Aline Bourgeois

System for measurement of the acoustic coefficient of reflection of submerged reflectors

System to measure the coefficients of reflection of submerged surfaces such as the seabed, by comparison of the intensities of acoustic impulses propagated between a source and a receptor, both of which are submerged, and that of the echoes of these impulses on the reflectors. For this purpose, there is provided a programmed calculator, an interface card to communicate with different units: an echo-sounder, a source triggering detector, a shot box to control the source and a recorder, a programmable attenuator, an acquisition card driven by the calculator and bearing an adjustable type filter and a circuit, also controlled by the calculator, for modifying the functioning parameters of the attenuation and of the filter, and fixing the starting parameters.

Method of improving the restoration of images supplied by a lateral sonar and a device for implementing same

A method and device for improving the restoration of the images obtained from samples taken from acoustic signals received by a lateral aimed sonar provides for the selection of samples from the signals received, the time interval between the samples selected being smaller at the beginning of the echo reception window than at the end thereof so as to improve the power of resolution of the sonar for directions of low obliqueness. The sampling may be carried out at a variable frequency or else at a very high fixed frequency, the selection then being made among all the samples obtained.

Qualitative And Quantitative Interpretation of 4D Pre-stack Synthetic Seismic Data

This paper presents an exhaustive time-lapse feasibility study where synthetic pre-stack seismic data are computed as function of reservoir production along elapsed time, and interpreted in a qualitative and quantitative ways. Geological model, based on a real gas storage site belonging to Gaz de France, is described in terms of lithofacies and converted to elastic models through elapsed time by using results of reservoir simulation together with a petroacoustic database. Two types of modeling (1D convolution and 3D fullwave) have been performed to obtain pre-stack data. Pre-stack joint inversion has next be used to derive P and S impedance cubes. Quantitative and qualitative interpretation conducted on both synthetic cubes and derived impedances enhance the lack of additional information given by the angle dimension and S-impedance in this particular case. Encouraging results come from the very good estimation of gas in place that can be obtained from P-impedance.

The Mesa Verde outcrop: Seismic modeling, processing, and interpretation

For a realistic geological model a 2D numerical elastic modeling is carried out. The objectives is, first, a better understanding of the information content of reflection seismic data and, second, to provide synthetic data for testing and evaluating the efficiency of processing and inversion techniques under known conditions. Of particular interest are the small-scale heterogeneities in view of reservoir delineation and characterization. The geologic model is constructed from the Campanian cross-section of the Mesa Verde outcrop (Colorado, USA). A subsurface seismic model buried in a 1D background medium is defined from the 2D Mesa Verde geologic model by assigning P-wave and S-wave velocities and densities to each geologic facies. This seismic model is used as input for a fully elastic seismic simulating a realistic marine survey of 280 shots. The synthetic seismograms are calculated by a finite-difference method. The data are processed in a conventional way; then a target-related post-processing is performed including stratigraphic inversion. Finally, a geologic significance is given to waveform and amplitude variations in the estimated P-impedances.

SEISMIC INVERSION AND THE IMPACT OF A PRIORI INFORMATION: Advanced Geophysical Modeling and Inversion Unification (AGU): Final Report of the GEOSCIENCE Project JOUF-0037

