Influence of Grain Size Distribution and Shape on GPR Waves -Study of Aeolian Dunes

International audienceSUMMARY Ground penetrating radar (GPR), a geophysical method based on electromagnetic (EM) wave propagation, can provide very detailed and continuous images of the internal structures of aeolian dunes. In order to model and explain the origin of observed reflections, we build a model of electric permittivity which accounts for the grain size distribution and shape. By modelling the propagation of GPR waves in frequency domain, we have shown that grain size and grain shape have an influence on GPR reflectivity and we confirmed it by adjusting the modelled data to the real GPR data acquired in arid zones

A17 Topographic Migration of GPR Data -Case Studies from Dry Sand Dunes and Active Fault Areas

International audienceSUMMARY Most Ground Penetrating Radar (GPR) measurements are performed on nearly flat areas. If strongly dipping reflections and/or diffractions are present in the GPR data, a classical migration processing step is needed in order to determine the geometries of shallow structures. Nevertheless, standard migration routine is not suitable for GPR data collected on areas showing variable and high topographic relief. To account for the topographic variations the GPR data are, in general, corrected by applying static shifts instead of using an appropriate topographic migration which would place the reflectors at their correct locations with the right dip angle. In this paper we present an overview of Kirchhoff migration and show the importance of topographic migration in the case where the depth of the target structures is of the same order as the relief variations. Examples of synthetic and real GPR data are shown to illustrate the efficiency of the topographic migration

