Understanding the driving forces and nature of intraplate seismicity remains a major unsolved problem in seismology. In the western Pyrenees, seismicity is concentrated in a narrow region that follows the boundary between the Axial Zone and the North Pyrenean Zone. Despite the presence of a permanent network in the region, the geometry of active faults, and their relationship with crustal structures, remain elusive, owing to significant earthquake location uncertainties. Here, we exploit data recorded by a large-N nodal array deployed in the Chainons Bearnais region during four weeks of 2022 in order to image crustal structures and characterize active faults. We automatically detected and picked P and S waves with PhaseNet, resulting in a catalog of over 500 events, half of which are located beneath the temporary deployment. Tomographic images obtained from the inversion of P and S arrival times provide detailed insight into the geometry of folds and thrusts in the sedimentary cover, as well as the presence of a main fault in the basement which dips northward with an angle of 65 degrees (Chainons Bearnais normal fault). Seismicity relocation within the 3D model obtained by tomography shows that earthquakes are concentrated along this main active fault, extending from the top of the basement to a depth of approximately 16 km. These results demonstrate that passive imaging approaches can offer cost-effective alternatives to traditional controlled source imaging for seismotectonic studies and natural resource exploration in regions with active seismicity.

<p>Envirosciences is developing an integrated multi-parameter low-cost monitoring station encompassing the geohazard and geophysical community needs. It consists in integrating co-located sensors ((meteorology, seismology, GNSS) on the same data acquisition card with modular configurations compatible with the EPOS - European Plate Observing System- specifications (sensor type, data sampling, noise level, data and metadata format). On-line data dissemination sand on-demand processing services are further being developed in order to propose advanced products such as GNSS position time series, advanced hydro-meteorological variables and seismic/micro-seismic catalogues.</p> <p>A dense network of 45 stations is currently being implemented in the Western and Central Pyrenees (South France). The network consists of a seismological, meteorological and geodetic (GNSS) antennas. The measurement network is semi-permanent with at least ten years of observation. It will allow to create catalogues of hydro-geomorphological and tectonic events, to document Pyrenean tectonic uplift, and to better constrain local micro-meteorology from the valley bottoms to the summit ridges (by combining co-localised measurements of classic meteorological parameters - wind, temperature, pressure, humidity, precipitation - and tomography of vertical water vapor profiles from GNSS delays).</p> <p>The objective of the presentation is to present the technological development of the station which combines several types of sensors (2 Hz seismometers, dual-frequency GNSS receivers and meteorological stations), a high-frequency geophysical digitization module, a communication module (WiFi or4G) and a power supply module (by solar energy or 220V). Softwares to control the station have been created, as well as software to supervise the database and codes to interpret the measurements.</p> <p>We will further present the processing worklows and the time series of data acquired since November 2022 on 8 measurement stations already deployed in the Pyrenees. By the end of 2023, the full network of 45 autonomous real-time stations will be deployed with inter-station distances of around 5 km.</p>

