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La communication concerne la thématique Sciences de la Terre, mais ni atmosphère, climat, hydrologie, ni sismologie stricto-sensu; Le traitement sismique sur la grille et sa communauté d'utilisateurs EGEODE

International audience; We present a sensitivity study applied to water front monitoring of an onshore oil reservoir, using a remote controlled source electromagnetic method (CSEM) with electric dipoles and a borehole‐to‐surface configuration. We have developed an optimized and parallelized code based on the method of moments, in order to study the influence of several static or time‐varying background uncertainties on the time‐lapse CSEM signal (also called 4‐D CSEM). Analysis of the relative and absolute variations in phase or quadrature of the time‐lapse signal induced by the fluid substitution process, inside the reservoir, has shown that the vertical electric dipole allows the shape of the water front to be monitored, while remaining less sensitive (compared to a horizontal electric source dipole) to the total volume of substituted fluid. We have examined the influence of missed anomalies (1‐D/3‐D), with more or less conductive properties, near to the ground surface or the reservoir, and with or without time‐varying properties. In most cases, the 4‐D signal behaves like a reliable filter, canceling almost all response anomalies. However, it can also lead to strong, local perturbations of the time‐lapse signal. We have also shown that in the presence of steel cased boreholes at the source location, or with outlying steel cased boreholes, the recording of exploitable data does not present insurmountable difficulties at low frequencies (∼1 Hz), and for a dense array of surface receivers. These positive results with CSEM monitoring suggest that minimal, coarse‐time 3‐D explorations should be used to ensure reliable interpretation of the monitored data.

Full Waveform Inversion (FWI) is a depth imaging technique that takes advantage of the full information contained in recorded seismic data. FWI provide high resolution images of subsurface properties, usually seismic velocities or related parameters, although in theory it could image any property used to formulate the wave equation. The computational cost of the methodology has historically limited its application to 3D acoustic approximations but recent developments in hardware capabilities have increased computer power to the point that more realistic approximations are viable. In this work the traditional acoustic approximation is extended to include elastic effects by introducing the elastic wave equation as the governing law that describes wave propagation. I have developed a software based on finite-differences to solve the elastic wave equation in 3D, which I applied in the development of a full-waveform inversion algorithm. The software is fully parallelised for both distributed and shared-memory systems. The first level of parallelisation distributes seismic sources across cluster nodes. Each node solves the 3D elastic wave equation in the whole computational domain. The second level of parallelisation takes advantage of present multi-core computer processor units (CPU) to decompose the computational domain into different volumes that are solved independently by each core. Such parallel design allows the algorithm to handle models of realistic sizes, increasing the computational times only a factor of two compared to those of 3D acoustic full-waveform inversion on the same mesh. I have also implemented a perfectly matched layer absorbing boundary condition to reproduce a semi-infinite model geometry and prevent spurious reflections from the model boundaries from contaminating the modelled wavefields. The inversion algorithm is based upon the adjoint-state method, which I reformulated for the wave equation that I implemented, which was based on particle-velocities and stresses, providing a comparison and demonstration of equivalence with previous developments. To examine the performance of the code, I have inverted several synthetic problems of increasing realism. I have principally used only pressure sources and receivers to assess the potential of the method's application to the most common industry surveys: streamer data for offshore and vertical geophones (only one component) for onshore exploration surveys. The results show that the imaged properties increase with the heterogeneity of the models, due to the increase in P-S-P conversions which provides the main source of information to invert shear-wave velocity models from pressure sources and receivers. It remains to demonstrate the inversion of field datasets and my future research project will focused on achieving this goal.

