Chap. 10] 



ELECTRICAL METHODS 



807 



ception coil about a horizontal axis until a minimum is obtained. The 

 current concentration may thus be located by measuring dip angles 

 along a profile at right angles to the strike. Contrary to low-frequency 

 vertical-loop methods, the loop field and the subsurface field are very 

 nearly in phase; elliptical polarization is negligible and sharp minima are 

 obtainable when the reception coil is tilted. The Radiore Company 

 employed a portable transmitter in the form of a circular coil with the 

 oscillator built into its base, supplied with plate voltage by a hand- 

 cranked alternator. ^"^ Receiving coils consisted of several tens of turns 

 of wire wound on bakelite hoops 1 to 2 feet in diameter, mounted on 

 transit heads in place of the telescope. 



If the magnetic field surrounding a subsurface current concentration 

 alone were present, its direction at any point A on a profile (see Fig. 10-124) 



Fig. 10-124. Construction of index curve. 



would be given by the vector T, and would coincide with the direction of 

 the plane of the detection coil in the minimum position. If normals were 

 drawn to this position at all points, they would intersect in the subsurface 

 conductor. However, the horizontal field Ho of the transmission loop 

 combines with the subsurface field T to form the res ultan t field vector R, 

 whose direction is that of the detection coil in the minimum position. 

 Therefore, the normals to the direction of the coil will intersect the vertical 

 at progressively deeper points CC as the distance of points A from the 

 point increases. The conductor may nevertheless be located by the 

 procedure of drawing an index curve: At any point {A) the normal to the 

 vector R or to the plane of the detection coil is drawn to the intersection 

 with the vertical at the point C. Through C a horizontal line is drawn to 



"• Illustrated in J. Jakosky, I.R.E. Proc, 16(10), 1344 (Oct., 1928). 



