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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
301 - 400 of 462 results
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Velocity continuation by finite differences
Authors A. Novais and J. Costa and R. PortugalWe have implemented a finite-difference algorithm for timeremigration of GPR data in FORTRAN 90 and C, and studied its theoretical properties in detail. For a number of synthetic models, out numerical experiments have been realized. For these examples, we obtained perfect agreement between the theoretical predictions and numerical results. The examples also prove the computational efficiency of the algorithm. Estimates of the true medium velocity can be obtained.
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Smoothing and automated picking of kinematic wavefield attributes
Authors Tilman Klüver and Jürgen MannTomographic methods for the determination of velocity models making use of kinematic wavefield attributes strongly depend on the accuracy of these attributes and their efficient extraction from the seismic prestack data. We use the Common-Reflection-Surface (CRS) method to estimate these attributes from the data. Our aim is to improve the quality of the attributes and to extract those values from the CRS output, which are necessary to perform a CRSbased tomography. This extraction should be highly automated and efficient as well as reliable. Both, smoothing and picking, make use of the same technique: application of locally valid statistics in small windows aligned with the reflection events. We discuss this approach in detail and apply both algorithms to a synthetic 3D dataset. The results clearly show the improved quality of the kinematic wavefield attributes and the stability of the picking process.
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Otimização Global: Simulated Annealing vs. Very Fast Simulated Annealing aplicados no problema de otimização do método SRC 2D
Authors L.M.K. Carmo, G. Garabito and e CostaThe final objective of the imaging methods of seismic reflections is to produce the best possible image of the geologic structure in subsurface. An image of the subsurface can be provided by mapping of data seismic for the zero-offset (ZO) domain, by means of pre and pos-stack depth migration processes. The common-reflection-surface (CRS) stack simulates a zero-offset section from multicoverage seismic data. Whereas conventional reflection imaging methods (e.g. the NMO/DMO stack or prestack migration) require a sufficiently accurate macrovelocity model to yield appropriate results, the CRS stack does not depend on a macrovelocity model; it only requires a priori knowledge of the near-surface velocity. The CRS staking surface depends on three search parameters: the emergence angle of the ZO ray, as well as two radii of the curvatures. These attributes are associated with the hypothetical normal wave and the normal-incidence-point wave. This paper present a comparative study of two global optimization methods - Simulated Annealing, Very Fast Simulated Annealing - in the problem of determination an optimal parameter triplet for the CRS stacking method.
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Beam Pre-stack Depth Migration Algorithm in a 3D PSDM Non-exclusive Reprocessing Project, Sergipe-Alagoas Basin
Authors Rachel Masters and Kevin SherwoodBeam Pre-stack Depth Migration has provided a fast and reliable method of delivering a depth migrated cube for non-exclusive reprocessing projects in Brasil. The method was tested on a 3D reprocessing project in Sergipe Alagoas Basin with impressive results. The Beam Pre-stack Depth Migration method was developed to overcome limitations in other commercial PSDM techniques and involves three main steps: decomposition, migration and reconstruction. This paper describes the process and presents results comparing pre-stack time migrated data with the beam pre-stack depth migration. The depth migrated data provides a much more accurate image of the subsurface, improving the chance of further discoveries in this new light oil province.
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Imageamento Sísmico de Alta Resolução de um Complexo de Lobos em um Sistema de Água Profunda
More LessThis work analyzes the depositional model of the Almirante Câmara system. Aiming to contribute to the seismic knowledge of modern turbidite systems. The focus is in seismic stratigraphy with emphasis on the characteristics of each seismic packages. The whole Almirante Câmara system is composed by a submarine canyon which acts as a sediment bypass zone; by a channel complex, developed within a trough which is controlled by faults and links the canion with a lobe complex. The interpretation of data was based on the internal configuration of the reflectors the external geometry of the seismic sets and the resulting depositional features. Halokinesis has strongly controlled the features here interpreted.
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Comparação de imageamentos sísmicos usando PSPI e RTM com registro de múltiplas fontes
Seismic imaging of hydrocarbon accumulations under structures with high acoustic impedance, such as saline and basaltic structures, has been of one the targets in the oil industry as a consequence of recent discoveries of interesting petroleum systems in structures like that. Signal-noise ratios of the seismic sections under those structures are usually lower than the one necessary for reliable interpretations. In this work, we study two seismic depth migration techniques, Reverse Time Migration (RTM) and Phase Shift Plus Interpolation (PSPI) of Areal Shot Records, in order to compare their performance on numerical seismic data generated on geological models which present exploration objectives located under high acoustic impedance layers. On our experiments, except for some small artifacts, the results produced by both techniques after depth migrating just one areal shot, almost do not differ and were excellent. It was possible to identify the main interfaces with confidence.
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CRS-stack-based seismic imaging for land data and complex near-surface conditions
Authors Zeno Heilmann and Ingo KoglinIn the current situation of rapidly growing demand in oil and gas, on-shore exploration, even under difficult conditions, becomes again more and more important. Unfortunately rough top-surface topography and a strongly varying weathering layer often result in poor data quality, which makes conventional data processing very difficult to apply. As recent case studies demonstrated, the Common-Reflection-Surface (CRS) stack produces reliable stack sections with high resolution and superior signal-to-noise ratio compared to conventional methods. Particularly for land data, the increased computational expense required by the generalized high-density velocity analysis preceding the CRS stacking process may be worthwhile. In order to define optimal spatial stacking operators, the CRS stack extracts for every sample of the zero offset (ZO) section an entire set of physically interpretable stacking parameters. These so-called kinematic wavefield attributes, obtained as a by-product of the data-driven stacking process, can be
applied to solve various dynamic and kinematic stacking, modeling, and inversion problems. By this means, a very flexible CRS-stack-based seismic reflection imaging workflow can be established. Besides the CRS stack itself, the main steps of this processing workflow are residual static correction, the determination of a macrovelocity model via tomographic inversion and limited aperture Kirchhoff migration. The presented extention of the CRS-stack-based imaging workflow provides support for arbitrary top-surface topography. The CRS stack is applied to the original prestack data without the need of any elevation statics. Finally, a redatuming procedure relates the CRS-stacked ZO section, the kinematic wavefield attribute sections, and the quality control sections to a chosen planar measurement level. Thus, an ideal input for a preliminary interpretation and subsequent CRS-stack-based processing steps is provided.
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Common-Reflection-Surface stack for OBS and VSP geometries and multi-component seismic reflection data
Authors Tim Boelsen and Jürgen MannThe 2D Common-Offset Common-Reflection-Surface (2D CO CRS) stack has so far mainly been applied to singlecomponent seismic data acquired along one straight line, e. g., conventional marine streamer surveys or land surveys with virtually flat measurement surfaces. A new hyperbolic traveltime formula for the 2D CO CRS stack that takes arbitrary top-surface topography into account is presented. This formula can then be used to derive stacking operators that are in principle able to handle a vertical seismic profile (VSP) acquisition geometry as well as reverse VSP and cross-well seismics. Moreover, the application of the 2D CO CRS stack to ocean bottom seismics (OBS) is discussed and successfully tested with a complex synthetic OBS data example. We also introduce an approach that allows to generate separate stacks of compressional and transversal waves from multi-component seismic reflection data. Based on the traveltime approximation for finite offset, the polarization is analyzed during the search for the optimum orientation and curvature of the CRS stacking operator. We apply this approach to a simple synthetic data set and obtain stacked sections and kinematic wavefield attribute sections separately for PP and PS reflection events. All concepts introduced here for the 2D case can be directly transferred to the more general 3D case.
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Image-wave remigration in elliptically anisotropic media
Authors R. Aleixo and J. SchleicherThe image-wave equations for the problems of depth and time remigration in elliptically isotropic media are secondorder partial differential equations similar to the acoustic wave equation. The propagation variable is the vertical velocity or the medium ellipticity rather than time. In this work, we derive these differential equations from the kinematic properties of anisotropic remigration. The objective is to enable the construction of subsurface images that correspond to different degrees of medium anisotropy. In this way, “anisotropy panels” can be obtained in a completely analogous way to velocity panels for a migration velocity analysis.
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Imaging of passive seismic sources by reverse modelling
Authors D. Gajewski and E. TessmerThe passive seismic method for reservoir monitoring and characterisation and monitoring of hydraulically induced fractures is developing into a main stream technology in the oil business. Current techniques rely on the fact that the recorded event is detectable at most of the stations of the recording array. Weak events, not visible in the individual seismogram of the array, are missed out. We present a new approach, where no picking of events in the seismograms of the recording array is required. The observed wave eld of the array is reversed in time and then considered as the boundary value for the reverse modelling. Assuming the correct velocity model, the reversely modelled wave eld focusses on the hypocenter of the seismic event. The origin time of the event is given by the time where maximum focussing is observed. The spatial extent of the focus resembles the resolution power of the recorded wave eld and the acquisition.
