3D land CSEM with a single transmitter position

International audienceAnthropogenic noise, cost and logistical constrains generally limit to the use of land CSEM to a singletransmitter position for the deep imaging of the electrical conductivity. As the inversion of CSEM data inthe near field using a single transmitter position suffers from critical sensitivity singularities, we proposeda robust inversion framework adapted to this ill-conditioned inversion problem. The framework reliesspecifically on a robust Gauss-Newton solver, several model parameter transformations to compensate forthe heterogeneous sensitivities, and on the reformulation of the CSEM data under the form of a pseudo-MT tensor. We describe here the approach used for modelling and inversion implemented in our codePOLYEM3D and the new pseudo-MT formulation. We illustrate its application on a pathological syntheticcase inspired from Grayver et al. (2013) and then show the application of the process to a real CSEMdataset acquired in the context of thermal water prospection

GEOTHERMAL EXPLORATION IN MARTINIQUE BASED ON MAGNETO-TELLURIC AND CONTROLLED-SOURCE EM SURVEY

International audienceResistivity imaging is a key input in many geothermal exploration programs, and particularly in volcanic environment. It is assumed that resistivity variations allow imaging the caprock, but also hydrothermal weathering and preferential geothermal fluid flow. The latter is generally a brine which drastically decreases the electrical resistivity and the temperature increase also lowers resistivity. Several authors have described resistivity signatures for some geothermal reservoirs context. Magnetotelluric (MT) is generally used to image resistivity variations down to a relevant investigation depth of several kilometers. Within the framework of a global French program oriented towards the development of geothermal energy, Martinique Island (Lesser Antilles, France) has been extensively investigated (from 2012 to 2013) through an integrated multi-method approach, with the aim to define precisely the potential geothermal structures previously highlighted. Amongst the common investigation methods deployed, we carried out three magnetotelluric (MT) surveys located above three out of the most promising geothermal areas of Martinique, namely the Anses d'Arlet (focus of this presentation), the Montagne Pelée and the Pitons du Carbet prospects. Based on natural source signal, the quality of broad band MT data depends on the source activity (lightning strikes and sun) during the survey but also on the local noise conditions. In many cases, because industrial activities have been developed close to the geothermal resource, the noise conditions make it difficult to obtain reliable MT tensors and therefore a good resistivity distribution. A common way to improve the data quality is to record longer time series and to apply robust processing with remote reference including noise filtering

3D land CSEM inversion in noisy environment with a single transmiter: inversion approach and application for geothermal water prospection

International audienceAnthropogenic noise, cost and logistical constrains generaly limit to the use of land CSEM to a few transmiter positions for the deep imaging of the electrical conductivity. The 3D inversion of CSEM data in the near field using a single transmiter position suffers from critical sensitivity singularities. We proposed a robust inversion framework adapted to this ill-conditioned inversion problem. The framework relies specificaly on a robust Gauss-Newton solver, model parameter transformations to compensate the heterogeneous sensitivies, and on the reformulation of the near field CSEM data under the form of a pseudo-MT tensor. We describe the approach used for modeling and inversion implemented in our code POLYEM3D and show the advantages of pseudo-MT tensor formulation. The strategy have been tested on a pathologic synthetic case inspired from grayver et al (2013), and then was successfully applied to a real CSEM dataset acquired in the context of thermal water prospection in a noisy environnement

Imagerie multi-fréquentielle d'un réservoir géothermal au Lamentin (Martinique, France) par méthode de Longue Electrode Mise-à-la-Masse

National audienceWithin the frame of geothermal exploration, a CSEM survey was performed at the Le Lamentin area (Martinique, French Indies) using 400m long energized metallic casings of two deep exploration boreholes as long electrodes for current injection (the so-called double Longue Electrode Mise-à-la-Masse setup, hereafter 2xLEMAM). Apparent resistivity maps were derived from the in-phase electric fields. Frequency dependent apparent resistivity maps and profiles reveal a very conductive area north of the Fort de France Bay connected to a known poly-phased geothermal system and shallow salt water intrusion. The most conductive body is proposed to be the geo-electrical signature of an active hydrothermal system, superimposed on the signature of a conductive fossilized geothermal system. It is spatially well correlated with high temperature borehole logs

High resolution quantitative seismic imaging of a strike-slip fault with small vertical o set in clay-rocks from underground galleries. Experimental Platform of Tournemire, France.

