In France, the monitoring of landfill cover after closure of the site is a local problem, since its tightness must be ensured over time. Leaks in the cover are a problem, as they allow water to ...infiltrate the stored waste. In order to locate such leaks, electrical resistivity tomography was used on an experimental site in which defects had been intentionally made in the cover. Repeated measurements taken on this site showed that the weather conditions preceding the measurements need to be taken into account, as they affect the water content in the cover material. They also showed that there are optimal weather conditions for detecting defects in the cover. A statistical analysis carried out on the electrical resistivity results for all surveys and cover material samples showed that the material was heterogeneous; this variability was mainly due to a difference in particle size (fines content) and in compaction.
This study has shown the capacity of electrical resistivity tomography to detect defects and heterogeneity in the cover material, indicating that it is a good means of monitoring the quality of landfill cover both when it is put in place and subsequently.
► Defects in landfill cover need to be detected. ► We monitored a clayey cover by means of electrical resistivity tomography. ► Variations of electrical resistivity had been related to the cover's heterogeneity. ► Its heterogeneity was due to a difference in particle size and compaction. ► The cover's characterization was easier during wet periods.
The Chauvet-Pont d’Arc Cave (Ardèche, France), famous for its remarkable rock art, also contains unique thermal-alterations such as rock spalling and color changes on the walls. These alterations ...resulted from intense fires that have not been observed in the other decorated caves thus far discovered. The functions of these unusual fires challenge archaeologists. To characterize these combustions, we used a numerical tool, previously validated with experimental data, to study the thermo-alterations in the Megaceros Gallery. This unprecedented approach in cave art research enabled us to assess the wood quantities and locations of the hearths responsible for the thermo-alterations. We report here that at least ten fires took place in the Megaceros Gallery while burning more than 170 kg of wood. Both simulation and
in situ
observations suggest that the branches were arranged in a tepee shape and purposefully positioned, some distance from the walls. This method therefore enables further analysis of the functions of these fires.
In 1994, three speleologists discovered the Chauvet–Pont d’Arc Cave, which contains singular thermal marks on walls deep in the cavity. These alterations arose from intense fires, and understanding ...their characteristics would help archaeologists suggest hypotheses about the function of such activities. In this context, three confined fires were conducted in a former underground quarry to reproduce thermo-alterations similar to those in the Chauvet–Pont d’Arc Cave and extract experimental data. Each fire involved approximately 135 kg of wood, which was continuously supplied by firemen for safety reasons (> 500°C) and burnt in the shape of a tepee 80 cm in diameter for 50 min. This paper presents the validation of a numerical model on this experimentation. The modelling requires coupling between the combustion and wall impact simulations. Thus, a link between the combustion code FireFOAM and the thermo-mechanical code Cast3m was created with Python scripts. The results from the simulation agree with the measurements and the observations. More specifically, the analysis is based on the temperatures, gas and particle concentrations, gas velocities, soot deposition, colour changes at the walls and areas likely to spall. These data were collected from thirty-seven measuring points covering the whole quarry. This validated tool will provide information about the features of the fires that occurred within the Chauvet–Pont d’Arc Cave.
SUMMARY
Water migration within the rock mass during a fire has not yet been studied. Here, an innovative high-resolution micro-ERT method is developed at a centimetric scale to measure this ...phenomenon. The method is two-step based: first, a correction of the electrode effect, and second, the correction of resistivity for rapid variations in temperature with high values. Water migration is successfully described within the wall of an underground limestone quarry with three phases: from fire ignition to 36 min, the surface dries to a 3-cm depth and a higher saturated layer forms deeper than 0.10 m; from 36 min to 46 min (fire supply stops), the rock dries out and water migrates from deeper parts of the rock wall, deeper than 20 cm, towards the surface; finally, after fire extinction, the water migrates through a readjustment phase until it returns to its initial state.
SUMMARY
The goal of data classification is to organize them into relevant groups using algorithms. In this study, two clustering algorithms are applied to classify a set of geophysical measurements ...performed around the Lascaux Cave (Dordogne, France). Based on a non-destructive geophysical method, electrical resistivity tomography (ERT), the data (resistivity values) are supposed to characterize the rock mass around the cave.
The rock mass must be divided into an optimal number of homogeneous domains with specific thermal properties to integrate them in the future thermo-aeraulic simulations of the cave. Since the data are georeferenced, a given resistivity cluster corresponds to a specific spatial domain of the rock mass.
