A new set of physical property measurements was undertaken on 29 peak‐ring samples from the IODP‐ICDP Expedition 364. Among the studied lithologies, the dominant one recovered in the peak ring consists of shocked granitoid rocks (19 samples). Porosity measurements with two independent methods (triple weight and 14 C‐PMMA porosity mapping) concur and bring new observations on the intensity and distribution of fracturing and porosity in these shocked target rocks. Characterization of the porous network is taken a step further with two other independent methods (electrical and permeability measurements). Electrical properties such as the cementation exponent (1.59 < m < 1.87) and the formation factor (21 < F < 103) do not compare with other granites from the published literature; they point at a type of porosity closer to clastic sedimentary rocks than to crystalline rocks. Permeabilities of the granitoid rocks range from 0.1 to 7.1 mD under an effective pressure of ∼10 MPa. Unlike other fresh to deformed and altered granitoid rocks from the literature compared in this study, this permeability appears to be relatively insensitive to increasing stress (up to ∼40 MPa), with implications for the nature of the porous network, again, behaving more like cemented clastic rocks than fractured crystalline rocks. Other analyzed lithologies include suevite and impact melt rocks. Relatively low permeability (10 −3 mD) measured in melt‐rich facies suggest that, at the matrix scale, these lithologies cutting through more permeable peak‐ring granitoid rocks may have been a barrier to fluid flow, with implications for hydrothermal systems.

Note: ɸ-porosity, ρb-bulk density, ρg-grain density, k-permeability, F-formation factor, m-cementation exponent, τ2-tortuosity, Cs-surface conductivity, Vp-acoustic velocity of compressional waves. Uncertainty for porosity, density, permeability, velocity and conductivity is 5%. Uncertainty for formation factor, cementation exponent and tortuosity is 8%). Lith 1 and Unit 1 after Morgan et al. (2017), Unit 2 after de Graaf et al. (2021, UIM-upper impact melt rock unit, LIMB-lower impact melt rock-bearing unit)) and Kaskes et al. (2021). Morgan, J. V., Gulick, S. P. S., Bralower, T. J., Chenot, E., Christeson, G. L., Claeys, P., et al. (2016). The formation of peak rings in large impact craters. Science, 354(6314), 878–882. https://doi.org/10.1126/science.aah6561 de Graaff, S. J., Kaskes, P., Déhais, T., Goderis, S., Vinciane, D., Ross, C. H., et al. (2021). New insights into the formation and emplacement of impact melt rocks within the Chicxulub impact structure, following the 2016 IODP-ICDP Expedition 364. Geological Society of America Bulletin. https://doi.org/doi: https://doi.org/10.1130/B35795.1 Kaskes, P., de Graaff, S. J., Feignon, J. G., Déhais, T., Goderis, S., Ferrière, L., et al. (2021). Formation of the crater suevite sequence from the Chicxulub peak ring: A petrographic, geochemical, and sedimentological characterization. Geological Society of America Bulletin. https://doi.org/https://doi.org/10.1130/B36020.1

The Lodève landslide is a slow moving (3 to 4 mm/yr) and deep (60 m) rotational instability, located in the South-East of France, 60 km North from Montpellier (Hérault department). It is located in the Lodève basin, a set of connected steep head valleys marking the southern limit of the karstic Larzac plateau, and particularly prone to hydraulically triggered landslides. The unstable slope was progressively formed by the erosion of the upper limestone and sandstone units. The local tectonics build up resulted in a series of vertical North/South faults and fissures, allowing the water to infiltrate down to the deeper Triassic clay and evaporite layers. During heavy rainfall events, an amount of the meteoritic water infiltrates along these flow paths, down to the clay and evaporite layers from the Norian and Rhaetian era, leading to the rapid recharge of the units, the onset of high pressure in the confined layers and the decrease of the cohesion of the rock material and of the shear strength. The Controlled Source Audio-frequency Magneto-Telluric (CSAMT) method is a low-impact, non-invasive active frequency domain electromagnetic sounding technique, deriving from the Magneto-Telluric (MT) method. An electromagnetic signal is produced a few km away from the studied site, and the electric and magnetic transfer functions of the plane wave signal are recorded at multiple frequencies, permitting the computation of far-field MT impedance tensor. CSAMT is characterized by a good vertical resolution and large depths of investigation, but poor sensitivity to the first tens of meters. For these reason, it is expected to be a good candidate method to conduct time-lapse studies in the context of pseudo-1D layered subsurface. CSAMT data were acquired at the landslide from November 2018 to March 2019 at 8 different stations. The landslide is assumed to be a pseudo-1D medium with a tilted flat surface topography. The aim was to observe the variations of electrical resistivity related to the hydrogeological response to the heavy rains observed during the monitoring period. Sensitivity tests were realized with the software custEM. Measurements were taken at ten fundamental frequencies ranged from 510 to 9600 Hz with a Phoenix’s System-2000.net equipment and were repeated every months except in February. The data quality is uneven from one station to another next. Most station showed significant variations in apparent resistivity. The observed variations were interpreted in a one-dimensional context, revealing lateral variations in the hydrogeological response of the slide. Complementary TDIP and DC data and high temporal geochemical and geophysical monitoring of properties at two boreholes were used to constraint the CSAMT interpretation.

