CHAPTER VI 



ELECTRICAL METHODS: Electromagnetic 



The electromagnetic methods measure directly the magnetic field asso- 

 ciated with the flow of current in the subsurface. The subsurface current 

 may be created by either of two techniques : ( 1 ) conductively, by flow 

 between two electrodes so positioned that the normal path of subsurface 

 current flow will include the area under investigation (either direct or 

 alternating current of any frequency may be employed for the energizing 

 current) ; or (2) inductively, by creating an alternating magnetic field at 

 the surface of the ground. 



The distribution of the current in the subsurface may be determined 

 by either of two techniques : (1) by magnetometric methods which measure 

 directly the strength and/or the direction of the magnetic field associated 

 with the flow of subsurface current. The subsurface current may be 

 either direct or low frequency alternating current. (2) By inductive 

 methods wherein a coil or large loop is positioned at the surface of the 

 ground. The measurements are conducted to determine one or more of 

 the following conditions : total induced E.M.F., from which the strength 

 of the alternating magnetic field may be calculated; induced E.M.F. in two 

 directions at right angles to each other, and their phase relationships; or 

 direction to the effective path of subsurface flow of current. 



PHYSICAL PRINCIPLES 



Magnetic Field Associated with Electric Current. — An electric cur- 

 rent flowing in a conductor produces a magnetic field which radiates out- 

 wardly from the conductor in 

 closed magnetic circuits. The / \ / \ 



lines of magnetic force surround- Q — 



ing an infinitely long, linear con- 

 ductor are shown in Figure 361. 

 The direction of the field is given 

 by the familiar right hand rule: ^ ^ 



namely, if the thumb of the right Fig- Sei.— Lines of magnetic force due to current 



, J • 1- 1 1- • • flowing in a linear conductor. A. side view; B, cross 



hand mdlCateS the direction in sectional view. (Minus sign indicates that current 



which the current is flowing, the '^ ^°"'"^ '^'° '''' ^^p^''-^ 



lines of force circle the conductor in the direction of the fingers of the 



closed hand. 



V7 



