32 METHODS OF GEOPHYSICAL EXPLORATION [Chap. 2 



practically abandoned on account of the limitations mentioned. In induc- 

 tive methods a horizontal or vertical loop may be employed. The former 

 afford the most effective coupling with horizontal subsurface conductors 

 and are used in the form of long cables and rectangular or circular loops. 

 Theoretically vertical loops would be more suitable for steeply dipping 

 bodies, but they are difficult to handle. For the measurement of the 

 electromagnetic field, the same procedures are used as described before 

 in connection with the electromagnetic-galvanic methods. 



Interpretation methods in electromagnetic-inductive exploration depend 

 to some extent on the purpose of the survey and the general shape and 

 disposition of the subsurface conductor. In wide and steeply dipping 

 conductive bodies the electromagnetic field is due primarily to currents 

 flowing along the edges. Hence, the horizontal component will show a 

 maximum and minimum, respectively, over the edges; a minimum in the 

 vertical component will occur over the center. Depth calculations of the 

 equivalent current concentrations are based on the relations discussed 

 before in connection with electromagnetic-galvanic methods. 



Inductive methods, using separate measurements of the in-phase and 

 out-of-phase components, have great interpretative advantages. The 

 in-phase component is largely due to the field-supplying loop, while ttie 

 out-of-phase components result from the induced subsurface current con- 

 centrations. In horizontally stratified ground, interpretation is usually 

 based on the out-of-phase components. If a very good conductor occurs 

 in horizontal position, the surface anomaly is largely due to the reflection 

 of the primary cable on the conductor, with a phase change of 180*. Depth 

 determinations of this conductor are, therefore, possible by locating the 

 * 'image" of the primary cable from the trend of the horizontal intensity 

 curve. Attempts have also been made to determine resistivities of sub- 

 surface formations and their variation with depth by using a horizontal 

 circular loop of gradually increased radius. The quadrature component 

 in this case depends on resistivity and frequency, depth penetration being 

 controlled by the radius of the loop. As in most other electrical meth- 

 ods, interpretation of results obtained with inductive methods is aided 

 greatly by model experiments. 



Radio methods. Since radio methods employ frequencies still higher 

 than the high-frequency-inductive methods, they are subject to the same 

 limitations. In one group of radio methods the effect of subsurface con- 

 ductors on the emission characteristics of a transmitter is observed. In 

 a second group a receiving arrangement is employed in addition to the 

 transmitter, and the variation of field intensity with location is measured. 

 This will give some idea of the absorption, interference, and reflection 

 that the waves may have undei^ne along their path from the traas- 