Imaging tectonic faults with small vertical offsets in argilittes (clay-rock) using geophysical methods is challenging. In the context of deep radioactive waste disposals, the presence of such faults has to be assessed since they can modify the rock confining properties. In the Tournemire Experimental Platform (TEP, France), fault zones with small vertical offsets and complex shape have been identified from underground works. However, 3D high-resolution surface seismic methods have shown limitations in this context that led us to consider the detection and characterization of the faults directly from underground works. We investigate here the potential of seismic full waveform inversion (FWI) applied in a transmission configuration to image the clay-rock medium in a horizontal plane between galleries, and compare it with first-arrival traveltime tomography (FATT). Our objective is to characterize seismic velocities of a block of argilittes crossed by a subvertical fault zone with a small vertical offset. The specific measurement configuration allows us to neglect the influence of the galleries on the wave propagation and to simplify the problem by considering a 2D isotropic horizontal imaging domain. Our FWI scheme relies on a robust adaptation of early-arrival waveform tomography. The results obtained with FATT and FWI are in accordance and both correlate with the geological observations from the gallery walls and boreholes. We show that even though various simplifications are done in the inversion scheme and only a part of the data is used, FWI allows to get higher resolution images than FATT, and is especially less sensitive to the incomplete illumination as it uses also diffracted energy. The results provided in this study highlight the complexity of the fault zone, showing a complex interaction of the main fault system with a secondary system composed of decimetric fractures associated with the presence of water

Characterization of intermediate depth reservoirs in complex coastal areas with CSEM method

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Characterization of Deep Geothermal Energy Resources in Low enthalpy sedimentary basins in Belgium using Electro-Magnetic Methods – CSEM and MT results

International audienceSedimentary basins in Northwest Europe have significant potential for low to medium enthalpy, deep geothermal energy resources. These resources are generally assessed using standard seismic exploration techniques to resolve geological structures. The ElectroMagnetic campaign carried-out in Mol area (Belgium) has shown that despite the presence of high level of industrialization, the resistivity of deep formations (>3km) can be recovered from MT and CSEM methods and hence provide very valuable information for the assessment of geothermal resources. 1. INTRODUCTION Sedimentary basins in Northwest Europe have significant potential for low to medium enthalpy, deep geothermal energy resources. These resources are generally assessed using standard seismic exploration techniques to resolve geological structures. However, the electrical resistivity parameter, which can be directly impacted by the presence of a geothermal reservoir is rarely investigated in such context. Therefore, the development of alternative and complementary exploration techniques such as Electromagnetic (EM) techniques may have an important role in reducing the cost and uncertainty associated with geothermal resource assessment

Establishment and analysis of a reference transcriptome for Spodoptera frugiperda

International audienceBackground Spodoptera frugiperda (Noctuidae) is a major agricultural pest throughout the American continent. The highly polyphagous larvae are frequently devastating crops of importance such as corn, sorghum, cotton and grass. In addition, the Sf9 cell line, widely used in biochemistry for in vitro protein production, is derived from S. frugiperda tissues. Many research groups are using S. frugiperda as a model organism to investigate questions such as plant adaptation, pest behavior or resistance to pesticides.ResultsIn this study, we constructed a reference transcriptome assembly (Sf_TR2012b) of RNA sequences obtained from more than 35 S. frugiperda developmental time-points and tissue samples. We assessed the quality of this reference transcriptome by annotating a ubiquitous gene family - ribosomal proteins - as well as gene families that have a more constrained spatio-temporal expression and are involved in development, immunity and olfaction. We also provide a time-course of expression that we used to characterize the transcriptional regulation of the gene families studied.ConclusionWe conclude that the Sf_TR2012b transcriptome is a valid reference transcriptome. While its reliability decreases for the detection and annotation of genes under strong transcriptional constraint we still recover a fair percentage of tissue-specific transcripts. That allowed us to explore the spatial and temporal expression of genes and to observe that some olfactory receptors are expressed in antennae and palps but also in other non related tissues such as fat bodies. Similarly, we observed an interesting interplay of gene families involved in immunity between fat bodies and antennae

The Genome Sequence of the Grape Phylloxera Provides Insights into the Evolution, Adaptation, and Invasion Routes of an Iconic Pest

