ELECTRICAL METHODS 



591 



and the operator again records the scale reading of the instrument. (4) The power is re- 

 versed and again passed into the ground and the scale reading recorded. (5) The power 

 flow is again interrupted and the normal scale reading of the instrument recorded. 

 The scale deflection is computed by taking the arithmetical difference between the 

 averages for power-on and power-off. The observed magnetic field strength is equal 

 to the product of the scale deflection by the calibration constant. Usually, the magnetic 

 reading is expressed as the quotient of the field strength divided by the current used in 

 energizing the ground at that set-up, i.e., field strength per unit current. 



In practice, the measurements are made by 



passing the current into the ground in a series of 

 pulses and measuring the compensated earth's field 

 during the off-current portion of the cycle and the 

 total field (the compensated earth's field plus the 

 electromagnetic field created by flow of the current) 

 during the on-current portion of the cycle. The on 

 and off portions of the cycle are made long enough 

 to allow steady-state conditions to be reached during 

 the measurement and short enough to allow the off- 

 current portion of the cycle to act as a control in 

 correcting for the earth's magnetic field variations. 

 A generator supplies a series of intermittent current 

 pulses to the two stake electrodes at a frequency of 

 about one pulse per second. The current circuit is 

 broken automatically by means of a high voltage 

 relay operated by the energizing current. The out- 

 put current is controlled by means of a commutator 

 brush system. 



The field strength may be plotted versus elec- 

 trode separation, traverse distance, or magnetometer 

 position, as desired. 



Figure 370 is a comparison of magnetometric 

 and surface resistivity data obtained in an investiga- 

 tion of shallow subsurface structure. The elimination of near-surface effects no doubt 

 accounts for the more consistent data obtained with the magnetometric method. 



Fig. 370. — Magnetometric and re- 

 sistivity curves to determine bedrock, 

 d) Wenner configuration resistivity 

 measurements, (2) magnetometric 

 measurements. 



Field Strength Measurements Using A.C. Energizing Means. — 



Current is applied to the ground between two electrodes connected with 

 an alternating current power supply. The general layout of field lines 

 is similar to that shown in Figure 367. However, the power supply in 

 this case may be much smaller and driven by a portable gasoline engine. 

 An alternator with a capacity of 500 watts will furnish sufficient power 

 for a 200 cycle supply, while approximately 1000 to 2000 watts should be 

 provided for a 25 to 50 cycle supply. 



Figure 371 shows a graph of mutual impedance as a function of fre- 

 quency for a conductively energized, homogeneous earth.* The plot has 

 \Z\ f 



been made of as a function of -^ , with an electrode spacing of 1000 



P P . , , 



feet. \Z\ is expressed in ohms, resistivity p in ohm-centimeters, and fre- 

 quency / in sec~^. 



* S. S. West, "Mutual Impedance of Collinear Grounded Wires," Geophysics, Vol. VIII, No. 2, 

 April, 1943, p. 161. 