Elastic full-waveform inversion is an ill-posed data-fitting procedure that is sensitive to noise, inaccuracies of the starting model, definition of multiparameter classes, and inaccurate modeling of wavefield amplitudes. We have investigated the performance of different minimization functionals as the least-squares norm [Formula: see text], the least-absolute-values norm [Formula: see text], and combinations of both (the Huber and so-called hybrid criteria) with reference to two noisy offshore (Valhall model) and onshore (overthrust model) synthetic data sets. The four minimization functionals were implemented in 2D elastic frequency-domain full-waveform inversion (FWI), where efficient multiscale strategies were designed by successive inversions of a few increasing frequencies. For the offshore and onshore case studies, the [Formula: see text]-norm provided the most reliable models for P- and S-wave velocities ([Formula: see text] and[Formula: see text]), even when strongly decimated data sets that correspond to few frequencies were used in the inversion and when outliers polluted the data. The [Formula: see text]-norm can provide reliable results in the presence of uniform white noise for [Formula: see text] and [Formula: see text] if the data redundancy is increased by refining the frequency sampling interval in the inversion at the expense of computational efficiency. The [Formula: see text]-norm and the Huber and hybrid criteria, unlike the [Formula: see text]-norm, allow for successful imaging of the [Formula: see text] model from noisy data in a soft-seabed environment, where the P-to-S-waves have a small footprint in the data. However, the Huber and hybrid criteria are sensitive to a threshold criterion that controls the transition between the criteria and that requires tedious trial-and-error investigations for reliable estimation. The [Formula: see text]-norm provides a robust alternative to the [Formula: see text]-norm for inverting decimated data sets in the framework of efficient frequency-domain FWI.

The Aghajari field is located in the Dezful Embayment, one of major structural zone in Zagros fold-thrust belt containing most of Iranian oil fields. The late Oligocene-early Miocene Asmari formation is one of the main reservoir rocks of SW Iran with several decades of production history from different oil fields in the region. The objective of the present Ph-D thesis is to improve the structural understanding of the Aghajari field using 3D seismic imaging for the first time, and well seismic. the lack of seismic response of the dipping horizons in the reservoir oil zone constitutes a major identified difficulty. Given the dip values of 30° to 45° of the reflectors in the pay zone, the P-P reflections are expected to appear mainly on the horizontal components of the VSP data, justifying the processing of the 3 components (3C). An innovative method of 3C VSP orientation was developed. The 3C VSP results confirm the presence of dipping reflectors, with dip values in agreement with surface seismic, on the eastern flank of the structure. Several new approaches of seismic imaging have been applied in order to investigate and improve the reservoir illumination. The “Beyond Dix” workflow of CGGVERITAS was applied to land seismic data for the first time; this process enables to build a depth/velocity model from the dips read on the PSTM image (pre-stack time migration), and the residual move out corrections. The depth migrated image is improved in the overburden, unfortunately not at reservoir level, due to low signal to noise, interference with multiples, thus poor dip and velocity determination in the reservoir interval. A new Kirchhoff PSTM prototype technique was developed in IFP, allowing a selection of azimuth sector, offset range and geological dip, with automatic optimization of the local dip. The results from this new technique show clear improvements in dip determination and signal to noise ratio of the migrated image. The additional study of another exploration anticline in Zagros, using surface seismic, well data, VSP and geological information illustrates how an integrated data analysis can improve the understanding of deep structures, and therefore help making more reliable drilling decisions, if achieved timely. In Zagros, Surface seismic is definitely a good tool for the structural exploration, but its application to fine reservoir studies has not been demonstrated, in spite of the efforts achieved in the present study.; Le Champ d'Aghajari est situé dans le Dezful, une des zones du Zagros structurée par d'importantes failles de charriage, et contenant la majeure partie des champs d'huile iraniens. La formation calcaire de l'Asmari, datant de la fin Oligocène à début Miocène, constitue l'une des principales couches réservoir du Sud-Ouest de l'Iran, avec un historique de production de plus d'un demi siècle pour plusieurs champs de cette région. L'objectif de cette thèse est de mieux comprendre la structure de ce champ, connu principalement par les puits de production,grâce à la première sismique 3D récemment acquise et en intégrant les informations fournies par la sismique de puits. Le problème principal est que le toit et la base du réservoir n'apparaît pas dans la zone à huile du réservoir, où les interfaces sont très pentés. Étant donné l'importance des pendages (environ 40°), les réflexions en sismique de puits zéro offset sont attendues principalement sur les composantes horizontales, ce qui rend impératif le traitement des 3 composantes. Une méthode originale d'orientation des 3 composantes est employée et les résultats confirment laprésence de réflecteurs pentés, en accord avec la sismique de surface sur le côté Est de la structure.Plusieurs nouvelles approches d'imagerie ont été appliquées afin de comprendre et résoudre les problèmes d'illumination de la zone réservoir. Nous avons appliqué pour la première fois en terrestre l'approche « Beyond Dix » de CGGVERITAS qui permet de construire un modèle profondeur à partir de pendages et courbure résiduelles move-out mesurée sur les images PSTM (pré-stack time migration). Les résultats migrés en profondeur sont plus précis dans le recouvrement mais la zone réservoir n'est pas améliorée. La raison en est queles images temps ne permettent pas de mesurer les pendages et courbures résiduelles dans la zone d'intérêt en raison d'une part de fort multiples et d'autre part d'u niveau de bruit important.Une nouvelle technique prototype de migration PSTM a été développée à l' IFP, permettant une sélection de l'azimut, du pendage géologique et la recherche automatique du meilleur pendage locale. L'application de ces techniques est très encourageante et montre qu'il est possible d'extraire les réflexions pentées en améliorant le rapport signal sur bruit de la migration elle-même.L'étude additionnelle d'exploration d'une autre structure anticlinale du Zagros, par utilisation combinée de sismique de surface, des données de puits, du PSV et de l'information géologique, illustre comment l'analyse intégrée des données permet d'améliorer la compréhension des structures profondes, et de contribuer à prendre de meilleures décisions de forage, dans la mesure où cette intégration est suffisamment rapide.Au final, dans le Zagros, la sismique de surface est définitivement une bonne méthode d'exploration structurale, mais son application aux études fines de réservoir n'a pas été démontrée, malgré les efforts effectués.