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Amazon Seismic Data Imaging with the WIT System
A line of land-data set from the Takutu sedimentary basin (Brazil) was processed as an example, not for comparison with other processing packages, to demonstrate once more the basic steps of the potential CRS-stack based data-driven imaging system aiming at to establish a work-flow for basin reevaluation. Based on the common-reflection-stack (CRS) attributes, a smooth macro-velocity model was determined via tomographic inversion for use on pre- and/or poststack depth migration was carried out. Poststack and prestack depth migration benefit from this approach. Also other CRS-stack based processing steps is being added in the system as, e.g. residual static corrections, limited-aperture migration based on the estimated projected Fresnel zone, determination of the geometrical spreading factor, and analysis of amplitude variation versus offset. Geological interpretation is to be carried out mainly on the bases of ZO and migrated sections, considering that all maps have the proper scale, axis exaggeration and size. From visual details of the panels, we can interpret thinning, and an anticline and faults can be mappe where plays of horsts, grabens and rollovers are indicated; on the other hand, the basement can not be simply traced. Also, the right hand part of the section needs more processing, and artifacts can be interpreted in the panels.
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High-resolution 3D tomographic model building using automatic, dense volumetric picking applied Offshore Brazil
Authors VOLKER DIRKS and STEPHEN PHAREZ AND VETLE VINJEPre-stack depth migration is increasingly being applied to larger exploration surveys and thus 3D tomography, as the key component of today’s velocity model building workflows, has to be performed on large data volumes within acceptable turnaround times. To provide high-resolution velocity models, dense volumetric picking is required, which in turn leads to even larger data volumes. It is therefore paramount to automate as many processing steps as possible to free the time of the geophysicist for the necessary QC. We present a modified workflow for high-resolution 3D tomography addressing the need for process automation. The flow is well suited to the geology in the Gulf of Mexico and offshore Brazil and we illustrate the flow with examples from the regions.
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Inverse Problem of Wave propagation in Weakly Lateral Heterogeneous Layered Medium
More LessA wave propagation generated by a boundary source into a weakly lateral heterogeneous medium (WLHM) occupying a half-space z > 0 and consisting of two different materials separated by an interface is considered in the acoustic approximation. In this paper WLHM means that the velocity of the wave propagation depends weakly on the horizontal coordinates x = (x1, x2) in comparison with the strong dependence on the vertical coordinate z. The density is assumed to be dependent only on z, and the shape of the interface is described by a function weakly dependent on x = (x1, x2). We consider the problem of the reconstruction of the velocity and density inside every component of the half-space and the shape of the interface between them from the knowledge of the medium response measured at z=0. We obtain a recurrent system of 1D inverse problems to find the first two terms in the decomposition in the refraction index n(z, x) (inverse power of the velocity of wave propagation) and the shape of the interface z = h( x) with respect to the small parameter, the ratio of the horizontal and vertical gradients of the velocity, along with the density. In the zero-order approximation we derive a system of nonlinear Volterra integral equations for determination of the density and zero-order term of the refraction index. Next, the first-order approximation the refraction index is determined as a solution of a coupled linear system of Volterratype integral equations. We demonstrate the effectiveness of the approach in numerical applications to the 2D case of wave propagation.
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Estimativa de anisotropia local através de dados sintéticos das ondas qP de walkaway VSP
Authors Ellen N. S. Gomes and Ivan Pšenčík e Jessé C. CostaInversion scheme for the determination of anisotropy in a vicinity of a receiver situated in a borehole from the data obtained during a multiazimuth multiple-source offset VSP experiment is studied. The data consist of vertical components of slowness vectors and of polarization vectors of direct and reflected qP waves. We analyze the effects of the way of choice of a reference medium and of the wave normal on the results of inversion. We also study sensitivity of the inversion scheme to the number and orientation of profiles, to the number of sources along the profiles and to the number and type of waves considered. Study of the stability of the inversion scheme indicates which parameters of the medium can be recovered reliably and which cannot.
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Inversão Elástica e correlação perfil-sísmica na detecção de arenitos com hidrocarbonetos
Authors Anderson Luiz Pimentel and Marcos Gallotti GuimarãesIn this work we inverted 4 angle stacks for acoustic impedance, poisson ratio and density using a simultaneous elastic inversion methodology. According with the results, there are a good match with inverted parameters and well derived parameters. Observing the crossplot of the severals petrophysical parameter derived from the well, we noted that low water saturation values are placed in a specific region on the crossplots. We can use this range of data for indentificaton of this good reservoir characteristics in the inverted seismic volumes.
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L-curve approach in cross-hole and VSP geophysical diffraction tomography
Authors Eduardo Telmo Fonseca Santos and Amin BassreiSince the inverse problem is ill-posed it is necessary to use some tool to reduce this deficiency. The tool that we choose is the regularization by derivative matrices. The L-curve was reintroduced in the literature of inverse problems by Hansen, and, in this work we use the L-curve for the selection of regularization factor in cross hole and VSP geophysical diffraction tomography. Simulations with synthetic data are presented, and the results validate the feasibility of the method.
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Migration with Gaussian beams
More LessA migration algorithm and computer codes based on the Gaussian beam method are developed for zero-offset and single short pre-stack configurations of the input data. Our approach preserves the main features of the classical Kirchhoff type migration (Schneider, 1978): for backward propagation of the recorded wave field the Green formula is used (in the time domain it is called the Kirchhoff formula); the image condition is based on the fact that the direct and backward fields are coherent in time on a reflector. Main point of our approach consists in replacing the corresponding Green function by its high-frequency asymptotics in terms of Gaussian beams (Popov, 1982).
Due to advantages of the Gaussian beam method, our approach does not face the caustic and two point problems in ray tracing, it automatically includes late arrivals and maximum amplitude of the wave field in the imaging process. As the requirement for the wave field to be coherent on a reflector is necessary but not sufficient condition, the stacking procedure is essential because it enables to remove random coherency in the migration domain.
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True-reflection imaging with amplitude correction in dip-angle domain
Authors Ru-Shan Wu and Mingqiu LuoWe compare the new method of amplitude correction in the local dip-angle domain with other correction schemes: the traditional vertical AGC, space-domain correction based on total illumination, and correction in other angle-domains, such as the scattering-angle and receiving-angle domains. For the total strength imaging, amplitude corrections can be considered as applying amplitude gain (AG) factors to the migrated images in prestack depth migration. We analyze the different approximations involved in these schemes and compare their results of amplitude correction for the migrated images of SEG-EAGE salt model. The advantages of the new scheme can be seen clearly. The image quality of subsalt structures is greatly improved and the image amplitudes, especially along the steep faults and the baseline are much more uniformly distributed. In the meanwhile the noises in the same region are depressed.
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Ninth International Congress of the Brazilian Geophysical Society Imageamento sísmico abaixo de camadas com altos contrastes de impedância
The seismic modeling has brought solutions for many problems faced in oil industry, like the imaging of hydrocarbons reservoir under structures with high acoustic impedance, case of saline and basaltic structures. In some sedimentary basins the high thickness of these layers appears as great problem to be considered. In this paper we use the multi source technique in order to show that thin layers under complex structures can continue being identified with good quality even when increase at five times its thickness. For this, we perform an acoustic seismic modeling of two velocity models.
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Optimal aperture in Kirchhoff common-angle time migration
Authors Cláudio Guerra and Tânia M.G. SantiagoRecently, much attention has been given in obtaining angle gathers and also amplitude versus angle (AVA) curves during the migration process. An attractive way to do this is provided by (time or depth) common-angle Kirchhoff migration (CAKM), which considers travel-time curves and weights that refer to the common-angle (instead of the conventional common-offset or commonshot) configuration. As occurs with any Kirchhoff-type procedure, the CAKM output strongly depends on the choice of the migration aperture, not only for reduction of computational costs, but also for the accuracy of results. In this work we shortly review the CAKM theory and address the problem of optimal selection of migration aperture. In this context, the concept of projected Fresnel Zone, which plays a prominent role in the estimation of an optimal migration aperture, is also reviewed and applied.
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Modified Kirchhoff prestack depth migration using the Gaussian beam operator as Green function – Theoretical and numerical results
Authors Carlos A. S. Ferreira and João C. R. CruzThe Gaussian Beam concept has been of great importance for works on modeling, migration and inversion, during the last two decades.This work then joins the flexibility of the true amplitude (diffraction stack) Kirchhoff migration process with the regularity of the high frequency Gaussian Beam description of the wavefield as Green function, in some simple numerical examples of geophysical exploration interest. Our process can be named as Kirchhoff-Gaussian Beam Prestack Depth Migration (KGB-PSDM) in a true amplitude sense.
