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9th International Congress of the Brazilian Geophysical Society
- Conference date: 11 Sep 2005 - 14 Sep 2005
- Location: Salvador, Bahia, Brazil
- Published: 11 September 2005
341 - 360 of 462 results
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Computer Graphics in 3D WaveFront Construction
Authors Lucas Freitas and Jorge StolWe describe an algorithm for simulating the propagation of a seismic wawefront, modeled by a mesh of triangles, through a general three-dimensional geophysical model, with automatic control of sampling density over the expanding front. We show that wavefront modeling is considerably simpli ed by using the Quad-edge mesh data structure. We also describe a control mechanism to eliminate samples when the front contracts.
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Estratégia para modelagem da resposta sísmica de reservatórios delgados
More LessThin bed reservoirs are very common in the Brazilians coast sedimentary basins and they are responsible for an expressive percentage of the oil production of this country. These reservoirs are in the limit or bellow the resolution power that is achieved by the seismic method, which makes interpretation for a correct characterization and oil reserves evaluation a difficult task. This work presents a seismic modeling strategy that is based on the finite-difference solution of the wave equation and is very appropriated for this kind of reservoirs. It also brings some solution to decrease the computational demand that is required when modeling, in a realistic scale is necessary. Trough a realistic example from Campos Basin, we demonstrate how an accurate seismic modeling may be useful to help in the interpretation of the seismic signature of such structures.
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Traveltime data profiles obtained using seismic ray tracing methods for the continental slope model parameterized by polynomials
More LessGeological structures known as continental shelf break, also called continental slope, are represented by seismic models characterized by two-dimensional, heterogeneous and isotropic velocity fields. By means of a polynomial parameterization of such fields, a smooth version of the model is generated with a strong variation near discontinuities that are, generally, associated to the seismic interfaces. In this kind of parameterization, all geometric and kinematics complications of the model are compacted in the polynomial coefficients, which are obtained by the least squares method. Traveltimes are calculated on the rays that represent paths for the seismic wave during its travel through the model. Seismic rays are connecting source positions to arrival points on the observation surface and they are traced by means of numerical approximations of ray equations (expansions of Taylor of second order). The calculated data tells us something about the geologic features of the considered seismic velocity field, such as: low lateral variation of velocity in one half of the model, predominance of low velocities in another half, break of model symmetry due to the increase of declivity of the layers in the accentuated slope region, increase of velocities with deep, etc. The same model is represented by different polynomial functions, it is observed that the increase of the polynomial degree produces an improvement on the representation when quantity and distribution of chosen points for polynomial adjustment are not altered. The used continental slope model comes from literature and, in this first approach, just its main features are considered. We believe that better results will be produced considering finer discrete models or polynomials of higher degree. Modeling traveltimes, such as it is done in this work, allows us the accomplishment of seismic inversions, in which the polynomials coefficients are the model parameters to be estimated.
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Modelagem Acústica e Elástica para a Bacia do Solimões
More LessSeismic modeling of paleozoic basins in the Amazon has its challenges for numerical simulation and interpretation. The low velocity layer, associated with the weathering zone, needs a more accurate rheological model; strong velocity constrasts at the Cretaceous-Paleozoic unconformity; highly fractured diabase layers, and reservoir rocks with thickness below seismic waves resolution. We present the results of acoustic and elastic wave propagation simulation in models with some of these features. Long period multiples between the free surface and Cretaceous-Paleozoic unconformity are marked features in the acoustic modeling. Strong P-S conversions and surface waves are main features in the elastic modeling. Surface waves present a special problem for numerical modeling due its dispersive nature which requires fine meshing. Besides, absorbing boundary of PML type do not have good performance for surface waves.
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Atualização da conversão tempo x profundidade no Campo de Albacora, Bacia de Campos
More LessThis paper discusses about the routine and techniques of time-depth conversion used during oil field development and management in Campos Basin. Each day, a lot of information is acquired and organized in order to improve the quality of time-depth conversion: new wells, seismic data, processing, image-ray conversion, pre-stack depth migration, geostatistical methods, etc. The point to be argued here is not what the best method is, but how we deal with it so that we can obtain the necessary information to make correct decisions about depths of horizontal wells and other forecasts, uncertainties about structures and its implications for reserve estimates.
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The role of bedding on the pulse shape change: Numerical modeling calibration with a physical experiment
Authors José Eduardo M. Lira and Webe J. MansurThe rocks in a sedimentary basin are made of several layers, as shown in well-logs, that go from some meters to millimeters in width, and maybe thinner. So, one could imagine that the seismic pulse is supposed to be reflected and refracted through hundreds to thousands of layers, most of them bellow the seismic resolution. These layers create a train of multiples that follows the main pulse with a very short time-lag among themselves. The summation of this train with the original pulse generates a new pulse shape which resembles the original pulse under the effect of absorption. This phenomenon was described in O’Doherty and Anstey (1971). The purpose of this work is to evaluate this effect through a numerical modeling calibrated with a physical experiment of ultrasonic-frequency wave propagation in samples composed of discs of aluminum and glass using a Finite Difference modeling of elastic wave equation.
