Neotectonics and current hydrologically-induced karst deformation. Case study of the Plateau de Carlern (Alpes-Maritimes, France)

International audienceThe astronomical and geodetic observatory OCA, located on the karst plateau of Calern (Caussols, France), has been monitoring earth deformation for several years. Two long baseline tiltmeters have been installed in a shaft in 2007, along with classical hydrogeological monitoring tools in order to investigate the relationship between current karst deformation and hydrology. Dye tests have shown that the plateau is drained towards the East, to the spring of Bramafam, except for its Western third. Karst tilting, as recorded by the tiltmeters, is linked to rainfall events. These instruments bring additional information to characterize different reservoirs: the deep karst aquifer of Bramafan with high amplitude oscillations of its water table, up to 100 m, the perched aquifer of Moustiques shaft whose response is attenuated, and several slope aquifers with reduced oscillations (Fontaniers, Castel Bon Pré). Tilt deformation reaches 8 μrad with a definite orientation between N90°E and N100°E. The best correlation between hydrology and tilts is observed for the deep aquifers. If the first autumn rainfall is ineffective on tilt, it recharges the epikarst and refills the reserves. The winter rains cause the water to flush towards the eastern deep aquifer and provokes a quick tilting of the plateau. Finally, the long term variations in tilt and water table show a very good correlation The relationship between current hydrological deformations and tectonics is also analyzed; broken and shifted speleothems in the shafts indicate a general shift of the plateau towards the south

Inferring field-scale properties of a fractured aquifer from ground surface deformation during a well test

International audienceFractured aquifers which bear valuable water resources are often difficult to characterize with classical hydrogeological tools due to their intrinsic heterogeneities. Here, we implement ground surface deformation tools (tiltmetry and optical leveling) to monitor groundwater pressure changes induced by a classical hydraulic test at the Ploemeur observatory. By jointly analyzing complementary time constraining data (tilt) and spatially constraining data (vertical displacement), our results strongly suggest that the use of these surface deformation observations allows for estimating storativity and structural properties (dip, root depth, lateral extension) of a large hydraulically active fracture, in good agreement with previous studies. Hence, we demonstrate that ground surface deformation is a useful addition to traditional hydrogeological techniques and opens possibilities for characterizing important large-scale properties of fractured aquifers with short-term well tests as a controlled forcing

Implementing ground surface deformation tools to characterize field-scale properties of a fractured aquifer during a short hydraulic test

International audienceIn naturally fractured reservoirs, fluid flow is governed by the structural and hydromechanical properties of fracturenetworks or conductive fault zones. In order to ensure a sustained exploitation of resources or to assess the safety ofunderground storage, it is necessary to evaluate these properties. As they generally form highly heterogeneous andanisotropic reservoirs, fractured media may be well characterized by means of several complementary experimentalmethods or sounding techniques. In this framework, the observation of ground deformation has been proved usefulto gain insight of a fractured reservoir’s geometry and hydraulic properties. Commonly, large conductive structureslike faults can be studied from surface deformation from satellite methods at monthly time scales, whereas meterscale fractures have to be examined under short-term in situ experiments using high accuracy intruments liketiltmeters or extensometers installed in boreholes or at the ground’s surface. To the best of our knowledge, thefeasability of a field scale ( 100 m) characterization of a fractured reservoir with geodetic tools in a short termexperiment has not yet been addressed. In the present study, we implement two complementary ground surfacegeodetic tools, namely tiltmetry and optical leveling, to monitor the deformation induced by a hydraulic recoverytest at the Ploemeur hydrological observatory (France). Employing a simple purely elastic modeling approach,we show that the joint use of time constraining data (tilt) and spatially constraining data (vertical displacement)makes it possible to evaluate the geometry (dip, root depth and lateral extent) and the storativity of a hydraulicallyactive fault zone, in good agreement with previous studies. Hence we demonstrate that the adequate use of twocomplementary ground surface deformation methods offer a rich insight of large conductive structure’s propertiesusing a single short term hydraulic load. Ground surface deformation tools therefore constitute an interestingaddition to traditional fractured media sounding technique

Free Core Nutation Parameters from Hydrostatic Long-Base Tiltmeter Records in Sainte Croix aux Mines (France)

International audienceThe resonance associated with the Free Core Nutation (FCN) has been widely studied in Very Long Base Interferometry (VLBI) network measurements and in superconducting gravity records, but few experiments have been done with tiltmeters. In this study we use records collected with a pair of about 100 m long hydrostatic silica tiltmeters, orthogonally installed in an abandoned silver mine at Sainte Croix aux Mines (Alsace, in North-Eastern France). Main difficulties in retrieving FCN parameters from tidal analysis arise from the weak amplitude of PSI1 tidal wave (the closest in frequency to the FCN), as well as from the inaccuracy of the available ocean loading correction. Moreover because of the closeness in frequency of the single constituents of the diurnal tidal band, long (>1 year) records are needed for resolving K1, PSI1 and PHI1 waves. Hence we analyze a 10-year dataset of tilt records, hich has preliminarily required a critical review and a relevant editing for making records suitable for tidal analysis and subsequent inversion of the tidal parameters. A Bayesian inversion is used for a preliminary retrieval of the FCN parameters