International audience; From a geophysical point of view, curvelets can be seen as the representation of local plane waves. They can be useful for a series of seismic processing tasks. Here we propose to use the decomposition in the curvelet domain in the context of velocity model scanning for prestack time migration. We start from an initial prestack time migration that we decompose into curvelet components. Each individual component is kinematically demigrated and then remigrated in a new velocity model. The demigration/migration sequence is reduced to a simple mapping of the curvelet coefficients and avoids the classical summations along isochrones. An application on a 2D real data set shows the advantages and the current limitations of the time demigration/migration approach.

The Aghajari field is located in the Dezful Embayment, one of major structural zone in Zagros fold-thrust belt containing most of Iranian oil fields. The late Oligocene-early Miocene Asmari formation is one of the main reservoir rocks of SW Iran with several decades of production history from different oil fields in the region. The objective of the present Ph-D thesis is to improve the structural understanding of the Aghajari field using 3D seismic imaging for the first time, and well seismic. the lack of seismic response of the dipping horizons in the reservoir oil zone constitutes a major identified difficulty. Given the dip values of 30° to 45° of the reflectors in the pay zone, the P-P reflections are expected to appear mainly on the horizontal components of the VSP data, justifying the processing of the 3 components (3C). An innovative method of 3C VSP orientation was developed. The 3C VSP results confirm the presence of dipping reflectors, with dip values in agreement with surface seismic, on the eastern flank of the structure. Several new approaches of seismic imaging have been applied in order to investigate and improve the reservoir illumination. The “Beyond Dix” workflow of CGGVERITAS was applied to land seismic data for the first time; this process enables to build a depth/velocity model from the dips read on the PSTM image (pre-stack time migration), and the residual move out corrections. The depth migrated image is improved in the overburden, unfortunately not at reservoir level, due to low signal to noise, interference with multiples, thus poor dip and velocity determination in the reservoir interval. A new Kirchhoff PSTM prototype technique was developed in IFP, allowing a selection of azimuth sector, offset range and geological dip, with automatic optimization of the local dip. The results from this new technique show clear improvements in dip determination and signal to noise ratio of the migrated image. The additional study of another exploration anticline in Zagros, using surface seismic, well data, VSP and geological information illustrates how an integrated data analysis can improve the understanding of deep structures, and therefore help making more reliable drilling decisions, if achieved timely. In Zagros, Surface seismic is definitely a good tool for the structural exploration, but its application to fine reservoir studies has not been demonstrated, in spite of the efforts achieved in the present study., Le Champ d'Aghajari est situé dans le Dezful, une des zones du Zagros structurée par d'importantes failles de charriage, et contenant la majeure partie des champs d'huile iraniens. La formation calcaire de l'Asmari, datant de la fin Oligocène à début Miocène, constitue l'une des principales couches réservoir du Sud-Ouest de l'Iran, avec un historique de production de plus d'un demi siècle pour plusieurs champs de cette région. L'objectif de cette thèse est de mieux comprendre la structure de ce champ, connu principalement par les puits de production,grâce à la première sismique 3D récemment acquise et en intégrant les informations fournies par la sismique de puits. Le problème principal est que le toit et la base du réservoir n'apparaît pas dans la zone à huile du réservoir, où les