Over the past decade, theory and experiments have shown that changes of reflection amplitude with offset (AVO) in multi-offset seismic records are strongly affected by relative changes of Poissons ratio. AVO analysis opens the possibility to distinguish ‘bright spots’ due to gas accumulation from those caused by other changes in lithology AVO analysis is generally carried out with ray tracing and the Zoeppritz equations as tools for forward modeling. Some inversion procedures have been developed to infer changes in the contrast of the elastic parameters. However, it was recognized that the quality of the available modeling and inversion techniques was insufficient. The alternative approach we propose for the modeling is (i) an extension of the reflectivity method to smoothly varying media, and (ii) a cost-efficient linearized finite-difference technique.In our study we have minimized structural difficulties in order to concentrate on a 2D target zone in a quasi-layered background media. Then small changes in elastic parameter contrasts can be estimated by inversion using a linear least-squares approach: the waveform match between the synthetic and the seismic multi-offset gathers is obtained by perturbing the model parameters in the target zone. We have also evaluated the impact of the use of a priori information. Finally a 2D seismic non-linear modeling has been carried out to understand what reservoir-scale information is contained in the seismic reflection data. Participants in this project were AGIP (Italian oil company), the Politecnico di Milano(POLIMI) and the Institut Francais du Petrole (IFP). Each partner of this project has contributed specialskills and experience. The task of AGIP, the industrial partner, was to analyse a real data ase with the maximum depth possible, and to extract all information on the elastic parameters.The role of the academic partner, Politecnico di Milano, was to improve elastic reflectivity modeling and to evaluate the uncertainties of both amplitude-versus-offset (AVO) studies and of linearized elastic inversion that are caused by uncertainties in the model of the overburden and by noise in the data. An effort was also made to implement the inversion technique in the (ω-k domain. The role of IFP was to develop both linearized elastic modeling and inversion techniques using finite differences, and to evaluate the inversion technique on a large-scale synthetic data set. In addition, a multidisciplinary team of IFP has carried out a numerical simulation of synthetic data bas d on controlled earth parameters followed by processing and interpretation. Part I deals with the impact of a priori information on seismic inversion.It was decided to improve the most accurate modeling technique, i.e., the well known reflectivity method, so that it better represents shorter offsets; subsequently it was extended to media that vary continuously with depth, and to media that vary smoothly in lateral direction. This was achieved by the hybridization of ray tracing and reflectivity methods. A powerful (and essentially exact) 3D modeling technique resulted, albeit limited to quasi ID media. The AGIP group was able to analyse a real data case that was suitable for this kind of modeling. This group has carried out a careful and systematic AVO analysis and a comparison between the elastically modeled data and the real data. The processing and interpretation steps are described below. Another important step was to determine error bars in linearized inversion. For this, it was useful to move the problem from the spatial domain to the frequency-wave number domain. This move opened the possibility to bette understand the ‘conditioning‘. The inversion in the ω-k domain (with some preliminary results) is presented below. It was necessary to understand not only linearized inversion, but the limits of AVO in general. To this end a complete study of the interaction between AVO and velocity errors was carried out. This thorough understanding is particularly important if the velocity is not as perfectly known as in the real data case we analysed. Part II describes a 2D linearized elastic inversion technique and its evaluation on synthetic data.A 2D linearized elastic modeling procedure based on the computation of the Jacobian matrix using finite differences was proposed, analysed and implemented. Its accuracy was tested for models containing a few scattering points. Besides linearized modeling, a linearized inversion technique using a least-squares formulation was implemented to find the elastic perturbations in the 2D target zone. The inversion was tested on seismic data generated from scattering points. Next a large-scale synthetic data set generated for a gas-bearing sand model by a full-wave elastic finite-difference code was inverted. We show that, by using a plane stratified reference medium and limited windows for parameters data centered on the reflected P-P waves, one can determine the band-limited perturbations of the P-impedance and - to a lesser degree - the band-limited perturbations of the S-impedance at reasonable effort. In addition we show that the inversion results are closer to the actual perturbations than the results obtained with more conventional seismic processing procedures. Part III describes a numerical simulation of synthetic data for the Mesaverde outcrop.The objective was to understand what geological information is embodied in the seismic data that can be used to delineate and characterize reservoirs. Both geological and elastic models were constructed from the Mesaverde outcrop study and from industrial logs of equivalent formations. Then (marine) synthetic data were generated using a full-wave 2D elastic propagation code. Conventional seismic data processing was applied to obtain a stacked section. This section was calibrated using impedance logs at theoretical well locations (this process corresponds to wavelet-extraction processing). Finally complementary direct and inverse stratigraphie impedance modeling techniques were applied to build the most likely impedance section and to give a geological significance to the impedance variations.

Linearized seismic inverse problem: An attractive method for a sharp description of stratigraphic Traps

We present a method for solving the linearized seismic acoustic inverse problem. 1D reference medium is sufficiently accurate allow the linearization, the computation can made much less than for non linear inversion use of an especially designed algorithm that 2D If a to be by we present here. A synthetic example illustrates the effectiveness of linearized inversion about a 1D reference medium. Such a method could be of great practical interest for a sharp description of stratigraphic traps in nearly flat areas.