ELECTRICAL METHODS 595 



where d is the asymptotic vakie of the depth of current penetration (the 

 depth to which one-half of the total current penetrates) as the electrode 

 spacing is increased.* 



Thus, the indicated optimum frequency which should be employed 

 when working to a depth of 3300 feet is one cycle per second, but it is 

 impractical to obtain sufficient induced power at this frequency to use 

 the method. On the other hand, a more useable frequency, such as one 

 hundred cycles per second, will have an indicated depth of penetration of 

 only 825 feet. Hence, methods employing alternating current having an 

 appreciable frequency are believed generally to have little economic value 

 except for shallow investigations. 



LOCATION OF CURRENT FLOW IN SHALLOW 

 CONDUCTORS BY DIRECTIONAL SEARCH COILS 



Instead of measuring the strength of a given component of the vary- 

 ing magnetic field associated with the flow of alternating current, the path 

 of the subsurface current flow may be located by use of directional or 

 direction-finding coils. This procedure utilizes a similar technique to that 

 employed in radio direction finding. A directional search coil, with its 

 necessary orientation and amplifying accessories, is readily portable and 

 allows rapid manipulation for reconnaissance surveys. 



Operating Principles of Directional Coils. — To obtain the most 

 accurate determination of the direction of the resultant field, the per cent 

 change in the magnitude of the E.M.F. for a small change in orientation 

 angle must be as large as possible. This condition may be expressed by 

 the relation 



1 dE 



— —rr — maxmium 



E d9 



where E is the induced E.M.F. and 6 is the angle made by the direction 

 of the magnetic field with the normal to the plane of the coil. But 



^~ dt 



4-(NAH cose) = — NA^ cos 

 dt dt 



and 



Hence, 



dE ^r^dH . 



1 dE 



* The Evjen table affords only a general survey, because the penetration for a par- 

 ticular frequency is dependent upon the materials constituting the subsurface. 



