796 



ELECTRICAL METHODS 



[Chai>. 10 



If more than one conductive sheet is present, the interpretation pro- 

 cedure makes use of the fact (proved by experiment) that the electro- 

 magnetic field beneath one conductive sheet is independent of its position. 

 Hence, when two sheets occur, their effect is equivalent to one sheet, 

 produced by dropping the upper sheet on the lower. The field of the 

 combined sheet is therefore given by the depth of the lower sheet and 

 the sum of the induction factors. The procedure of depth determination 

 has been described in detail by Sundberg and Hedstrom. 



In the central-ring induction method the fundamental theory is the same 

 as that discussed above, except that the 180° component practically 

 vanishes and the quadrature component predominates. The magnetic 

 field inside the loop is proportional to the conductivity of a portion of 



Conductive lager 



Image of Cable 



Fig. 10-113. Combination of primary and secondary fields for perfect conductor. 



ground whose depth is roughly equal to the radius R of the loop. The 

 arrangement used by Koenigsberger and his associates is illustrated in 

 Fig. 10-105, and references to the pertinent literature are given on page 782. 

 Only the vertical component is measured. The magnetic field in the loop 

 is affected by the ground conductivity and differs from the field of a loop 

 suspended in air. 



As shown in eq. (10-58), the field near a straight cable is composed of 

 (1) the direct vector, (2) the vector due to the image of the cable, and (3) 

 a vector arising from finite conductivity of the bed. Since the first two 

 are in opposition and in quadrature with the third vector, the vertical 

 component for any medium below may be written Z — Zp -\- jZq , where 



Sundberg and Hedstrom, World PetroL Congr. Proc, B(I), 107 (1934). 



