816 



ELECTRICAL METHODS 



[Chai>. 10 



brines in mines. Fritsch"^ found that broadcast signals could be received 

 in subterranean cavities, that reception and penetration was better for 

 waVes above 600 m wave length than for waves of 200 m or less, that the 

 propagation of radio waves was controlled more by geologic factors than 

 by the length of the path, and that the absorption in the direction of 

 strike was greater than at right angles thereto. E. Cloos noted dead 

 spots of radio reception on faults and observed reception anomahes on 

 formation contacts. Such contacts may act as carriers or shields for the 

 radiation, depending on their conductivity and geometric disposition. 



Fig. 10-130. Heterodyne receiver for measuring relative field strength (after Stern). 



Quantitative measurements of intensity, with constant frequency and 

 constant amplitude excitation across faults, have indicated a character- 

 istic anomaly. If a transmitter is set up on the down-dip side of a normal 

 fault with the antenna slightly sloping toward the fault, the anomalous 

 intensity shows a maximum near the suboutcrop, flanked by two minima. 

 On quartz veins a similar phenomenon has been observed, the steep gradi- 

 ent portion of the curve between the intensity maximum and the far 

 minimum corresponding to the vein intersection with the surface. This 

 indicates that a shallow phenomenon is involved. Frequencies used in 

 this work were around 1900 kc. (158 m wave length); according to the 

 theory discussed on page 811 their depth penetration is rather small. It 

 can happen, of course, that fissures and beds of good conductivity act as 

 carriers, thus makmg energy available at more remote points horizontally 

 and vertically. 



115 TjOC, cii, 



"6 Amer. J. Sci., 28(166), 255 (1934). 



1" Wm. M. Barret, U.S. Patent No. 2,172,688. 



