Hélène Guyard

Three-dimensional magnetic resonance imaging for groundwater

The surface nuclear magnetic resonance method (SNMR) is an established geophysical tool routinely used for investigating one-dimensional (1D) and sometimes 2D subsurface water-saturated formations. We have expanded the tool by developing a 3D application. 3D-SNMR is a large-scale method that allows magnetic resonance imaging of groundwater down to about 80?m. Similar to most surface geophysical methods, 3D-SNMR has limited resolution, but it is effective for investigating water-saturated geological formations larger than several tens of meters. Because the performance of the method depends on variable survey conditions, we cannot estimate it in general. For demonstration purposes, we present an example of numerical modeling under fixed conditions. Results show that under certain conditions it is possible to detect a water volume as small as 500?m3 and the detection threshold depends on the ambient electromagnetic noise magnitude and on the location of the target volume relative to the SNMR loops. The 3D-SNMR method was used to investigate accumulated water within the T?te Rousse glacier (French Alps). Inversion of the field measurements made it possible to locate the principal reservoir in the central part of the glacier and estimate the volume of accumulated water. These results were verified by 20 boreholes installed after the 3D-SNMR results were obtained and by pumping water out of the glacier. Very good correspondence between the 3D-SNMR and borehole results was observed.

On the inclusion of ground-based gravity measurements to the calibration process of a global rainfall-discharge reservoir model: case of the Durzon karst system (Larzac, southern France)

This work examines the relevance of the inclusion of ground-based gravity data in the calibration process of a global rainfall-discharge reservoir model. The analysis is performed for the Durzon karst system (Larzac, France). The first part of the study focuses on the hydrological interpretation of the ground-based gravity measurements. The second part of the study investigates further the information content of the gravity data with respect to water storage dynamics modelling. The gravity-derived information is found unable to either reduce equifinality of the single-objective, discharge-based model calibration process or enhance model performance through assimilation.

Experimental study of domestic waste material using magnetic resonance measurements

In this paper, we present results of a laboratory and in situ study of a domestic waste landfill using magnetic resonance measurements. For our study, we used a laboratory Earths field nuclear magnetic resonance (NMR) instrument developed at LTHE and a large-scale commercial magnetic resonance sounding (MRS) system NUMISLITE from IRIS Instruments. We show that NMR could be a tool for investigating different processes in water-saturated waste samples. Our results show that domestic waste material contains ferromagnetic or paramagnetic particles that perturb the homogeneity of the geomagnetic field at a microscopic scale and render an NMR signal short. Consequently, only the spin echo technique can be applied for measuring. At a macroscopic scale, waste and different buried objects may also perturb the natural geomagnetic field. While investigating the landfill, we observed that magnetic anomalies (±2500 nT) are localized around some cells. This is probably linked to the presence of a higher percentage of metallic objects within the waste disposal. Our first appraisal of the possibility of investigating water-saturated waste in a laboratory using an Earths field NMR instrument shows that, with existing instruments, waste samples can be studied when the dry density of waste is less than approximately 450 kg/m3. Because the relaxation times of magnetic resonance signals in landfill may be short (T2 < 100 ms and T2*<10 ms), existing large-scale MRS instrumentation is not adapted to the investigation of domestic waste landfills.

Experimental Verification of 3D Magnetic Resonance Tomography (MRT) in the Luitel Peat Bog, France

The goal of this study is the experimental verification of the newly developed 3D magnetic resonance tomography (MRT) inversion code. The Luitel site was chosen because this peat bog was previously well studied with different geophysical methods. We compared 3D MRT inversion results with a ground penetrating radar profile (GPR) and a coring to check the bedrock position. For the water content, we used a common middle point measurement (CMP). We showed that the 3D MRT inversion results are stable. The geometry of the peat bog is well resolved. A calibration with a coring improves the precision of MRT inversion. We have found that 3D MRT derived water contents show smaller value than CMP point. However, these differences may appear by the fact that the CMP measurements are ad hoc measures while the MRT measurements are volumetric. Therefore, in this case, it will be necessary to verify our results by some complementary measurements in different parts of the bog. Finally, the vegetation distribution seems to confirm the water content distribution given by MRT inversion.