Imagerie de la proche surface par géoradar

Abstract : Ground-penetrating radar (GPR) is a geophysical method based on thepropagation, reflection and scattering of high frequency (from 10 MHz to 1 GHz)electromagnetic (EM) waves in the earth. This method is currently used to image thesubsurface. The depth of investigation depends on the EM attenuation of the mediumand the frequency used and varies from 2 to 50 m. Our interest for the GPR methodstarted during a mutual study with CEA in order to adapt the seismic processing to theradar data. Since 1994, many studies (in 1 and 2D) have been carried out by ourLaboratory Imagerie Tectonique.The radar attenuation is closely related to the dielectric permittivity of the medium,which is, in general, a complex quantity and also shows a frequency dependence. Inorder to model for dielectric losses (or radar attenuation) we consider a complex powerfunction of frequency for the dielectric permittivity. This approach allows us to developan original method of determining the quality factor Q of radar waves.During the propagation of a radar signal through a medium with a complex dielectricresponse depending on frequency we observe two effects: the amplitude of the signaldecreases (absorption) and its phase changes (dispersion), consequently the radarsignal is not stationary. To account for both effects of attenuation we have alsodeveloped methods (in 1D and 2D) of inverse Q imaging of GPR data. This later studyhas been inspired from a similar study carried out before in the case of 1D imaging ofseismic waves.Several acquisitions of GPR data have been performed for tectonic objectives(locating active faults, Mulhouse-Bâle, over the Wellington fault in New Zealand andover Nîmes and Moyenne Durance faults) and sedimentological objectives as well(over sand dunes in Tchad and Mauritanie). The aims were to image the geometry ofsedimentary deposits and identify possible fault segments under vegetation cover andto understand and explain the origin and the nature of the reflections observed insidethe active or old dunes.Surface objects such as: power lines, metallic fences, trees and corners of buildingsare strong reflectors and can produce strong scattering that masks the reflections fromthe subsurface in GPR data. After recognizing these diffractions (by a simplegeometrical modeling), we model them by using a zero offset imaging technique inFourier domain. Once the modeling of the air diffractions is performed, one can usethem as a mask to remove the real surface diffractions observed in the real GPR data.A recent study of modeling reflections, in the case of dry sand, shows that amongstthe propagation effects (absorption, dispersion and geometrical spreading) the mostimportant is the geometrical spreading. On the other hand using a simple mixingformula between the dielectric constant of the dry sand and the water content of themoist sand (e), we highlight the sensitivity of the reflection coefficient to smallchanges in water content (e) into the sand. That explains the strong reflectionscoming from the base of present-day dune.Résumé : Le géoradar est une méthode de prospection géophysique fondée surl’analyse de la propagation, de la diffraction et de la réflexion des ondesélectromagnétiques (EM) haute fréquence (de 10 MHz à 1 GHz. Il est aujourd’huilargement utilisé pour l’étude de la subsurface. La profondeur d’investigation dépendde l’atténuation du milieu et de la fréquence utilisée, elle varie de 2 à 50 m. Nous avonsabordé le domaine du géoradar à l’occasion d’une étude menée en collaboration avecle CEA pour adapter le traitement sismique aux données radar. Depuis 1994, plusieursétudes (en 1 et 2D) ont été entreprises par notre Laboratoire d’Imagerie Tectonique.L’atténuation radar est fortement reliée à la permittivité diélectrique totale du milieuqui, en général, est une quantité complexe et montre aussi une dépendance enfréquence. Afin de modéliser les pertes diélectriques (ou l’atténuation radar) nousconsidérons une fonction complexe de puissance en fréquence pour la permittivitédiélectrique. Cette approche nous a permis de développer une méthode originale dedétermination du facteur de qualité Q des ondes radar.Lors de la propagation du signal radar dans un milieu à une reponse diélectriquecomplexe et dépendante de la fréquence nous observons deux phénomènes: l’amplitude du signal diminue (absorption) et sa forme change (dispersion), parconséquent le signal radar n’est pas stationnaire. Afin de tenir compte de cesphénomènes nous avons aussi développé des méthodes d’imagerie radar (filtreinverse-Q à 1 et 2D) dans le cas d’un milieu absorbant et dispersif. Cette dernièreétude a été inspirée par une étude similaire faite auparavant dans le cas de l'imagerieen 1D des ondes sismiques.Des acquisitions ont été réalisées à la fois sur des objectifs tectoniques (recherchede failles actives; Mulhouse-Bâle, sur la faille de Wellington en Nouvelle Zélande et surles failles de Nîmes et de la Moyenne Durance) et sédimentologiques (sur les dunesde sable; Tchad et Mauritanie). L’objectif était d’imager par géoradar la géométrie desdépôts sédimentaires et d’identifier les failles susceptibles de les décaler et decomprendre et d’expliquer la nature et l’origine des surfaces de discontinuités àl’intérieur d’une dune active ou ancienne.Les objets de surface tels que les lignes et les poteaux électriques, les clôturesmétalliques, les arbres et les coins des immeubles peuvent produire de fortesdiffractions aériennes qui, à leur tour, peuvent masquer les réflexions radar primairesprovenant du sous-sol. Apres avoir reconnu ces diffractions (par une simplemodélisation géométrique) nous les modélisons en utilisant une méthode d’imagerie2D en domaine de Fourier. Ensuite nous comparons les données réelles auxdiffractions synthétiques afin d’éliminer ces dernières des données réelles.Une étude de la modélisation des réflexions radar dans le cas du sable sec montreque parmi les effets de la propagation (absorption, dispersion et divergencesphérique) le plus important est la divergence sphérique. D’autre part, en utilisant uneformule simple de mélange entre la constante diélectrique des sols sableux et lateneur en eau (e) dans l’échantillon, nous mettons en évidence la sensibilité ducoefficient de réflexion due aux faibles changements de la teneur en eau (e) dans lesable. Ce qui explique les fortes réflexions provenant de la base de la dune actuelle

Electrical structure of the Himalaya of Central Nepal: high conductivity around the mid-crustal ramp along the MHT