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Some aspects on 2.5D true amplitude Kirchhoff common-angle time migration
Authors Tânia M.G. Santiago, Cláudio Guerra and Martin TygelThe analysis of amplitude variation with offset or reflection angle is of great importance in the oil industry to predict hydrocarbon presence in the reservoir. This analysis is applied to both reservoir and exploration areas. The usual transformation method from offset to reflection angle, for common-offset migrated data, involves errors that may produce unreliable results. Thus, it becomes necessary to find alternative procedures to obtain appropriate data to provide more reliable curves of amplitude versus reflection angle (AVA). This work considers 2.5D true amplitude Kirchhoff-time migration in the common-angle domain, and its application to synthetic seismic data. It also examines the influence of seismic acquisition and migration aperture on the migration results. Comparison between AVA curves obtained from common-angle and common-offset migrated data confirms that the former is a more reliable procedure.
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Migração RTM de registros de múltiplas fontes aplicada a estruturas geológicas complexas
Reverse Time Migration (RTM) is among the most indicated tools for depth imaging of complex geological structures. Its assumptions are simple enough to make it recommended in cases where severe lateral velocity variations are present. However, its raditional approach, based on depth migrating each and every common shot gather, not rarely demands immense computational efforts. In order to make RTM projects feasible, some authors have proposed alternative schemes in which multi sources records are migrated. In this work we exhibit the result obtained depth migrating a multi source record generated summing up 760 common shot gathers of a seismic acquisition simulation on Marmousi model.
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Phase-shift anisotropic depth migration using controlled illumination: Stability in relation to addition of random noise
The present work studies the stability of a depth migration for elastic vertical transverse isotropic media (VTI), using the concept of controlled illumination. In the proposed method the areal shots obtained from multicomponent records are extrapolated using ase-shift techniques. Through the weighted addition of delayed shots we synthesize appropriate areal shots, which increases the accuracy of the seismic imaging in the area of interest. The computational cost of the present method is much lesser, when compared to the cost of migrating all the records, since only a few areal shots are necessary to image the area selected by the interpreters. The proposed method was tested on a typical numerical 2D model from the San Alberto field in Bolivia exposed to noisy conditions created by a random noise generator. Even with lower signal to noise ratio (SNR) we can correctly migrate anticline structures under a thick anisotropic shale layer.
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Cálculo da matriz de tempo de trânsito por rotação de fase em meios Localmente Tranversalmente Isotrópicos (LTI)
More LessWe propose an algorithm to compute the travel time matrix of the waves with the largest energy for prestack depth migration in an anisotropic medium with polar local symmetry or locally transversally isotropic (LTI). The algorithm is based in the phase shift concept to extrapolate the wave field. The irregularities that normally appear in the travel time matrix, computed by other methods, such as, ray tracing, when the medium has large velocity variations, do not appear with the same intensity in the proposed method. The algorithm is robust and its extension to the 3D case is straightforward.
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Use of reciprocity in a double plane wave Kirchhoff depth migration
Authors R.K. Seifoullaev and P.L. Stoffa and M.K. SenSlant stacking of typical marine and end-on land shot gathers generally produces high quality traces for positive ray-parameters while those for the negative ray-parameters are often noisy due to limited aperture. Use of such data in migration results in images with limited or missing negative dips. In this paper we employ the well-known reciprocity principle in an efficient manner such that a split-spread gather can be formed from existing one-sided offset data. This can be achieved by physically re-gathering the common shot gather data into common receiver gathers and collecting the data for a fixed surface reference position into the equivalent of a split spread gather. This approach encounters problems for irregular shot and receiver geometries. It can also cause processing difficulties since large volumes of intermediate data may need to be stored for purposes of collection into the common surface reference gathers. Thus, the intermediate volume of data to be used for pre stack depth imaging are significantly increased. We show that for plane wave pre stack imaging, reciprocity can be taken into account as the original (one sided offset) gathers are used in the construction of the plane wave transform. We demonstrate using a coupled ray-parameter Kirchhoff migration that the plane wave migrated data that include reciprocity are better imaged than the data that do not include reciprocity.
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Migrac¸ ao Kirchhoff em Profundidade de Sec¸ oes de Ondas Planas
More LessPlane wave migration has been employed to increase the ef ciency of wave eld extrapolation procedure. The point source records are synthesized into a plane source prior the shot pro le migration and the resulting plane wave data can be migrated using the same scheme as the one for convencional shot-record migration. Plane wave migration is one such method to reduce migration, because the extrapolations needs not be done for all the individual shot records, but for the plane wave shot record only.In this paper we has developed a version of plane wave migration method based on the Kirchhoff integral. Integral methods are easier than the wave equation methods to adapt to different recording geometries and also allow large and small models to be treat the same way. And also, with the aid of the Asymptotic Ray Theory (ART), integral methods can be extended to handle arbitrary velocity structures without any dip limitation.`A synthectic example is used to test the theory and the Kirchhoff plane wave depth migration
has produced images comparables to those from plane wave migration.
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Método Híbbrido de Migraçäo Pré-empilhamento em Duas Etapas com Interpolaçäo
Authors Gary C. Aldunate and Reynam C. PestanaThis paper shows the application of Phase Shift Plus Interpolation Split-Step (PSPISS) wave eld extrapolation coupled with the excitation-time imaging condition to perform prestack seismic depth migration of common-shot pro les.The core of hybrid migration methods is the use of differents propagators for the source and receiver wave elds. The source can be forward extrapolation using the traveltimes computation from the eikonal solution. Here we do this with the eikonal nite difference solution (TGRID) or with a geometrical method called FAST method. However, the receiver extrapolation is carried out by the PSPI Split-Step operator (PSPISS), that is the conventional splitstep method using multiple velocity references. The source traveltime computation uses a smooth velocity and for the receivers extrapolation the true velocity eld. We named these method as PSPISS-H with the traveltimes obtained by TGRID or FAST. A dataset from Gulf of Mexico was used to test the hybrid migration methods. The migration results shows a good image in depth. The PSPISS-H methods had better computacional performance compared with the wave equation PSPISS method, but in both cases having a processing expense time proporcional to the number of reference velocities.
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Migração Pré-empilhamento de Dados de Fontes de Ondas planas
Authors Celso A. Moreira Neto and Reynam PestanaIn this paper, we developed a migration methods for plane wave source data seeking to obtain good quality of subsurface images a low computational cost. The prestack migration implemented in this work is accomplished about constant ray parameter section,
obtained through the decomposition of the order receiver common gather data taking the
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Migração RTM orientada ao alvo
In this paper, we introduce a reverse time migration (RTM) scheme using the concept of controlled illumination in order to improve the seismic imaging of exploration targets localized under high complex geological structures, such as areas subject to intense compress- ional regimes. Synthesis operators are specially designed from upward wavefield propagations in order to guarantee the seismic illumination around the areas of interest. The use of the acoustic wave equation without high frequency approximations and its solution through finite differences techniques make our method adequate to cope with strong lateral velocity variation and steep dipping events. We perform some numerical experiments on a typical model of San Alberto field in Bolivia. Reservoirs located under intense tectonic deformed layers and high dipping interfaces were properly imaged using less the 5% of the time to migrate using the conventional scheme, in which each and every shot is migrated individually.
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Migração RTM orientada ao alvo: experimentos numéricos em modelos envolvendo tectônica salífera
In this work we illustrate the stability of our scheme for target oriented imaging of complex geological structures in relation to velocity field variations. The scheme is based on two powerful concepts: wavefront synthesis and reverse time migration (RTM). In order to guarantee an adequate seismic illumination around the targets, appropriate synthesis operators are obtained by upward wavefield extrapolations from a predefined source localized along a horizontal line located in the area of interest. Convolutions between those operators and shot records give rise to areal shot records, which are depth migrated using the imaging condition of maximum energy traveltimes. The final seismic section is obtained by summing up the migrated sections of all areal shots records. In order to illustrate the robustness of our method, we performed numerical experiments on three 2-D velocity models obtained from the SEG/EAGE 3-D SALT DOME Model. Except for some numerical artifacts, the reverse time migrations of just one areal shot record were able to achieve accurate depth images. The stability of the method concerning the smoothness of the velocity field was also investigated.
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Double Plane Wave Kirchhoff Depth Migration
Authors P.L. Stoffa, M.K. Sen, R.K. Seifoullaev, R. Pestana and J.T. FokkemaWe present a migration method in the coupled ray-parameter domain that is fast and efficient for seismic data that are densely sampled in the source-receiver configuration space. The method is based on slant stacking over both shot positions and receiver positions (or offsets) for all the recorded data. If the data acquisition geometry permits, both in-line and cross-line source positions and receiver positions (or offsets) can be incorporated into a multidimensional phase velocity space which is regular even for randomly positioned input data. By noting the maximum time dips that are present in the shot and receiver gathers and constant offset sections, the number of plane waves required can be estimated and this generally results in a data reduction of at least one and possibly two orders of magnitude. The required travel time computations for depth imaging are independent for each particular plane wave component and thus can be used for either the source or the receiver plane waves during extrapolation in phase space, reducing considerably the computational burden. Even so, each source and receiver plane wave component must be combined with all other receiver and source components for a complete diffraction summation. Since only vertical delay times are required, many travel time techniques can be employed and the problems with multi-pathing and first arrivals are either reduced or eliminated. Further, the shot plane wave integral can be pruned to concentrate the image on selected targets. In this way the computation time can be further reduced and the technique lends itself naturally to a velocity modeling scheme where for example, horizontal and then steeply dipping events are gradually introduced into the analysis. Of course this imaging scheme can be implemented in parallel using a distributed architecture like a PC cluster to compute various plane wave sections since they are independent of each other. The common ray-parameter image gathers can be used exactly like common angle image gathers for residual migration velocity analysis. The migration method lends itself to imaging in anisotropic media since phase space is the natural domain for such an analysis.