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Modelagem acústica por divisão em domínios independentes usando transformada wavelet packet
More LessThis work compares the wave equation solution in different domains: the spatial domain and the wavelet packet transform domain. Since there is a sparse representation of the wavefield in this domain, it is possible to explore this sparseness to increase the
efficiency of the modeling algorithms. Comparing to our previous work (Loureiro et al. 2003), we improved the wavefield extrapolation in the wavelet domain, by expanding from a single level into a second wavelet decomposition level, while taking the more general
wavelet packet approach, which is very appropriate to parallel implementation. The results were compared to the finite difference method.
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Crustal thickness variations in western Venezuela from deep seismic observations
Authors M. Bezada, M. Schmitz, A. Levander, M. Jácome and Fr. AudemardSeismic measurements were carried out in northwestern Venezuela including both, offshore and land refraction and wide angle reflection data. Seismic sections were constructed from land shots recorded on portable single channel stations as well as from airgun shots recorded by broadband seismological stations on land. Modeling of the land profile indicates the existence of a pronounced crustal thinning in the northeastern Falcón area. Additional seismic lines help to delineate the spatial distribution of the thinning area. The observations suggest that its western end occurs just west of Aracua while it opens to the east. The seismic observations discussed here support theories of diachronic opening of the Falcón Basin in an east-west fashion.
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Gaussian beams and Fresnel volume elements
Authors Carlos A. S. Ferreira, German Garabito and Pedro Chira OlivaThe Gaussian beam concept was originally introduced in the seismological literature by Russian and by Czech researchers in the beginning of the 80's, in order to investigate certain limitations of the zero order ray theory, up to now the standard method to study the propagation of a seismic wavefield in smooth geological models. This paper investigates then a possible relationship of this concept with the Fresnel volume elements. This restriction permits that certain parameters that control the half-widths of the beams present an analytical expression, based on the knowledge of the Fresnel volume elements, common on modeling of seismic wavefield, and that can be fully determined by dynamic raytracing (DRT).
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Modelagem Sísmica do Reservatório Turbidítico do Campo de Namorado, Bacia de Campos (RJ), Brasil
More LessEsse artigo tem por finalidade discutir problemas de resolução sísmica vertical do reservatório turbidítico do Campo de Namorado. Nesse trabalho foi feita uma modelagem sísmica usando o método da convolução. O pulso escolhido foi o de Ricker freqüências de 40 e 80 Hz. Espera-se que este trabalho contribua para o aprimoramento do conhecimento da área em que se enquadra esse estudo, e auxilie geólogos e geofísicos na interpretação de seções sísmicas na prospecção de hidrocarbonetos, principalmente nesse.
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Modelagem acústica e implementação da superposição das condições de Dirichlet e Neumann nas fronteiras do modelo
Authors Cíntia Matsumura and Viatcheslav I. PriimenkoIn numerical modeling of wave propagation, the size of the model is limited by the available computer memory. This restriction, introduces artificial boundaries, which produces unwanted reflections that contaminate the solution. Then it is highly desirable to be able to eliminate these reflections. In this paper we make a comparative analysis using different artificial boundaries for the acoustic wave propagation case. The results show that reflections from the boundaries of a model may be completely eliminated by adding together the solution of the Dirichlet and Neumann problems. The results obtained are compared with ones, calculated using another types of artificial boundary conditions.
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Modelagem sísmica SH viscoelástica anisotrópica
Authors Roberto H. M. dos Santos and Marco A. B. BotelhoElastic solids and viscoelastic fuids differ widely in their characteristics of deformation. The deformed solids elastically return to the undeformed state as soon as the applied loads are removed. Viscous fluids present no trend to retorn to their original configuration after the deformation. Moreover, the elastic tensions are directly related with the deformations; in the case of, in a viscous fluid, the tensions (except for the hidrostatic components) are related with the deformation speed. The behavior of a material that presents a combination of both characteristics, elastic and viscoelastic, is called viscoelastic behavior. In this work we initially analyze the effect of attenuation the propagation of SH wave in a isotropic viscoelastic media. We compare the seismograms
generated with the viscoelastic equation and with acoustic equation, using the same isotropic acustic velocity field.
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Zero-offset modelling and simulated migration by one-way wavefront construction
Authors Einar Iversen and Joachim MispelAssuming a zero-offset configuration of sources/receivers, we present an approach to forward modelling based on a one-way wavefront construction process progressing upward from a selected target reflector. Subsequent simulation of migration amplitudes along this target reflector yields reliable estimates of amplitudes from a zero-offset depth migration and a quick (but less accurate) indication of small-offset depth migration amplitudes. The one-way wavefront construction process is based on theory permitting quantities related to the two-way wave propagation (accumulated reflection/transmission coefficients, geometric spreading, and phase shift due to caustics) to be continued step by step in the upward direction of normal-incidence rays. The upward continuation yields in addition the isochron curvature matrix, which is essential for the migration amplitude simulation. Numerical tests demonstrate that one-way wavefront construction is far more efficient than conventional two-point raytracing, especially when the number of sources/receivers is large. This makes it feasible to compare several scenarios with respect to model parameters in reasonable time.