Apport de l’inclinométrie dans lacaractérisation de la dynamique saisonnièredes aquifères fracturés et de leur recharge

National audienceLes aquifères cristallins fracturés représentent une importanteressource en eau, mais sont généralement difficiles à caractériserdu fait de leur hétérogénéité et de la dépendance de leur propriétéshydrodynamiques à l’échelle d’investigation. En particulier, un enjeumajeur pour l’étude et l’exploitation pérenne de ces réservoirsest la description des processus, des temps caractéristiques et desflux de leur recharge naturelle. Dans ce travail, nous exploronsle potentiel de l’analyse couplée de mesures inclinométriques etpiézométriques long-terme pour répondre à cet enjeu. En effet,les inclinomètres permettent de mesurer d’infimes déformationsde la surface du sol et sont très sensibles aux flux d’eau souterraine,même lointains. Nous présentons les données pluriannuellesdes inclinomètres longue-base de l’Observatoire hydrogéologiquede Ploemeur, Morbihan (site SOERE, réseau H+), dont la grandestabilité rend possible le suivi continu des cycles saisonniers del’aquifère fracturé. Puis, nous proposons une interprétation conjointedes données inclinométriques et piézométriques à l’aide d’unmodèle hydromécanique 1D simple, qui suffit toutefois d’expliquerde manière satisfaisante les observations et le fonctionnementsaisonnier au premier ordre du système. Nous montrons ainsique l’inclinomètre est sensible non seulement aux propriétés hydrodynamiqueslarge échelle de l’aquifère, mais aussi aux flux dela recharge ainsi que sa modalité (ponctuelle versus diffuse). Cesrésultats révèlent de façon inédite que le suivi long-terme de ladéformation en surface apporte une information très complémentaireà la réponse piézométrique d’un aquifère fracturé, permettantainsi d’introduire une précieuse contrainte qualitative et quantitativesur sa recharge

New insights on fractures deformation from tiltmeter data measured inside the Fontaine de Vaucluse karst system

International audienceTilt fluctuations can potentially reflect the response of hydrosystems to important rainfall. In this context, long baseline tiltmeters have been installed in an underground tunnel penetrating the Fontaine de Vaucluse karst to study the medium deformation related to solicitations exerted by water infiltrating the hydrosystem. The instruments monitor the tilt as well as its spatial variation. Northward tilts reaching a 1 μrad amplitude are observed consecutively to rainfalls. The tilt amplitude is highly correlated with the Fontaine de Vaucluse outlet flow fluctuations. The measured tilt signal is also relatively homogeneous over a 150 m length. Different types of structure likely to produce such observations are tested in order to identify their location with respect to the tiltmeters, their dimension as well as the amount of water level variation in the structure. Following rainfalls, the infiltration of water modifies the pore pressure, inducing a medium deformation. The hypothesis of an homogeneous surface loading on the Vaucluse plateau is first refuted since the related tilt is much lower than the one measured. The water supplied by rainfalls has to accumulate in discontinuities in order to generate a higher tilt. So, the deformation related to a pressure exerted on a fracture filled by water is assessed. A first study reveals the interest of the tilt homogeneity information that constrains strongly the fracture properties. Thus, the fracture must be located at a distance more than a few hundreds metres from the tiltmeters in order to produce a tilt homogeneous in space. If the fracture is initially dry, it must also be filled on a height higher than 150 m consecutive to a rainfall in order to generate a tilt amplitude in the same magnitude as the one measured. Then, we explore the influence of water level variations on the tilt produced by a fracture located at the interface between the saturated and unsaturated zones, which are thereby permanently flooded. Since several parameters of that model satisfactorily explain the field observations, we discuss how simultaneous geodetical observations could provide complementary information that would further constrain the geometry of the structure at the origin of the medium deformation

High resolution tiltmeters and strainmeters based on extrinsic fiber Fabbry-Perot interferometry: the LINES project

