Estimating storage properties of aquifer with magnetic resonance sounding: a field verification in northern Cambodia of the gravitational water apparent cutoff time concept

摘要

Magnetic Resonance Sounding (MRS) has already showed its capability for estimating some aquifer properties, but very little work has been carried out concerning the storage properties used for calculating groundwater reserve and recharge. For unconfined aquifer, this storage property (i.e., the specific yield) is often estimated from the MRS water content. However, a recent study carried out in a poor sandstone aquifer of Northern Cambodia found that the MRS pore-size related parameter is probably more appropriate because the MRS water content can be dominated by water which does not contribute to the specific yield. A main output of this study is that a threshold value (so-called apparent cutoff time ACT) of the MRS decay time T_2* can be defined for discriminating gravitational water (T_2*> 130 ms) from capillary and bound waters. Since measuring such short MRS signal is quite common, a validation of this result is compulsory to avoid the misuse of the MRS water content. This paper presents an experiment that we set up to check the validity of the ACT approach. We compared two MRS carried out at the same location in a clayey-sand layer 15 metres thick. The first MRS was carried out with a static water level located at 1.7 metres below the ground level. It revealed a layer with a water content of 3.8% and T_2.*= 120 ms. According to the ACT approach, this short value of T_2* indicates water that is not part of the specific yield and that can not be released by pumping. Thus, we carried out a second MRS after pumping and lowering the water level to an average of 15 metres. As predicted by the ACT approach,we observed that thersecond MRS signal was the same as the one obtained before pumping. To cross-check our observation, we carried out a pumping test at the same location. We found a very low value of specific yield, confirming that the main part of the MRS signal is generated by water which was not drained by the pumping. We calculated that the volume of water removed from the aquifer by the pumping was far too low to generate a measurable magnetic resonance signal. From our experiment, we conclude that the ACT approach can be successfully used to estimate the specific yield of poor aquifers, and to avoid a misuse of the MRS water content which can lead to strong overestimates of aquifer reserve and recharge.