A LOW ACTIVITY SPECTROMETRIC GAMMA-GAMMA BOREHOLE LOGGING TOOL FOR THE COAL INDUSTRY

A logging tool based on the spectrometric gamma-gamma technique was tested in water-filled boreholes of nominal diameter 96 mm in a coal deposit. The tool was centralized and employed a y-ray source of very low activity (1.8 MBq). The tests proved that the tool could be used to delineate the coal seams in boreholes and measure the ash content of coal. The logging tool was tested separately using: (i) Cs-137; and (ii) Ba-133, and with a 37 x 75 mm NaI(Tl) scintillation detector. Values of 2.5 and 1.9% ash were obtained for the r.m.s. deviation between the laboratory and the gamma-gamma prediction. The tool is sensitive to changes in the Z(eq) number of the matrix surrounding the borehole and could find application in the metalliferous mining industry for ore body delineation. Due to its very low gamma-ray source activity and light weight, the tool is ideal for use with portable logging systems.