Chap. 2] METHODS OF GEOPHYSICAL EXPLORATION 31 



Numerous methods are available for measuring the electromagnetic field. 

 The simplest procedures involve a determination of direction only or of 

 the absolute values of the horizontal and vertical components by using a 

 vacuum-tube voltmeter in the output circuit of an amplifier connected to 

 a reception frame. A determination of the in-phase and quadrature com- 

 ponents of the field is possible by the use of the compensators previously 

 described (see Fig. 2-14). These are connected by a pair of leads to the 

 source of the primary power and thus determine the electromagnetic field 

 in terms of the current and phase of the primary circuit. Finally, the 

 ratios of field intensities and their phase differences at successive points 

 may be measured by dual coil arrangements and "ratiometer" bridges 

 (see Fig. 2-14). 



Interpretation of results obtained with the electromagnetic-galvanic 

 method is both qualitative and quantitative. In approximation, lines of 

 equal direction of the field vector (in horizontal projection) may be as- 

 sumed to represent flux lines which are attracted by bodies of good 

 conductivity, and vice versa. If the current is concentrated in a good 

 conductor and flows along its strike, then the horizontal component will 

 have a maximum above the current concentration. The vertical com- 

 ponent will be zero at a location immediately above the concentration and 

 will have a maximum and minimum, respectively, on either side of the 

 concentration, their distance being equal to twice the depth. 



Indications are also obtained from induction currents concentrated along 

 the edges of a subsurface body. Thereforp, in the case of an ore body of 

 some width, the horizontal intensity will show not merely a maximum over 

 the center but a maximum and minimum, respectively, over the edges. 

 This, strictly speaking, comes under the heading of electromagnetic- 

 inductive methods described in the next paragraph. 



Electromagnetic-inductive methods. In inductive procedures power is 

 supplied to the ground by insulated loops which will cause induction cur- 

 rents to flow in subsurface conductive bodies. An advantage of inductive 

 methods is the ease with which power may be transferred into the ground 

 when the surface formations are poor conductors. Since currents induced 

 in the subsurface conductors are dependent on frequency, interpretative 

 advantages may be gained by regulating the frequency. 



However, there are limitations to this procedure. Too low frequencies 

 will reduce the strength of the induced currents, while too high frequencies 

 lack depth penetration and cause interference from near-surface noncom- 

 mercial conductors and from topography. Frequencies usually applied 

 in mining, range from 250 to 1000 cycles; in structural exploration the 

 range is from 25 to 200 cycles. The other extreme is represented by the 

 high-frequency methods using several tens of kilocycles which have been 



