592 



EXPLORATION GEOPHYSICS 



Note the asymptotic behavior of \Z\ as the frequency becomes very 

 large or very small, and the minimum of impedance at about 10^ cycles. 



.001 .002 



.6 1.0 



.006 .01 .02 .oe .1 .2 



7r 



Fig. 371. — Mutual impedance/resistivity as a function of frequency/resistivity 

 for a homogeneous earth. (.Geophysics, VIII, 2, p. 161.) 



INDUCTIVE MEASUREMENTS 

 Search Coil Measuring Apparatus 



The term search coil, as used here, applies to a stationary coil com- 

 prising one or more turns of wire wound in any shape, usually circular 

 or rectangular. The physical dimensions of the coil preferably are such 

 as to allow it to be readily portable. The terminals of the coil are con- 

 nected to a potential measuring device. This type of search coil can be 

 used in magnetic field strength measurements only when the earth is 

 energized with a varying current, e.g., an alternating current. 



The operation of a stationary search coil depends on the fact that 

 when an alternating current is caused to flow through the earth, an alter- 

 nating magnetic field is produced whose magnitude and direction depend 

 on: (1) strength of the energizing current, (2) orientation between the 

 plane of the search coil and the line joining the energizing electrodes, and 

 (3) the subsurface materials and structure. 



The alternating magnetic field induces an E.M.F. in the search coil, 

 and data on the magnitude of the induced E.M.F. at a series of stations 

 in a given area are diagnostic with respect to the paths of current flow 

 in the subsurface. The magnitude of the induced E.M.F. is given by 

 the equation: 



(5) 



^~ dt 



where <^ is the normal component of the magnetic flux threading or cut- 

 ting the coil. 



Consider, for example, that an alternating current flows in the earth 