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.

Summary This paper focuses on the relationship between rainfalls, groundwater flow and in-situ deformation within a landslide body. In relation to more traditional surface measurements, a set of innovative, in-situ and high-frequency monitoring techniques of both hydrochemical and geophysical processes has been deployed in two nearby shallow boreholes. The experimental site is located near Lodève at Pegairolles de l'Escalette, Languedoc (France), an hour west of Montpellier. In this region, the deformation process is linked to intense climatic events such as torrential rains (Cevenol events) and the associated dissolution of evaporites. With a relatively simple geological context and a unique triggering factor, these landslides are natural observatories to study the impact of climatic events on slope processes. Downhole data collected over 5 years from the integrated set of permanent borehole instruments will be presented to illustrate the spatial and temporal dynamics of landslides processes near Lodève, shedding light on the relationships between rainfalls, ground water flow and deformation. The data recorded over time provide constraints to derive physical and numerical models describing the landslide hydrodynamic behavior. Also, the high-frequency monitoring may lead to the identification of geophysical proxys of deformation, which could be used as landslide precursors in the future.

To evaluate the performance of downhole and surface geophysical monitoring methods, a series of shallow gas injection-monitoring experiments has been performed in a coastal saline aquifer at Maguelone, France. The recorded data include pressure measurements with a Westbay multilevel completion and CO2 saturation at an observation well derived from electrical resistivity with a modified Waxman-Smits (MWS) model. In this work, the aim is to develop a simulation model capturing the gas transport behavior and consistent with field data. For this purpose, the simulation of the CO2 injection experiment is carried out with two conceptual models, a homogeneous model and a heterogeneous model treated with multiple realization Monte Carlo simulations. Numerical simulator TOUGH2 with the equation of state module EOS7C is used for the simulations. Comparison of the model results with field data suggests that the pressure responses are captured with relatively good accuracy. Similarly, the model also provides an overall reasonable agreement and correct order of magnitude for predicted gas saturation values. However, as the heterogeneity pattern in the field data remains largely unknown, the model predictions can only be used to capture the mean behavior as well as to provide insights into how heterogeneity can influence the system behavior, by means of sensitivity analyses of the influence of heterogeneities on individual realizations. (C) 2016 Elsevier Ltd. All rights reserved.

Seismic amplitude anomalies have been observed at the Maguelone site in the course of sub-surface gas injection experiments. To allow the interpretation of the seismic monitoring in terms of physical parameter changes, various modelling attempts were conducted that all share the elastic wave theory principles. We show in this presentation that we cannot reproduce the experimental results with this theory, thus some form of anelastic energy dissipation would be required.

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.

Viability of modelling gas transport in shallow injection-monitoring experiment field at Maguelone,France

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.

In this paper, TOUGH2/EOS7CA model is used to simulate the shallow injection-monitoring experiment carried out at Maguelone, France, during 2012 and 2013. The ultimate objective of the work is to improve our understanding of gas transport in the shallow subsurface as well as to develop and validate the model to monitor it. This work represents first results towards modelling the nitrogen and CO2 injection experiments carried out. The pressure data from the first injection experiments in summer 2012 is used as basis for comparison. Work is presently going on to incorporate the experimental data into the numerical simulation further.

An experimental setup for shallow subsurface hydrogeophysical monitoring has been installed at the Maguelone site, located along the Mediterranean lido of the Gulf of Lions near Montpellier, France. This experimental site was developed in the context of MUSTANG EC project (FP7). SIMEx (Shallow Injection Monitoring Experiment) is a unique opportunity to test in a cost effective manner a full suite of coordinated monitoring techniques, either from surface or downhole. The field spread includes an injection hole, a logging hole, a downhole hydrodynamic observatory based on a pore fluid sampling completion from WestBay (SWS), a downhole seismic observatory, plus surface seismic observatories. This coordinated set of observatories should lead to the design of integrated sensors and methods for the monitoring of gas injection in deeper reservoirs. More recently, nitrogen injection was undertaken to measure the site response to gas injection. Nitrogen was chosen because of the reducing nature of the in-situ environment present in the shallow subsurface at Maguelone, precluding oxygen injection to avoid massive bacterial developments. The next phase of SIMEx will be that of CO2 injection using similar surface and downhole hydrogeophysical monitoring.