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

Sparse laterally constrained inversion of surface wave dispersion curves via minimum gradient support regularization

We present a 1D laterally constrained inversion of surface wave dispersion curves based on the minimum gradient support regularization, which allows solutions with tunable sharpness in both vertical and horizontal directions. The forward modelling consists of a finite elements approach incorporated in a flexible non-parametric gradient-based inversion scheme, which has already demonstrated good stability and convergence capabilities when tested on other kinds of data. Our deterministic inversion procedure is performed in the shear-wave velocity log-space as we noticed that the associated Jacobian shows a reduced model dependency, and this, in turn, decreases the risks of local non-convexity. We show several synthetics and one field example to demonstrate the effectiveness and the applicability of the proposed approach

Structurally constrained inversion by means of a Minimum Gradient Support regularizer: examples of FD-EMI data inversion constrained by GPR reflection data

Many geophysical inverse problems are known to be ill-posed and, thus, requiring some kind of regularization in order to provide a unique and stable solution. A possible approach to overcome the inversion ill-posedness consists in constraining the position of the model interfaces. For a grid-based parameterization, such a structurally-constrained inversion can be implemented by adopting the usual smooth regularization scheme in which the local weight of the regularization is reduced where an interface is expected. By doing so, sharp contrasts are promoted at interface locations while standard smoothness constraints keep affecting the other regions of the model. In this work, we present a structurally-constrained approach and test it on the inversion of frequency-domain electromagnetic induction (FD-EMI) data using a regularization approach based on the Minimum Gradient Support (MGS) stabilizer, which is capable to promote sharp transitions everywhere in the model, i.e., also in areas where no structural a priori information is available. Using 1D and 2D synthetic data examples, we compare the proposed approach to a structurally-constrained smooth inversion as well as to more standard (i.e., not structurally-constrained) smooth and sharp inversions. Our results demonstrate that the proposed approach helps in finding a better and more reliable reconstruction of the subsurface electrical conductivity distribution, including its structural characteristics. Furthermore, we demonstrate that it allows to promote sharp parameter variations in areas where no structural information are available. Lastly, we apply our structurally-constrained scheme to FD-EMI field data collected at a field site in Eastern Germany to image the thickness of peat deposits along two selected profiles. In this field example, we use collocated constant offset ground-penetrating radar (GPR) data to derive structural a priori information to constrain the inversion of the FD-EMI data. The results of this case study demonstrate the effectiveness and flexibility of the proposed approach

3DMCD/1DLCI inversion of an FD-EMI dataset: the case study of the paleolandscape of Bourbourg/Loon-Plage (France)

An electromagnetic survey supports the archaeological field evaluation. Data calibration with electric sounding is necessary before inversion. The 1D LCI inversion is accurate enough if the horizontal variation is smooth. The 3D MCD inversion considerably reduces lateral effects. To date, the 3D/pseudo-3D is not required in the field evaluation phase

Enhancing FD-EMI in-phase observations by means of 3D inversion: the case of Orcines, France

FD-EMI in-phase data were collected at a Gallo-roman site in Orcines, France. 3D inversion unveils features, which are not visible in the recorded in-phase maps

Neotectonic Activity in the Low-Strain Broken Foreland (Santa Bárbara System) of the North-Western Argentinean Andes (26°S)

