594 EXPLORATION GEOPHYSICS 



that the current has a maximum value of 10 amperes and a frequency of 5 cycles per 



second. That is, 



7=10 cos Zir • 5t 



and 



dl 



-J- = — IOOtt sin IOtt* 



at 



Assume that the coil has 1000 turns and an area of 5 square feet and assume also that 

 the coil is located midway between the stake electrodes. 



Substitution of the assumed values into Equation 6 yields 



£=-^^•1000 



= (55 + To )(-"«'' =■""'") 



= .00192 sin lOirt 

 or 



£m.x = .00192 volts = 1.92 millivolts 



Field Procedure. — Field strength measurements with search coils are 

 conducted by measuring the potential generated in the coil at various 

 locations in the area. The induced alternating potentials usually are 

 measured by use of a calibrated constant gain amplifier and vacuum tube 

 voltmeter. A fixed or systematic orientation of the coil, with respect 

 to the energizing electrodes, must be employed throughout the meas- 

 urements. The simple procedure usually employed is to maintain the 

 coil so ifs plane is vertical and oriented parallel to the straight line join- 

 ing the two energizing electrodes. 



Limitations of A.C. Energizing Methods 



From the preceding numerical example it will be seen that the voltage 

 induced in a search coil is extremely small when a low frequency is 

 employed for energizing the ground. If a higher frequency energizing 

 current is employed, the induced voltage in the search coil will be pro- 

 portionally increased, but the effective depth of penetration decreases 

 because the subsurface distribution of current will be altered due to self 

 inductance. That is, the alternating current flowing into the earth induces 

 a voltage in the earth in such a direction as to oppose the flow of the 

 current. The effect of this induced voltage is greatest at a depth where 

 the normal current is small and hence the potential gradient is also small. 

 This results in a crowding of the current towards the surface. 



Evjen t gives the current penetration for different frequencies as 

 follows : 



t H. M. Evjen, "Electrical Methods in Geophysical Exploration," Geologie en Mijnbouw 

 (Jaargang No. 1) pp. 2-8, January, 1939. 



