ELECTRICAL METHODS 587 



flow is deeper than that for the homogeneous condition, the magnetic field 

 strength will lie along a curve lower in value; and if the effective current 

 flow is shallower than that of the homogeneous condition, the magnetic 

 field strength will be greater than the homogeneous curve values. By 

 plotting a family of such curves, the relative depths to the marker or 

 contact may be determined rapidly. 



LOCATION OF SUBSURFACE CURRENT FLOW BY 

 FIELD STRENGTH MEASUREMENTS 



Methods for measuring the field created by current flowing through 

 the earth between grounded electrodes are conveniently classified into two 

 groups: namely, (1) methods using direct current energizing and (2) 

 methods using alternating current energizing. The chief practical dif- 

 ference between these groups is the apparatus used to detect or measure 

 the field strength. The usual detecting device in the first group (steady- 

 state field) may be a magnetometer or fluxmeter which measures the 

 magnetic field strength directly. The usual detector in the second group is 

 a search coil which measures the field strength indirectly by means of the 

 E.M.F. induced in it by the alternating field. 



Field Strength Measurements Using D.C. Energizing. — The most 

 direct method of studying the distribution of current is to employ a 

 sensitive magnetometer for measuring the strength of the horizontal com- 

 ponent of the magnetic field created by a direct current flowing through the 

 subsurface. The voltage requirements will vary from 200 to 900 volts, 

 depending upon the near-surface conductivity, and the current required 

 will vary from 1.0 to 50 amperes, depending upon the depth of investiga- 

 tion. The greater the current employed in the measurements, the less the 

 sensitivity requirements of the magnetometer and the less the disturbance 

 errors introduced by diurnal variations in the earth's magnetic field. A 

 suitable indicating or recording ammeter and voltmeter should be provided 

 for determining the value of current flowing in the subsurface at the 

 instant the magnetometer reading is made. 



Energizing Apparatus 



Current is applied to the ground between two electrodes connected 

 with a suitable power supply. The separation of the two energizing elec- 

 trodes may be held constant where lateral studies are to be made, or it may 

 be varied to increasingly greater separations to obtain increasingly greater 

 effective depths of penetration for vertical structural studies. For deep 

 structural studies, a 100-horsepower gasoline engine direct connected to a 

 1000-volt, 50-ampere DC generator may be employed. A truck-mounted 

 power supply of this type is shown in Figure 366. 



The general layout of equipment and field lines is illustrated in Figure 