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Velocity/Depth Model Building leading to 3D Prestack Depth Migration in the Santos Basin - a case study
Authors Graham Hodgkiss and Jason SunIn this paper, we present a workflow for performing 3D prestack depth migration using data from the Santos Basin. Deep water data from this region is characterised by a sediment layer of varying thickness exhibiting severe structural dips and faulting. This overlies a sequence of halite/anhydrite layers which, in turn, sit above potentially prospective hydrocardon bearing intervals. Each ‘macro’ lithologic unit presents challenges for successful depth imaging and, specifically, accurate velocity depth model building requires a phased approach in which the workflow is tailored to specific regions of the subsurface. In this case history, descriptions are given of the methodology used to construct velocity depth models covering areas in excess of 1000 sqkm. Results of this process are presented together with images from the subsequent prestack migration. These are contrasted with previous results obtained from 2D and 3D work. Subsequent work involved an analysis of anisotropy as a potential factor during the model building process and its associated impact on the final image. The importance of an integrated model building, interpretation and visualization environment is illustrated using examples from the crucial stages during the project.
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Areal Shot Records: Migração Reversa no Tempo e a Influência dos Critérios para a Determinação das Matrizes de Tempo de Trânsito
Authors D.M. Bulcăo, Soares Filho, D.M. Bulcăo, A. Mansur and W.J. MansurThis paper presents a methodology that applied the superposition of different seismograms, called Areal Shot Records, to provide the data to use Reverse Time Migration (RTM) to form a depth image. A new image condition is also described, and results obtained are discussed. Synthetic results for Marmousi Model are presented.
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True-amplitude migration { the traveltime-based strategy
Authors C. Vanelle, M. Spinner, T. Hertweck, C. Jäger and D. GajewskiTrue-amplitude migration of the Kirchho type is a task of high computational e ort. A substantial part of this e ort is spent on the calculation of proper weight functions to countermand the e ect of geometrical spreading in the data. The generation of the weights is usually very time consuming. Also, the weights must be stored. Together with the traveltime tables which are needed for the stacking surfaces, this leads to large demands in computer storage in addition to the high requirements in CPU time. In this paper we propose a strategy to compute the weight functions directly from coarsely-gridded traveltimes. Together with a fast and accurate method for the interpolation of the traveltimes onto the required fine migration grid, this leads to considerable savings in CPU time as well as storage. Application to a complex synthetic data set demonstrates the high quality of our approach.
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True Amplitude One-Way Waves
Authors Gabriela Melo and Jörg SchleicherIn homogeneous media, the two-way wave operator (or just wave operator) can be substituted by the product of two one-way wave operators that form the standard factoring of the wave equation. These one-way operators generate two one-way wave equations, which are fast tools for modeling and migration since they allow us to separate the two-way wave (or full wave), which is the solution of the full wave equation, into two one-way waves, a downgoing and an upgoing one. Since these one-way waves are solutions
of the wave equation in homogeneous media, they satisfy the same approximate (ray-theory) differential equations, eikonal and transport equations, as does the full wave, that is, their traveltime and amplitudes agree in first order approximation with those of the full wave. Since these oneway wave equations produce correct traveltimes even in inhomogeneous media, they have been used in wave equation migration WEM (Claerbout, 1971; 1985). However, in this case, they do not correctly treat reflection amplitudes.
Therefore, using them, only kinematically correct migrated images are obtained. In this paper, we study how the oneway wave operators need to be changed to make the oneway waves produce the same amplitudes of the full wave and keep the traveltime agreement.
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Modelagem Sísmica Elástica 3D: Estudo de Caso na Bacia do Amazonas
Authors A. Bulcăo, Soares Filho, Gomes. A.L.O., J. Schmidt, J. Queiroz and Rosa FilhoThe main focus of this work is concentrated on the Seismic Modeling schemes for 3D problems, using the Elastic Wave Equation. The computer program elaborated in this work use parallel processing resources and runs on microcomputer clusters, in order to reach the necessary computational power to operate the threedimensional simulations concerned to the petroleum industry. Synthetic results for a velocity model with real proportions for the Amazonian basin are presented and compared with real seismic data.
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Numerical and analytical amplitude comparison on acoustic modeling
Authors Luiz Alberto Santos and Djalma Manoel Soares FilhoThis paper presents the comparison of amplitudes response of P wave calculated by a finite difference algorithm for 2D acoustic modeling, 2D AM, and analytically calculated amplitudes. For the comparison a simple model containing 3 horizontal layers was built. It was given a single shot in the middle part of the model and direct and reflected P wave behavior were analyzed. It was observed that geometric spreading amplitudes calculated by 2D acoustic finite difference algorithm exhibited smaller values than the ones calculated with cylindrical divergence formula. The same behavior was observed on reflected amplitudes when compared with Zoepprittz equation results. Both, direct and reflected waves calculated with finite difference routine are linearly related to analytical amplitudes. Such linear relation permits us to improve numerical results in studies that ask for more accuracy.
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Integrating seismic modeling and basin modeling
More LessIn this work basin model outputs of an onshore sedimentary sequence were submitted to seismic modeling embodying three different algorithms: Kirchhoff-Helmholtz, Acoustic and Elastic, the last two based on finite difference. The geological model was built using Petromod (IES) basin simulation program. Porosity, density and fluid saturation were exported from Petromod and such data were applied in Willie equation to estimate P velocities. P velocities were used to perform Kirchhoff-Helmholtz and Acoustic finite difference based algorithms. To perform elastic finite difference modeling, shear wave velocity was estimated by Vs/Vp x poisson expected behavior, and density was applied directly from Petromod exported file.
All methods are equally suitable for structural interpretation support. Noise level in Kirchhoff-Helmholtz model is lower than the other two methods. Finite difference methods are indicated, mainly the elastic one, for dynamic studies. Meanwhile, comparison among real seismic section and synthetic seismic ones mainly on onshore surveys are significantly different. It is caused by scale differences between basin scale parametrization and seismic simulation one.
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Modelamento Sísmico Assintótico Utilizando Diferenças Finitas
Authors Matheus Pila and Lúcio Tunes Santos and Amélia NovaisO modelamento sísmico é uma técnica para simular a propagaçäo de ondas no subsolo terrestre. Uma das maneiras de fazer isso é através dos métodos assintóticos, que computam o campo de onda por partes, decompondo a equaçáo da onda em outras duas equaçöes: Iconal e Transporte. Neste trabalho, apresentamos um método de diferenças finitas (DF) adaptativo que é aplicado na equaçäo iconal, para calcularmos o tempo de trânsito. O método adaptativo é altamente eficiente, no entanto isso pode levar a um alto custo quando aplicado em regiöes que contenham alto contraste.
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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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Seismic characteristics of gas hydrate system at the Hydrate Ridge, offshore Oregon
Authors Dhananjay Kumar, Mrinal K. Sen and and Nathan L. BangsSeismic wave velocities vary in the presence of gas hydrate and free gas in the sediments. Seismic properties (velocities) of the gas-hydrate bearing sediments allow us to identify the presence of gas hydrates, to study their character, formation and distribution, and to estimate the amount of gas hydrate and/or free gas that may be present in the sediments. Accuracy in the estimation of distribution and saturation of gas hydrates and free gas depends on the interval velocities of P- and S-waves. We have carried out an interactive velocity analysis of P- and converted S-waves in the tau-p (intercept time – ray parameters) domain,
which directly gives the interval velocities. This requires multicomponent seismic data. A two-ship seismic experiment was carried out (to record multicomponent seismic data) in summer 2002 at the Hydrate Ridge to map the gas hydrate. Our approach to multicomponent velocity analysis comprises three steps: 1) P-wave velocity analysis, 2) PP to PS event correlation, and 3) Swave velocity analysis. PP to PS correlation is performed using synthetic seismograms. Observed velocities are matched with modeled velocities to estimate gas hydrate saturation. P- and S-wave velocities are modeled with a “Modified Wood equation” which is a modification of Wood equation with a rock physics model and an empirical relation, respectively. We present results from the multicomponent ocean bottom seismometer data recorded at the Hydrate Ridge, offshore Oregon. The Pwave velocity is found to be more sensitive to the saturation of gas hydrates and free gas than S-wave velocity. Gas hydrate is estimated to be upto 7% of rock volume (12% of pore space). The S-wave velocity does not show an anomalous increase in the hydrate-bearing sediments. Thus we conclude that hydrate does not cement sediment grains enough to affect shear properties. It is more likely that the hydrates are formed within the pore space in this region.