This study aims to compare two different clustering methods, the Hierarchical Agglomerative Clustering (HAC) and the K-means methods, on the resistivity data set. Thus, the objective of this study is to determine which of the two methods leads to a partition of the massif with an optimal number of classes, allowing us to find the geological structures partially known thanks to previous studies. The results of these methods are analyzed in light of two different indices, the Silhouette Index (SI) and the Coefficient of Variation (CV). Based on these indices alone, K-means might seem to be the best algorithm, but a fine analysis of each cluster shows that the HAC method gives better results. Indeed, only the HAC method highlights areas already known to be either waterlogged or clayey. Our partial geological knowledge, which is of paramount importance in such a study, also supports the choice of the HAC method.
The study of karst features under a detrital cover is difficult to obtain using punctual traditional tools. This paper presents a 3D geostatistical modeling of ERT (Electrical Resistivity Tomography) ...data to describe undercover karst features. A case study was carried out on the karstic site of the prehistoric decorated Lascaux cave (France). Geophysical measurements were used to define the limit between outcropping Coniacian or Santonian limestones (southwest) and clayey sands (northeast), with a main orientation of N140°. A geometrical description of the scarp was also constructed; pinnacles and notches were found under the clayey sand detrital formation. By combining 3D ERT with geomorphological and geological observations, the geometry of the stratigraphic limit between the Coniacian and Santonian limestone could be determined. This stratigraphic limit separates two domains, one is a potential aquifer, and acts as a feeder for the intermittent spring at the cave entrance, while the other is less permeable, resulting in a permeability contrast with the later rock type. All these observations help define the geological cave environment and ensure better protection for the paintings.
•Undercover karst features are found with 3D ERT geostatistical model.•Geomorphological features found comprises shelf, pinnacles and scarps.•The geological limits are redrawn around the ornamented Lascaux cave.•The top of the Lascaux cave develops within Santonian limestone.•The rest of the cave develops in Coniacian, e.g. the Hall of the Bulls.
The hydrodynamics of karst terrain are highly complex due to the diverse fractures and reservoirs within limestone formations. The time delay between rainfall events and subsequent flow into ...reservoirs exhibits significant variability. However, these hydrological processes are not easily visualized in karst topography. Subsurface geophysics, specifically 2D time-lapse electrical resistivity tomography (ERT), provides an effective method for studying the relationships between hydrological and geophysical features. In our research, we adopted ERT in the Karst Critical Zone (KCZ) to visualize specific karstic zones, including cave galleries, water storage reservoirs, wetting fronts, soil layers, and potential preferential flow paths down to a depth of 20 m. To capture spatial and seasonal variations in resistivity, we presented a comprehensive approach by combining sixteen inversion models obtained between February 2020 and September 2022 above the Villars Cave in SW-France—a well-known prehistoric cave. We used a multi-dimensional statistical technique called Hierarchical Agglomerative Clustering (HAC) to create a composite model that divided the synthetic ERT image into eight clusters representing different karst critical zones. The ERT image clearly visualized the cave gallery with high resistivity values that remained consistent throughout the seasons. Our analysis revealed a close seasonal relationship between water excess and resistivity variations in most infiltration zones, with time delays increasing with depth. The karst reservoirs, located at significant depths compared to other clusters, displayed sensitivity to changes in water excess but were primarily affected by fluctuations in water conductivity, particularly during summer or dry periods. These findings have significant implications for predicting rainwater infiltration pathways into caves, thereby assisting in the conservation and preservation of prehistoric caves and their cultural heritage.
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•ERT identifies different karst critical zones, even at depths of 20 meters.•Employing HAC analysis enables the assessment of seasonal variations within different karst zones associated with excess water.•Visualizing water flow pathways contributes to the conservation and preservation of prehistoric caves.
The Lascaux cave, located in southwest France, is one of the most important prehistoric cave in the world that shows Paleolithic paintings. This study aims to characterize the structure of the ...weathered epikarst setting located above the cave using Time-Lapse Electrical Resistivity Tomography (ERT) combined with local hydrogeological and climatic environmental data. Twenty ERT profiles were carried out for two years and helped us to record the seasonal and spatial variations of the electrical resistivity of the hydraulic upstream area of the Lascaux cave. The 20 interpreted resistivity models were merged into a single synthetic model using a multidimensional statistical method (Hierarchical Agglomerative Clustering). The individual blocks from the synthetic model associated with a similar resistivity variability were gathered into 7 clusters.
