More than 80 Mm3per year are pumped into the Roussillon plain coastal aquifer located between the Pyrenean massif to the South and West, and the Mediterranean to the East. This is a 350 m thick Pliocene multilayered aquifer, with sandy layers embedded in low-permeability clayey material and topped by Quaternary alluvial deposits. The groundwater resource is primarily used for drinking water and irrigation. For more than 40 years, this aquifer has been undergoing a piezometric level decline due to pumping, while water demand is expected to increase with ongoing climate change, sea level rise and increasing demand in water use. Consequently, the Roussillon aquifer is likely to suffer from sea water intrusion and marine submersion in the near future. As part of the Dem'Eaux Roussillon project, a set of downhole geophysical profiles was recorded at three drill sites, in Saint Cyprien and Barcares along the Mediterranean shore and at Pollestres, 14 km inland. Downhole petrophysical data (NMR porosity and permeability, acoustic velocities, electrical resistivity and spectral gamma natural radioactivity) contribute to better define the penetrated structure. Core petrophysical measurements were also made to support and calibrate these analyses and, in particular, to provide a dm-scale description of the subsurface pore fluid electrical conductivity along the length of each hole. A combined analysis of the latter with porosity and permeability points at incipient and m-scale intrusion processes along discreate horizons. Repeated downhole measurements overtime and fluid sampling provide a means to follow the dynamics of these intrusion processes found to be more acute at the Barcarès site to the North than close to the Pyrenean, at Saint Cyprien. While high-resolution permanent downhole geophysical observatories have been installed to measure at high frequency both formation electrical resistivity and temperature, a real time management of groundwater resources should contribute to improve aquifer water quality in the future. These observatories datasets will be analyzed and modeled on the basis of smaller scale petrophysical data, providing both an assessment of water quality evolution in terms of salinity from resistivity and quantity in terms of flow rate from temperature.

More than 80 Mm3per year are pumped into the Roussillon plain coastal aquifer located between the Pyrenean massif to the South and West, and the Mediterranean to the East. This is a 350 m thick Pliocene multilayered aquifer, with sandy layers embedded in low-permeability clayey material and topped by Quaternary alluvial deposits. The groundwater resource is primarily used for drinking water and irrigation. For more than 40 years, this aquifer has been undergoing a piezometric level decline due to pumping, while water demand is expected to increase with ongoing climate change, sea level rise and increasing demand in water use. Consequently, the Roussillon aquifer is likely to suffer from sea water intrusion and marine submersion in the near future. As part of the Dem'Eaux Roussillon project, a set of downhole geophysical profiles was recorded at three drill sites, in Saint Cyprien and Barcares along the Mediterranean shore and at Pollestres, 14 km inland. Downhole petrophysical data (NMR porosity and permeability, acoustic velocities, electrical resistivity and spectral gamma natural radioactivity) contribute to better define the penetrated structure. Core petrophysical measurements were also made to support and calibrate these analyses and, in particular, to provide a dm-scale description of the subsurface pore fluid electrical conductivity along the length of each hole. A combined analysis of the latter with porosity and permeability points at incipient and m-scale intrusion processes along discreate horizons. Repeated downhole measurements overtime and fluid sampling provide a means to follow the dynamics of these intrusion processes found to be more acute at the Barcarès site to the North than close to the Pyrenean, at Saint Cyprien. While high-resolution permanent downhole geophysical observatories have been installed to measure at high frequency both formation electrical resistivity and temperature, a real time management of groundwater resources should contribute to improve aquifer water quality in the future. These observatories datasets will be analyzed and modeled on the basis of smaller scale petrophysical data, providing both an assessment of water quality evolution in terms of salinity from resistivity and quantity in terms of flow rate from temperature.

The Balaruc hydrothermal system is fed both from surrounding karstic carbonates with fresh water outpouring in the nearby Thau lagoon with the Vise source, and at depth along deep regional faults with local springs up to 50°C. This hydrothermal system was cored and logged in 2020-2021 down to 765m depth at Balaruc-les-Bains, 200m to the NE of the Vise source. During the project, the Vise source underwent a reversal in November 2020, with the Thau lagoon salty water being drained deep into the subsurface, shedding light into the complex processes affecting the Balaruc hydrothermal system. The DEM’EAUX THAU project is aiming at a better understanding of this complex geological and hydrological system for a more sustainable use of this resource. A set of downhole geophysical data and borehole wall images was recorded from near surface to 756m depth in 4 vertical holes, only a few meters apart at surface. While mm-scale images reveal the detailed geological structure, petrophysical data (acoustic velocities, electrical resistivity and natural gamma) contribute to better define the penetrated structure, yielding porosity and permeability. Acoustic velocities provide a base to analyze the vertical (VSP) and walk-away seismic profiles shot to replace these holes in the regional geological structure. In turn, core petrophysical measurements are being made to support these analyses and, in particular, to provide a dm-scale description of the subsurface pore fluid salinity. In addition, the physical and chemical properties of the borehole fluid were characterized with an Idronaute probe, showing the impact of the Vise source reversal from measurements before and after. Similarly, electrical resistivity profiles were recorded over time and during downhole pumping tests, emphasizing the hydraulic vertical connectivity. On that basis, pore fluids dynamics are being described from time-lapse downhole logging measurements and the emplacement of permanent downhole geophysical observatories consisting in (i) an optical fiber for temperature and (ii) a flute for electrical resistivity of the formation. In the future, a second optical fiber already in place will be used for acoustic probing of subsurface fluid flow from Digital Acoustic Scanning (DAS).