Bedrock geometry, geological discontinuities, geotechnical units and shear surfaces/bands control the deformation patterns and the mechanisms of slope instabilities. Seismic P-wave refraction tomography is useful to detect these features because P-wave velocity significantly decreases in fractured and weathered rocks relative to consolidated ones, and because lateral changes of velocity can highlight alternation of dipping fracture zones and consolidated rocks. Acquiring this information at high spatial resolution is of paramount importance to model landslide behaviour. The Viella slope instability (Hautes-Pyrénées, France) is a complex and deep-seated (> 80 m) landslide which has reactivated in Spring 2018 as a consequence of both a 100-yr return period flash flood (Bastan torrent) which affects the lower part of the slope, and a major rockslide (> 100.000 m3) modifying the stress conditions in the upper part. The landslide, which covers an area of ca. 650 000 m², is primarily composed of schists with different degrees of weathering, forming several kinematic units with surface velocities in the range [0.5 – 5] mm.month-1. Many buildings and infrastructures (roads, bridge) are progressively damaged (cracks, progressive tilting) and scarps and lobes develop at the surface delineating the kinematic units. In order to model the evolution of the landslide and design possible mitigation measures (drainage, slope reprofiling), a 3D seismic survey has been carried out in summer 2020. The survey was designed to provide a highly detailed velocity model untill 100 m depth, highlighting possible lithological and mechanical contrasts as well as water preferential flow paths. The acquisition was carried out using 71 3C miniaturized seismic sensors buried at ca. 30 cm in the ground and spaced with an average intertrace of 70 m in accordance with slope topography. IGU16HR-3C 5Hz SmartSolo geophones of the DENSAR service (EOST) were used. The seismic array was recording continuously from June, 22nd to July, 21st 2020 at a sampling rate of 500 Hz. 370 controlled seismic sources were triggered at 122 locations using blank 12-gauge shotgun cartridges, Seismic Impulse Source Systemshots, 90-kg Propelled Energy Generator shots and a Mechatronics Lightning source generating P and S-waves with mono-frequency and sweep signals between 5 and 60 Hz of maximum 80 s length. We present the results of this active P-wave traveltime tomography. We first discuss the quality of the recorded signals related to each different type of source, given the noise and attenuation conditions at Viella. Because the signals were challenging to detect a methodology based on manual picking was used, supported by automatic detection tools and considerations regarding the network geometry in an a priori velocity model. The P-wave model was obtained using the inversion library pyGIMLI, which permits an accurate description of the topography, and provides a spatial discretization adapted to the problem. To supplement and constrain the interpretation of the P-wave velocity model, borehole information as well as a 3D resistivity model of the zone are available. With regards to these data, the geometric features and physical parameters of the main geological structures of the landslide are discussed.

Seismic monitoring of southwestern France began in the 1960s, and homogeneous coverage by observation networks has been in place since the 1990s. The accumulation of data now allows a refined understanding of regional seismicity, not only on its spatial aspects, but also on the regularity of the earthquake distribution over time. This paper is both a review of the work carried out on the subject, and a progress report on the current knowledge of the regional seismicity in its seismotectonic context. With the support of maps, the available catalogs are exploited at different nested scales, from the region as a whole to the numerous clusters that characterize the seismicity of southwestern France, and more specifically that of the Pyrenees. An exhaustive study of these Pyrenean clusters and their temporal behavior is proposed, allowing in particular a better description of the prominent seismicity stripe to the northwest of the range.

PreviousNext No Access18th International Conference on Ground Penetrating Radar, Golden, Colorado, 14–19 June 20203D GPR investigations of a highly porous limestoneAuthors: Dominique RoussetGuy SenechalNatacha BelowicheJean-Paul RolandoGérard MassonnatDominique RoussetUniversité de Pau et des Pays de l’Adour, E2S UPPA, CNRS, TOTAL, Pau, FranceSearch for more papers by this author, Guy SenechalUniversité de Pau et des Pays de l’Adour, E2S UPPA, CNRS, TOTAL, Pau, FranceSearch for more papers by this author, Natacha BelowicheUniversité de Pau et des Pays de l’Adour, E2S UPPA, CNRS, TOTAL, Pau, FranceSearch for more papers by this author, Jean-Paul RolandoCentre Scientifique et Technique Jean Féger, TOTAL, Pau, FranceSearch for more papers by this author, and Gérard MassonnatCentre Scientifique et Technique Jean Féger, TOTAL, Pau, FranceSearch for more papers by this authorhttps://doi.org/10.1190/gpr2020-081.1 SectionsAboutPDF/ePub ToolsAdd to favoritesDownload CitationsTrack CitationsPermissions ShareFacebookTwitterLinked InRedditEmail Abstract The Font d’Armand quarry in Sussargues, Southern France, offers the opportunity to carry out geological surveys on fresh outcrops as well as to perform geophysical measurements to investigate the inner structure of the formation. We present two kinds of surveys in different but close areas: the first one is a full 3D 100 MHz cross-well travel time inversion using 13 wells on a 30 m x 30 m x 30 m cube, the second one is a 3D 250 MHz reflection survey on a 43 m x 30 m x 8 m area. The rocks are composed of tidal carbonate dunes of Middle Burdigalian ages (ca 18 Myr). Investigated rocks are located in the saturated zone and thanks to the high but non homogeneous porosity they exhibit good permittivity contrasts. The 3D traveltime inversion gives a low resolution view of the EM waves velocity field. We can eventually derive a positive correlation between EM slowness and permeability using permeability measurements made on cores, allowing us to provide hydrogeologists with a 3D permeability field. The 3D reflection has been performed using 250 MHz shielded antennas with a trace spacing of 10 cm and a line spacing of 25 cm. Initial processing consists in direct waves attenuation, band pass filtering and gain recovery. Final imaging has been done using a constant velocity 3D migration. Results show the detailed inner dune structure of the Sussargues limestone with much more details than visual inspection of sections in the quarry. Keywords: permeability, heterogeneous, electromagnetic, tomography, attenuationPermalink: https://doi.org/10.1190/gpr2020-081.1FiguresReferencesRelatedDetails 18th International Conference on Ground Penetrating Radar, Golden, Colorado, 14–19 June 2020ISSN (online):2159-6832Copyright: 2020 Pages: 455 publication data© 2020 Published in electronic format with permission by the Society of Exploration GeophysicistsPublisher:Society of Exploration Geophysicists HistoryPublished Online: 11 Nov 2020 CITATION INFORMATION Dominique Rousset, Guy Senechal, Natacha Belowiche, Jean-Paul Rolando, and Gérard Massonnat, (2020), "3D GPR investigations of a highly porous limestone," SEG Global Meeting Abstracts : 307-310. https://doi.org/10.1190/gpr2020-081.1 Plain-Language Summary KeywordspermeabilityheterogeneouselectromagnetictomographyattenuationPDF DownloadLoading ...