We combined the resistivity temporal variations with climatic and hydrogeological data to propose a geo-electrical model that relates to a conceptual geological model. We provide a geological interpretation for each cluster regarding epikarst features. The superficial clusters (no 1 & 2) are linked to effective rainfall and trees, probably a fractured limestone. Another two clusters (no 6 & 7) are linked to detrital formations (sand and clay respectively). The cluster 3 may correspond to a marly limestone that forms a non-permeable horizon. Finally, the electrical behavior of the last two clusters (no 4 & 5) is correlated with the variation of flow rate; they may be a privileged feed zone of the flow in the cave.
•ERT survey is a reliable approach to detect epikarstic features.•Resistivity temporal variability correlates with the flow rate.•Cluster analysis of time-lapse resistivity helps to better identify the ERT model.•Largest variations of resistivity are observed within a privileged feed zone.•A geoelectrical model can be established from the resistivity variations analysis.
•The δ18Op (rainfall δ18O) d-excess, and moisture sources reveal decadal trends in SW-France.•Seasonal and interannual changes of δ18Op not only respond to atmospheric temperature but also ...significantly to moisture sources.•The impact of moisture sources and temperature on δ18Op were quantitatively evaluated.
In the mid-high latitude region, variations of stable isotopic compositions of atmospheric precipitation (δ18Op and δDp) were commonly regarded as reflecting the “temperature effect”. However, some studies have indicated that changes in moisture sources are important controlling factors for δ18Op. To clarify whether there are connections between δ18Op and variations of moisture sources in Southwest France (SW-France), whose implications for speleothem δ18O are of great importance, we have used among the longest isotopic time-series from SW-France (Le Mas and Villars stations) and a 5 days’ reconstruction of air mass history during the 1997–2016 A.D period based on the HYSPLIT tracking model. We found the percentage of initial moisture sources (PIMS) as important factors controlling the oxygen isotope composition of precipitation in SW-France, whether monthly or inter-annual timescale was considered. Additionally, we observed that the δ18Op preserved the signal of local temperature, supporting the “temperature effect”, while no evidence for its “amount effect” has been observed. These quantified links between PIMS/local-temperature and δ18Op appear useful references to understand the link between stable oxygen isotopes and climate parameters. Our long-term monitoring of δ18Op, d-excess, and moisture sources reveals decadal trends, highlighting a tight coupling in hydrologic systems and relatively fast changes on rainfall sources controlled by atmospheric circulations in SW-France.
Inland aquifers are connected to the ocean through permeable coastal sediments. The mixing zone between sea water and fresh-groundwater forms a so-called subterranean estuary. The dynamics and the ...shape of this interface influence fluxes of fresh water to the coastal zone. On tidal sandy beaches, tide and waves actions lead to the formation of an intertidal saline circulation cell that complements the structure of the subterranean estuary between sea water and fresh water. The upper beach corresponds to the recharge zone, where large volumes sea water penetrate the sediment at high tide, whereas the lower beach corresponds to the discharge zone of older pore water that remains in the sand for several tidal cycles. The objective of our study was to characterize the shape and the evolution of this saline cell in a high-energy macro-tidal beach, the Truc Vert beach (SW France). In order to assess the distribution of saline, brackish and fresh water in this high-energy beach aquifer, we conducted electrical resistivity tomography (ERT) measurements from the lower beach face to the sand dune in winter 2012–2013. We also measured water table elevation and salinity in three piezometers situated at the base of the dune and in pore waters at low tide on tidal cross-shore transects. Results show that the intertidal saline plume can be identified and localized with the ERT method. The fresh-saline water interface is almost vertical in the upper beach. The saline circulation cell occupies the upper part of the tidal beach over a thickness of 5 m deep. Below this saline cell, a brackish water flows to the ocean: about 30% of fresh water was found in pore water in the lower part of the beach. Shifting of the vertical front between saline and fresh waters of about 2–5 m occurred during the spring–neap tidal cycle. This migration is small relative to the beach size, suggesting that the transition zone is relatively stable. The variations in salinity of the circulation cell suggest that the position of the saline plume is controlled by the spring tide level. Our results differ slightly from studies based on modelling and piezometric measurement made in less exposed micro-tidal sandy beaches. Our data would allow to better constrain numerical models that would enable to quantify water residence time and solute fluxes in subterranean estuaries of high-energy beaches.
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