A B S T R A C TWe study a new curvilinear scheme for wave propagation modelling in presence of topography. The discrete scheme takes advantage of recent developments in areoacoustics. Our new scheme relies on the conventional grid coupled with optimized filters to remove numerical noise in case of strong material heterogeneity. We used non-centred stencils for free surface implementation and optimized the explicit RungeKutta scheme for the time differencing. We performed a complete theoretical stability and dispersion analysis of the discrete scheme. Finally, we illustrate the numerical accuracy of the new scheme by intensive experiments.

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We present in this work an exhaustive seismic study which was performed on a subset of a wide azimuth Ocean Bottom Cable seismic acquisition shot over a carbonate field from the U.A.E. A first challenge was to reprocess these data ensuring that the azimuthal information is properly preserved all along the sequence. An additional challenge is related to the very shallow water environment which brings complexity like dispersive ground-roll and water layer related guided waves, multiples which can be generated at sea-surface, sea-floor or internally, near-surface heterogeneities. On top of that the high velocities which are encountered in these carbonate formations are detrimental to a high resolution description of reservoirs. Two main issues are presented: Wide azimuth processing: the whole sequence was designed in order to preserve the azimuthal information. Multiples were attenuated in a cascaded approach including 3D predictive deconvolution and wide azimuth high resolution radon filtering. The Common Offset Vector strategy was followed at the imaging step to build 3D Common Image Gathers versus offset and azimuth, which enables to apply an azimuthally variant residual velocity correction and then stack all azimuthal contributions in an optimal manner. This azimuth friendly processing delivers gathers which are ready for azimuthal amplitude analysis. Heterogeneities and fracture characterization: we tested a newly developed technique for imaging diffractions and compared it to the azimuthal amplitude analysis performed on Common Imaged Gathers. This pilot processing study enabled to set up an operational wide azimuth processing sequence on this dataset, several processing scenarii could be tested and compared. In particular the sequence for attenuation of multiples was considered efficient, bringing a lot of character to the final result. The implementation of multi-azimuth Diffraction Imaging over this carbonate reservoir is proving very encouraging. Numerous detected features display geologically meaningful characteristics, some of them being not visible on conventional structural attributes. This study would be among the first applications of the methodology with tangible results providing more static and dynamic fracture data become available for calibration.

TX 75083-3836, U.S.A., fax +1-972-952-9435. AbstractA major challenge in carbonates environments is to map heterogeneities which have a strong impact on oil and gas production. For example in many carbonate reservoirs, matrix porosity contains the oil in place but the permeability is mainly provided by fracture corridors. In some carbonate reservoirs, such as Kashagan field in Kazakhstan, the oil in place is essentially contained primarily in karstic caves and conduits. Therefore being able to precisely locate these heterogeneities and possibly characterize their properties is essential in these environments. Various techniques have been developed recently to locate and characterize these heterogeneities, by detailed analysis of the elastic signals recorded in seismic experiments. Among them one can think of the scattering index presented by Willis et al (2004) or all the imaging techniques developed recently under the generic name of interferometry. The first step in all processing or inversion procedures is to be able to accurately simulate the forward problem, and it is therefore of paramount importance to be able to accurately model the scattering related to fractures and caves. The numerical and computer constrains even on very large clusters limit the resolution at which a model can be described. The current practice for finite difference modeling is to use 20m grid cells for acoustic modeling and spatial discretization leading to cells 10m in size for elastic modeling. In these cases, heterogeneity associated with fracturing on smaller scales must be upscaled and distributed in an equivalent medium, which will.reproduce varaiations in traveltimes and changes in reflection coefficient but will neglect the scattered wavefields that are the subject of the recently developed methods for characterizing fracture distributions. In this paper, we present a full waveform modeling technique based on a new approach of finite differences which allows simultaneous simulation of both macro-and micro-scale heterogeneities. This algorithm is based on a multi grid approach that models the seismic response of a shot point over a large size model containing micro scale heterogeneities in a reasonable time frame (i.e., several hours on a modern cluster). We show various examples of the signals related to fractures and their importance in terms of heterogeneities interpretation, and we also examine results of simulations using 3-D modeling based on the Born approximation to help interpret and explain the synthetic seismograms.These new numerical results will have significant advantages over previous results that rely on approaches that replace the detailed spatial variations of elastic properties with media with altered velocities and densities.