1990 Volume 31 Issue 4 Pages 185-194
A measurement system was assembled for easily and rapidly detecting full features of the fissures buried in the ground, and the applicability of the system was tested in the field.
Geological features analyzable by the system are: distribution of formations or rock layers, locations of their boundaries, locations of fault fissure zones, degrees of fissuring along such zones, locations of open fissures and the amount of radon-222 accumulated in the surface layer. This amount reflects the scale of fissures, and an abnormal increase in the amount can be a significant indicator in prospecting subterranean heat, uranium, natural gas and oil resources and in earthquake and landslide prediction studies.
Direct object of measurement by the system is the dose of the primary gamma rays emitted by three radionuclides, 214Bi, 208Tl, and 40K contained in the surface layer within a cone about 30cm in maximum depth and about 1m in basal radius. The gamma-ray dose is measured by means of a 5-channel pulse-height analyzer equipped with a NaI (Tl) crystal detector 12.7cm in diameter and height. This system can shorten the measurement time to a half as that practiced in the method of Adams and another.
The primary gamma-ray dose of each of the three nuclides indicates the concentration of each nuclide in the surface layer, as a principle. This dose, however, has a difficulty for practical application as it changes with geometrical conditions of ground surface, various man-made factors and diverse meteorological factors. A new technique was then proposed to make use of the ratio of primary gamma-ray doses of two different nucliedes as an indicator for overcoming this difficulty. This technique could thus effectively eliminate most of the said hindering factors and enabled quantitative analyses of needed geological features.
A steep slope mountain area and a crowded city area, which both involved difficult conditions for application, were subjected to field applicability test. The existence of active faults inferred in those areas was examined. The existence of open fissures, their locations, changes of fissure openings and local changes of fissuring degree could be detected by the new system in both areas. The existence of accompanying closed fissures developed in parallel and local variations of their state of fracturing could also be detected.