Background: Although native to North America, the invasion of the aphid-like grape phylloxera Daktulosphaira vitifoliae across the globe altered the course of grape cultivation. For the past 150 years, viticulture relied on grafting-resistant North American Vitis species as rootstocks, thereby limiting genetic stocks tolerant to other stressors such as pathogens and climate change. Limited understanding of the insect genetics resulted in successive outbreaks across the globe when rootstocks failed. Here we report the 294-Mb genome of D. vitifoliae as a basic tool to understand host plant manipulation, nutritional endosymbiosis, and enhance global viticulture. Results: Using a combination of genome, RNA, and population resequencing, we found grape phylloxera showed high duplication rates since its common ancestor with aphids, but similarity in most metabolic genes, despite lacking obligate nutritional symbioses and feeding from parenchyma. Similarly, no enrichment occurred in development genes in relation to viviparity. However, phylloxera evolved > 2700 unique genes that resemble putative effectors and are active during feeding. Population sequencing revealed the global invasion began from the upper Mississippi River in North America, spread to Europe and from there to the rest of the world. Conclusions: The grape phylloxera genome reveals genetic architecture relative to the evolution of nutritional endosymbiosis, viviparity, and herbivory. The extraordinary expansion in effector genes also suggests novel adaptations to plant feeding and how insects induce complex plant phenotypes, for instance galls. Finally, our understanding of the origin of this invasive species and its genome provide genetics resources to alleviate rootstock bottlenecks restricting the advancement of viticulture

Modélisation physique à échelle réduite pour l'adaptation de l'inversion des formes d'ondes sismiques au génie civil et à la subsurface.

Full Wave Inversion (FWI) is a multiparameter quantitative method of seismic imaging that is currently booming. Taking into account all propagation phenomena recorded, especially the surface waves, makes it a promising technique for near surface applications on geotechnical and environmental issues. However, adapting a seismic method validated on synthetic data to the context of near surface or on deep imaging applications remains a difficult task. What we are proposing here is a reduced scale physical modelling approach using a laboratory of non contact laser-ultrasonic measurement, which will have been set up as part of this thesis. The various elements of the laboratory were validated for reduced scale seismic modelling, and particular attention has been paid to the simulation of the source. The acquired data has been validated by comparison with data obtained by numerical modelling using algorithmes provided by Géoazur within the context of the ANR SEISCOPE project. In a second step, we evaluated the potential of the use of FWI for near surface applications by using a joint approach of numerical modelling and experimental modelling. By this dual approach, we have thus shown that FWI allows to obtain information on the morphology of underground cavities by taking into account surface waves. Furthermore, on the various multilayer models we studied, the FWI was able to exploit the information on the most superficial areas supplied by dispersive surface waves.L'inversion des formes d'ondes (FWI) est une méthode d'imagerie sismique quantitative multiparamètre actuellement en plein essor. La prise en compte de l'ensemble des phénomènes de propagation enregistrés, en particulier des ondes de surface, en font une technique prometteuse pour des applications de la subsurface sur des problématiques géotechniques ou environnementales. Cependant, l'adaptation au contexte de la subsurface d'une méthode sismique validée numériquement ou sur des applications d'imagerie profonde est une tâche difficile. Nous proposons ici une approche par modélisation physique à échelle réduite à l'aide d'un laboratoire de Mesures laser-Ultrasonore Sans Contact (MUSC) mis en place dans le cadre de cette thèse. Les différents éléments du laboratoire ont été caractérisés pour la modélisation sismique à échelle réduite et une attention particulière a été portée à la simulation de la source. Les données acquises ont été validées par confrontation avec des données obtenues par modélisation numérique à l'aide des codes fournis par Géoazur dans le cadre du projet ANR SEISCOPE. Dans un deuxième temps, le potentiel de la FWI pour des applications de la subsurface a été évalué par l'approche conjointe de la modélisation numérique et de la modélisation expérimentale. Nous avons ainsi pu montrer par cette double approche que la FWI permet d'obtenir une information sur la morphologie de cavités souterraines grâce à la prise en compte des ondes de surface. Par ailleurs, sur les différents modèles multicouches étudiés, l'information sur les zones les plus superficielles fournie par les ondes de surface dispersives a pu être exploitée par la FWI