Noise removal in MRS applications: field cases and filtering strategies

International audienceThe usefulness and reliability of magnetic resonance information to characterize water bearing geological structures has been widely demonstrated these last two decades all over the world and many future applications just begin. The main limitation of MRS applicability is its sensitivity to the electromagnetic noise which results in a long and site dependent measuring duration, and generally prove to be impossible in urban conditions. Many improvements have been performed all along the development of MRS technology. Nowadays, numerous mono and multi-channel processing schemes have been published, but efficiency remains site and time dependent. We have reviewed data from various contexts and compared the noise removal efficiency and impact of the filtering on synthetic signal added to real noise data. We also used methods derived from magneto-telluric to study the structure of the noise and present a continuous EM field monitoring during a storm event in mountain where we performed a MRS survey. We observed that the reconstruction of natural noise is a percentage of the ambient noise, the ratio is almost stable. Despite this observation of stable removal performance, it means that when the level of noise is multiplied by 10 to 100 and more… it is better to stop measuring MRS and wait for a quiet period of time

Assessing groundwater resources in coastal area: a case study in Myanmar

Coastal aquifers are of a great relevance for human needs because coastal areas are often densely populated. To meet the needs of the people living in coastal area and to assess the groundwater resources for the future, we need to know the main characteristics of these aquifers. A preliminary hydrogeological assessment is conducted in order to point out the main questions encountered. Then, a geophysical survey is designed to answer these questions. Finally, the economical impact of integrated hydro-geophyscial approach is calculated. This paper presents the main results of a survey conducted in Myanmar. We found that the joint use of hydrogeological data (boreholes and pumping tests) with appropriate geophysics (magnetic resonance sounding and electrical resistively measurements) improves significantly the knowledge of coastal groundwater resources. The presence of groundwater, its available quantity but also its salinity can be reasonably estimated from surface geophysical measurements

Monitoring water accumulation in a glacier using time lapse magnetic resonance surveys

International audienceSince the catastrophic subglacial lake outburst flood in 1892, the risk of a new event in the glacier of Tête Rousse, in the Alps (close to the Mont Blanc) has been thoroughly studied until now (Vincent et al., 2010, 2012). In the last 5 years, the combination of several geophysical technics has provided valuable input for the glaciologists to better understand the structure and the evolution of sub-glacial liquid water (Garambois et al, 2015). Ground penetrating radar which has proven for long to be a very efficient tool in glacial environment has been used here, providing fine imaging of internal structures, bed rock depth estimate, crevasses and the top of the main cavity. In addition, Magnetic resonance has been performed in 2009, confirming the existence of the liquid water volume, and applied in 2010 along a tight array of loops to provide a 3 D image and an estimate of the total water volume. Indeed, this latter parameter is of major importance to evaluate the level of risk

Suivi temporel [2007-2014] par géoradar et résonance magnétique protonique d'une poche d'eau localisée au sein du glacier polythermique de Tête Rousse (massif du Mont Blanc)

National audienceWe present Ground Penetrating radar and Surface Nuclear magnetic resonance investigations of an englacial water pocket embedded in the polythermal glacier of Tête Rousse. Besides glacier bed and crevasses characterization, these data allowed the monitoring of the cavity whose geometry changed after artificial water drainages were performed. Introduction Le régime thermique des glaciers tempérés dépend de plusieurs facteurs climatiques et locaux, parmi lesquels l'altitude, l'accumulation de neige, l'exposition et la géométrie 3D (Gilbert et al., 2012). Dans ce contexte, la présence de lacs supra-glaciaires, pro-glaciaires ou intra-glacaires génère un risque important pour les infrastructures et les habitants localisés dans les vallées en aval, notamment en cas de drainage abrupt et soudain de ces lacs. Parmi ceux-ci, les lacs intra-glaciaires posent un problème crucial car ils sont invisibles depuis la surface et donc difficiles à détecter. Un cas emblématique s'est produit en 1892 à Saint-Gervais (Alpes françaises), où une vague composée de plusieurs centaines de milliers de m 3 d'un mélange d'eau, de boue et de blocs rocheux a atteint la ville quelques minutes après la rupture d'une cavité remplie d'eau au sein du glacier de Tête Rousse (Vincent et al., 2010), faisant 175 victimes. Le cas présenté ci-après détaille les investigations géophysiques réalisées au sein du glacier de Tête Rousse à partir de 2007, originellement dédiées à l'évaluation du maintien de la maintenance d'une galerie drainante creusée en 1905. En 2007, les investigations géoradar ont révélé la présence d'une forte réflectivité localisée à 40 mètres de profondeur au sein du glacier, qui potentiellement pouvait être due à une cavité remplie d'eau. Des campagnes additionnelles basées sur les méthodes de Résonance Magnétique Protonique (RMP) et géoradar ont permis entre 2009 et 2010 de valider l'hypothèse d'une cavité remplie d'eau, hypothèse définitivement confirmée par 11 forages de reconnaissance réalisés en juillet 2010 (Vincent et al., 2010). Cette découverte a conduit les autorités en charge de la gestion du risque à décider d'un drainage de la poche, opération réalisée en 2010 et renouvelée en 2011 et 2012, et d'un suivi temporel notamment géophysique (Figure 1). De nombreuses mesures météorologiques, hydrologiques et géophysiques ont été combinées afin de comprendre la formation et l'évolution de la poche suite aux drainages. Ces travaux ont fait l'objet de nombreuses publications portées sur l'étude de la formation de la poche d'eau et de son évolution (Vincent et al., 2012, 2015), sur les données RMP (Legchenko et al., 2011, 2014) et sur les données géoradar (Garambois et al., 2015). Nous proposons ici de revenir sur ces reconnaissances géophysiques et de leurs apports et limites respectives, depuis la découverte de la poche d'eau en 2007 jusqu'aux reconnaissances effectués jusqu'en 2014