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Anti-Multiple Processing on offshore Brazil Deep Water Seismic Data Using 3D Surface-Related Multiple Modeling
Authors A. Pica, G. Poulain, B. David, M. Magesan, S. Baldock, T. Weisser, P. Hugonnet, S. Pharez and and Ph. HerrmannMarine seismic data acquired over sea floors made of submarine canyons are characterized by the presence of extremely complicated patterns in the shape of the reflected multiple energy. Despite the fact that data-driven SRME techniques do not require any a priori knowledge of the subsurface (reflectivity, structures and velocities), it is sometimes difficult in 3D to perform the reconstruction of the missing data or the missing multiple contributions needed when applying this technique, as the method theoretically requires a shot location at each receiver location. In the following instead, we present a model-based surface-related multiple modeling technique (SRMM), which is free from any constraint relating to shot position (including OBC) and distribution, which was applied on marine seismic data acquired offshore Brazil over a series of deep submarine canyons.
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Multiple removal strategy for deep and shallow water
Authors Riaz Alá’i and Eric VerschuurThis paper discusses the removal strategy of surfacerelated multiples in marine situation with different water depths. Multiples can make interpretation of primary target structures very difficult if they have not been removed from the recorded data in a very early stage. Therefore it is important to define strategies for optimal attenuation of surface-related multiples in various environments. Surface-related multiple elimination (SRME) has been applied very successfully to various marine datasets, but is known to have difficulties with shallow water environment, due to the fact that missing near offsets cannot be reconstructed in a reliable manner. Therefore, a combined methodology with multi-gate predictive deconvolution (MGPD) is suggested to cover all application areas. Examples on synthetic and field data are shown to support this strategy.
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Comparação de Resultados do Filtro de Velocidade com Operador WHLP-CRS na Atenuação de Múltiplas em Dados Sintéticos
Authors Edson C. Cruz and Lourenildo W. B. LeiteThe geological motivation of this work is the imaging of sedimentary basin structures of the Amazon region, where the generation and accumulation of hydrocarbons is related to the existence of diabase sills. The seismic motivation is the fact that these intrusive rocks present a great impedance contrast with respect to the host rock, what gives rise to external and internal multiples, with primary-like amplitudes. The seismic signal of the multiples can predominate over the primary reflection signals from deeper interfaces, making difficult the processing, interpretation and imaging of seismic sections. We present a practical problem of attenuation of multiples in common-shot (CS) sections by the comparison of two methods. The first method considers the combination of the Wiener-Hopf-Levinson (WHLP) theory and the common-reflection-surface (CRS) stacking techniques,, where the operator is exclusively designed in the space-time domain. The second method is a velocity dependent filter (w-k), applied after the CRS stacking, where the operator is exclusively designed in the frequency-wavenumber domain.
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Teoria da Expansão em Série de Debye para o Guia de Onda de Pekeris
Authors Luiz G. Guimarães and Rosana B. SantiagoMostramos neste trabalho que a aplicação da teoria da expansão de Debye ao problema do guia de onda de Pekeris, nos permite analisar de forma simples o papel das reflexões múltiplas nos modos ressonantes e não ressonantes que se propagam ao longo do guia.
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Quadratic Normal Moveouts in Isotropic Media: A Quick Tutorial
Authors Martin Tygel and Lúcio Tunes SantosWe present an organized and didactic tutorial on the formulation and derivation of the generalized quadratic normal moveouts in isotropic media. General 2D/3D expressions, with the inclusion of topographic as well as inhomogeneous velocities are reviewed and discussed.
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Polarization of plane waves in viscoelastic anisotropic media
Authors Vlastislav Cervený and Ivan PsencıkHomogeneous and inhomogeneous time-harmonic plane waves propagating in unbounded viscoelastic anisotropic media are generally elliptically polarized. Exceptions are P and S waves propagating along some specific directions, along which they are
linearly polarized. A typical case are SH waves propagating in a plane of symmetry of a viscoelastic anisotropic medium. Two most important characteristics of the polarization are the orientation of the axes of the polarization ellipse and its eccentricity. They both usually vary considerably with the direction of wavefront propagation, and with varying strength of inhomogeneity of the considered plane wave. The orientation of the polarization ellipse generally differs from the direction of wavefront propagation, being usually closer to the direction of the energy flux. The eccentricity of the polarization ellipse depends particularly strongly on the inhomogeneity of the plane wave. For homogeneous plane waves, the polarization is usually nearly linear, with large eccentricity.
The eccentricity decreases with increasing inhomogeneity of the wave. For strongly inhomogeneous plane waves, the polarization ellipse becomes nearly circular, eccentricity being very small. The eccentricity of the polarization ellipse usually also decreases in a vicinity of singular directions.
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Aproximações Hiperbólicas de Tempos de Trânsito 3-D para Reflexões e Difrações
Authors Charles Lima, German Garabito and Pedro ChiraThe Zero-Offset (ZO) Common-Reflection-Surface (CRS) stacking technique is a macro-model independent seismic reflection imaging method that simulates a ZO volume or section from multi-coverage reflection pre-stack data. This method has been established as an improvement and alternative of the conventional Normal-Moveotu/Dip-Moveout (NMO/DMO) processing. Over the past years it has been sucessfully applied both to 2-D and 3-D synthetic and real seismic data. It provides important wavefield attributes or parameters for several applications, e.g. migration, inversion and interpretation. It uses as operator a second-order hyperbolic traveltime approximation in the vicinity of a central ray. In 3-D, for a normal or ZO central ray, this operator depends on eight parameters that are determined by means of coherence analysis procedures. In this work, we examine the 3-D ZO CRS operator for reflection and diffraction events with its respective true traveltimes. The results of these comparisons demonstrate that the 3-D ZO CRS operator has a good fit with the true traveltime surface.
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Common-Reflection-Surface (CRS) approaches for stacking of 3-D seismic data
Authors Pedro Chira-Oliva and João Carlos R. CruzThe Common-Reflection-Surface (CRS) stack is a macromodel independent seismic imaging method that, in many important situations, improve the imaging quality with respect to the conventional methods (e.g. Normal- Moveout/Dip-Moveout (NMO/DMO) stack). The CRS method has been sucessfully applied both 2-D synthetic and real seismic data. Recently, it was also tested for full and narrow azimuth 3-D data with satisfactory results,showing its advantages over conventional methods. By using optimized search strategies, the CRS parameters are estimated by means of a coherence analysis procedure. The estimated 3-D CRS parameters (fourteen parameters for the finite-offset (FO) central ray and eight parameters for the zero-offset (ZO) central ray) are used in the hyperbolic traveltime approach to stack the 3-D multi-coverage seismic data, providing as results a highresolution simulated FO or ZO volume, and coherence and parameter volumes. The 3-D CRS approach is also specialized in order to approximate diffraction traveltimes.
In this case the central ray is a diffraction ray, and the stack formalism depends on ten parameters (FO case) and five parameters (ZO case). In this work, we present the formalism and examples of applications of the 3-D CRS stacking operator for reflection and diffraction events. We consider two cases for the central ray, ZO and FO, respectively. We show special formulas for applications of the 3-D CRS attributes to determine, e.g. the geometrical spreading (GS) factor and projected Fresnel zones, important to define the aperture for stacking and migration.
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Using WCDP ("Wave Analogue Common Depth Point") to Generate Depth Interval Velocity for Migration
A successful prestack depth migration depends on a good depth interval velocity. The WCDP technique provides a reliable geological model after a composition based on interpretation of a series of time sections, which can be generated in a very short execution time when compared with the traditional prestack time migration. It will be discussed how this methodology works in order to obtain the geological model and the depth interval velocity field.
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Horizon Velocity Analysis Using OCO Rays
More LessIn this paper, I present a new method for horizon velocity analysis, which is able to find the RMS velocity that maps an horizon between two different common offset sections. The method introduces the concept of OCO rays and is based on the premise that the media presents a smooth velocity variation. The method was implemented for the 2D case and was applied to determine the velocity field in a synthetic dataset.