interfaces sont très pentés. Étant donné l'importance des pendages (environ 40°), les réflexions en sismique de puits zéro offset sont attendues principalement sur les composantes horizontales, ce qui rend impératif le traitement des 3 composantes. Une méthode originale d'orientation des 3 composantes est employée et les résultats confirment laprésence de réflecteurs pentés, en accord avec la sismique de surface sur le côté Est de la structure.Plusieurs nouvelles approches d'imagerie ont été appliquées afin de comprendre et résoudre les problèmes d'illumination de la zone réservoir. Nous avons appliqué pour la première fois en terrestre l'approche « Beyond Dix » de CGGVERITAS qui permet de construire un modèle profondeur à partir de pendages et courbure résiduelles move-out mesurée sur les images PSTM (pré-stack time migration). Les résultats migrés en profondeur sont plus précis dans le recouvrement mais la zone réservoir n'est pas améliorée. La raison en est queles images temps ne permettent pas de mesurer les pendages et courbures résiduelles dans la zone d'intérêt en raison d'une part de fort multiples et d'autre part d'u niveau de bruit important.Une nouvelle technique prototype de migration PSTM a été développée à l' IFP, permettant une sélection de l'azimut, du pendage géologique et la recherche automatique du meilleur pendage locale. L'application de ces techniques est très encourageante et montre qu'il est possible d'extraire les réflexions pentées en améliorant le rapport signal sur bruit de la migration elle-même.L'étude additionnelle d'exploration d'une autre structure anticlinale du Zagros, par utilisation combinée de sismique de surface, des données de puits, du PSV et de l'information géologique, illustre comment l'analyse intégrée des données permet d'améliorer la compréhension des structures profondes, et de contribuer à prendre de meilleures décisions de forage, dans la mesure où cette intégration est suffisamment rapide.Au final, dans le Zagros, la sismique de surface est définitivement une bonne méthode d'exploration structurale, mais son application aux études fines de réservoir n'a pas été démontrée, malgré les efforts effectués.

International audience

International audience

International audience; It has been shown on an 'ideal' synthetic dataset that PP/PS-stereotomography can estimate an accurate velocity model without any pairing of PP- and PS-events. The P-wave velocity model is first estimated using PP data and then, fixing this velocity field, the S-wave velocity is estimated using the PS data. This method needed to be evaluated further and we present here the first application of PP/PS-stereotomography to a real dataset: the 2D East-West Mahogany OBC line (Gulf of Mexico). We are here confronted with data which do not fit our working assumptions: coherent noise (due to an approximate separation of PP- and PS-events and some remaining multiples), probably some anisotropy and 3D effects. With a careful selection of the stereotomographic picks, which allows one to decrease the effect of the picked coherent noise by the automatic picker, our application can demonstrate the relevance of our approach in the upper part of the profile, where anisotropy and 3D effects might be low. We can thus estimate, without any pairing of PP- and PS-events, a velocity field which provides not only flat common image gathers, but also PP- and PS-depth migrated images located at the same positions. For the deeper part of the profile, a significant shift in depth appears. In addition to possible anisotropy, 3D effects and a more complex velocity field ('salt body'), this is due to the quality of the PZ- and X-components profiles: The PZ-component profile where the PP-stereotomographic picking is performed, is polluted by conflicting converted or multiple events and the X-component profile, where the PS-stereotomographic picking is performed, is highly noisy. This study emphasizes the need to develop accurate selection criteria for the stereotomographic picks

International audience