Suivi temporel [2007-2014] par géoradar et résonance magnétique protonique d'une poche d'eau localisée au sein du glacier polythermique de Tête Rousse (massif du Mont Blanc)

National audienceWe present Ground Penetrating radar and Surface Nuclear magnetic resonance investigations of an englacial water pocket embedded in the polythermal glacier of Tête Rousse. Besides glacier bed and crevasses characterization, these data allowed the monitoring of the cavity whose geometry changed after artificial water drainages were performed. Introduction Le régime thermique des glaciers tempérés dépend de plusieurs facteurs climatiques et locaux, parmi lesquels l'altitude, l'accumulation de neige, l'exposition et la géométrie 3D (Gilbert et al., 2012). Dans ce contexte, la présence de lacs supra-glaciaires, pro-glaciaires ou intra-glacaires génère un risque important pour les infrastructures et les habitants localisés dans les vallées en aval, notamment en cas de drainage abrupt et soudain de ces lacs. Parmi ceux-ci, les lacs intra-glaciaires posent un problème crucial car ils sont invisibles depuis la surface et donc difficiles à détecter. Un cas emblématique s'est produit en 1892 à Saint-Gervais (Alpes françaises), où une vague composée de plusieurs centaines de milliers de m 3 d'un mélange d'eau, de boue et de blocs rocheux a atteint la ville quelques minutes après la rupture d'une cavité remplie d'eau au sein du glacier de Tête Rousse (Vincent et al., 2010), faisant 175 victimes. Le cas présenté ci-après détaille les investigations géophysiques réalisées au sein du glacier de Tête Rousse à partir de 2007, originellement dédiées à l'évaluation du maintien de la maintenance d'une galerie drainante creusée en 1905. En 2007, les investigations géoradar ont révélé la présence d'une forte réflectivité localisée à 40 mètres de profondeur au sein du glacier, qui potentiellement pouvait être due à une cavité remplie d'eau. Des campagnes additionnelles basées sur les méthodes de Résonance Magnétique Protonique (RMP) et géoradar ont permis entre 2009 et 2010 de valider l'hypothèse d'une cavité remplie d'eau, hypothèse définitivement confirmée par 11 forages de reconnaissance réalisés en juillet 2010 (Vincent et al., 2010). Cette découverte a conduit les autorités en charge de la gestion du risque à décider d'un drainage de la poche, opération réalisée en 2010 et renouvelée en 2011 et 2012, et d'un suivi temporel notamment géophysique (Figure 1). De nombreuses mesures météorologiques, hydrologiques et géophysiques ont été combinées afin de comprendre la formation et l'évolution de la poche suite aux drainages. Ces travaux ont fait l'objet de nombreuses publications portées sur l'étude de la formation de la poche d'eau et de son évolution (Vincent et al., 2012, 2015), sur les données RMP (Legchenko et al., 2011, 2014) et sur les données géoradar (Garambois et al., 2015). Nous proposons ici de revenir sur ces reconnaissances géophysiques et de leurs apports et limites respectives, depuis la découverte de la poche d'eau en 2007 jusqu'aux reconnaissances effectués jusqu'en 2014

3-D magnetotelluric inversion with coast effect modeling to assess the geothermal potential of Anses d'Arlet (Martinique, Lesser Antilles)