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Finite Difference Elastic Modeling in 2.5D
Authors F. Silva Neto, J. Costa and A. Novais and B. BarbosaFinite difference modeling of elastic wavefields in 2.5D is described in the velocity-stress formulation for isotropic media. The 2.5D modeling computes the 3-D elastic wavefield in a medium which is translation invariant in one coordinate direction. The approach is appealing due reduced storage and computing time when compared to full 3-D finite difference elastic modeling. The scheme handle inhomogeneities in mass density and elastic moduli, includes free-surface and perfect matched layers as absorbing
boundaries. High order finite difference operator allows the use of a coarse mesh, reducing the storage even more without producing numerical dispersion and numerical anisotropy. Numerical experiments show the accuracy of the scheme and its computational efficiency. The method can also be extended to include anisotropy.
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Wave propagation modeling in heterogeneous, porous media
More LessBiot’s equations are used for modeling wave propagation in 2-D heterogeneous porous media through a second-order accurate time, fourth-order accurate space, staggered-grid finite-difference scheme, based on Madariaga-Virieux formulation. For implementing the numerical scheme, Biot’s equations are reformulated into a first-order system of four equations of motion and four constitutive equations, expressed explicitly in particle-velocity components, stresses and pore-fluid pressure. Results from poroelastic and an equivalent Levander’s elastic modeling are compared to illustrate Biot’s poroelastic effect and then the amount of additional information available for seismic and sonic inversion.
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Experimental 4C seismic survey offshore Eastern Brazil
This paper aims to present and discuss geophysical targets, survey planning and acquisition issues for a 4C experimental seismic survey designed for technology evaluation offshore Eastern Brazilian Margin, on Campos and Santos Basins, two of the most prolific oil basins in Brazil. The 4C job, which included both 2D and 3D surveys, was conducted from December 2004 to April 2005. The water depths encountered ranged from 300 m to 1,850 m over the Brazilian Continental Margin, establishing the world’s deepest 3D-4C survey ever acquired. The areas selected for this campaign were chosen based on technical criteria that took into account the potential of the 4C Seismic Technology to deal with specific exploratory and reservoir characterization problems, thus enabling a technology’s proof of concept, while providing further useful geophysical information from both PP and PS-wave modes. Processing workflow and preliminary results shall also be briefly discussed herein.
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Multicomponent Technology: Reducing Risk and Creating Opportunity
Authors S.L. Roche, S.L. Roche, Gibson J., B. Mattocks, S. Ronen, S. and Watt and H.J. WattsThree-dimensional (3D) compressional-wave (P-wave) seismic methods routinely provide faithful structural images of the earth's subsurface but often lack the ability to provide complete and distinct information about roch properties. Shear-wave (S-wave) data, in conert with P-wave data (multicomponent), can provide additional information to reduce risk and create new opportunities. Significant advances in the acquisition, processing and interpretation of multicomponent data have served to make this technology more viable but acceptance remains limited. More widespread acceptance of multicomponent methods is critically dependent on continuing demonstrations of value from field projects and on the development of interpretation tools and work processes conducive to evaluating multimode data. This paper provides an overview of multicomponent technology and focuses on the techniques and advances that are improving viability and includes theory and practical examples of how multicomponent technology is improving the ability of the industry to find and produce more oil and gas.
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The first 4C surveys acquired on the Eastern Brazilian Margin
In this paper, we discuss the first 4C surveys acquired on the Eastern Brazilian Margin. All aspects will be reviewed, from the feasibility study which first indicated the potential value of multicomponent technology in this region, to the objectives, through the illumination analysis used in the survey design process, to the seismic crew which acquired the data, and finally through the field operation itself. Acquisition will be completed in April, and data processing is expected to be finished by September. We plan to present and discuss the fully processed results when delivering this paper at the congress in September.
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A Deep Water 4C 4D Permanent Installation Pilot
Authors Michael C. Hodge, Christophe Massacand and Helge TeiglandThis poster presentation outlines some of the challenges faced with the installation of a buried 4C-4D seismic cable over a deep-water oil-producing field in the Gulf of Mexico. A single 6km 4C cable was laid on the seafloor and tied back to recording equipment on the platform. The MARS field utilizes a “Tethered Leg Platform” design because its location in over 1000 metres of water. The 4C installation design had to take into account the harsh environmental conditions of the Gulf such as frequent hurricanes and its infamous loop currents that move the platform topside by up to 50 metres laterally. Detailed finite element analysis was carried out to ensure that the 4C riser cable did not mechanically interfere with the many existing risers that hung in catenary from the TLP’s deck level to the seabed. The 4C seismic cable was installed underneath 5 existing risers in catenary. Parts of the cable were trenched so that comparisons could be made between 4C stations that were buried and those that were not. These are presented in this poster
together with an analysis of recorded data from special source lines that were shot at 45 degrees to the cable azimuth.
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