Chap. 12] MISCELLANEOUS GEOPHYSICAL METHODS 875 



Radioactive mineral concentrations, not only in sedimentary but also 

 in igneous rocks, may exceed their surroundings in radioactivity by 1000 

 to 100,000 times. Radioactive ores low in uranium oxide (from 0.3 to 0.5 

 per cent) have a radium content of the order of 10~* g Ra g~\ while those 

 of high concentration (80 per cent uranium oxide) reach 10" g Ra g" . 

 Of interest is the observation of B^hounek that the range of a radium 

 deposit is fairly limited.^ The pitchblende deposit in Joachimstal could 

 riot be detected at distances exceeding 1000 feet. It is further significant 

 that the anomalies caused by local concentrations of radioactive products 

 (faults, and the like, see below) may exceed those due to radioactive 

 ores. Hence, radioactivity prospecting is more useful for detailing local 

 concentrations than for finding radioactive ores by reconnaissance. 



Likewise, the delineation of rocks of different radioactivities, the separa- 

 tion of sedimentary from igneous rocks, and similar applications based on 

 the data given in Tables 80 and 81 will be possible only in exceptional 

 cases because of interferences from such local concentrations. On the 



Table 81 

 RADIOACTIVITIES OF SEDIMENTARY ROCKS^ 



^ Largely after Born, loc. cit. 



other hand, radioactivity methods can be quite useful for locating faults, 

 fissures, and other openings along which radioactive products have been 

 deposited or radioactive waters are circulating. Radon is readily absorbed 

 by water much in the same manner as is carbon dioxide; it also has a 

 tendency to accumulate in porous and shattered rocks, that is, in and near 

 fractured and faulted zones. Experiments indicate that, to be radio- 

 active, it is not necessary for such concentration channels to remain open; 

 mineral dikes deposited in fault zones exhibit as much and sometimes more 

 radioactivity than do open fracture zones. Organic matter appears to 

 have a considerable affinity for radon. For instance, oil absorbs from 

 40-50 times as much radon as does water (at temperatures from 20°-60°C.). 

 Certain spring sediments containing vegetable matter are more radioactive 

 than are the rocks on which they are deposited. Deep-sea ooze rich in 

 animal remains is more radioactive than are ordinary sediments. Radio- 

 active ores are frequently found in beds rich in carbonaceous matter and 



8 Fr. Behounek, Phys. Zeit., 28, 333-342 (1927). 