ABSTRACTA novel traveltime tomographic approach is applied to anisotropic media, limited to 2D geometry at present. A general anisotropic Eikonal solver based on a discontinuous Galerkin method is combined with an efficient adjoint formulation for multiparameter least‐squares inversion. This new approach is tested considering synthetic crosshole ground‐penetrating radar data. The configuration of the ground‐penetrating radar survey is inspired by a real experiment done on layered carbonate media disturbed by the presence of a deep gallery, which induces a localized high‐electromagnetic contrast. This made it possible to define a well‐adapted general workflow in this context. We notably show that, under the elliptical anisotropic assumption, the parametrization based on vertical and horizontal velocities provides less biased results than those obtained by considering the vertical velocity and the relevant Thomsen parameter . The initial vertical and horizontal velocity models are identical and built from an isotropic inversion. The presence of the high‐contrast gallery generates a weak diffraction pattern, which is taken into account in our tomography approach. It also creates potential artefacts due to the model discretization, which are mitigated by a model regularization term within the definition of the misfit function. This general workflow is then applied to the real experiment dataset. The vertical and horizontal velocity images provide similar structures as those previously obtained by isotropic full waveform inversion, complemented by an image of a rather weak elliptical anisotropy.

Crosshole ground-penetrating radar (GPR) is applied in areas that require a very detailed subsurface characterization. Analysis of such data typically relies on tomographic inversion approaches providing an image of subsurface parameters. We have developed an approach for processing the reflected energy in crosshole GPR data and applied it on GPR data acquired in different sedimentary settings. Our approach includes muting of the first arrivals, separating the up- and the downgoing wavefield components, and backpropagating the reflected energy by a generalized Kirchhoff migration scheme. We obtain a reflection image that contains information on the location of electromagnetic property contrasts, thus outlining subsurface architecture in the interborehole plane. In combination with velocity models derived from different tomographic approaches, these images allow for a more detailed interpretation of subsurface structures without the need to acquire additional field data. In particular, a combined interpretation of the reflection image and the tomographic velocity model improves the ability to locate layer boundaries and to distinguish different subsurface units. To support our interpretations of our field data examples, we compare our crosshole reflection results with independent information, including borehole logs and surface GPR data.

Surface Ground-Penetrating Radar (GPR) data have been acquired along the floor as well as along the vertical walls of a tunnel inside a karstic limestone reservoir in Rustrel. Geological study previously demonstrated the existence of stratification planes with an average dip of 25° to the south and numerous subvertical fault planes. The mono-offset GPR profile analysis acquired along the vertical wall of the tunnel demonstrates the presence of dipping reflectors that can be followed as deep as 16 m from the acquisition surface with 250 MHz nominal antennas. The position of these reflectors coincides with observations of faults recorded in a report written during the tunnel excavations.

