766 ELECTRICAL METHODS [Chap. 10 



3. Interpretation procedures applied in electromagnetic-galvanic pros- 

 pecting vary with the completeness and nature of observed field parame- 

 ters. If only the direction of the field is measured (Elbof method), inter- 

 pretation has to be largely qualitative. The direction of the field obtained 

 with a vertical pickup coil, or the strike and dip of the ellipse of polariza- 



_^^ tion determined with a coil rotatable about 



\ ^/ \^ both a horizontal and a vertical axis, is a 



^!>f ^ function of (a) the normal ground field due to 



y^ \ the regular current distribution between two 



>^ \ ^ point or line electrodes, (b) the field produced 



>^ \ by subsurface current concentrations, and (c) 



-•^ i the field of the generator leads. The normal 



^ 4 field can be calculated from relations previously 



Fig. 10-89. Magnetic field givg^ for the potential field. It is expressed 



due to currents between two „, /— ^ -„ . , 



•electrodes. by'' H = VX' + Y' = /(l/n - l/r2).10-' 



(Gauss), where the total current / is in amperes 

 and r is in meters (see Fig. 10-89). The field direction is given by tan a = 

 Y/X in the horizontal plane and by tan <p = Z/H in the vertical plane, 

 where the components due to subsurface conductors follow from eq. (10- 

 46a). A similar relation applies if the field of the cable is used for com- 

 parison as in the Haalck method (see page 765). 



When the field intensities are measured at low frequency, phase shifts 

 are small and the observed anomalies may be compared to advantage with 

 fields calculated for simple geometric bodies as shown below. This pro- 

 cedure is likewise applicable when the components have been measured 

 separately with a compensator and normal fields and cable effects have 

 been deducted. 



The electromagnetic anomalies of subsurface bodies may be determined 

 with sufficient approximation by assuming equivalent current concentra- 

 tions in such bodies. Hence, if the width of an ore bodj'^ is small compared 

 with its depth, the total field is T = 21' /r (see Fig. 10-92a), where r is 

 the distance to any point at the surface and /' is the current in the con- 

 ductor. In the following equations, let d be the depth of a current con- 

 centration and X, Y, and Z be the components of the electromagnetic field 

 (where X is the horizontal strike component, Y the horizontal component 

 at right angles to the strike, and Z the vertical component). Further, 

 let y be in the direction at right angles to the strike, x in the direction 

 parallel with the strike, and (p be the angle between y and r. Then the 



"J. Koenigsberger, Phys. Zeit., 28, 342 (1937). A. Graf, Zeit. Geophys., 5(8), 

 331 (1929); and Beitr. angew. Geophys., 1(3), 286 (1931). 



