Geophysical study to characterize input karst water circulation in the saulges cave (Mayenne, France)

Introduction The landscape around the Saulges caves is a limestone plateau cut by the valley of the river Erve. Some of the caves in the sides of this valley contain prehistoric art (Pigeaud, 2004; Pigeaud et al., 2006) and should therefore be protected from water trickling down the walls. This study is part of a program: “Occupations paléolithiques de la vallée de l’Erve” conducted by UMR 6566 for CNRS “CReAAH”, coordinated by Jean-Laurent Monnier. Geophysics could be useful in finding new c..

Caractérisation structurale de morphologies karstiques superficielles et suivi temporel de l'infiltration à l'aide des méthodes électriques et sismiques

All the sites studied in this thesis, which are situated in tabular regions, allowed us to observe and analyze from a geophysical point of view a diversity of morphologies associated with karst plateaus and karsts undercover: outflow mounds and suffosion sinkholes in eastern France, buried and dissolution sinkholes in the Causses du Quercy, epikarst and weathering in the Causse du Larzac. The electrical resistivity and seismic refraction tomographies were very useful for characterizing the geometry of such objects. The contribution of 3D seismic refraction has been significant for imaging fracturing and weathering features in the epikarst. Electrical and seismic time-lapse measurements was also realized on a karst aquifer in south France and on two epikarstic areas on the Causse du Larzac. Time-lapse resistivity tomography was not very effective, but time-lapse seismic refraction tomography shows a high potential to identify infiltration and storage areas, particularly in the epikarst. New results were obtained: (1) the understanding of karstic morphologies by analyzing their structure, (2) the structure and functioning of the epikarst, (3) the effectiveness of electrical and seismic methods for structural imaging and water saturation monitoring in karstic systems.Tous les sites étudiés lors de cette thèse, qui se situent tous dans des régions tabulaires ou monoclinales, ont permis d'observer et d'analyser d'un point de vue géophysique une diversité de morphologies associées aux karsts de plateaux et aux karsts sous couverture : tertres émissifs et dolines de suffusion dans l'Est de la France, dolines de dissolution et dolines masquées dans les Causses du Quercy, épikarst et altération sur le Causse du Larzac. Les tomographies de résistivité électrique et de sismique réfraction ont en effet été très utiles pour caractériser la géométrie de ces objets. L'apport de la sismique réfraction 3D s'est révélée considérable pour l'imagerie de la fracturation et de l'altération dans l'épikarst. Un suivi temporel électrique et sismique a également été mis en place sur un aquifère karstique en Corrèze et sur deux secteurs épikarstiques sur le Causse du Larzac. Si le suivi par tomographie de résistivité ne s'est pas avéré très efficace, le suivi par tomographie de sismique réfraction montre un potentiel conséquent pour identifier les zones de stockage et les zones d'infiltration, particulièrement dans l'épikarst. Des résultats nouveaux ont été obtenus dans : (1) la compréhension des morphologies karstiques par l'analyse de leur structure, (2) la structure et le fonctionnement de l'épikarst, (3) l'efficacité des méthodes électriques et sismiques pour l'imagerie structurale et le suivi temporel en milieu karstique

Caractérisation structurale de morphologies karstiques superficielles et suivi temporel de l'infiltration à l'aide des méthodes électriques et sismiques

All the sites studied in this thesis, which are situated in tabular regions, allowed us to observe and analyze from a geophysical point of view a diversity of morphologies associated with karst plateaus and karsts undercover: outflow mounds and suffosion sinkholes in eastern France, buried and dissolution sinkholes in the Causses du Quercy, epikarst and weathering in the Causse du Larzac. The electrical resistivity and seismic refraction tomographies were very useful for characterizing the geometry of such objects. The contribution of 3D seismic refraction has been significant for imaging fracturing and weathering features in the epikarst. Electrical and seismic time-lapse measurements was also realized on a karst aquifer in south France and on two epikarstic areas on the Causse du Larzac. Time-lapse resistivity tomography was not very effective, but time-lapse seismic refraction tomography shows a high potential to identify infiltration and storage areas, particularly in the epikarst. New results were obtained: (1) the understanding of karstic morphologies by analyzing their structure, (2) the structure and functioning of the epikarst, (3) the effectiveness of electrical and seismic methods for structural imaging and water saturation monitoring in karstic systems.Tous les sites étudiés lors de cette thèse, qui se situent tous dans des régions tabulaires ou monoclinales, ont permis d'observer et d'analyser d'un point de vue géophysique une diversité de morphologies associées aux karsts de plateaux et aux karsts sous couverture : tertres émissifs et dolines de suffusion dans l'Est de la France, dolines de dissolution et dolines masquées dans les Causses du Quercy, épikarst et altération sur le Causse du Larzac. Les tomographies de résistivité électrique et de sismique réfraction ont en effet été très utiles pour caractériser la géométrie de ces objets. L'apport de la sismique réfraction 3D s'est révélée considérable pour l'imagerie de la fracturation et de l'altération dans l'épikarst. Un suivi temporel électrique et sismique a également été mis en place sur un aquifère karstique en Corrèze et sur deux secteurs épikarstiques sur le Causse du Larzac. Si le suivi par tomographie de résistivité ne s'est pas avéré très efficace, le suivi par tomographie de sismique réfraction montre un potentiel conséquent pour identifier les zones de stockage et les zones d'infiltration, particulièrement dans l'épikarst. Des résultats nouveaux ont été obtenus dans : (1) la compréhension des morphologies karstiques par l'analyse de leur structure, (2) la structure et le fonctionnement de l'épikarst, (3) l'efficacité des méthodes électriques et sismiques pour l'imagerie structurale et le suivi temporel en milieu karstique

