A major hope for discovering subsurface uranium ore is that measurable concentrations of the radioactive gas 222Rn can be recognized near the surface of the earth. Integrated measurements, made over several weeks time, show promise of giving greater reproducibility than short-term measurements, which are more subject to meteorological variability.
The use of improved methods of integrated radon measurements — free of 220Rn, of thermal track fading, and of moisture-condensation effects — allow readings to be made that generally are highly stable over time. At a site 16 km north of Thoreau, NM, readings at 60 cm depth taken over a 13-month interval for a set of 55 positions give different, but nearly constant monthly readings at each position, the typical standard deviation being 22%. Superimposed on that stable pattern have been three periods during which spatially grouped radon readings increased by a factor of two or more over their normal values. The simplest tenable description of the source of the increases are sporadic puffs of upflowing gas, originating at as yet unknown depths. The measurements are consistent with an upward velocity of flow of ∼ 10−3 cm/s. If this velocity is maintained to depth it is still insufficient to transport detectable amounts of radon from the ore body which is at 90 m depth, but it would be sufficient to reveal ore at 50 m or less.
Down-hole measurements of permeability yield values that generally are too low to allow signals to be delivered from the ore body by any of the mechanisms that have been modeled. Occasional localized regions of adequately high permeability have been found, but their orientation and extent have not been measured.
Although it is improbable that this ore body was discovered by distant transport of radon, the possible existence of occasional gas flow in the earth gives encouragement that at some sites discovery will be aided by such flow.
