IRIS Instruments (France)

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Summary Under the pressure of climate change, reduced precipitation, including snowfall, which makes a major contribution to groundwater recharge, inevitably leads to lower water levels in both alluvial and crystalline basement aquifers. The ground water available is actually getting deeper and deeper. What technical approaches are available to support integrated management of groundwater resources? This paper illustrates the use of a nodal electrical resistivity tomography (ERT) system in Italy aiming for helping the setting of potential zones for drilling. The survey covers an area of about 95 hectares for maximizing the exploration depth down to about 425 meters from ground level. Together with geological knowledge of the area, newly collected measurements, make it possible to highlight mineralogical/textural, geological-structural variations and tectonic discontinuities in terms of subsurface electrical resistivity. The area is characterized by the outcrop of predominantly Tertiary (pre-Orogenetic) and only minimally Quaternary (post-Orogenetic) rocks.The complex geology and the poor access forced the scientist to choose a unconventional method based on the FullWaver system and ErtLab Studio software. The ERT and induced polarization survey results gave enough evidences to the stake holders for selecting three potential zones.

<p>A telescope based on scintillators technology has been installed at the footstep of giant cliffs for assessing massive rocks falls. Such an experience is a first in the “world of muography”. It was made possible thanks to the national geological service, the BRGM, in particular its regional branch based in Reunion Island. Muography was chosen because it allows to access the density variation in time and space, in a passive way, by collecting in the telescope the muons which crossed the rock. The telescope has been installed for up to 6 months at the footwall of the giant cliff.  The rainy season was chosen as the acquisition window to be able to follow the density variations that occur in the massif during rainy events.</p><p>The telescope is composed of 3 parallel ~1m<sup>2</sup> active detection planes recording the positions and the precise time of the particles hits. The detector readout has been developed on the early concept of connected “smart sensors”. It allows an optimized selection of the particles hits to perform their tracking. A post-processing analysis will translate the recorded tracking properties into a detailed image of the target within the acceptance of the detector.</p><p>La Reunion Island, located East of Madagascar, is composed of three shield volcanoes among with la Fournaise which is still active. The volcanic cirques are subjected to large-scale rock-falls (>10,000 m<sup>3</sup>). </p><p>On the top of the wall (or Rempart), the target of our muography experiment, the decompression cracks are concentrated on a strip often equivalent to 10% of the height of the cliff. The cliff can be higher than 1,000 m. These cracks, sometimes more than a meter wide, delimit the rock scales likely to be crumbled. The origin of these cracks, which are almost vertical on the surface, is linked to the natural decompression of the massif by the vacuum. The geometry of these cracks at depth is not well known, but it is likely that they acquire a slightly concave shape, bringing them closer to the wall and cutting out large scales at the crest of the Rempart. The volume of rock falling highly depends on the depth of these cracks.</p><p>Our experiment is focused on the Maïdo Rempart overlooking the western part of the Cirque of Mafate where the formations of the ancient volcanic outcrop in 1,000 m high scarps. We have installed a Muons telescope at “l’Ilet de Roche-Plate”, a small village located at the foot of active scree cones at the foot of the Maïdo Rempart. This innovative experiment follows a fire that occurred on the top of the Rempart at the end of 2020, which led to an increase in falling blocks and a potential acceleration in the opening of cracks. One of the issues is to better delimit the volume of "fractured" rocks and if possible, to identify the depth of the decompression cracks that delimit the scales likely to fall. </p>

PreviousNext No AccessSymposium on the Application of Geophysics to Engineering and Environmental Problems 2021Dense 3D ERT to better understand deep landslide structuresAuthors: Julien GanceCatherine TruffertMyriam LajaunieJean-Philippe MaletOrlando LeiteFabrice VermeerschJulien GanceIRIS Instruments, Catherine TruffertIRIS Instruments, Myriam LajaunieEOST France, Jean-Philippe MaletEOST France, Orlando LeiteIRIS Instruments, and Fabrice VermeerschIRIS Instrumentshttps://doi.org/10.4133/sageep.33-081 SectionsAboutPDF/ePub ToolsAdd to favoritesDownload CitationsTrack CitationsPermissions ShareFacebookTwitterLinked InRedditEmail Abstract This paper demonstrates the relevance of 3D Electrical Resistivity Tomography to better understand complex landslide structures. Keywords: apparent resistivity, tomography, inversion, resistivity, porosityPermalink: https://doi.org/10.4133/sageep.33-081FiguresReferencesRelatedDetails Symposium on the Application of Geophysics to Engineering and Environmental Problems 2021ISSN (online):1554-8015Copyright: 2021 Pages: 349 publication data© 2021 Published in electronic format with permission by the Society of Exploration Geophysicists and Environment and Engineering Geophysical SocietyPublisher:Environmental & Engineering Geophysical Society HistoryPublished: 11 Jun 2021 CITATION INFORMATION Julien Gance, Catherine Truffert, Myriam Lajaunie, Jean-Philippe Malet, Orlando Leite, and Fabrice Vermeersch, (2021), "Dense 3D ERT to better understand deep landslide structures," Symposium on the Application of Geophysics to Engineering and Environmental Problems Proceedings : 162-165. https://doi.org/10.4133/sageep.33-081 Plain-Language Summary Keywordsapparent resistivitytomographyinversionresistivityporosityPDF DownloadLoading ...

