626 



EXPLORATION GEOPHYSICS 



after which it decreases until a zero dip angle is obtained when vertically 

 over the conductor. As the readings are continued beyond a point over 

 the conductor, the same condition results, except that the dips are in the 

 opposite direction. This is illustrated by the arrows in the diagram marked 

 "dips across conductor," and by the dip curve. 



The illustration shows a circular traverse for the direction-finding coil. 

 The distance between the two coils is constant; hence, the primary field 

 has a constant intensity, and the change in the resultant angle is due only 



BC OE IrCHt JK 



iiTiTmTT/rmTir 



DIPS ACROSS 



CONDUCTOR 



(PRIMARY FIELD j 



.SECONOfkRY FIELD 



A BVQV E_|fG H I J K 

 I 



Fig. 393. — Plan view of a linear conductor situated in the magnetic field of the energizing 

 coil. (A.I.M.E. Geophysical Prospecting, Tech. Paper 134.) 



to variation in intensity of the secondary field and its change in emergence 

 angle. In practice, however, the traverses are taken along straight lines 

 perpendicular to the conductor. (The relatively great distance between 

 energizing and direction-finding coils allows this to be done without any 

 appreciable error.) 



When working over a conductor of considerable length and uniform 

 depth, the ratio of primary to secondary field varies with the distance be- 

 tween the direction-finding and the energizing apparatus. This is usually 

 due to the smaller attenuation of the induced current when traveling along 

 the conductor as compared to the attenuation of the primary field. As a 

 result, the secondary field vector may be relatively large at considerable 



