ELECTRICAL METHODS 627 



distances from the energizer, while when very close to the energizer it may 

 be completely masked by the strong primary field. 



Since it is necessary that a certain minimum ratio exist between the 

 field strengths of the primary and secondary fields in order that a readable 

 dip angle may be obtained, it is evident that the deeper a conductive ore 

 body lies, the longer must be its effective length for optimum operating 

 conditions. After a given power input has been reached, increasing the 

 power of the energizing system in order to impart more energy to the 

 current induced in the conductor will not change conditions materially, 

 inasmuch as the primary field is increased in proportion. At all times, 

 however, enough power must be supplied to the earth to penetrate to the 

 desired depth and induce currents of such magnitude that the magnetic 

 fields of the induced currents travel through the overburden and reach 

 the surface with sufficient intensity for producing a detectable effect on 

 the direction-finding apparatus, 



A high frequency energizer with an output of about 10 watts will be 

 found to have an effective depth range of 25 to 50 feet in dry desert 

 countries when working on favorable type conductors, i.e., a vein or other 

 elongated conductive mineralized zone. If the 500-cycle apparatus is used, 

 a good practical rule, in ordinary work where the mineralized zones are 

 imbedded in damp areas or under water level, has been to employ power 

 supply of at least three watts per foot of depth; i.e., when working to a 

 depth of 100 feet, the energizing coil should supply a power output of at 

 least 300 to 350 watts. It should be noted that due to the low power factor 

 of the coil, a high volt-ampere product may be necessary to obtain this 

 power output, unless proper means (such as a resonating condenser) are 

 employed for power-factor correction. 



Interpretation of Data. — Owing to the distortion of the primary 

 field or improper alignment of energizing and receiving equipment, it often 

 happens that a small (usually less than 10 degrees) "dip" or improper 

 strike direction is obtained. These are called phantom dips or strikes 

 and are readily recognized by the experienced operator. Such dips are 

 frequently obtained when the energizing and direction-finding coils are 

 located on a ridge, in a narrow valley or canyon, or at the edge of a 

 deep cut or precipice. Usually the greater the distance between the energiz- 

 ing and the direction-finding coils, the greater is the wave-front distortion. 

 The lower the frequency employed, however, the less becomes the distortion 

 both of the primary and the secondary fields, and the distortion may usually 

 be neglected at frequencies of 1000 cycles or less. 



Because this topographic effect increases when higher frequencies are 

 employed, the use of radio frequencies will yield "indications" over many 

 ridges, or valleys, and all electrical inhomogeneities existing close to the sur- 

 face; such "indications" are, of course, of little value in indicating the struc- 

 tural conditions at depth and they must be carefully evaluated by proper 