Twelve broadband magnetotelluric (MT) soundings were performed across the Himalaya of Central Nepal in 1996 in order to determine the electrical structure of the crust and its relation to geological structures and active tectonics. The MT impedance tensors were obtained for frequencies between 0.001 and 500 Hz. The 2‐D section, derived from joint inversion of TE‐ and TM mode after RRI and Groom/Bailey decomposition, shows high conductivity in the foreland basin (∼30 Ω.m) that contrasts with the resistive Indian basement (>300 Ω.m) and Lesser Himalaya (>1000 Ω.m). In addition, our MT sounding reveals a major conductive feature beneath the front of the Higher Himalaya, also characterized by intense microseismic activity, and the position of a mid‐crustal ramp along the major active thrust fault (MHT). This high conductivity zone probably reflects metamorphic fluids, released during underthrusting of the Indian basement and pervading well connected microcracks induced by interseismic stress build‐up, or distributed brittle deformation around the ramp

Influence of grain size, shape and compaction on georadar waves: example of an Aeolian dune

Many Ground Penetrating Radar (GPR) profiles acquired in dry aeolian environment have shown good reflectivity inside present-day dunes. We show that the origin of this reflectivity is related to changes in grain size distribution, packing and/or grain shape in a sandy material. We integrate these three parameters into analytical models for bulk permittivity in order to predict the reflections and the velocity of GPR waves. We consider two GPR cross-sections acquired over Aeolian dunes in the Chadian desert. The 2D migration of GPR data suggests that dunes contain different kinds of bounding surfaces. We discuss and model three kinds of reflections using reasonable geological hypothesis about Aeolian sedimentation processes. The propagation and the reflection of radar waves are calculated using the 1D wavelet modelling method in spectral domain. The results of the forward modelling are in good accordance with real observed data

GPR measurements to assess the Emeelt active fault's characteristics in a highly smooth topographic context, Mongolia

International audienceTo estimate the seismic hazard, the geometry (dip, length and orientation) and the dynamics (type of displacements and amplitude) of the faults in the area of interest need to be understood. In this paper, in addition to geomorphologic observations, we present the results of two ground penetrating radar (GPR) campaigns conducted in 2010 and 2011 along the Emeelt fault in the vicinity of Ulaanbaatar, capital of Mongolia, located in an intracontinental region with low deformation rate that induces long recurrence time between large earthquakes. As the geomorphology induced by the fault activity has been highly smoothed by erosion processes since the last event, the fault location and geometry is difficult to determine precisely. However, by using GPR first, a non-destructive and fast investigation, the fault and the sedimentary deposits near the surface can be characterized and the results can be used for the choice of trench location. GPR was performed with a 50 MHz antenna over 2-D lines and with a 500 MHz antenna for pseudo-3-D surveys. The 500 MHz GPR profiles show a good consistency with the trench observations, dug next to the pseudo-3-D surveys. The 3-D 500 MHz GPR imaging of a palaeochannel crossed by the fault allowed us to estimate its lateral displacement to be about 2 m. This is consistent with a right lateral strike-slip displacement induced by an earthquake around magnitude 7 or several around magnitude 6. The 2-D 50 MHz profiles, recorded perpendicular to the fault, show a strong reflection dipping to the NE, which corresponds to the fault plane. Those profiles provided complementary information on the fault such as its location at shallow depth, its dip angle (from 23 • to 35 •) and define its lateral extension. Central Asia is known for its high level of seismic hazards, especially Mongolia, which has been one of the most seismically active intracontinental regions in the world with four large earthquakes (magnitude around 8) along its active faults in the western part of the country during the last century (Khilko et al. 1985). The deformation in Mongolia is located between compressive structures related to the collision and penetration of the Indian plate into the Eurasian plate and extensive structures in the north of the country related with the Baykal rift (Tapponnier & Molnar 1979; Baljinnyam et al. 1993; Schlupp 1996; Bayasgalan & Jackson 1999). The seismic activity observed in the vicinity of Ulaanbaatar (UB), capital of Mongolia, is relatively low compared to the activity observed in western Mongolia. Nevertheless, since 2005, the seismic activity around UB not only has increased, but is also organized (see Fig. 1) at the west of UB along two perpendicular directions, which determine two active faults: Emeelt fault, discovered in 2008 (NNW-SSE direction, 25-km-long minimum and situated about 10 km W of UB) and Hustai fault (WSW–ENE direction, 80 km long, with its NE tip at less than 20 km west of UB); their length and morphology indicate that they can produce earthquakes of magnitude 6.5–7.5 (Schlupp et al. 2012). Most of the Mongolian population (1.2 million over 3 million) is concentrated at UB, which is the main political and economical centre of the country. Hence, the study of seismic hazard and the estimation of the probability of future destructive earthquakes are of primary importance for the country (Dugarmaa et al. 2006). Since the last large earthquake, the faults geomorphology has been highly smoothed by erosional processes and the exact location of the fault plane surface rupture is thus hidden within a several metre wide strip. The GPR method has been proven to give good and useful results to characterize faults by identifying offsets of radar reflections (Malik et al. 2007; Christie et al. 2009; Yalçiner et al. 2013) an