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Seismic Velocity Quality Control: from picking to regional gathering
More LessMost of technicians who participated in Seismic Reflection Processing or Interpretation have one story about seismic velocity. In Seismic Processing sequence there are few capital chances to strongly decrease the quality of a seismic section. Among then we may point out errors in acquisition geometry and errors in seismic velocity interpretation. Seismic velocity is a very important attribute derived from seismic data that have immediate implications in seismic section quality and subsequently in petroleum E&P projects. Poor quality seismic section with a bad depth conversion holds enough conditions to reach an E&P project failure. Bad seismic velocities might result from interpretation by inexperienced geophysicists, or obtained from bad seismic data quality. Seismic Velocity Interpretation is a difficult job that requires experienced technicians. As a general rule, in petroleum major companies as much as in the petroleum service companies, this job is delegated to inexperienced technicians. Sometimes the technician involved in velocity interpretation has knowledge about seismic but has not enough knowledge about the local or regional Geology. One perception about seismic velocity is that every interpreter has a fingerprint: in a word, seismic velocity interpretation is an art form.
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Influence of seismic anisotropy in NMO correction
Authors M.B.C. Silva, M. B. C., C. Rodriguez, C. and Fontoura and S. A. B.This aim of this work is to investigate the errors generated when VTI anisotropy is not considered for normal moveout correction of seismic data. Synthetic data (with different degrees of anisotropy) were processed and the errors on layer depth and NMO velocity were quantified. The results show that NMO velocity is more sensible to δ than ε (as expected) and that water depth and anisotropic layer thickness did not influence the velocity error in any of the tests.
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Dip-correction for coherence-based migration velocity analysis
Authors Ricardo Biloti and J¨org SchleicherMigration velocity analysis (MVA) is a seismic processing step that aims at translating the velocity information that is contained in the residual moveout in an image gather after migration with an erroneous velocity model into velocity updates. In this paper, we extend the original coherencebased MVA approach to dipping reflectors. We devise a new MVA technique, where the reflector dip is treated as an additional search parameter that is to be detected together with the velocity updating factor. A numerical example demonstrates that the additional search parameter can indeed be helpful to improve the quality of the velocity updates.
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Interação Magnetométrica e Gamaespectrométrica com abrangência das Razões U/Th, U/K e Th/K no Mapeamento Geológico da Folha Quixadá, Estado do Ceará, Brasil
More LessThis paper presents results from a 1:250,000 scale integration of the magnetometric and gamaspectrometric airborne data in northeast Brasil, central region of Ceará State, with the objective of assisting regional geologic and tectonic-structural problems. This airborne magnetometric and gama-spectrometric was executed by LASA in Itatira Project for organizations led for the NUCLEBRÁS, in 1977. This portion of Ceará comprises broad sectors of key complex Proterozoic geologic units:Cruzeta Complex, Ceará Complex, Independencia Unit, Tamboril Santa Quiteria Complex and Granitoids. The magnetometric and gama-spectrometric study established three anomalous areas in the region of Quixadá that correlate well with several geologic formations and tectonic features found in the area.
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Estrutura Crustal na Região Central do Brasil: aplicação do método de Função do Receptor
Authors Mônica M.M. Costa and Jesus BerrocalThis work intends contributing with more information to improve the already existent models about the crustal structure in the collision zone between Tocantins Province and São Francisco Craton, the two most important tectonic units of Central Brazilian geologic history.
Receiver Function (RF) method was chosen to curry on this study. RF uses the phases that are generated through the refraction of teleseismic P waves in Moho and other discontinuities of the crust. To isolate the information about the local effects of the recorded waveforms and get a function that only depends on the structure below the station, we use the deconvolution process, both in frequency and time domain, choosing the result that produced the best responses. Due to the smaller velocity of Ps phase compared with the refracted P, the difference between the arrival times of P and Ps refracted phases will measure the depth of the discontinuity where occurred the refraction. In the case of Moho, the result was the thickness of the crust beneath the recorder station.
At present, the receiver functions were already computed with telesseismic data of CV1B, CV2B and CV3B stations located in Cavalcante, GO, region, and preliminary results of crustal thickness beneath that region have been found. The results show the smaller thickness were gotten in the events with azimuths between 135º and 180º (SSE), which were related to an apparent anomalous region in the crust below of these stations, coinciding with the model considered by Soares et al (2004). In this region the crust, that generally presents a thickness among 41 to 43 km, started to have thickness of approximately 37 km. These values show an irregular Moho below Cavalcante, just waited because the complex geotectonic of the region. The improvements of these results are going to help in the construction of a refined model for the area.
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Two-dimensional seismic refraction model of central Brazil crust
Authors José E. P. Soares, Jesus Berrocal and Reinhardt A. FuckA two-dimensional model of central Brazil crust and upper mantle was obtained from travel-time interpretation of deep seismic refraction data from Porangatu, and Cavalcante lines. Moho is an irregular interface from 36 km to 44 km deep. Mean crustal VP and VP/VS are, respectively, 6.6 km/s and 1.74 under Araguaia Belt, 6.5 km/s and 1.71 beneath Goiás Magmatic Arc, 6.4 km/s and 1.70 below Goiás Massif, as well as 6.4 km/s and 1.69 beneath the foreland fold-and-thrust belt, and western São Francisco Craton. The upper mantle presents VP of 8.0 km/s under Porangatu line, and 8.3 km/s beneath Cavalcante line. Seismic features allow identifying: i) Neoproterozoic sutures related to a westwards subduction of São Francisco plate, and to an eastwards subduction of Amazon plate; ii) delamination of mafic-ultramafic root beneath Goiás Magmatic Arc; iii) thick skin tectonics in the foreland fold-and-thrust belt of northern Brasília Belt.
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Identificação das principais feições geológicas do SE do Brasil através do estudo das anomalias magnéticas crustais – resultados preliminares
Authors Cosme F. Ponte Neto and Lays H. F. OliveiraNeste trabalho foram determinadas as anomalias do campo geomagnético, de origem crustal, na região sudeste do Brasil, entre os paralelos 13°S e 25°S e os meridianos 39W e 55W, com o objetivo de correlacionar os padrões destas anomalias com as feições das principais províncias tectônicas da região. Foram utilizados dados da rede geomagnética do Observatório Nacional [Lima et al., 2001].
A análise dos resultados indicou uma anomalia positiva na porção sul do craton do São Francisco, sugerindo a presença de uma estrutura, em subsuperfície, maciça ou em platô (as anomalias apresentam padrões da isovalores concêntricos e simétricos). A fonte desta anomalia apresenta contraste de suscetibilidade magnética bem localizado espacialmente e valores de magnetização remanescente com intensidades da ordem de 40 A/m, inclinação magnética de -20° e atingindo profundidades da ordem de 35 km. Na parte sudoeste da área estuda, sobre a bacia do Paraná, existe uma anomalia positiva com fonte não pontual, provavelmente associada aos derrames de basalto da bacia do Paraná. A parte leste da área apresenta anomalias negativas, também de caráter não pontual, provavelmente associadas aos limites orientais da plataforma continental sul americana.
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Determinação de estruturas crustais com função do receptor na estação sismológica de Rio Claro, RCLB
More LessThis paper presents the analysis of receiver functions in the Rio Claro Seismological Station. The station is in operation since October 2002, and in this period the recorded teleseisms allowed a preliminary evaluation of receiver functions. The depths obtained for the Moho discontinuity range from 42 to 47 km. The values of 47 km were calculated with events from SE and the values of 42 km were calculated with teleseisms arriving from NW and SW backazimuths.
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ESTUDOS DOS EFEITOS DO MANTO DE INTEMPERISMO SOBRE ANOMALIAS ( VLF ) GERADAS POR CORPOS INCLINADOS ATRAVÉS DE MODELAGEM NUMÉRICA
Authors Gilberto Emanoel Reis Vogado and Om Prakash VermaWe have studied the effects of a partially conductive overburden on VLF anomaliesof an inclined prism placed in a highly resistive half space. This study is carried out through numerical modelling using Finite Element techniques. Two electrical target-overburden contact situations are modelled: (1) no contact between them we called this stuation as “Inductive Overburden” and (2) a galvanic contact between them is called “Galvanic Overburden” . Ellipticity anomalies are highly prone to overburden effects in the both stuations, while tilt angle suffers less. Galvanic overburden affects much more anomalies than the inductive overburden. In the presence of the galvanic overburden initially an increase in the anomaly is observed, which decreses later on with the increase in the conductance of the overburden. Due is the presence of the overburden the Argand diagrams suffer rotation in the anticlock wise direction.
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O Uso do Geóide no Estudo de Mecanismos de Compensação Isostática
More LessIn this paper, we present some equations for the determination of the models of isostatic compensation, where geoid anomalies can be applied. This technique is based on the admittance function, which is related with geoid anomalies in the frequency domain by a simple mathematical formulation.
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Paleomagnetism of the post-Paleozoic alkaline magmatism in the Brazilian Platform: discussing ages and plate displacements
More LessA review of the available paleomagnetic data on the post-Paleozoic alkaline rocks in the Brazilian Platform in combination with other South American poles of same age allow the proposition of a new apparent polar wander path (APWP) revealing rotations of the plate associated with the emplacement of alkaline provinces. By means of an analysis of the magnetization polarity of the rocks some inferences of the relative ages of the igneous complexes were possible, as the investigated time interval comprises the long normal polarity interval of the Cretaceous (Cretaceous Normal Superchron). Absolute reconstructions of the drift movement of South America was achieved by means of paleomagnetic rotations and longitude control through the sea floor magnetic anomalies.