2pWithin the framework of a global French program towards development of renewable energies, Martinique Island (Lesser Antilles, France) has been extensively investigated (from 2012 to 2013) through an integrated multi-disciplinary approach, with the aim to identify precisely the potential geothermal resources previously highlighted (Gadalia et al., 2014). Among the investigation methods deployed (geological, geochemical and hydrogeological), we carried out three magnetotelluric (MT) surve ys at three of the four most promising areas of Martinique, namely the Anses d'Arlet, the Montagne Pelée and the Pitons du Carbet prospects. A total of 32 MT tensors were collected in the Anses d'Arlet area in the frequency range 1000 Hz to 10-2-10-3Hz togetherwith TEM soundings for potential static shift correction. A 3-D MT inversion of the full tensor was carried out including the coast effect. The 3-D resistivity model reveals a major resistive body elongated in the NNO -SSE direction, corresponding to the main structural volcanic axis of the area, and interpreted as a deep intrusion almost reaching the surface. Based on geological observations -an alteration zone located between Anses d'Arlet and Petite Anse-the shallow conductive layer identified eastward is interpreted as the remaining of an old cap-rock partly eroded that becomes thicker southwards. The latter could be related with the altered core of the Morne Larcher. Other studies allowed the reconstructing of the geothermal system evolution to its present and moderate apparent activi ty mainly located south of the resistive anomaly. This intrusion could act as heat source for the geothermal system. This sector is identified as the most interesting for further exploration wells. In order to better understand structures highlighted by the real data distribution and test the strategy to integrate correction coefficients for the coast effect (modeled separately) during the inversion, we designed a forward model using the s ame bathymetry, topography and MT sounding distribution of the survey. Impedance tensors were calculated for the 32 sites. The geometry of the 3-D structure has been designed quasi-independently of the data distribution to assess the impact of un-appropriate acquisition grid. The 3-D inversion was run with these synthetic data (Hautot et al, 2000, 2007). The results indicate that the general structure is recovered for the first 1000 m. Deeper, and southward, the absence of MT soundings surrounding the resistive body prevented a good lateral constraint, causing its disappearance. To the north, the conductive body "pseudo caprock" is well defined, especially on layer 5 (Figure 2), were the apex of a supposed geothermal reservoir could be identified (sites 3 and 6). Deeper the eastern border remains very well constrained by the eastern MT synthetic soundings

3-D Magnetotelluric Investigations for geothermal exploration in Martinique (Lesser Antilles). Characteristic Deep Resistivity Structures, and Shallow Resistivity Distribution Matching Heliborne TEM Results

Within the framework of a global French program oriented towards the development of renewable energies, Martinique Island (Lesser Antilles, France) has been extensively investigated (from 2012 to 2013) through an integrated multi-methods approach, with the aim to define precisely the potential geothermal ressources, previously highlighted (Sanjuan et al., 2003). Amongst the common investigation methods deployed, we carried out three magnetotelluric (MT) surveys located above three of the most promising geothermal fields of Martinique, namely the Anses d'Arlet, the Montagne Pel{\'e}e and the Pitons du Carbet prospects. A total of about 100 MT stations were acquired showing single or multi-dimensional behaviors and static shift effects. After processing data with remote reference, 3-D MT inversions of the four complex elements of MT impedance tensor without pre-static-shift correction, have been performed for each sector, providing three 3-D resistivity models down to about 12 to 30 km depth. The sea coast effect has been taken into account in the 3-D inversion through generation of a 3-D resistivity model including the bathymetry around Martinique from the coast up to a distance of 200 km. The forward response of the model is used to calculate coast effect coefficients that are applied to the calculated MT response during the 3-D inversion process for comparison with the observed data. 3-D resistivity models of each sector, which are inherited from different geological history, show 3-D resistivity distribution and specificities related to its volcanological history. In particular, the geothermal field related to the Montagne Pel{\'e}e strato-volcano, is characterized by a quasi ubiquitous conductive layer and quite monotonic typical resistivity distribution making interpretation difficult in terms of geothermal targets. At the opposite, the resistivity distribution of Anse d'Arlet area is radically different and geothermal target is thought to be connected to a not so deep resistive intrusion elongated along a main structural axis. Beside these interesting deep structures, we demonstrate, after analyzing the results of the recent heliborne TEM survey covering the whole Martinique, that surface resistivity distribution obtained from 3-D inversion reproduce faithfully the resistivity distribution observed by TEM. In spite of a very different sampling scale, this comparison illustrates the ability of 3-D MT inversion to take into account and reproduce static shift effects in the sub-surface resistivity distribution.Comment: Wordl Geothermal Congress 2015, Apr 2015, Melbourne, Australi