Uplift in the broken Andean foreland of the Argentine Santa Bárbara System (SBS) is associated with the contractional reactivationof basement anisotropies, similar to those reported from the thick-skinned Cretaceous-Eocene Laramide province of NorthAmerica. Fault scarps, deformed Quaternary deposits and landforms, disrupted drainage patterns, and medium-sizedearthquakes within the SBS suggest that movement along these structures may be a recurring phenomenon, with yet to bedefined repeat intervals and rupture lengths. In contrast to the Subandes thrust belt farther north, where eastward-migratingdeformation has generated a well-defined thrust front, the SBS records spatiotemporally disparate deformation along structuresthat are only known to the first order. We present herein the results of geomorphic desktop analyses, structural fieldobservations, and 2D electrical resistivity tomography and seismic-refraction tomography surveys and an interpretation ofseismic reflection profiles across suspected fault scarps in the sedimentary basins adjacent to the Candelaria Range (CR)basement uplift, in the south-central part of the SBS. Our analysis in the CR piedmont areas reveals consistency between theresults of near-surface electrical resistivity and seismic-refraction tomography surveys, the locations of prominent fault scarps,and structural geometries at greater depth imaged by seismic reflection data. We suggest that this deformation is driven bydeep-seated blind thrusting beneath the CR and associated regional warping, while shortening involving Mesozoic and Cenozoicsedimentary strata in the adjacent basins was accommodated by layer-parallel folding and flexural-slip faults that cut throughQuaternary landforms and deposits at the surface.Fil: Arnous, Ahmad. Instituto Miguel Lillo, Universidad Nacional de Tucumán; Argentina. University Of Postdam; AlemaniaFil: Zeckra, Martin. University Of Postdam; AlemaniaFil: Venerdini, Agostina Lia. Universidad Nacional de San Juan. Facultad de Ciencias Exactas, Físicas y Naturales. Departamento de Geofísica y Astronomía; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - San Juan. Centro de Investigaciones de la Geosfera y Biosfera. Universidad Nacional de San Juan. Facultad de Ciencias Exactas Físicas y Naturales. Centro de Investigaciones de la Geosfera y Biosfera; ArgentinaFil: Alvarado, Patricia Monica. Universidad Nacional de San Juan. Facultad de Ciencias Exactas, Físicas y Naturales. Departamento de Geofísica y Astronomía; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - San Juan. Centro de Investigaciones de la Geosfera y Biosfera. Universidad Nacional de San Juan. Facultad de Ciencias Exactas Físicas y Naturales. Centro de Investigaciones de la Geosfera y Biosfera; ArgentinaFil: Arrowsmith, Ramón. School Of Earth And Space Exploration, Arizona State Un; Estados UnidosFil: Guillemoteau, Julien. University Of Postdam; AlemaniaFil: Landgraf, Angela. University Of Postdam; AlemaniaFil: Gutiérrez, Antonio. Instituto Miguel Lillo, Universidad Nacional de Tucumán; ArgentinaFil: Strecker, Manfred R.. University Of Postdam; Alemani

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

Fast approximate 2D inversion of airborne TEM data: Born approximation and empirical approach

International audienceAirborne transient electromagnetic surveying provides data sections with a sufficient coverage to perform 2D imaging of electrical conductivity within the ground. Full 2D inversion using numerical modeling with finite differences or finite elements is still a time-consuming method to process the large amount of data acquired during an airborne survey. 2D structures increase the complexity of eddy current patterns within the ground. Consequently, fast approximate imaging using 2D sensitivities of equivalent homogeneous media is not sufficient and causes strong artefacts in the resulting model. To overcome this problem, one prefers to use 1D inversion or 3D inversion using local sensitivity to process this kind of data. However, we consider a fast 2D inversion to be reachable. By estimating numerically the 2D sensitivity caused by 2D perturbations and showing that it differs considerably from the ones derived from homogeneous media, we propose an empirical model for in-loop configuration which describes the numerical 2D sensitivity. By applying this method to synthetic data, we show that it eliminates 2D artefacts which are often encountered when using approximate inverse methods based on the theory of equivalent homogeneous half-space. An application to real in-loop data illustrates this improvement for imaging a dipping layer of conductive graphite deposits in Canada. This method is relatively fast. It could provide a better understanding of the ground during the survey and would allow geophysicists to better manage the whole campaign

Methodology for airborne electromagnetic imaging

Ce travail de thèse apporte des éléments méthodologiques pour l'interprétation rapide de données électromagnétiques transitoires (TEM) aéroportées. Dans un premier temps, nous nous sommes intéressés au problème de l'inversion 1D qui est, à ce jour, un traitement standard. Nous nous sommes ensuite focalisés sur le problème de l'inversion 2D rapide. En nous basant sur les résultats de modélisations numériques, nous proposons un modèle empirique de sensibilité pour effectuer une inversion 2D rapide. Les tests effectués sur des données synthétiques et réelles s'avèrent très prometteurs. Ensuite, nous étudions analytiquement l'effet de la topographie. Pour un demi-espace homogène incliné, nous montrons que la direction des courants induits dans le sous-sol dépend de l'angle de l'interface air/sol. Nous proposons alors une méthode pour prendre en compte cet effet. Enfin, nous testons nos méthodes de traitement sur des données VTEM acquises dans le bassin de Franceville au Gabon.This PhD work provides methodological tools for the fast interpretation of airborne transient electromagnetic data (ATEM). As our first step, we focused on the problem of 1D ground imaging, which is currently a standard treatment. Then, we focused on the fast 2D imaging problem. We proposed an empirical sensitivity function that allows fast 2D inversion. The results of the 2D empirical inversion when applied on both synthetic and real data sets are very promising. In the third part, we analyzed the effect of topography on the airborne EM data. For an inclined homogeneous half-space, we show that the direction of the ground induced current is parallel to the surface air/ground. Then, we proposed a method to take into account this effect. Finally, the tools developed during this PhD have been applied to a real VTEM data set acquired over the basin of Franceville, in Gabon