Chap. 12] 



MISCELLANEOUS GEOPHYSICAL METHODS 



885 



All the measurements discussed above were made by testing the activity 

 of soil air. According to Lane, a fracture zone could be located in Michi- 

 gan by an analysis of the radioactivity of well Waters. ^^ Measurements 

 of the penetrating radiation are reported to show fault zones much less 

 distinctly than do observations of the alpha radiation.^^ 



If a fault is covered by a layer of alluvium, the thickness of cover may 

 be calculated by assuming a linear source of concentration, Co , located 



fiO meters 



Fig. 12-7. Effect of mineralized dike and contact zone on radon content of soirair 



(after Mueller). 



in the boundary between impervious rock and cover. The gas ascends 

 through the latter by diffusion. A maximum concentration, Cm, , is located 

 directly above the source and is given by 



dir 

 '2h' 



(12-3) 



where d is the sampling depth and h the thickness of the cover. At a dis- 

 tance X = 0.76 h from the maximum, the concentration (activity) has 

 dropped to one-half its maximum value. 



II. HYDROCARBON (SOIL AND GAS) ANALYSIS 



Methods for the detection of hydrocarbons in the surface soil are desig- 

 nated in the literature either as geochemical or geophysical. Their classifi- 



" Science, 79, No. 2040, 34 (Feb., 1934). 



" Ambronn, op. cit., p. 131. 



»» Koenigsberger, Zeit. Geophys., 4(2), 76-83 (1928). 