Water accumulation dynamics of Tête Rousse glacier revealed by 3D-SNMR measurements

Between 2009 and 2018, we applied the three-dimensional Surface Nuclear Magnetic Resonance method (3D-SNMR) to the monitoring of water accumulation in the Tête Rousse glacier in French Alps. 3D-SNMR data allow for non-invasive observations of water and ice dynamics following three drainages of the accumulated water (2010, 2011 and 2012). We detected two principal reservoir in respectively the downhill and uphill zones of the glacier. After each drainage, meltwater refilled the downhill reservoir but its volume reduced. In 2018, the downhill reservoir contained about 10% of the volume relative that measured in 2010. In 2018, the water volume in the uphill reservoir was about 26,000 m 3. It makes about the half of the volume considered dangerous in 2009 when first detected in the downhill reservoir (50,000 m 3). The total volume assessed for the entire glacier in 2018 was about 31,500 m 3 against 24,800 m 3 measured in 2014. The increase in the volumes of the uphill zone accompanies a change in the geometry of the downhill cavern

Hydrogeological experience in the use of MRS

International audienceNowadays, MRS contribution to characterize aquifers in common conditions down to about 100 meters deep is highly valuable in rocks that exhibit hydraulic behaviour of non-consolidated aquifer at the sounding scale (e.g. sediments, weathered and fissured hard-rocks, densely fissured or highly interstitial porous carbonates). In rocks that exhibit behaviour of fractured aquifer (e.g. low density fractured crystalline basements and limestone, karsts) MRS is a useful complementary method but is not always effective for common engineering studies. In magnetic rocks MRS measurements are often impossible. On the one hand, field experiences reveal that MRS is useful to characterize aquifers. (1) The geometry of saturated aquifer can be estimated in 1D case. Interpolation in-between 1D soundings also reveals 2D geometry if the size of MRS loops is smaller than about half the heterogeneity size. (2) Links between MRS water content and aquifer total porosity and storativity have been found. (3) The transmissivity is accurately estimated from MRS parameters in several geological contexts, using the appropriate conversion equation. (4) Thanks to the integrative property of the sounding, the spatial scale of measurement is considered by hydrogeologists as appropriate for aquifer characterization and modelling. (5) The aquifer characterization is improved when MRS is used in the framework of a hydrogeological methodology and jointly with complementary geophysical methods. On the other hand, field experiences reveal the main limitations encountered in the use of MRS with the actual instrumentation. (1) MRS is not yet self-sufficient to characterize aquifers as the MRS output parameters still need to be compared to hydrogeological properties to achieve quantitative estimation of transmissivity. Storage related parameters are not yet quantitatively accessible from MRS. (2) The electromagnetic noise lowers the signal to noise ratio and makes urban areas but also low interstitial porosity and poorly fractured rocks difficult to survey. (3) Heterogeneity of the magnetic properties of rocks makes measurements impossible or interpretations wrong. (4) Electrically conductive layers reduce the investigation depth of MRS in salty water and clayey environments

Efficiency of joint use of MRS and VES to characterize coastal aquifer in Myanmar

International audienceThe productivity and the water quality of coastal aquifers can be highly heterogeneous in a complex environment. The characterization of these aquifers can be improved by hydrogeological and complementary geophysical surveys. Such an integrated approach is developed in a non-consolidated coastal aquifer in Myanmar (previously named Burma). A preliminary hydrogeological survey is conducted to know better the targeted aquifers. Then, 25 sites are selected to characterize aquifers through borehole drillings and pumping tests implementation. In the same sites, magnetic resonance soundings (MRS) and vertical electrical soundings (VES) are carried out. Geophysical results are compared to hydrogeological data, and geophysical parameters are used to characterize aquifers using conversion equations. Finally, combining the analysis of technical and economical impacts of geophysics, a methodology is proposed to characterize non-consolidated coastal aquifers. Depth and thickness of saturated zone is determined by means of MRS in 68% of the sites (evaluated with 34 soundings). The average accuracy of confined storativity estimated with MRS is ± 6% (evaluated over 7 pumping tests) whereas the average accuracy of transmissivity estimation with MRS is ± 45% (evaluated using 15 pumping tests). To reduce uncertainty in VES interpretation, the aquifer geometry estimated with MRS is used as a fixed parameter in VES inversion. The accuracy of groundwater electrical conductivity evaluation from 15 VES is enough to estimate the risk of water salinity. In addition, the maximum depth of penetration of the MRS depends on the rocks' electrical resistivity and is between 20 and 80 m at the study area