Field experiments to test neutron interaction techniques in boreholes in relatively dry, low porosity rock

Abstract

Depending on the technique, there are a number of problems associated with in situ neutron activation elemental analyses in boreholes. Several techniques of using delayed and capture gamma rays for in situ borehole spectral analysis were tried in a relatively dry gabbro. Using copper as a test element the best results using a delayed gamma-ray method were obtained by independent uniform neutron irradiation of the borehole followed by a delay time of about 16 hours before spectral analysis. The results obtained for several other elements measured by both delayed and capture gamma-ray methods, however, showed poor correlation with the chemical analyses. The change in the nonthermal compared with thermal neutron flux density in the rock appears to be the most serious cause of the poor correlation observed. This in turn affects the gamma-ray peak intensities and energies. Errors in the driller's log, poor correspondence between the core chemistry and the sample, and interstation interference also contributed substantially to the poor results.