812 



ELECTRICAL METHODS 



[Chap. 10 



Kentucky and found that radio broadcast waves could not be picked up 

 through more than 100 feet of overburden with a loop aerial, and through 

 no more than 300 feet with a long antenna. Reception dropped almost to 

 zero past 20 or 30 kilocycles. 



Radio methods fall into two groups: (1) methods in which subsurface 

 effects are measured by their reaction on antenna emission; (2) methods 

 involving a determination of signal strength, that is, measurement at the 

 receiving end. 



5000 



■^3500 

 %3000 

 5 2500 

 t 2000 



I 

 I 



f500 



1000 



500 







10 fo^ 10^ i(y 10^ 10^ 10^ 

 Frequencij. Circles per second 



Fig. 10-127. Half -value 

 thicknesses of absorption, for 

 various resistivities and di- 

 electric constants (after Joyce). 



B. Transmission Measurements 



1. The quarter-wave method makes use of 

 the effect of wave reflection on the emis- 

 sion characteristics of the antenna. If the 

 reflected wave is in phase with the trans- 

 mitted wave, the antenna has maximum 

 emission and the antenna current is at a 

 maximum. This occurs when the distance 

 of a reflecting surface is equal to one-quar- 

 ter of the wave length or an odd multiple 

 thereof [d = (2n + l)X/4]. The depth can 

 therefore be determined by varying the fre- 

 quency of the transmitter and by plotting 

 the antenna current as a function of /. 

 Hummel has pointed out""^ that the maxima 



and minima in the transmission intensity 

 are not so sharp as would be expected from 

 this simple scheme, since, near the antenna, 

 the static and induction portions of the field cannot be neglected. The 

 effect of a sheet of good conductivity in a poorly conductive medium 

 upon the antenna emission is equivalent to the action of a reflected doublet 

 at twice the depth obtainable from eq. (10-67a). 



A theoretical curve expressing the variation of effective field strength 

 with variation of the distance of the reflecting bed is shown in Fig. 10-128a. 

 Contrary to the siynplified theory, the peak at X/4 is very indistinct, and 

 the minimum at 2 • X/4 and the maximum at 3 • X/4 are more pronounced. 

 Theory and experiment are in very good agreement, as is shown by Fig. 

 10-1286. The quarter- wave method is reported to have been tried in 

 Southwest Africa for the location of ground water in desert areas. These 

 experiments were interrupted by the World War in 1914, and little has 

 become known since then about the application of the method in prospect- 



"^ Zeit. Geophys., loc. cit. 



