ELECTRICAL METHODS 625 



When the equipment has been set up with the energizing coil vertical 

 and "pointing toward" the direction-finding coil, the operator of the 

 direction-finding coil knows that if he obtains any angle or dip other than 

 zero (measuring from the vertical) and a strike not pointing toward the 

 energizing coil, some disturbing influence is present. This disturbing in- 

 fluence may be another field caused by induced current flowing in an 

 underground conductive mass. 



Orientation of Search Coil Under Influence of Primary 

 and Secondary Fields 



If the direction-finding coil is cut by two electromagnetic fields which 

 are in phase and of the same frequency, the position of the coil for maxi- 

 mum or minimum signal strength will be determined by a single resultant 

 of the two fields. For example, if one field is horizontal and the other 

 field is tilted so that it makes an angle of 60° with the horizontal, the 

 direction-finding coil, under proper conditions, "points" somewhere be- 

 tween these two vectors, the exact direction depending on the relative 

 strengths of the two fields and their phase relationship. If the fields were 

 of equal strength, and in phase, the resultant vector would lie equidistant 

 between them. 



The elementary conditions prevailing in actual operation are illustrated 

 in Figure 393 which is a plan view of a conductor of considerable length 

 and small diameter placed so as to be in the field of the energizing coil. 

 The direction-finding coil now has two fields linking it. 



At position C (lower right-hand part of the figure) the component 

 fields would exert the following effects. Since the energizing coil is vertical, 

 the primary magnetic field will tend to cause the direction-finding coil to 

 give the loudest signal when it, too, is vertical. However, for proper in 

 phase conditions, the field surrounding the subsurface conductor will tend 

 to produce the loudest signal when in the position shown by the resultant 

 vector. If the direction-finding coil is moved to the position F, which 

 is directly above the conductor, both the primary and the secondary fields 

 will induce the loudest signal in the coil. As the coil is moved beyond the 

 vertical position, the direction of the dip angle changes as shown by the 

 vectors G, H, etc. Thus, it can be seen that as a traverse is taken across a 

 conductor through which an induced current is flowing, a series of dips 

 will be obtained on the direction-finding coil. 



For purposes of illustration, assume that a series of readings is being 

 taken on a circular traverse across the surface of the ground above the 

 conductor, as shown in the plan view. As the distance from the con- 

 ductor increases, the secondary field vectors are not sufficiently strong to 

 give a noticeable deflection to the angle of the direction-finding coil. The 

 resultant direction for all practical purposes is vertical, or a zero dip. As 

 the direction-finding coil is moved along the traverse toward the conductor, 

 the dip angle becomes increasingly larger until a maximum dip is reached, 



