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Davies M.A.,ARKeX Ltd | Martin J.,CGG Veritas
KazGeo 2010 - 1st International Geosciences Conference for Kazakhstan: Where Geoscience Meets the Silk Road | Year: 2010

This paper demonstrates the use of Full Tensor Gravity Gradient Imaging (GGI) as an exploration tool within a salt basin province. In May 2010 Gravity Gradient data was acquired over 9000 km2 of the Gabon Atlantic Margin to; (1) compare and contrast GGI with 'conventional' gravity observations, (2) better image carbonate / salt structures, (3) demonstrate the ability of GGI to image structures in ultra deepwater and (4), demonstrate the shelf life of a GGI dataset by incorporating the data into a joint Seismic Velocity Modelling workflow, thus providing a better image of the subsalt structure. This paper will report on the outcome of the study and draw parallels between the Gabon dataset and areas in Kazakhstan where GGI would aid salt basin modelling. This will be done using a GGI feasibility study approach, where the 3D input model is based on seismic data acquired by CGGV over the Northern Caspian area.

Gholami Y.,CGG Veritas | Brossier R.,Joseph Fourier University | Operto S.,University of Nice Sophia Antipolis | Ribodetti A.,University of Nice Sophia Antipolis | Virieux J.,Joseph Fourier University
Geophysics | Year: 2013

In most geologic environments, accounting for anisotropy is necessary to perform acoustic full waveform inversion (FWI) of wide-azimuth and wide-aperture seismic data because of the potential dependence of wave speeds on the direction of the wave propagation. In the framework of multiparameter FWI, the subsurface parameterization controls the influence of the different parameter classes on the modeled seismic data as a function of the scattering angle and hence the resolution with which the parameters can be reconstructed and the potential trade-off between different parameters. We have evaluated a numerical procedure based on computation of the scattering patterns of the different parameters to assess the sensitivity of the seismic data to different parameterizations of vertical transverse isotropic media in the acoustic approximation. Among the different categories we have tested, a monoparametric FWI was found for imaging one wave speed with a broad wavenumber content, keeping the Thomsen parameters fixed, which have a small influence on the data relative to the wave speed. This raises the question of the initial information required in the background models of the Thomsen parameters to perform reliable monoparameter FWI. Alternatively, simultaneously inverting the horizontal and vertical wave speeds introduces limited trade-off effects because these wave speeds have significant influence on the data for distinct ranges of scattering angles, while the influence of the Thomsen parameter δ remains weak. With such parameterization, the short-to-intermediate wavelengths of the vertical velocity are updated from the short-tointermediate scattering angles, while the long-to-intermediate wavelengths of the horizontal velocity are updated from the wide-to-intermediate scattering angles. We concluded that the choice of the subsurface parameterization can be driven by the acquisition geometry, which controls the scattering-angle coverage and hence the resolving power of FWI, and by the accuracy of the available initial FWI models. © 2013 Society of Exploration Geophysicists.

Elis V.R.,University of Sao Paulo | Pejon O.J.,University of Sao Paulo | Hidalgo-Gato M.C.,CGG Veritas | Ustra A.T.,University of Sao Paulo
Near Surface Geoscience 2013 | Year: 2013

A domestic waste disposal site was installed in above a "voçoroca" area (natural depression zone produced by intense erosion). The waste was disposed without any technical procedure to retain contaminants at a recharge area of Botucatu Aquifer (sandstones). The aquifer is seriously affected by the contaminated water flow in the permeable substrate. This paper aims to study the geometry of the waste deposit and the contamination plume produced by the wastes using Resistivity and Induced Polarization methods. Six survey lines were carried out - five of them crossing the wastes and one external from the deposit. Geoelectrical data were interpreted with inversion software that generated resistivity and chargeability sections. Normalized chargeability sections were computed using the interpreted values. The geoelectric sections were analyzed together with wells information to help the characterization of waste area and to evaluate soil and groundwater contamination.

Prieux V.,University of Nice Sophia Antipolis | Lambare G.,CGG Veritas | Operto S.,University of Nice Sophia Antipolis | Virieux J.,CNRS Institute of Earth Sciences
Geophysical Prospecting | Year: 2013

Building an accurate initial velocity model for full waveform inversion (FWI) is a key issue to guarantee convergence of full waveform inversion towards the global minimum of a misfit function. In this study, we assess joint refraction and reflection stereotomography as a tool to build a reliable starting model for frequency-domain full waveform inversion from long-offset (i.e., wide-aperture) data. Stereotomography is a slope tomographic method that is based on the inversion of traveltimes and slopes of locally-coherent events in a data cube. One advantage of stereotomography compared to conventional traveltime reflection tomography is the semi-automatic picking procedure of locally-coherent events, which is easier than the picking of continuous events, and can lead to a higher density of picks. While conventional applications of stereotomography only consider short-offset reflected waves, we assess the benefits provided by the joint inversion of reflected and refracted arrivals. Introduction of the refracted waves allows the construction of a starting model that kinematically fits the first arrivals, a necessary requirement for full waveform inversion. In a similar way to frequency-domain full waveform inversion, we design a multiscale approach of stereotomography, which proceeds hierarchically from the wide-aperture to the short-aperture components of the data, to reduce the non-linearity of the stereotomographic inversion of long-offset data. This workflow which combines stereotomography and full waveform inversion, is applied to synthetic and real data case studies for the Valhall oil-field target. The synthetic results show that the joint refraction and reflection stereotomography for a 24-km maximum offset data set provides a more reliable initial model for full waveform inversion than reflection stereotomography performed for a 4-km maximum offset data set, in particular in low-velocity gas layers and in the deep part of a structure below a reservoir. Application of joint stereotomography, full waveform inversion and reverse-time migration to real data reveals that the FWI models and the reverse-time migration images computed from the stereotomography model shares several features with FWI velocity models and migrated images computed from an anisotropic reflection-traveltime tomography model, although stereotomography was performed in the isotropic approximation. Implementation of anisotropy in joint refraction and reflection stereotomography of long-offset data is a key issue to further improve the accuracy of the method. © 2012 European Association of Geoscientists & Engineers.

A sweep generator is employed to generate a sweep to be used by a seismic vibrator device for generating a desired target output spectrum, wherein the frequency sweep is designed so as to comply with one or more constraints imposed by the seismic vibrator device and/or imposed by the environment in which the device is to be used. In one embodiment, a sweep generator determines a sweep for achieving a desired target output spectrum by a given seismic vibrator device in compliance with at least a pump flow constraint imposed by the seismic vibrator device. In another embodiment, a sweep generator determines a sweep for achieving a desired target output spectrum by a given seismic vibrator device in compliance with all of multiple operational constraints of the seismic vibrator device, such as both mass displacement and pump flow constraints. Environmental constraints may also be accounted for in certain embodiments.

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