Assessment of doline geometry using geophysics on the Quercy plateau karst (South France).

In the protection of karstic aquifers, the role played by dolines is of interest as these shallow structures can be preferential infiltration pathways for groundwater recharge. In this study, three depressions in South France (Orniac, Lot) were identified and characterized using combined geophysical methods. Apparent conductivity mapping allows precise definition of dolines corresponding to topographic depressions. It also reveals other dolines that are hidden by soil cover. Electrical resistivity and seismic refraction tomography provide detailed cross-sections. Even though the dolines studied are close one to another, they exhibit great variability in shape and content. Moreover, time-lapse data show that the most saturated regions are on the edges of the first doline. Such information helps to evaluate human impacts and forecast potential groundwater contamination. Copyright © 2011 John Wiley & Sons, Ltd

Caractérisation structurale de morphologies karstiques superficielles et suivi temporel de l infiltration à l aide des méthodes électriques et sismiques

Tous les sites étudiés lors de cette thèse, qui se situent tous dans des régions tabulaires ou monoclinales, ont permis d observer et d analyser d un point de vue géophysique une diversité de morphologies associées aux karsts de plateaux et aux karsts sous couverture : tertres émissifs et dolines de suffosion dans l Est de la France, dolines de dissolution et dolines masquées dans les Causses du Quercy, épikarst et altération sur le Causse du Larzac. Les tomographies de résistivité électrique et de sismique réfraction ont en effet été très utiles pour caractériser la géométrie de ces objets. L apport de la sismique réfraction 3D s est révélée considérable pour l imagerie de la fracturation et de l altération dans l épikarst. Un suivi temporel électrique et sismique a également été mis en place sur un aquifère karstique en Corrèze et sur deux secteurs épikarstiques sur le Causse du Larzac. Si le suivi par tomographie de résistivité ne s est pas avéré très efficace, le suivi par tomographie de sismique réfraction montre un potentiel conséquent pour identifier les zones de stockage et les zones d infiltration, particulièrement dans l épikarst. Des résultats nouveaux ont été obtenus dans : (1) la compréhension des morphologies karstiques par l analyse de leur structure, (2) la structure et le fonctionnement de l épikarst, (3) l efficacité des méthodes électriques et sismiques pour l imagerie structurale et le suivi temporel en milieu karstique.PARIS-BIUSJ-Sci.Terre recherche (751052114) / SudocSudocFranceF

Assessment of water resources to support the development of irrigation in northwest Cambodia: a water budget approach

International audienceWater availability is the primary constraint on the improvement of food security in rural areas in northwestern Cambodia. A 4-year study was carried out in the upper Stung Sreng watershed to assess water resources. Four sub-watersheds with different land cover types, ranging in size from 1.5 to 185 km2, were monitored using dedicated weather stations and rain- and streamgauges. Geophysics and observation boreholes were used to characterize aquifers. Rainwater is mostly split into evapotranspiration (annual mean of 54% rainfall) and streamflow components (49%), because groundwater recharge is low (1%). Thus, rainwater and streamflow are the main sources for irrigation development. Groundwater can be used only in specific locations for low water-demand crops. A total of 186 household ponds and three village-scale dams were built and 31 wells were installed. The household pond was determined to be the best solution for irrigation development because of its simple management