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Moho depth estimative at Balbina lake area, Amazonas, Brazil
Authors Cristiano Chimpliganond and Maria Fernanda Novo BarbosaThe Balbina Reservoir belongs to Eletronorte hydroelectric power company and is located at Amazonas State, Brazil. It’s volume is of 17,5 km3 and the dam has 42 m high, and the triggered seismicity started 2,5 years after the start of impoundment of the lake (1987). The broadband seismic station BALB was installed at the area in April 1998, equipped with a Guralp 40T sensor and a Reftek data logger sampling continuously at 100 samples per second. The Balbina Lake is located at the Amazonian Craton, between the geochronological provinces Amazonia Central (2.5 Ga) and Ventuari-Tapajós (1.95-1.8 Ga). The first comprise Paleoproterozoic felsic to intermediate volcanic rocks and granitoids, and the later granite-gnaisses (Tassinari & Macambira, 2004).
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Determinação dos parâmetros de fonte do sismo de 10/03/98 de Porto dos Gaúchos/MT, por análise espectral de sinais
More LessBrune’s source model is employed to determine the source parameters (stress-drop, seismic moment, peak frequency) and the seismic moment magnitude of the Porto dos Gauchos/MT main shock, occurred on March 10, 1998. The source parameters were determined from the spectral analysis of digital waveforms recorded by seven stations of a regional seismographic network. For all stations were determined displacement spectrum and from those the source parameters. The final values were taken as the average of the results. Three different types of magnitudes are presented: body wave magnitude (mb = 5.1), macrosseismic magnitude (mb = 5.1) and seismic moment magnitude (Mw = 5.2).
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Evolution of the Areado/MG seismic sequence - started in January, 2004
More LessA microearthquake seismic sequence, in course in the neighborhoods of the little Town of Areado, located in the southern part of Minas Gerais State is studied. This sequence started on January 21, with a 3.4 regional magnitude felt with a MMI V. After that, a local seismograph network with up to six stations detected more than nine hundred microearthquakes, 458 of these located using data of at least three components stations. A composite focal mechanism indicates a dextral transcurrent fault in NNE-SSW direction, in accordance with a hypocentral distribution of the microearthquakes. Results of preliminary studies of that sequence were presented in the first Symposium of the Brazilian Geophysical Society (SBGf) occurred in São Paulo in September 2004. In these studies, it was presented the analysis results of the data generated by the seismic network from January 28 to May 28. This paper intends to update the results presented on 2004 SBGf Symposium, in a more conclusive way, showing seismicity and evolution from January 28, 2004 to May 28, 2005.
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ESTUDO DE REFRAÇÃO SÍSMICA NA REGIÃO QUE ABRANGE A ESTAÇÃO SISMOGRÁFICA DE ANGRA DOS REIS (ESAR)
Authors Sabrina de Souza Lima Pierobou and Jesus BerrocalThe project studies the structural characteristics of the main layers of the crust in the region of the Seismographic Station of Angra dos Reis (ESAR), through the method of deep seismic refraction (RSP), defining the value of some seismological parameters, which are used for epicentral determination of the local seismic activity.
To complete the study of crust (it wasn’t possible to get all the RSP points wanted), the method Receiver Function was used to characterize the structure of the crust under ESAR. The results of both methods were used in the program TVEL that helped modeling the missing part of the RSP line. This model allowed to get relations between distance and the time S-P of the arrival of these waves.
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Estudos preliminares da anisotropia sísmica em Senador Sá-CE
In the João Câmara and Açu areas, NE Brazil, the Precambrian fabric has been interpreted as predominantly controlling the crustal seismic anisotropy. This interpretation is due the polarization of the fastest S wave splitting which is parallel to Precambrian lineaments. The seismic activity in Senador Sá occurred between two shear zones. Initially, two stations presented the NE and NNE polarization; and delay time 8.16 ms/km and 2.56 ms/km. The investigation of regional lineaments shows that these features present a NE trend, which is compatible with our preliminary seismic anisotropy study. A detailed study using other stations located between the shear zones could shed further light on this issue and confirm or not the Precambrian-controlled seismic anisotropy in NE Brazil.
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Mecanismo Focal do Terremoto Principal de 10 de Março de 1998, Porto dos Gaúchos, Mato Grosso, Brasil
Authors Takato Nakayoshi and Vasile I MarzaThe active earthquake area of Porto dos Gaúchos, Mato Grosso state, Brazil, hereinafter called the Seismogenic Zone of Porto dos Gaúchos (whose Portuguese acronym is ZSPG) is placed at junction of the southern boundary of the Amazonian Craton with the northern part of the Parecis Basin. This area represents one of the most outstanding active seismic areas of Brazil intraplate seismicity, where it occurred in 1955 the largest known Brazilian earthquake and where recently, on March 23, 2005, happened another significant earthquake with magnitude 5.0 mb (USGS). The ZSPG quake of March 10, 1998, reaching a magnitude of 5.3 mb(ISC) is the second largest event of the ZSPG and the largest seismic event during the recent times of digital seismology, hence supplying a great amount of useful data due to the large number of seismographic stations recording it. Using the polarities of the incident P waves at 12 world-wide seismographic stations it was possible to workout the focal mechanism of the 1998 mainshock. This mechanism shows a predominant strike slip fault plane with a small reverse component. The preferred slipping plane has a strike of 303°, coincident with the kilometer extension lineaments (faults and shear zones) observed in the Amazonian Craton and with the structure of the Parecis Basin and also matching early aftershock lineament of the 1998 mainshock. The inferred fault mechanism is compatible with a model presenting an approximately EW compression coinciding with the direction of the predominant regional tectonic stresses acting in the South America Tectonic Plate.
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Representation of the Earth’s Gravity Field in the Southern part of the African Plate
Authors Ahmed M. A. Meguid and Eder C. MolinaA new digital gravity field model (SAGM2005) is presented, representing the free air anomaly and the height anomaly in the southern part of the African plate. A total of 12,203 gravity observations together with 5,486 free air GRACE-only data points were used in the process of Least-Squares Collocations (LSC) in order to carry the present representation out. A new computer algorithm was written, as a Matlab application, to calculate the observed covariance function as a primary step of the collocation procedure. The LSC method was applied as described by Moritz (1980) assuming a constant standard error of 3 mGal to the free air data points. The present work aimed to show the effect of introducing GRACE-only data with land observations on the precision of the calculated digital models, which represent the gravity field components in the studied area with spatial resolution of 0.1°, which is quite suitable to be used in regional studies.
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On gravity influence in fault patterns formation and westward displacement of the lithosphere
More LessMotivated by the lack of a unified theory for the driving force that causes plates movements on Earth and based on the success of some simple models to predict general features of Earth's seismic development we introduce outer gravitational fields as the possible energy providers for such enormous and permanent phenomena. Some known results on faults preferential orientations might give support to the hypothesis. Further experimental base for the consideration of external gravitational fields is supplied by the almost null tectonic activity nearby Earth's poles.
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Aspectos da evolução termo-mecânica da Bacia do Parnaíba
More LessIn this paper, we intend to contribute to the a better assessment of the thermo-mechanical evolution of the Parnaíba Basin and to investigate its capability for hydrocarbons generation and accumulation. The study of intracontinental sedimentary basins, such as Parnaíba, is also useful for the understanding of the thermo-mecanical evolution of the continental lithosphere. Based on the non-uniform lithospheric stretching model of Royden and Keen (1980), backstripped subsidence and paleotemperatures curves were constructed for 20 PETROBRÁS boreholes using the program BASTA (Friedinger, 1988) modified by De Sousa (1996).
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Comportamento da Anomalia Magnética do Atlântico Sul para os últimos 105 anos
Authors Gelvam A. Hartmann and Igor I. G. PaccaIn this paper, we discuss the characteristics of the South Atlantic Magnetic Anomaly (SAMA) for the last century. Some authors relate the origin of the SAMA to the offset of the eccentric dipole of the Earth’s magnetic field. There are several ways for defining the SAMA geographical location. We used in this work the minimum intensity values of the field. The change in the westward drift of the Anomaly may correspond to changes in the motion of the fluid in the outer core. The intensity curve shows three intervals with different behavior for the studied period. It may be speculated that these changes can be related to geomagnetic jerks. However, the SAMA and other similar anomalies should be studied in the thousands and the million years time scales in order to improve an interpretation in terms of the Geodynamo Theory.
