818 ELECTRICAL METHODS [Chap. 10 



If the receiver has been kept constant and two wave lengths, Xi and X2 , 

 have been found to produce successive minima in reception, then the 

 depth 



For practical appUcations, the above formulas are only approximate, 

 since the wave lengths in air and in rock are not the same. The two 

 paths are usually traversed with different speeds and different wave 

 lengths. This leads to more complex formulas, which have been derived 

 by Petrowsky. 



The depth to a horizontal reflection surface can lastly be determined by 

 taking a "bearing" of the image of the antenna with a coil-aerial receiver, 

 so that d = {a tan <p)/2. 



3. The polarization of radio waves and its relation to the electrical 

 properties and disposition of near-surface beds was discussed on page 653. 

 The ratio of major and minor axis of the ellipse and its tilt angle may be 

 measured by a receiving set with rotatable double L antenna. The polari- 

 zation is chiefly dependent on depth of ground water level near surface 

 moisture and varies with wave length (see Fig. 10-20). With the excep- 

 tion of Feldman's recent studies,"^ Uttle has been done along this line 

 since Hack's fundamental investigations in 1908. Measurements of 

 polarization, when combined with relative or semiabsolute determinations 

 of electrical and magnetic field components, are likely to place the radio 

 methods of electrical prospecting on a more quantitative basis and to 

 make them more usable than they are at present. 



X. TREASURE AND PIPE FINDERS 



These devices and methods utilize electrical prospecting procedures for 

 locating objects buried at shallow depth. Obviously they can be applied 

 only if differences in conductivity exist, that is, if buried objects are con- 

 ductive or enclosed in metal cases. Often the duration of burial makes 

 considerable difference in detectability, since the chemical action of the 

 surface waters creates a zone of low resistivity around the conductor, which 

 increases its effective electrical size. 



A. Treasure and Pipe Finders with Separate Excitation 



A method for locating pipe by separate excitation, that is, an adaptation 

 of the electromagnetic method, was mentioned on page 765 (Darley pipe 



121 Jjoc. cit. 



1" I.R.E. Proc, 21, 790 (June, 1933). 



"» Ann. Phys., 27, 43-63 (1908). 



