764 ELECTRICAL METHODS [Chap. 10 



The more complex arrangements measure the field components by ampli- 

 tude and phase, or as in-phase (real) and out-of-phase (imaginary) com- 

 ponents. Fields at successive points may also be measured relatively in 

 regard to amplitude ratio and phase difference. Following is a summary 

 of observed electromagnetic field parameters: 



Strike and dip of the ellipse of polarization. 



Absolute values of intensity and intensity components. 



Semi-absolute determination of intensity components in reference to 

 amplitude and phase of the primary current. 



Out-of-phase field components in terms of corresponding in-phase com- 

 ponents. 



Field ratios and phase differences in successive intervals. 



Ratios of in-phase components at successive points. 



Potential methods of electrical prospecting are preferred whenever pri- 

 mary power may be readily applied by contact and when the objects 

 sought are not very good conductors. Electromagnetic-galvanic methods 

 are suitable where bodies of good conductivity are to be located, where 

 surface beds of good conductivity would produce too much screening effect 

 on potential methods," where requirements of depth penetration are not 

 too great, and where contact of the primary electrodes with ground is 

 readily possible. When this is difficult (as in deserts, on the ice of lakes, 

 and the like) inductive methods must be applied. 



A. Electromagnetic Methods with Galvanic Power Supply 



1. Electrode arrangements are like those used in equipotential-line or 

 potential-profile methods. Line electrodes are laid out at right angles to 

 the strike so that maximum distortion of the current lines occurs; profiles 

 are run at right angles to the strike. Where large amounts of ground are 

 to be covered and where the uniformity of the primary field does not play 

 an important part (as in field-ratiometer measurements), point electrodes 

 are preferred. 



A long cable is laid out each way from the generator and grounded at 

 the ends with a number of pins or wire screens. The line must be in the 

 direction of strike so that profiles can be measured at right angles thereto. 

 The field produced by the line connecting the electrodes to the generator 

 may be reduced by carrying the leads around the area in a square or rec- 

 tangle or by taking the measurements on the outside of a short current 

 basis. For the simpler layouts, the field due to the leads may be calcu- 



" See J. N. Hummel, Zeit. Geophys., 7(5/6), 258 (1931). 



