The detectability of water by NMR considering the instrumental dead time - a laboratory analysis of unconsolidated materials
Résumé
The geophysical technique of magnetic resonance sounding (MRS) is directly sensitive to groundwater but a part of groundwater is not detected by MRS because of an instrumental dead time, during which the signal is not recorded. In order to estimate the influence of physical and hydrous properties of the porous media on the amount of water that cannot be detected because of this instrumental dead time, laboratory nuclear magnetic resonance (NMR) measurements on synthetic samples have been performed. The laboratory measurements allow recording the signal without dead time and offer the possibility of easily controlling physical properties and hydrous conditions of the media. The effect of a dead time similar to that of the common MRS instrumentation (NUMIS equipment) has been modelled on the signals and the amount of undetectable water has been estimated for different grain size distributions, different clay contents and different saturation degrees. Results show that the amount of the dead-time-related undetectable water is controlled by the distribution of the decay time T2. Both the average and the uniformity of T2 influence the percentage of undetectable water. Variations of median grain size, sorting of grains, clay content and/or saturation degree cause variations in decay time distributions. Less water is missed when median T2 is long and/or when the distribution of T2 is narrow. Generally the fraction of undetectable water decreases when enlarging or better sorting the grains, when decreasing the clay content, as well as when increasing the saturation degree. However, a median grain size lower than 0.2 mm or clay contents higher than 20% may also lead to decreasing fractions of undetectable water in comparison to respectively higher median grain size and lower clay contents because of the narrowing of the decay time distribution.