The use of indirect electrical techniques is gaining interest for monitoring the corrosion of steel in concrete as they do not require any connection to the rebar. In this paper, we provide insights into the physical aspects of the indirect galvanostatic pulse (GP) method in the Wenner configuration. Considering uniform corrosion, the instantaneous ohmic drop is decreased due to the presence of the rebar, which acts as a short-circuit. However, we observed that this phenomenon is independent of the electrochemical parameters of the Butler–Volmer equation. They are, however, responsible for the nonlinear decrease of the current that polarizes the rebar over time, especially for a passive rebar due to its high polarization resistance. This evolution of the resulting potential difference with time is explained by the increase of the potential difference related to concrete resistance and the global decrease of the potential difference related to the polarization resistance of the rebar. The indirect GP technique is then fundamentally different than the conventional one in three-electrode configuration, as here the steady-state potential is not only representative of polarization resistance but also of concrete resistance. Considering non-uniform corrosion, the presence of a small anodic area disturbs the current distribution in the material. This is essentially due to the different capability of anodic and cathodic areas to consume the impressed current, resulting in slowing down the evolution of the transient potential as compared to uniform corrosion. Hence, highly corroding areas have a greater effect on the transient potential than on the steady-state one. The use of this temporal evolution is thus recommended to qualitatively detect anodic areas. For the estimation of their length and position, which is one of the main current problematic issue when performing any measurement on reinforced concrete (RC) structures with conventional techniques, we suggest adjusting the probe spacing to modulate the sensitivity of the technique.

The MUST2 (MUon Survey Tomography based on Micromegas detectors for Unreachable Sites Technology) camera is based on a thin Time Projection Chamber read by a resistive Micromegas. This innovative combination presents interesting distinctive features compared to existing muon detection technologies. It allows a wide angular acceptance of the detector with a low weight and compact volume, well adapted for confined spaces or underground operation. The current work presents the results obtained during the calibration measurements at the reference site, the Low Background Noise Laboratory (LBNL). Preliminary results from field measurement campaign carried out at the dam overlooking the village of Saint-Saturnin-les-Apt (South-East of France) are presented and discussed.

Within the framework of a groundwater survey for camel breeding purposes, electrical resistivity soundings have been carried out for delineating the depth and the lateral extension of an aquifer layer in the Dhar Nema area located in the South‐East part of Mauritania. The geology basically consists in sand and sandstones, with clay occurrences. In the middle part of the aquifer, the DC resistivity soundings could point out the presence of water. However, in the edge parts, they could not clearly make the difference between dry rocks and fresh water aquifer layers. On the contrary, the magnetic resonance soundings could identify the presence of water at depths of 60 to more than 100m, and gave estimations of the values of the porosity and of the permeability. The low EM noise levels and the significant quantity of water existing into the ground explain the good quality of the data. As a whole, more than 60 MRS soundings have been carried out in the area of approximately 150 × 100 km, and permitted to delineate the limits of the aquifer layer. The first drill‐hole confirms the depths determined with the magnetic resonance soundings. A campaign of new drill‐holes with pumping tests is planed to ascertain the groundwater resources of this area.

The Magnetic Resonance Sounding method (MRS) has been used in the past years with success in various geological and geographical contexts for groundwater surveys. This method has indeed the ability of directly detecting the presence of water through the excitation of the hydrogen protons of water molecules.The frequency to which the H protons react depends on the magnitude of the Earth magnetic field, while the intensity of the excitation determines the depth of investigation. The amplitude of the magnetic field generated in return by the water of a layer is proportional to the porosity of this layer, and the time constant of the relaxation curve is linked to the mean pore size of the material, that is to say tightly related to its permeability.A loop laid on the surface of the ground is used for both transmitting the excitation pulse and measuring the response of the H protons. The linear relation between the measured signal and the layer porosity permits to interpret the ID sounding as soon as the readings have been collected in the field.The main applications of this method concern the determination of the water level and of the total quantity of water available down to 100 to 150 m depths. Magnetic Resonance Soundings can also help to select the best place for drilling, to predict a yield using a calibration, and to determine the geometry of an aquifer layer for hydrogeological modellingA set of field examples acquired in various countries (Africa, Asia, Europe) points out both the advantages and the limitations of this method and suggests the place it should take among other geophysical methods in the methodology of groundwater investigations.

In the second sentence of section 5.5 (Inversion of a sphere response)the infinite conductive space is given incorrectly as 5 Ω m. This value should be 50 Ω m.