Extraction Automatique des Réflexions, Modélisation des Diffractions et Migration des Données de Sismique Profonde ECORS.

The classical method of seismic reflection developed by industry for the study of the sedimentary part of subsurface becomes nowadays an indispensible tool for the investigation of the upper and lower the crust. The work realized during this thesis is devoted to the development of novels techniques in seismic data processing in order to facilitate the interpretation of deep seismic images acquired during the ECORS program. The interpretation of deep seismic reflection data usually involves a manual picking of the laterally coherent reflections observed on adjacent traces. Such method is highly subjective because the deep reflections are segmented and often contaminated by incoherent noises. We have developed a method based on local (tau-p) transforms that permits the extraction of all coherent reflections in an objective manner and preserves the original appearance of the signal. The diffractions on the deep seismic images can complicate the interpretation since they are difficult to distinguish from dipping reflections. We propose a simple method which permits to evaluate the diffraction on the seismic section. Applying a coherency filter to the data, we separate both the sub horizontal reflections and the Fresnel zone of hyperbolic events. Assuming that this section represents the reflectivity of the crust, we diffract this model using several RMS velocities. Among the synthetic sections, we choose the section that best fits the original one. The conventional seismic migration methods do not give satisfactory results for the deep seismic data. We have developed a migration method based on the combination of coherency filter with hyperbola-summation in (tau-p) plane. This method improves significantly the signal/noise ratio because, contrary to classical methods it migrates only the seismic signal.La méthode de sismique réflexion verticale, développé par l’industrie pour étudier la partie sédimentaire du sous-sol, est devenue un outil nécessaire à l’étude de la croute continental et océanique. Le mémoire est consacré au développement de méthodes nouvelles de traitement sismique dans le but de faciliter l’interprétation des images sismiques de la croute obtenue dans le cadre du programme ECORCS. L’interprétation des images de la sismique profonde repose sur un pointé manuel qui consiste à souligner de manière subjective les réflexions cohérentes sur un nombre de traces voisines. Ce pointé se révèle délicat car les réflexions profondes apparaissent sous forme de courts segments et elles sont fréquemment contaminées par le bruit incohérent. Nous avons développé une méthode fiable basée sur la transformation (tau-p) locale permettant d’extraire de manière objective toute les réflexions cohérentes sur un certain nombre de traces et de conserver apparence original du signal sur la section. Les diffractions présentes sur les images de sismique profonde peuvent compliquer l’interprétation car elles sont difficiles à distinguer des réflexions pentées. Nous proposons une méthode simple permettant d’évaluer l’importance des diffractions sur une section sismique. En appliquant un filtre de cohérence aux données, nous isolons les réflexions subhorizontales ainsi que la zone de Fresnel des événements hyperboliques. En supposons que cette section représente le modèle de la croute, nous diffractons ce modèle selon plusieurs vitesses RMS. Parmi les sections synthétiques ainsi obtenues, nous déterminons celle qui ressemble le plus aux données originales. Les techniques conventionnelles de migration ne donnent pas de résultats satisfaisants sur les données de sismique profonde. Nous avons élaboré une méthode basée sur la combinaison du filtre de cohérence avec la migration par sommation le long des hyperboles de diffraction dans le plan (tau-p). Cette méthode présente l’avantage, par rapport aux méthodes classiques, d’augmenter le rapport signal/ bruit et de ne migrer que le signal sismique

