THE LOOP METHOD IN EXPLORING BURIED STRUCTURES 267 



In this respect also the region of Bukhara and Khiva on the right bank of the 

 Armu-Dar'ia is not without interest. 



Of considerable help would be the use of the loop method on the exten- 

 sive, but little studied, Ust'-Urt plateau, where the loop method gave favour- 

 able results with deposits showing large-scale anisotropism. 



The loop method should also be used to a greater extent in tracing faults 

 in disturbed zones. When carrying out experimental work it would be possible 

 to recommend for this purpose objectives such as the Emba salt domes. 



In connection with the proposed modernization of the method, in partic- 

 ular for extending its depth and effectiveness of study and the use of a 

 small dimension frame instead of a large diameter loop, the loop method 

 can be very helpful in a nuinber of wooded regions in Western Siberia; 

 for example, regions to the east of the Chelyabinsk graben. 



For the further development of modifications of the loop method, we can 

 also recommend observations at a fixed frequency with increase in the disper- 

 sions of the feed electrodes A and B, for work on a fixed length of AB line 

 with changing frequency of the feed current (1, 2, 3 c/s etc.). This would 

 make it possible to follow the change in vectors of the loops with increase 

 (or decrease) in the depth of the study. 



At the present time, in view of the absence of a theory for alternating 

 electromagnetic fields in inchned anisotropic media, this suggestion can 

 only be based on an evaluation of the depth of penetration of electromagnetic 

 oscillations at various frequencies in a homogeneous, isotropic unbounded 

 medium. 



If we assume the resistance of such an idealized medium to be equal to 

 the resistance of the clays of the Maikop formation (^ = 3 . 10^ Q. . cm), 

 then the depth of penetration of the oscillation with a frequency 1 c/s is 

 about 2 km, and for an oscillation with frequency 10 times greater about 

 640 m. Consequently, the range of depths with change in the frequency 

 from 1 to 10 c/s is sufficiently great to serve as an indication of the possi- 

 bility of increasing the depth of investigation by reducing the frequency of 

 the feed current, as suggested above. 



However, to interpret field observations it is necessary to have a devel- 

 oped theory for an alternating electromagnetic field in anisotropic media, 

 and in particular a calculation of the actual depth of penetration of the 

 alternating magnetic field of a given (low) frequency in inclined anisotropic 

 deposits. This makes it possible to evaluate in the first place the depth of 

 the proposed modification of the loop method and to calculate the range of 

 depths with change in frequency of the feed current, and in the second place, 

 to establish the resolving power of the method in the presence of a hetero- 



