Éditeur(s) : HAL CCSD Oxford University Press (OUP)
Résumé : International audience
Previous work has demonstrated that geoelectrical measurements, acquired either along the Earth’s surface or in boreholes, can be sensitive to the presence of fractures. However, a lack of numerical approaches that are well suited to modelling electric current flow in fractured media prevents us from systematically exploring the links between geoelectrical measurements and fractured rock properties. To address this issue, we present a highly computationally efficient methodology for the numerical simulation of geoelectrical data in 2.5-D in complex fractured domains. Our approach is based upon a discrete-dual-porosity formulation, whereby the fractures and rock matrix are treated separately and coupled through the exchange of electric current between them. We first validate our methodology against standard analytical and finite-element solutions. Subsequent use of the approach to simulate geoelectrical data for a variety of different fracture configurations demonstrates the sensitivity of these data to important parameters such as the fracture density, depth, and orientation.
ISSN: 0956-540X

hal-01622151
https://hal.archives-ouvertes.fr/hal-01622151
https://hal.archives-ouvertes.fr/hal-01622151/document
https://hal.archives-ouvertes.fr/hal-01622151/file/sanzGJI2017.pdf
DOI : 10.1093/gji/ggx080

Éditeur(s) : HAL CCSD
Résumé : International audience
Understand the dynamics of the fresh-salt water interface in aquifers is a key issue to comprehend mixing processand to quantity the discharge of nutrients in to coastal areas. In order to go beyond the current knowledge inthis issue an experimental site has been set up at the alluvial aquifer Riera Argentona (Barcelona – Spain). Thesite comprises 16 shallow piezometers installed between 30 and 90 m from the seashore, with depths rangingbetween 15 and 25 meters. The seawater interface is being monitored using several techniques, the combinationof which will help us to understand the spatial and temporal behaviour of the mixing zone and the geochemicalprocesses occurring there. Specially the deepest piezometers are equipped with electrodes in order to performcross-hole electrical resistivity tomography (CHERT). In addition, all piezometers are also equipped with FiberOptic cable to perform distributed temperature measurements. Two single steel armoured fibre optic cable lines ofaround 600m length were installed in all boreholes. The objective is to use the cable both as passive and activetemperature sensor. The first is being done for the continuous monitoring of temperature whereas; the secondprovides a higher temperature resolution used to monitor field experiments.Periodic CHERT measurements are carried out between the piezometer equipped with electrodes, resultingin parallel and perpendicular vertical cross sections of the site resistivity. The position of the fresh-salt waterinterface can be identified due to the resistivity contrast between the saline and fresh water. Preliminary resultsof periodic distributed temperature measurements will be also be used to monitor the position of the mixing zonethanks to the contrast and seasonal temperature changes. Periodic down-hole EC profiles will be used to validatethe method.AcknowledgementsThis work was funded by the projects CGL2013-48869-C2-1 y CGL2013-48869-C2-2-R of the Spanish Government.We would like to thank SIMMAR (Serveis Integrals de Manteniment del Maresme) and the ConsellComarcal del Maresme in the construction of the research site
European Geosciences Union General Assembly 2016
Vienne, Austria

insu-01308757
https://hal-insu.archives-ouvertes.fr/insu-01308757

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
The main goal of the monitoring stage of Carbon Capture and Storage (CCS) is to obtain an accurate estimation of the subsurface CO2 accumulation and to detect any possible leakage. Laboratory experiments are necessary to investigate the small scale processes governing the CO2–brine–rock interaction. They also provide a means to calibrate the results coming from field scale geophysical methods. In this work we set up an experimental system which is able to perform Electrical Resistivity Tomography (ERT) measurements on centimeter-scale rock samples at various P–T conditions. We present the results of two new experiments related to CO2 monitoring, performed on a cylindrical (4 × 8 cm) Fontainebleau rock sample. In the first one, we have quantified the CO2 saturation at different volume fractions, representing zones from a deep saline aquifer with varying degrees of saturation. In the second one, we have monitored and quantified the effect of CO2 dissolution in the brine at a pressure of 40 bar during eight days, emulating the invasion of CO2 into a shallow aquifer. Results highlight the importance of accounting for the contribution of surface conductivity in highly CO2-saturated regions, even in clay-free rocks, and also for brine conductivity variation due to CO2 dissolution. Ignoring any of these effects will end up in a CO2 saturation underestimation. We present a modified CO2 saturation equation to account for these two influences.
ISSN: 0926-9851

hal-01356323
https://hal.archives-ouvertes.fr/hal-01356323
DOI : 10.1016/j.jappgeo.2016.03.039

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
An original karstic system in a coastal alluvial plain located in a carbonate environment (Cassis, Provence -- SE France) was studied using a multi-method approach (electrical resistivity tomography, gravimetric, passive seismic survey), combined with geomorphologic methods, surface observations and drilling. Limestone dissolution and/or cavity collapse led to the development of a polje, which was filled rapidly by erosion of Aptian marls. The combination of several dating and paleoenvironmental proxies indicates that the polje was filled during a glacial period (MIS 6, 8 or 10, i.e. in the 130-360 ka age range). This discovery has implications for the understanding of karst processes. The connection between the polje and the three km-long Bestouan underwater karstic conduit with submarine outlet is strongly suggested by sedimentological studies and geophysical prospections.
ISSN: 0169-555X

hal-01054334
https://hal.archives-ouvertes.fr/hal-01054334
DOI : 10.1016/j.geomorph.2014.02.021

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
Active faults from the Campo de Dalías (SE Betic Cordillera) allow us to constrain the deformation styles involved in the development of segmented oblique-slip faults. This sector constitutes the widest outcrop of Plio-Quaternary sediments in the northern boundary of the Alboran Sea. It has emerged since the Late Pliocene, and therefore provides recent deformation markers that are not disturbed by erosive processes. The faults started to grow during the Pleistocene, reactivating previous hybrid joints, with a normal-dextral slip. We present a detailed map of the largest fault in the area, the Loma del Viento fault, comprising six onshore segments. Based on field work and aerial photography, the distributions of the contiguous joints have been mapped, and the joints reactivated as faults are identified. Some of these fault segments are hard-linked, and fault slip enhances toward the linkage sectors between them with associated sedimentary depocenters. An electrical tomography profile reveals the wedge geometry of a unit of Pleistocene conglomerates and red silts that were coevally deposited during the fault movement. Long-term slip rate in the central part of the fault is estimated at 0.07 ± 0.03 mm/y. In addition, a seismic crisis nucleated close to the Loma del Viento fault during November 2010 was recorded. Moment tensor analysis of the two mainshocks (Mw 3.5 and 4.2) provides a focal solution indicating a N120°E striking right-lateral strike-slip fault. The corrugated morphology of the Loma del Viento fault may have influenced its seismic behavior. Some of the fault segments are oblique to the general motion of the fault. These oblique segments would provide higher resistance against the general fault motion and could lock the fault, leading to accumulate elastic energy.
ISSN: 0040-1951

hal-00750357
https://hal.archives-ouvertes.fr/hal-00750357
DOI : 10.1016/j.tecto.2011.12.001