NEW METHODS OF ELECTRICAL EXPLORATION IN SIBERIA 207 



by an infinitely long supply line AB. If the medium under investigation is 

 horizontally homogeneous, then this field is always constant in value and 

 direction. On the other hand, on the surface of a horizontally heterogeneous 

 medium, the field shows variations both in value and in direction. These 

 anomalies are associated with features of the geological structure of the 

 medium under investigation, especially with the change in the total longi- 

 tudinal condvictivity of the sedimentary succession lying on a non-conducting 

 foundation. 



Thus, the field of telluric currents, as distinct from gravitational and 

 magnetic fields of the earth, does not depend on the structure and petro- 

 graphic composition of the rocks underlying the friable deposits, and under 

 favourable conditions reflects the basic, features of the relief of the reflecting 

 highly resistant level. 



Exploratory observations of telluric currents are made at various points 

 of the studied area at the same time as observations at the stationary base 

 point. The distance between the base and field stations does not usvially 

 exceed 30-35 km. 



The dimensions of the measuring devices are selected independently of 

 the depth of the electrically reflecting horizon and in most cases are limited 

 to 500-1000 m. This offers possibilities for using the method of telluric 

 current in deep surveys, especially in difficultly accessible locations. 



By processing the tellurograms obtained simultaneously at the base and 

 field stations, the so-called parameter K associated in a simple way with the 

 ratio of the areas of synchronized closed hodographs* of the field of telluric 

 currents at the base and field points. The parameter K is the ratio of the 

 average field intensities of the telluric currents at the points of observation. 

 Using the value of the parameter K and taking the average intensity E of 

 the field of the telluric current at the base point as being equal to any arbitrary 

 value, it is possible to calculate the value of £" at a number of field points and 

 construct a map of E (a map of the average intensity of the field of the 

 telluric currents) which represents the main results. In large-scale surveys, 

 in addition to the E map, maps are drawn for other values, of auxifiary 

 importance. 



The geological interpretation of the E map is based on the idea that the 

 decrease in E values is caused by an increase in the total longitudinal 

 conductivity of the layer above the marker horizon and, consequently, under 

 conditions of a constant geoelectrical cross-section by the downward dip of 

 the high -resistance marker horizon. The regions of increased valwes of E 



* Distance-time curves. 



