 

 tation of the data. 



(b) Methods Applicable Under Moving Field Conditions. 



These include all methods which employ alternating or variable current 

 of sufficiently high frequency to produce significant inductive phenomena. 

 Analysis of the data obtained with these methods must take into account 

 such variables as phase shift, polarization ellipse, impedance (instead of 

 resistivity), redistribution of current caused by inductive effects, etc. All 

 methods employing medium and high frequency alternating currents and 

 transient currents fall within this classification. The typical medium 

 frequency alternating current methods are those in which frequencies 

 ranging from about 50 to 500 cycles per second are employed. The typical 

 high frequency methods employ alternating currents having frequencies 

 from a few thousand cycles per second to the radio frequencies. 



(2) Electromagnetic Methods 



This group includes those methods wherein the properties of the magnetic 

 field associated with the flow of an electric current are utilized. Two groups 

 are distinguished : 



(a) Methods Employing Conductive Energising Means. 



In these methods, direct or alternating current is passed into the 

 ground between two electrodes, and its subsurface distribution is studied 

 by means of the magnetic field associated with the flow of current. If 

 medium or high frequency alternating current is utilized, a direction- 

 finding or search coil with appropriate amplifying and phase compen- 

 sating apparatus may be employed. If direct current or low frequency 

 alternating current is used for energizing the ground, various forms of 

 variometers, magnetometers, etc., may be used for measuring the magnetic 

 field associated with this flow of current. 



(b) Methods Employing Electromagnetic Energizing Means. 



The characteristic feature of this group is that an alternating current 

 is induced in subsurface bodies by passing high or medium frequency 

 alternating current through an energizing coil or loop mounted on the 

 surface of the earth and oriented, usually, in a horizontal or vertical 

 position. Studies of the subsurface distribution of current may be made 

 by different means : such as, search coils and surface electrodes. 