Full waveform inversion (FWI) of seismic or Ground Penetrating Radar data provides high-resolution quantitative images of the constitutive parameters of the rock/soil which control seismic/GPR wave propagation. We developed a 2D inversion tool in the frequency domain adapted to the multi-parameter physics controlling GPR propagation in isotropic non dispersive media, i.e. dielectric permittivity and electrical conductivity. This inversion engine was previously tested using synthetic 2D data to mitigate the trade-off between the two parameter classes. In this paper, we present the required processing techniques and first inversion results obtained on a real GPR dataset acquired in carbonates with a cross-hole configuration. The presence of the 2 m diameter underground gallery at depth constitutes a nice target to test the robustness, efficiency and resolution of the inversion in such high-contrasts context. Starting from a time tomographic image for the dielectric permittivity and from a homogeneous conductivity, we show that FWI is efficient to retrieve high resolution images of dielectric permittivity but struggles with electrical conductivity. As a quality control, we compare real and synthetic radargrams computed from the tomography and final images, showing the efficiency of the process to reconstruct some events but also underlying some issues, particularly on large incidence angles amplitude traces.

In this paper, we present ground‐penetrating radar (GPR) investigations performed along a 3.7 km long tunnel located inside a lower Cretaceous limestone massif of south‐eastern France. This fractured massif is mainly characterized by water circulation and karstic structures. This kind of geological formation contains a large part of the fresh underground water resources of the world and is also considered as an analogue of Middle East oil reservoirs. Since tunnel walls are covered by thick reinforced concrete, direct geological observations are impossible.After some preliminary tests, the entire tunnel was investigated using 250 MHz shielded antennas. Data are generally of very good quality, with reflection time up to 400 ns (down to 18 m under the tunnel floor with a velocity of 9 cm/ns). We correlate the GPR signal along the tunnel with surface geological observations: the upper part of the investigated formation (Bedoulian) displays prominent stratigraphic reflectors while the lower part (Barremian) does not. Numerous diffractions are observed in both formations and can be related to karstic features.These investigations allow to better constrain the geological context along the tunnel, necessary for future hydrogeological studies. We conclude that this tunnel offers a unique opportunity of performing GPR measurements within a karstified limestone massif.

Seismic anisotropy of a fractured karstic limestone massif in sub-parallel underground galleries is studied. As the fractures are mostly vertically oriented, the seismic properties of the massif are approximated by horizontal transverse isotropy (HTI). Several data inversion methods were applied to a seismic dataset of arrival-times of P and S-waves.The applied methods include: isotropic tomography, simple cosine function fit, homogeneous Monte-Carlo anisotropic inversion for the parameters of horizontal transverse isotropy and anisotropic tomography for tilted transversely isotropic bodies. All methods lead to the conclusion that there is indeed an anisotropy present in the rock massif and confirm the direction of maximum velocity parallel to the direction of fracturing. Strong anisotropy of about 15% is found in the studied area. Repeated measurements show variations of the P-wave parameters, but not of the S-wave parameters, which is reflecting a change in water saturation. (C) 2013 Elsevier B.V. All rights reserved.

This paper highlights the efficiency and complementarity of a light package of geophysical techniques to study the structure of karst Unsaturated Zone (UZ) in typical Mediterranean environment where soil cover is thin or absent. Both selected techniques, 2D Ground Penetrating Radar (GPR) and Electrical Resistivity Tomography (ERT), are widely used in environmental studies and their application is accessible for a lot of scientists/engineers. However, GPR or ERT alone is not able to provide an enhanced characterization of geological features in karst media. In the present study, GPR results supply a near surface high resolution imaging and thus can provide relevant geological information such as stratifications and fractures. Despite the quality of the results GPR's investigation depth remains limited to around 12m. Apparent and inverted resistivity provided by ERT surveys shows strong lateral and vertical variations. These variations can inform about general geological structuring and feature orientation. ERT is able to prospect down to 40m but it's a low resolution integrative technique. In the study area the investigated limestone is a commonly electrical resistive formation (more than 2000Ω.m). However deeper than 5–7m, the ERT profiles reveal several zones of moderate resistivity (around 900Ω.m). In these zones a stratification change corresponding to slanted bedding is clearly identified by GPR results. The combination of both GPR and ERT results can allow a well-established geological interpretation. These moderate resistivity zones with slanted beddings can explain the presence of a perennial water flow point 35m below the surface of the studied site within the underground gallery of the Low-Noise Underground Laboratory (LSBB).