Technical Note: Analytical Solution for Well Water Response to Earth Tides in Leaky Aquifers with Storage and Compressibility in the Aquitard

International audienceAbstract. In recent years, there has been a growing interest in utilizing the groundwater response to Earth tides as a means to estimate subsurface properties. However, existing analytical models have been insufficient in accurately capturing realistic physical conditions. This study presents a new analytical solution to calculate groundwater response to Earth tide strains, including storage and compressibility of the aquitard, borehole storage and skin effects. We investigate the effects of aquifer and aquitard parameters on well water response to Earth tides at two dominant frequencies (O1 and M2) and compare our results with hydraulic parameters obtained from a pumping test. Inversion of the six hydro-geomechanical parameters from amplitude response and phase shift of both semi-diurnal and diurnal tides provides relevant information about aquifer transmissivity, storativity, well skin effect, aquitard hydraulic conductivity and diffusivity. The new model is able to reproduce previously unexplained observations of the amplitude and frequency responses. We emphasize the usefulness in developing relevant methodology to use the groundwater response to natural drivers for characterizing hydrogeological systems

Mapeamento de reservas de águas subterrâneas no noroeste do Camboja com uso combinado de dados litológicos e sondagens eletromagnéticas de domínio de tempo e por ressonância magnética

International audienceLack of access to water is the primary constraint to development in rural areas of northwestern Cambodia. Communities lack water for both domestic and irrigation purposes. To provide access to drinking water, governmental and aid agencies have focused on drilling shallow boreholes but they have not had a clear understanding of groundwater potential. The goal of this study has been to improve hydrogeological knowledge of two districts in Oddar Meanchey Province by analyzing borehole lithologs and geophysical data sets. The comparison of 55 time-domain electromagnetic (TEM) soundings and lithologs, as well as 66 magnetic-resonance soundings (MRS) with TEM soundings, allows a better understanding of the links between geology, electrical resistivity and hydrogeological parameters such as the specific yield (Sy) derived from MRS. The main findings are that water inflow and Sy are more related to electrical resistivity and elevation than to the litholog description. Indeed, conductive media are associated with a null value of Sy, whereas resistive rocks at low elevation are always linked to strictly positive Sy. A new methodology was developed to create maps of groundwater reserves based on 612 TEM soundings and the observed relationship between resistivity and Sy. TEM soundings were inverted using a quasi-3D modeling approach called ‘spatially constrained inversion’. Such maps will, no doubt, be very useful for borehole siting and in the economic development of the province because they clearly distinguish areas of high groundwater-reserves potential from areas that lack reserves

Application of combined time‐lapse seismic refraction and electrical resistivity tomography to the analysis of infiltration and dissolution processes in the epikarst of the Causse du Larzac (France)

International audienceIn geophysical and hydrological studies, it can be challenging to achieve a complete understanding of infiltration processes in the upper zone of a karstic aquifer since this type of medium is highly heterogeneous and may contain a perched aquifer. In an effort to further investigate such aquifers, time‐lapse electrical resistivity tomography and time‐lapse seismic refraction tomography were carried out at three different epochs and at two sites on a dolostone plateau of Southern France. The first site has a sinkhole, whereas the second covers a less efficiently drained area, which is characterized by temporary lakes during periods of heavy rain.These studies show that shallow time‐lapse electrical resistivity tomography and time‐lapse seismic refraction tomography analyses are correlated with hydrological data because resistivities and propagation velocities decrease with increasing rainfall. Nevertheless, the Biot–Gassmann relationship does not provide an adequate explanation for the strong variations in velocity observed in the upper parts of the models. This could be explained by the dissolution process called ghost‐rock weathering, which directly attacks the dolostone rock frame of the studied area. The assessment of such processes can lead to an improved understanding of velocity variations and to the localization of dissolution processes that may affect karstic landscapes or even lead to their collapse

Особенности морфологии эпигенетических минералов продуктивного пласта ЮС[2] нефтяного месторождения

International audienceThe karst environment is one of the most challenging in terms of groundwater, engineering and environmental issues. Geophysical methods can provide useful subsurface information in karst regions concerning , for instance, hazard estimation or groundwater exploration and vulnerability assessment. However, a karst area remains a very difficult environment for any geophysical exploration; selection of the best-suited geophysical method is not always straightforward, due to the highly variable and unpredictable target characteristics. The state of the art is presented, in terms of the contributions made by geophysical methods to karst-system exploration, based on extensive analysis of the published scientific results. This report is an overview and should be used as a preliminary meth-odological approach, rather than a guideline