The Pegairolles-de-l'Escalette landslide is located in the central part of the Languedoc Region (Southern France). It corresponds to a deep-seated landslide (> 50 m of thickness) with extremely slow slip displacement (3-4 mm/year). In this area, the current landslide activity is associated with intense precipitation events (300-500 mm in a few days) and the related dissolution of Triassic evaporite layers at depth. Considering a relatively simple geological context and a landslide mainly controlled by slope hydrogeology, this site constitutes a natural observatory to study the impact of large rain events on slope kinematics. This work is based on complete and original instrumentation deployed in two nearby boreholes since 2012. The landslide is investigated down to 65 m depth by two in situ permanent observatories for geophysical (electrical resistivity and deformation, metric sensor spacing, daily and seasonal data acquisition) and hydro-geochemical monitoring (pressure, pH, temperature, electrical conductivity, fluid sampling - 4 depths, monthly data acquisition). The data recorded during the five years of investigations help us to characterize the active slip zones in the underground as well as the seasonal dynamics of the different hydrogeological units within the slope. We also observe a heavy rainfall event characterized by both electrical resistivity and geochemical changes down hole, which suggest different time responses of the system to the meteorological solicitations. These first results point out the relevance of the downhole monitoring to progress towards a better understanding of internal landslide processes in relation to climate forcing.

A shallow field experimental site for CO2 injection was established at Maguelone (Languedoc, France), in order to test in an integrated manner a suite of surface and downhole hydrogeophysical monitoring methods. The objective is to improve monitoring of gas transport in the shallow subsurface and to determine the sensitivity of CO2 monitoring systems for leakage detection. The site offers a natural laboratory to study the processes associated with CO2 injection in a clastic and clay-rich context saturated with saline fluids. Prior to CO2 injection, three nitrogen (N-2) injections were undertaken in 2012 to measure the site response to neutral gas injection. In 2013, a volume of 111 m(3) (mass of 220 kg) of CO2 was injected during 3.5 h at 15 m depth. During each experiment, the gas plumes were successfully detected from pressure monitoring, time-lapse induction logging and downhole resistivity monitoring with dipole-dipole array. Increases in resistivity are attributed to free gas propagation (either N-2 or CO2) whereas decreases in resistivity correlate with CO2 dissolution in the pore fluid. Chemical analyses confirm this hypothesis with a decrease in pH and an increase in the concentration of dissolved species in the latter case. (C) 2015 Elsevier Ltd. All rights reserved.

CO2 geological storage remains a recent research field and many questions are still open, particularly for saline formations, which are expected to provide over time a larger storage capacity than depleted hydrocarbon reservoirs. The Maguelone shallow experimental site for shallow CO2 injection (Mediterranean coastline, Gulf of Lions, France) has been developed to study in an integrated manner surface and in-situ (downhole) monitoring methods. The presence of two small reservoirs with impermeable boundaries (R1: 13-16 m and R2: 8-9 m) provides an opportunity to study a saline formation for gas geological storage both in the field and in a laboratory context. During the shallow injection experiment (~48 m3 of CO2 was injected over ~2 hours on December 4, 2014), traces of the CO2 plume were detected by time-lapse downhole and surface electrical resistivity monitoring techniques, although some of the injected CO2 appeared to leak along the new injection hole, which should be corrected in the future with additional cementing around the new holes.

Adequate hydrogeophysical monitoring of CO2 geological storage remains a challenge as different parameters might be modified during storage. That implies to compare real-time measurements to an adequate baseline. At the Maguelone shallow experimental site a representative baseline for electrical resistivity was built from a large number of downhole geophysical measurements. At this coastal site this issue is particularly important due to the production of biogenic gas from the subsurface sediments. For this, a modified petrophysical model based on the Waxman-Smits model is proposed to estimate gas saturation found to vary from 2 to 7% within shallow sand layers.