Our study focuses on the potential usefulness of surface geophysical data to constrain the water content within an alluvial aquifer. On a study area where two wells have been drilled, we have performed several geophysical measurements, including ground penetrating radar, DC resistivity prospecting, seismic refraction survey and magnetic resonance soundings. From these data, we estimated several parameters, namely, the water height in the deposits, the effective porosity, the water content, the permeability, and the transmissivity of alluvial deposits. These physical parameters allow us to characterize the alluvial deposits in order to constrain the estimation of the potential water flow. The lithology and water flow rate known from the wells enabled us to compare geophysical results obtained in a high water flow rate zone to those in a low water flow rate zone. Correlation has been found between the water flow rate observed in both wells and the geophysical data obtained in the vicinity of these wells.

A seismic time reversal experiment based on Time Reversal Mirror (TRM) technique was conducted in the mesoscopically scaled medium at the LSBB Laboratory, France. Two sets of 50 Hz geophones were distributed at one meter intervals in two horizontal and parallel galleries 100 m apart, buried 250 m below the surface. The shot source used was a 4 kg sledgehammer. Analysis shows that elastic seismic energy is refocused in space and time to the shot locations with good accuracy. The refocusing ability of seismic energy to the shot locations is roughly achieved for the direct field, and with excellent quality, for the early and later coda. Hyper-focussing is achieved at the shot points as a consequence of the fine scale randomly heterogeneous medium between the galleries. TRM experiment is sensitive to the roughness of the mirror used. Roughness induces a slight experimental discrepancy between recording and re-emitting directions degrading the quality of the reversal process.

The technique of time-reversal acoustics was applied to image a bottle filled with saline, using an eight element Vivaldi antenna array with frequency bandwidth 2 to 8 GHz. At these short length scales, a smooth three-dimensional image of the bottle was obtained, with the usual limitations imposed by limited offset and frequency. Time snapshots of the wavefield evolution, in reversed time are presented for two real data sets. The first, shows the focusing for the single target of the bottle, while the second demonstrates the principle for two targets. (C) 2008 Elsevier B.V. All rights reserved.

Abstract : The Jiroft area, located in a semi-arid zone along the Halil Rud in southeastern Iran, seems to have been an important cultural center during the 3rd millennium ВС. This paper presents the preliminary results of the first two years (2004 and 2005) of a geoarchae- ological program carried out at the invitation of Prof. Madjidzadeh. The Quaternary geomorphological dynamics in the valley are studied, and we present the preliminary results of a geophysical prospection (electro-magnetism, seismic refraction, radar) carried out in January 2005. Climatic aridity, seismicity and large, sudden floods are the most important risks for human societies in the Jiroft basin. Artesian wells and abundant phreatic water are of primary importance for human settlements even though the water is salty. The reconstruction ofpaleoalluvialfans and fossil riverbeds under several meters ofHolocene deposits, should come first in our attempt to understand the paleogeography of the valley.

We present here a complete depth-migrated line-drawing of the ECORS Pyrenees profile. The main feature of the migrated section is an important diffraction zone generated by the North Pyrenean Fault. We image several inhomogeneities in both the Iberian and the European upper crust. Some of them can be linked to Proterozoic materials that have been affected by Hercynian tectonics. Deeper, around 25 s two-way time (TWT), several dipping reflectors initially identified below the Aquitaine basin migrate to the south. They form the northern limit of the Iberian lower crust dipping beneath a wedge consisting of European crust and upper mantle, a previously described but now better imaged feature of the lithosphere beneath the Pyrenees.