874 



MISCELLANEOUS GEOPHYSICAL METHODS 



[Chap. 12 



cm ) of substance. The radioactivity of gases is generally characterized 

 by the amount of Ra-emanation present. 



Table 79 gives a few representative figures for the radioactivities of soil, 

 water, and air. Table 80 gives a number of average values for the radium 

 and thorium contents of igneous and metamorphic rocks. 



The radioactivity of igneous rocks increases with an increase in SiOa 

 content. Basic rocks are generally less radioactive than are acidic rocks, 

 although there are considerable regional and local variations. The average 



Table 79 

 REPRESENTATIVE FIGURES FOR THE RADIOACTIVITIES OF SOIL, 



WATER, AND AIR 



Soil 



Water 



Soil air. . . . 

 Atmos. air 



U 



(g/cm3) 



7-10-« 



Ra 



(g/cm3) 



2.3-10-12 



10-1^ to 10-i« 



Th 

 (g/cm3) 



1.4-10-* 



10-7 to 10-» 



Em. 



(Curie/ 



cm') 



2- 10-" 



io-i« 



10-18 



Penetrating Radiation 

 (ions/cm Vsec.) 



3-5 I" 

 (up to 20 I on ra- 

 dium deposits) 



about 4 I on land 

 0-2 I at sea 



" I = ions. 



Table 80 

 AVERAGE RADIOACTIVITIES OF IGNEOUS ROCKS« 



Rock 



Granites 



Quartz-porphyry 



Syenite 



Diorite 



Trachyte 



Porphyrite 



Rock 



Gabbro, norite 



Diabase, dolerite 



Basalt 



Basalt, high values 



Basalt, low values 



Recent lavas 



Gneiss 



Ra Th 



(10-12 g/g-i) (10-«g/g-i) 



1.3 



1.0 



1.4 



5 



0.5 



2-20 



2.1 



5.0 



2.2 

 5.6 

 15 

 4 



8.7 



* After Kirsch, loc. cit; A. Born, Lehrb. Geophys, p. 26. 



for acidic igneous rocks is about 3-10"^^ g Ra g~^; for basic rocks the 

 average is about 1 • 10~^^ and may reach 0.5 • 10~^^ for extremely basic con- 

 stituents. Effusive rocks are more radioactive than are plutonic rocks; 

 the radioactivity of metamorphic rocks is largely dependent on whether 

 they are derived from igneous or sedimentary rocks. 



The average radioactivity of sediments is comparable to that of the 

 basic igneous rocks, as shown in Table 81. This tabulation does not in- 

 clude the (recent) deep-sea sediments which range from about 10 to 

 40 10"'' gRa. 