The objective of the CO(2)FieldLab project was to demonstrate that adequate monitoring methods can be deployed to document potential leaks of CO2 from subsurface reservoirs. For this, a shallow injection experiment through permeable sediment was designed and conducted at Svelvik (Norway) in September 2011. The goal was to produce a CO2 leak in order to assess the relative sensitivity of several geochemical and geophysical CO2 monitoring tools. A total mass of 1.67 tons of CO2 was injected at a depth of 20 m through a 45 degrees inclined well over a 6 day period. Time-lapse induction logging and downhole resistivity monitoring from a permanent dipole-dipole array observatory prove to be very sensitive to the presence of CO2 in the shallow subsurface after the start of injection. Electrical resistivity response correlates with progressive CO2 dissolution in groundwater, with resistivity and pH values decreasing due to the increase of bicarbonate and dissolved species. The CO2 plume is channelled by enhanced permeability layers, with updip migration of the gas plume to the North. Other processes responsible for resistivity changes were fresh/salt water mixing, tidal effects and rainfall. (C) 2014 Elsevier Ltd. All rights reserved.

The objective of the CO2 Field Lab project is to determine the sensitivity of monitoring systems to detect shallow CO2 migration and surface leakage. To achieve its objectives, the project comprises two controlled releases of CO2 into the shallow and very shallow subsurface in a Norwegian aquifer in the Svelvik ridge (Figure 1).

The downhole imaGeau observatory provides an opportunity to deploy a vertical string of sensors probing outward into the reservoir in order to study changes over time of pore fluid electrical conductivity (hence salinity) and/or saturation. This in-situ set-up is based on near-field and high resolution (in space and time) measurements of formation electrical resistivity in aquifers. This paper presents results related to the salinization of a coastal aquifer exploited for the city of Hossegor (SW France). The subsurface measuring device (SMD) observatory has been located in the near vicinity of the city pumping station for domestic use, where increasing water salinity have been measured over the past 10 years. More than 9 months of daily probing show not only seasonal changes changes but also an heterogeneous profile of pore fluid salinity, far from the first order model expected from the Ghyben-Herzberg gravity model. These more detailed data will lead to more adequate aquifer management strategy in Hossegor. As a consequence, the technology deployed by imaGeau demonstrates to provide an answer to a series of hydrogeological issues such as salt water intrusion in coastal aquifers, pollution studies whether in a petrochemical or dump site context, or else pollution prevention or remediation.

The CO2FieldLab shallow injection experiment is a mutual effort from several research groups and organizations (SINTEF, NGI, BRGM, BGS, CNRS, imaGeau and Schlumberger). The objective is to create a downhole leakage of CO2 in order to determine sensitivity of CO2 monitoring systems with respect to CO2 distribution and leakage detection. The CO2 displacement in the subsurface and at the surface was monitored with an exhaustive set of techniques. For this, the field Laboratory for monitoring CO2 migration and leakage was established in glacial deposit that forms Svelvik ridge, 50 km south of Oslo. The shallow injection experiment was conducted in fall 2011 with a monitoring plan designed to spatially and temporally monitor the expected plume development. The monitoring equipment was distributed around the 20m deep injection point of an inclined well. An integrated set of surface and downhole strategies was deployed across a 64m2 square monitoring area. The data recorded by CNRS are presented here. The CNRS techniques included: permanent downhole electrical resistivity observatory, time-lapse downhole induction and sonic logging. The CO2 plume was successfully detected and characterized by these methods even if the trajectory of migrating CO2 deviated from the monitoring predictions.

The Maguelone experimental site is located along the Mediterranean lido of the Gulf of Lions passive margin, 10 km south from Montpellier. Limited to the north by a coastal lagoon and to the south by the Mediterranean sea, this site offers a natural laboratory to study porous coastal reservoirs in a clastic and clay-rich context saturated with mostly saline fluids. Drilled and cored in 2003 for sedimentological purpose, the Maguelone site crosses from surface to 9 m depth a thin Holocene sequence constituted with lagoonal sediments made mostly of dark green clays. This sequence forms an impermeable seal overlying unconformably a Pliocene sequence consisting mainly of relatively homogeneous fine grained continental deposits (clays, silts, and clayey silts). The clayey fraction is relatively high all along, making those deposits poorly permeable. In this sequence, a single remarkable depositional unit located from 14 to 17 m, consists of porous and permeable conglomerates and sands interpreted as fluvial deposits. Sedimentary and geophysical measurements suggest a high permeability for this 3 m-thick reservoir. Hydrogen sulphite (H2S) (possibly resulting from lagoonal organic matter decomposition and compaction) encountered during coring operations near 15 m depth confirmed the presence of a small reservoir at this depth.