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Numerical analysis of electromagnetoelastic waves propagation in heterogeneous media
Authors Lucas A. P. Gomes, Viatcheslav Priimenko and Mikhail VishnevskiiInteraction of electromagnetic fields with elastic bodies is the subject of many theoretical investigations in mechanics of continua for the last decades. Some variants of direct and inverse problems have been studied leading into the determination of some characteristics of a medium. It is very interesting to study this phenomenon due to the possibility of applying this theory to geophysical prospecting and study of earthquake sources. It is well known that when an electrical-conducting elastic body oscillates in an electromagnetic field, variations of the electrical and magnetic fields are observed as a result of this motion. Similar processes are also observed when seismic waves propagate in the Earth’s crust. Variations of the seismic and electromagnetic fields arising in this case are called electromagnetoelastic waves. Our work is dedicated to investigation of such interaction. The model considered here is based on a simple variant of combination of the Lamé and Maxwell equations. We form the basic equations for the description of some variant of this coupling and consider mathematical and numerical models of this process.
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CONSIDERAÇÕES SOBRE EVENTOS TERMOCINEMÁTICOS ASSOCIADO AO SOERGUIMENTO DE ÁREAS PRÉ CAMBRIANAS E A SUA RELAÇÃO COM A SUBSIDÊNCIA DA BACIA DO PARANÁ: TRAÇOS DE FISSÃO E PALEOTENSÕES
The Precambrian basement of Southeast Brazil along the northeast board of Paraná basin is characterized by many bittle reactivations of shear zones that developed during the end of the Brasiliano orogeny. This brittle activity occurred several times during the Phanerozoic, resulting in additional fault movements and uplift of the Pre-cambrian areas. To constrain the uplift (fission tracks) and paleostress phases of the Pre – cambrian areas, in the northeast Paraná basin margin, and establishment to relation with the subsidence events of the Paraná basin. The results of this study consists of two main domains, which record different episodes of uplift and reactivation of faults: The first domain is denominated Sorocaba, it is located near of the Paraná basin and consists of Neoproterozoic rocks and Paleozoic sedimentary units bounded by Campinas fault zone (NE), Jundiuvira fault zone (WNW) and Moreiras fault zone (NE); The second domain consists of Pre-cambrian rocks, Granites rocks and Tertiary sedimentary units, characterized by three principal faults Jundiuvira fault zone (EW), Camanducaia and Extrema fault zone (NE). This with many evidences of reativaction, where was recognized tectonic rocks as pseudotachylytes cataclasites and fault gouges. Based on fission track data, it is associated uplift of basement rocks around the NE margin of the Paraná Basin in southeast Brazil was related to the tectonic subsidence and consequent deposition of the Paraná basin. The evolution of this basin seems to have been strongly influenced by compressive and extensional events as suggested by Millani (1997) that developed during the subduction of Panthalassan plate under the southwestern Gondwana margin (Ramos,1988) These pulses are recorded as important unconformities in the stratigraphic framework of the Parana basin and the elucidation of the uplift and denudation histories of the basement shield is crucial to the understanding these tectonics effects.
Fission track studies (correct ages) indicate the correlation with significant events of cooling in 193Ma (Triassic); 126Ma (Early Cretacic); 89Ma (Late Cretacic) and 69Ma (Late Cretacic). The thermal histories are indicate tectonic uplift117 the Oligocene too.
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Recent progress in mesospheric gravity wave studies using nightglow imaging systems
During the past four decades a variety of optical remote sensing techniques have revealed a rich spectrum of wave activity in the upper atmosphere. Many of these perturbations, with periodicites ranging from ~5 min to several hours and horizontal scales of a few ten's of km to several thousands km, are due to freely propagating acoustic-gravity waves and forced tidal oscillations. Optical observations of the spatial and temporal characteristics of these waves in the mesosphere and lower thermosphere (MLT) region (~80-100 km) are facilitated by several naturally occurring, vertically distinct nightglow layers. This paper describes the use of state-of-the-art ground-based CCD imaging techniques to detect these waves in intensity and temperature. All-sky (180°) image measurements are used to illustrate the characteristics of small-scale, short period (< 1 hour) waves and to investigate their seasonal propagation and impact on the MLT region. These results will be contrasted with measurements of mesospheric temperature made using a separate imaging system capable of determining induced temperature amplitudes of much larger-scale wave motions and investigating night-to-night and seasonal variability in mesospheric temperature.
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A statistical study of plasma bubble zonal drift velocity variations with geomagnetic activity and events of corotating and transient solar wind streams
Authors P.M. Terra, P. M., C.G.M. Brum, C. G. M., J.H.A. Sobral, J. H. A., M.A. Abdu, M. A., J.R. Souza and H. TakahashiA study of the plasma bubble zonal drift velocities is carried out using experimental airglow data registered at Cachoeira Paulista (22.5° S, 45° W, dip 30° S) during the period from October to March, between 1980-1994. This study is based upon 109 nights of zonal scanning photometer measurements of OI 630 nm airglow. The zonal velocity magnitudes of the ionospheric bubbles are investigated taking into account the geomagnetic effects. The geomagnetic activity condition in this study is based on the 3-hour geomagnetic index Kp. A detailed analysis of these plasma bubble velocities, as a function of geomagnetic activity, shows that the velocity variations expressed by Vfinal – Vinitial are smaller during quiet nights than during disturbed nights. In addition, it is noted that the mean velocities present the highest (lowest) values during quiet (disturbed) nights of solar maximum (minimum). Also it is observed that the tendency of the plasma bubbles is to appear earlier during corotating stream events than transient or no stream periods.
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Muon and Neutron Observations in Connection with the Corotating Interaction Region
Ground cosmic ray observations are used for studying several kinds of interplanetary structures. The cosmic ray data has different responses to each kind of interplanetary structure. This article has as objective to study cosmic ray muon and neutron signatures due to the passage of Corotating Interaction Region (CIR) in the interplanetary medium, and identify the signatures in the cosmic ray data due to these events. The cosmic ray muon data used in this work are recorded by the multidirectional muon detector installed at INPE’s Observatório espacial do Sul – OES/CRSPE/INPE-MCT, in São Martinho da Serra (Brazil) and the neutron monitor installed in Newark (USA). The CIR events were selected in the period from 2001 to 2004.
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Some important factors that affect the Doppler properties of vertical m-size echoes in the low latitude E region
Authors J.-P. St.-Maurice and R. K. ChoudharyMeter size irregularities are routinely studied with radars in the equatorial and low latitude regions. In both instances echoes from the E region (90 to 120 km altitude) are a common occurrence. The resulting echoes are labeled as so-called Type I or Type II according to their spectral signature. In this paper we show that the phase velocity of Type I echoes increases with decreasing altitude owing to thermal feedback effects taking place in the growth process. We also show that Type II echoes can be influenced by atmospheric neutral winds to the point of revealing the presence of Kelvin-Helmholtz billows, as shown by a recently studied example taken from the Gadanki radar in India.
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Observações de ondas de gravidade mesosféricas de baixas frequências via keograma da aeroluminescência OH IR
Authors Delano Gobbi, Fábio Augusto Vargas and Hisao TakahashiContinuos observation of airglow emissions obtained by an all-sky imager (INPE), installed at Cachoeira Paulista (230 S), have been used to investigate the characteristics on the major low frequencies oscillations. The images of the Near Infrared OH emissions produce keograms constructed from a north-south and east-west slice from the middle of every image joined over the whole night. The keograms are a convenient way to reveal wave activities over the whole night, on the other hand, the individual images are the best way to see short period gravity waves.
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F-region nocturnal zonal plasma drift velocities of the brazilian equatorial ionosphere during maximum solar
This study focused on the nocturnal zonal plasma drift velocities of ionosphere (F region) at equatorial latitude regions of the Brazil. The zonal plasma drifts result from equatorial latitude electrodynamics with influences from the E and F region conductivities and neutral wind shears in altitude and latitude. The analysis is interpreted using the simple plasma drift model (Eccles, 1998a,b) and monochromatic digital images of OI630nm emission, which allow direct visualization of ionospheric bubbles. The digital images were obtained by optical imager system operating at São João do Cariri (equatorial geographic latitudes: 7.4ºS; 36.5ºW; 19oS dip latitude) from January to December 2001 (maximum solar activity). In this period, 48 quiet nights ( KP<24o) presented the signature of ionospheric bubbles and the zonal drift velocities were calculated (observational results). The geophysical conditions of each quiet night were considered in equations of the simple plasma drift model in order to obtain the theoretical zonal plasma drift velocities between 18LT and 6LT. The theoretical and observational results were grouped seasonally. The theoretical results showed two humps around 20LT-21LT and 2LT–4LT that were present in observational results.
Observational results were smaller (greater) than theoretical results before (after) 22LT-23LT being earlier in the Summer. The discrepancies between the theoretical and observed results apparently were due to
the neutral wind magnitudes provided by HWM93 empirical model. The Summer 2001 season presented the theoretical and observed results in better agreement considering the bars of variance.
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