456 EXPLORATION GEOPHYSICS 



effects and conductive ore bodies) are the differences of potentials caused 

 by currents covering large areas, in fact the entire earth. 



The use of these natural earth currents,! or telluric currents,^ in geo- 

 physical prospecting has so far been meager, § since investigations of the 

 phenomena have themselves been very limited, although knowledge of their 

 existence is far from recent. 



These currents circulate about the earth in large "sheets" covering vast 

 sections of the earth's surface ; generally they consist of four huge current 

 whorls covering the whole globe. Between two points, the component of 

 the difference of potential arising from telluric current is approximately 

 proportional to the separation of the points. However the telluric field at 

 a given point varies so in intensity and direction (particularly during the 

 daylight hours) that the daily average field is probably zero. Over shorter 

 periods of time (of the order of an hour) definite facts can be established ; 

 for example, the field has two maxima and two minima per day, and its 

 average direction is generally that of the magnetic meridian for the middle 

 latitudes while almost perpendicular to the magnetic meridian for lower 

 latitudes. 



That telluric currents do flow in such large sheets and that telluric 

 phenomena between distant points are interrelated is demonstrated by 



simultaneous recordings made by crews at 

 ANISOTROPIC sYNCLiNE AND ANTICLINE wldcly scparatcd polnts on the earth — for 

 Th. orrowi tehtmaticoiiy indicot* th» examplc, southcm France and Madagascar. ff 



Problems to which telluric currents may 

 possibly be applied are, in general, those 

 anomalies which cause a horizontally flowing 

 current to be disturbed. A salt dome will 

 deflect the current, since the salt plug is non- 

 FiG. 273.-F10W of the telluric conductive. This upward deflection of the 



current in an anisotropic folded telluric currcnt causcs a Concentration of cur- 

 ground. (Geophysics, Vol. AliI, 



No. 3, July, 1948, p. 401.) j-cut near the surface, with a resultant higher 



telluric potential vector over the salt dome. 

 Folded formations, which are generally anisotropic, will show an increased 

 telluric vector at the peak of an anticlinal or domic structure (Figure 273), 

 since the current tends to flow along the bedding planes rather than per- 

 pendicular to them. Faults are detectable if the resistivities, size and posi- 

 tions of the beds cause a concentration of the current alone: the fault. 



t R. S. Dahlberg, Jr., "An Investigation of Natural Earth Currents," Geophysics, Vol. X, No. 

 4, October, 1945. 



t E. Boissonnas and E. G. Leonardon, "Geophysical Exploration by Telluric Currents," Geo- 

 physics, Vol. XIII, No. 3, July, 1948, pp. 387-403. 



§ M. Schlumberger, "Method and Apparatus for Electrical Underground Prospecting," U. S. 

 Patents No. 2,240,520, May 6, 1941, and 2,284,990, June 2, 1942. 



J. A. Marchand, "Geophysical Prospecting," U. S. Patent No. 2,263,097, Nov. 18, 1941. 

 tt Boissonnas and Leonardon, loc. cit. 