Automatic extraction of reflections, modeling of diffractions and migration of ECORS deep seismic reflection data

La méthode de sismique réflexion verticale, développé par l’industrie pour étudier la partie sédimentaire du sous-sol, est devenue un outil nécessaire à l’étude de la croute continental et océanique. Le mémoire est consacré au développement de méthodes nouvelles de traitement sismique dans le but de faciliter l’interprétation des images sismiques de la croute obtenue dans le cadre du programme ECORCS. L’interprétation des images de la sismique profonde repose sur un pointé manuel qui consiste à souligner de manière subjective les réflexions cohérentes sur un nombre de traces voisines. Ce pointé se révèle délicat car les réflexions profondes apparaissent sous forme de courts segments et elles sont fréquemment contaminées par le bruit incohérent. Nous avons développé une méthode fiable basée sur la transformation (-p) locale permettant d’extraire de manière objective toute les réflexions cohérentes sur un certain nombre de traces et de conserver apparence original du signal sur la section. Les diffractions présentes sur les images de sismique profonde peuvent compliquer l’interprétation car elles sont difficiles à distinguer des réflexions pentées. Nous proposons une méthode simple permettant d’évaluer l’importance des diffractions sur une section sismique. En appliquant un filtre de cohérence aux données, nous isolons les réflexions subhorizontales ainsi que la zone de Fresnel des événements hyperboliques. En supposons que cette section représente le modèle de la croute, nous diffractons ce modèle selon plusieurs vitesses RMS. Parmi les sections synthétiques ainsi obtenues, nous déterminons celle qui ressemble le plus aux données originales. Les techniques conventionnelles de migration ne donnent pas de résultats satisfaisants sur les données de sismique profonde. Nous avons élaboré une méthode basée sur la combinaison du filtre de cohérence avec la migration par sommation le long des hyperboles de diffraction dans le plan (-p). Cette présente l’avantage, par rapport aux méthodes classiques, d’augmenter le rapport signal/ bruit et de ne migrer que le signal sismique.The classical method of seismic reflection developed by industry for the study of the sedimentary part of subsurface becomes nowadays an indispensible tool for the investigation of the upper and lower the crust. The work realized during this thesis is devoted to the development of novels techniques in seismic data processing in order to facilitate the interpretation of deep seismic images acquired during the ECORS program. The interpretation of deep seismic reflection data usually involves a manual picking of the laterally coherent reflections observed on adjacent traces. Such method is highly subjective because the deep reflections are segmented and often contaminated by incoherent noises. We have developed a method based on local (-p) transforms that permits the extraction of all coherent reflections in an objective manner and preserves the original appearance of the signal. The diffractions on the deep seismic images can complicate the interpretation since they are difficult to distinguish from dipping reflections. We propose a simple method which permits to evaluate the diffraction on the seismic section. Applying a coherency filter to the data, we separate both the sub horizontal reflections and the Fresnel zone of hyperbolic events. Assuming that this section represents the reflectivity of the crust, we diffract this model using several RMS velocities. Among the synthetic sections, we choose the section that best fits the original one. The conventional seismic migration methods do not give satisfactory results for the deep seismic data. We have developed a migration method based on the combination of coherency filter with hyperbola-summation in (-p) plane. This method improves significantly the signal/noise ratio because, contrary to classical methods it migrates only the seismic signal

Extraction automatique des reflexions, modelisation des diffractions et migration des donnees de sismique reflexion profonde ECORS

SIGLECNRS T Bordereau / INIST-CNRS - Institut de l'Information Scientifique et TechniqueFRFranc