Anatoli Legchenko

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.

The contribution of MRS and resistivity methods to the interpretation of actual evapo-transpiration measurements: a case study in metamorphic context in north Benin

A quantitative budget estimate of actual evapo-transpiration is a key issue for enhanced hydrological modelling in northern Benin. Actual evapo-transpiration is estimated using large aperture scintillometer equipment, devoted to sensible heat flux measurements. However, a previous study reported that the actual evapo-transpiration cycle is not fully understood. Indeed, the actual evapo-transpiration depends strongly on several factors such as climate, vegetation pattern, soil water storage and human activities. The respective contributions of the aquifer and vadose zone to the actual evapo-transpiration budget are not known. When using piezometric variations of the water table, the aquifer contribution is not easy to quantify since the specific yield may vary in the investigated area, located in a metamorphic rock environment. In the present study, we investigate whether significant differences in the aquifers specific yield could exist within the large aperture scintillometer measurement area, leading to different actual evapo-transpiration water losses. We use joint frequency electromagnetic resistivity mapping, geological surveys and magnetic resonance sounding (MRS) to delineate the effective porosity of the regolith around the scintillometre measurement area. Thirteen MRS soundings implemented in key areas reveal a clear classification of the main geological units on the basis of their water content. The MRS water content varies between 1.5-3% for amphibolite and micaschists formations to more than 12% for quartzitic fractured formations, whereas the MRS relaxation time T1 is less discriminating (150-250 ms), indicating a small variation in pore size. Then, as a first modelling exercise, we assumed that the MRS water content (the effective porosity) maximizes the specific yield. The actual evapo-transpiration budget given by a previous study (Guyot et al. 2009) is then re-interpreted using geophysical data: we found that a) the measured water table depletion can explain the actual evapo-transpiration value providing enough water for the transpiration process and b) the significant discrepancies in actual evapo-transpiration signals observed between the eastern and western parts of the watershed can be explained by the respective effective porosity of the geological units. Even if further research is needed to link MRS water content to the specific yield and to evaluate a possible role of the deep vadose zone, the hydrogeophysical mapping presented in this study highlights the role of the MRS method for providing relevant information to understand hydrological processes in this complicated geological context of north Benin.

Glimpse into the design of MRS instrument

The Magnetic Resonance Sounding method (MRS) was developed in the former USSR in the late 1970s. Nowadays, available MRS instruments are more compact and reliable, and enormous progress has been made in electronics, computers and materials. Therefore, we can hope that it may be possible to increase the maximum depth of water detection and to improve the resolution of the method by using a larger current in the loop. Quite naturally, the questions arise: what are the practical limits of the MRS method and how much should be transmitting power to get the maximum depth of investigation? In this paper, we analyse the depth of groundwater detection and the vertical resolution of the MRS assuming different loops and different power levels of the current generator. The originality of our approach consists of a joint analysis of the maximum depth of investigation using acceptable loop voltage and the modifications in the instrument design necessary for the improvements. We show that even under very favourable conditions it would be difficult to get significant improvement in the depth of investigation using currently available instruments. For example, under favourable noise conditions when rocks have low electrical conductivity and are non-magnetic, a 20 m thick water saturated sand layer can be detected at a depth of about 325 m using an existing instrument (4 kV of the loop voltage) with a 400 × 400 m2 square loop. A 20% increase in the detection depth (390 m instead of 325 m) requires more powerful electronic equipment (16 kV instead of 4 kV) thus rendering the MRS system larger and heavier. However, using a 16 kV instrument allows us to increase the resolution depth by about 80% (from 120 m to 215 m). When rocks are electrically conductive, the screening of the MRS signal limits the depth of investigation and allows for only minor improvements even with a much more powerful current generator.

Investigating water distribution in the Luitel Lake peat bog using MRS, ERT and GPR

In this paper we present the results of a geophysical study of water distribution in the peat bog at Luitel Lake. The goal of the study was to determine water distribution within the bog, which is part of a protected nature preserve. The small peat bog (17 ha) provides a good test site for developing and testing surface geophysical methods. For this study we used magnetic resonance sounding (MRS), electrical resistivity tomography (ERT) and ground-penetrating radar (GPR). Because the water distribution in the bog is a 2D target for MRS, we had to develop a measuring procedure and 2D inversion routine for MRS. The fieldwork consisted in establishing seventeen MRS stations and conducting three ERT profiles and one GPR line. The MRS, ERT and GPR results on the reservoir geometry correlated well with each other. Pine and birch trees cover most of the bog surface but they have not yet populated the centre of the bog, the location where the maximum water content was observed. This result agrees well with vegetation distribution in the study area: at the centre of the bog, vegetation is typical of a swampy environment but outside the centre the vegetation is typical of a forest. According to MRS, the water content of the peat formation at the centre of the bog is 60–70%, whereas GPR estimated the water content to be between 64–70%. Outside the centre, MRS showed the water content of the peat to be about 30%.

Water in karst hydrosystems unsaturated zone; MRS evidences within an integrated hydrogeophysical approach.

Karst unsaturated zone (UZ) is recognized to play a key role in karst hydrosystems recharge and contaminant attenuation processes. Its characteristics are identified as an important factor for karst groundwater vulnerability assessment and resource management. A large scale hydrogeophysical experiment was undertaken over the last 4 years combining several surface-based geophysical methods and measuring techniques in a unique experimental site within the Fontaine de Vaucluse (FDV) hydrosystem in southern France. The aim was to apprehend the UZ structure and functioning of typical urgonian karstified mediterranean limestone. Geological structure is being well recognized by electrical, electromagnetic and seismic measurements. Magnetic Resonance Sounding (MRS) results clearly identify water presence and evidence seasonal variations within the karst UZ. Due to these results a conceptual hydrogeological model is proposed. On May-June 2014 a drilling campaign is being programmed and a cross-validation of the MRS results is also planned.

Joint Use of Geophysical and Hydrological Methods to Characterize Structures and Flow Geometry in a Complex Aquifer

Characterizing the hydrogeological functioning of complex crystalline aquifers requires a precise assessment of their structural and hydrodynamic heterogeneities. This study shows how the joint use of TDEM, ERT and MRS enables to characterize both alterites and weathered-fissured zones. Moreover, with complementary data such as water table measurements and hydraulic tests, the characterization of the fractured zone is also possible. A conceptual hydrogeological model is then proposed from a case study in India, thanks to this joint use of geophysical and hydrological data.

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.

Usefulness of Magnetic Resonance Soundings to Constrain Groundwater Modelling

The unconfined aquifer in southwest Niger was intensively studied over the last decades. However, the transient numerical modelling that was conducted over the area suggested that additional field storativity and hydraulic conductivity measurements are needed. To check the capability of magnetic resonance sounding (MRS) to improve the knowledge of this aquifer, a survey was conducted in Niger in December 2005. We found that (1) the depth to the water table is estimated by MRS with an acceptable accuracy to survey poorly documented areas. (2) After a parameterization process, the transmissivity is accurately estimated from MRS. (3) MRS water content is linked to the total porosity calculated with the Bretjinski formula. However, MRS water content still needs to be parameterized with new measurements to quantify the specific yield and the total porosity. We finally found that MRS is useful to better constrain groundwater modelling of such large unconfined aquifer. (1) MRS depth to the water table can be used as head boundary in the numerical model (2) The range of realistic transmissivity and storativity values that could be used in the model can be estimated from MRS.