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

Abstract

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 (socalled 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 the second 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. of groundwater resource in many poor aquifers even though they are the only water source in many areas of Africa and Asia. Poor aquifers can potentially supply small pumping abstraction to local communities (typically 10 to 100 m 3 per day) and they have not yet been deeply studied. Nowadays, increasing the knowledge of their storage properties is essential.Drilling boreholes and carrying out hydraulic testing is the conventional method for estimating aquifer properties because it is an in situ measurement, thus based on a ground truth.

Integration of hydrogeophysics and remote sensing with coupled hydrological models

“…In hard rocks, the success of MRS survey is still limited but could be improved by optimizing: (i) the MRS loops and the instrumentation (larger pulse moment, better noise rejection capability and shorter dead-time); (ii) the retrieval of the storage parameters, namely the separation of gravitational water from capillary water based on MRS measurements. The latter issue showed recently a progress with the introduction of the concept of apparent cutoff time, which relates the MRS decay time with the aquifer storage property (Vouillamoz et al, 2012b(Vouillamoz et al, , 2014a. In parallel, laboratory studies were also performed to establish better relationships between aquifer and MRS parameters (Stingaciu et al, 2010;Boucher et al, 2011;Yaramanci, 2011, 2013), showing that MRS capacity can also be extended to the investigation of water percolation and storage in the vadose zone.…”