In-situ gamma-ray spectrometric study of weathered volcanic rocks in Hong Kong

摘要

In-situ gamma-ray spectrometry (GRS) measurements were conducted at 35 sites in Hong Kong where volcanic rocks with varying extent of weathering were exposed. Elemental analyses using X-ray fluorescence spectrometry and inductively coupled plasma-mass spectrometry were carried out on samples collected from these 35 plus 22 other locations to assess the feasibility of using the GRS method to quantify the extent of weathering. The Parker weathering index, varying within a range of 0.0-0.8 for the samples studied, was used as a geochemically based reference scheme for correlating the gamma-ray spectrometric results with the extent of weathering. For the former 35 sites, the concentrations of the three major radioelements, K, U and Th, determined by in-situ GRS were compared to laboratory-determined values from the samples. The study reveal that no significant change occurs to the contents of the three radioelements during the initial state of weathering. But once the rocks become highly weathered, further progression of weathering is accompanied by a systematic removal of K and an increased dispersion of U and Th. The results show that K content, which is indicative of the extent of weathering, can be retrieved reliably with the gamma-ray spectrometry technique. The study has given support to the potential use of the downhole spectral gamma method for evaluation of weathering grade and the detection of subsurface clay-rich levels.