Preliminary investigations on the use of radiation transmission measurements in petrophysics

Pilot measurements were performed to explore the possibilities of neutron radiation transmission measurements in the quantitative determination of the amount of extra water in porous concrete. The experiments were performed by using the 14 MeV neutron generator of the Department as a neutron source and plastic scintillation detectors. To expedite the interpretation and quantification of the results, a regular geometry was chosen for the sample with two rectangular concrete blocks above each other and a layer of water in between in a tray. The water content in the concrete was simulated by using different water heights in the tray between the concrete blocks. The measurements were compared with a simple first-flight analytical model of radiation attenuation as well as with more involved, advanced Monte-Carlo calculations with MCNP. With these latter calculations, a number of effects could be investigated separately, namely the contribution of multiply scattered neutrons (by simulating different energy cut-off in the detector), and the difference between having the water in the tray or distributed evenly in the concrete block. A noticeable difference was found between the measurements and the various calculations. The main trends are well understood in a qualitative way, but the quantitative differences require more experimental work and measurements