612 EXPLORATION GEOPHYSICS 



magnetic flux anomaly. These isanomalic contours may be determined 

 by using two search coils or loops which are opposingly connected. Each 

 coil contains about 500 turns of fine wire wound on a four- to six-foot 

 frame. The work can be carried out by the usual field procedure of map- 

 ping the location of points of equal magnetic strength, i.e., by leaving 

 one coil at a station and then moving the second coil until a position 

 is found where the induced potential between the two coils is a minimum ; 

 these two locations are marked and transferred to a plot of the area. 

 The first coil is now moved along the traverse line beyond the second 

 coil to the next null point of reading and the process repeated. In an 

 alternate procedure, the second coil is moved to the position initially 

 occupied by the first coil, and the first coil is moved forward to search for 

 a new null point. 



Precautions must be taken to have the two search coils as nearly iden- 

 tical electrically and magnetically as possible. The comparing circuit 

 and amplifier should be well shielded and placed midway in the shielded 

 two-circuit conductor connecting the two search coils. Proper electrical 

 balance of the two coils can be readily checked during the progress of the 

 survey by placing one coil exactly over the other. If their outputs are 

 connected in opposition, no signal should be noted in the headphones or 

 vacuum tube voltmeter. 



The location of these points of equal magnetic flux is usually accom- 

 plished by keeping both coils in a horizontal position. In one field 

 procedure, two operators are usually required, each one "wearing" a 

 coil. The coil is suspended in a horizontal position by means of cross- 

 straps which hang over the shoulders of the operator. The coil should 

 be about waist high to allow the operator freedom in walking. 



The method is relatively simple in application, and changes in the 

 energizing current usually do not affect the comparisons. The chief 

 limitation of this method is the fact that measurements are practically 

 confined within the central portion of the area bounded by the large 

 energizing ground loop. Within this area, the primary field (that created 

 by the flow of energizing current through the coil) has its maximum 

 value and the secondary field (that created by the flow of induced cur- 

 rent) is superimposed on the primary field. Since the primary field is 

 usually many thousands of times stronger than the secondary field 

 associated with a subsurface conductive zone, it is often difficult (except 

 when good conductive bodies lie close to the surface) to make reliable 

 measurements which will show such small variations in the total or 

 resultant field. 



Other balancing techniques may be employed. If desired, the relative 

 field strengths at two different points may be determined by keeping one 

 coil horizontal and then tilting the other coil until the signal is at a mini- 

 mum, t This technique necessitates that the tilted coil be the one located 



t Karl Sundberg, "Method and Device for Detecting and Locating Ores in an Electromagnetic 

 Way," U. S. Patent 1,678,489, issued July 24, 1928. 