International audienceMeasurements of strain and vibrations due to seismic and volcanic processes are mandatory for the understanding and the monitoring of the behavior of these systems. In the future, risk mitigation will depend on our capability to detect in a reliable way small precursors of large seismic and volcanic events and to assess the seismic/aseismic spatial and temporal distribution and evolution of crustal strain in these unstable systems.The robustness of strain and motion detection is primary linked to measurement accuracy, but also to the number and repartition of instrument. This implies that instrument cost and maintenance are essential for the development of networks. To date, only GPS sensors are robust enough to be deployed for long periods of time with limited problems of maintenance. Tiltmeters and strainmeters capabilities are often plagued by numerous technical problems limiting their usefulness. On the basis of existing or prototype sensors, we develop new instruments (seismometers, tiltmeters, strainmeters) using an interferometric motion measurement. Both Laser source and fringe analysis are connected to the mechanical sensor with long optic fiber (100 m - 10 km) depending on applications (volcanoes, sea bottom, boreholes) The fiber signal transmission is a major improvement by comparison with usual electric wires (cost, data channels, lightning, weight). Also, the absence of embedded electronic on the sensor is a guarantee for reliability and toughness. The proposed optical cell is an extrinsic all-fiber Fabry-Perot type interferometer (EFFPI). While being intrinsically insensitive to external perturbations to the sensing arm such as from stress/strain and temperature variations, the EFFPI is, however, extremely sensitive to changes in its sensing cavity length caused by parameters such as displacement, strain, and mechanical deformation along the optical axis. Coupled to well-advanced associated technologies in terms of laser sources (stability, output power), optical fiber (quality, low losses, couplers, connectics), photodetection (bandwidth, gain, low-noise) and real-time interferometric signal demodulation, this interferometer is today a mature device whose performance potential can be exploited in in-situ environmental monitoring of seismic activities (earthquakes and volcanoes) and in predicting the related risks. In the framework of the LINES project, we develop three types of mechanical sensors: a long baseline tiltmeter based on hydrostatic levelling, a borehole tiltmeter based on a simple pendulum and a seismometer for detecting vibrations at frequencies higher than 1 Hz. A common building principle is an external laser source and phase detector: as this part of the tool is remotely connected through an optic fiber to the underground sensor, this overcomes most of electric, power and maintenance problem occurring with non-optical devices. Moreover, this allows simple analog data transmission for a real-time network monitoring. We will show preliminary results suggesting that a rapid transition between laboratory prototypes and field instruments is likely

Optical long range instruments for high resolution monitoring of geohazard in harsh environment

International audienceIn many areas with high telluric hazard, our understanding of the seismogenic and mechanical processes is limited by the difficulty or impossibility to deploy arrays of high performance seismometers or of other geophsyical sensors, due to high cost and/or difficulty of the installation and maintenance. This led us to develope several optical instruments: the idea was to separate, by a long optical fiber, the robust mechanical part of the sensor from the opto-electronic part, which interrogates the sensor and measures its internal deformation with Fabry-Perot (FP) interferometry. Our IPGP-ESEO team has thus constructed innovative optical seismometers and strainmeters, complemented by borehole and long base optical tiltmeters from Geoscience Montpellier and ENS. Here we present the ESEO-IPGP instruments, and their successful installation in the field: one seismometer at the top of La Soufrière volcano (1.5 km long fiber), 10/2019 (ANR HIPERSIS ); one ocean-bottom seismometer offshore Les Saintes, 06/2021 (FIBROSAINTES, Interreg Caraïbe PREST) (5 km long fiber buried by a plow GEOAZUR); and one strainmeter on the Stromboli volcano (09/2021)

Onland and Offshore Extrinsic Fabry–Pérot Optical Seismometer at the End of a Long Fiber

International audienceWe report here the design, performance, and in situ demonstration, on‐land and offshore, of an innovative high‐resolution low‐cost optical (laser) seismometer. The instrument was developed within the Laser Interferometry for Earth Strain project (French Agence Nationale de la Recherche [ANR] program), and first tested at the low‐noise underground laboratory Laboratoire Souterrain à Bas Bruit (LSBB, France). It is based on Fabry–Pérot optical interferometry between the extremity of a probing optical fiber and a reflecting mirror secured to the mobile mass of a passive 2 Hz geophone. The detection technique is based on the wavelength modulation of the laser diode (1310 nm), which allows the separation of the optical power into two signals in quadrature, thanks to an heterodyne technique. The relative displacement of the mobile mass is retrieved in real time by the phase unwrapping of these two signals. At LSBB, the fiber was 3 km long. It recorded many teleseismic earthquakes and a few regional ones, and resolves the low‐seismic noise of the Earth for periods up to 6 s, presenting an acceleration noise floor lower than 1  ng/Hz in the 0.3–5 Hz range. A three‐component version of this fiber‐based interferometric 2 Hz geophone has been recently constructed, shielded in a hyperbaric container, and installed offshore for test in Brittany (France) in April 2018, with an improved control system. Its record of the marine ambient noise matches those of a collocated commercial broadband seismometer for periods up to 50 s. This opens promising perspectives for large‐scale ocean bottom instrumentation with up to 50‐kilometer‐long optical lines; an installation is planned for 2020, off Guadeloupe, with a 5‐kilometer‐long fiber cable. It may also prove useful for installations in other challenging and exposed environments, such as deep hot boreholes, active volcanoes, unstable landslides, for real‐time monitoring in regions with high natural hazard, but also for seismic monitoring of geoindustries