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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
251 - 300 of 462 results
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Repeatability issues of time-lapse marine seismic data
By Ola EikenRepeatability is a key issue in seismic monitoring, as it defines the noise threshold which a time-lapse change needs to exceed. Good measures of repeatability are non-trivial to define, and while NRMS (Normalized Root Mean Square) is widely used today, more than one parameter is needed to describe various aspects of the data. The background noise level and horizontal positioning variations are basic causes of non-repeatability which need to be controlled in data acquisition. Source- and receiver positions may be controlled by towing many streamers in parallel and having overlapping swaths, by keeping the source on a pre-defined line, and by laterally steerable streamers. Timing variations, caused by water layer variations, may be corrected for by data dependent matching, provided the position variations are not too large. Optimizing the choice of sampling parameters, as streamer separation, number of streamers, amount of swath overlap, cable steering or not, may be guided by repeatability modeling. A simplified scheme taking the statistical distribution of position variations into account is suggested.
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Estimativa de porosidade e saturação a partir de dados sísmicos 4D
More LessWe present a methodology for porosity and saturation estimation from a 4D P wave impedance data using bayesian probability theory. The proposed methodology is tested on synthetic data, considering two different states of knowledge about the reservoir at legacy data (pre production). We first assume that the initial saturation (Swi) of reservoir is well known. In this case we can infer the porosity values from the legacy data and water saturation from the monitor data, assuming that porosity is not changed due to production. In the second case, we assume that exists some degree of uncertainty about the initial saturation. For this case, we introduce an attribute called “porosity perturbation”, exploring the strong coupling which exists between porosity and saturation. Porosity perturbations can be computed using any reasonable reference state of initial saturation to yield an indirect measure of saturation changes over the reservoir. This approach is applicable to situations where both porosity and pressure do not vary significantly.
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Contextualizando a importância da repetibilidade na sísmica 4D
More LessPetrobras is beginning its 4D seismic investments in Brazil. As this is yet a non-disseminated technology in our company, common sense leads us to study and try to reproduce the successful experiences of other oil companies worldwide. North Sea is the oil province where are the major 4D seismic investments in the whole world, with very high incomes. As everybody knows, Statoil (Norway) has continuously contracted 4D seismic acquisitions for many oil fields. Its success is due, mainly, to the high repeatability level in successive acquisitions for a certain field, what was essential in obtaining a high quality 4D response. When learning with this experience, it’s important to have in mind the geological and geophysical characteristics of that basin, as well as sea conditions during the North Sea acquisition season. The objective of this paper is to relate the importance of repeatability in 4D seismic to the peculiarities of oil fields to be imaged, in order to point the differences between North Sea (Norway) and Campos Basin (Brazil) and to suggest a strategy adapted to the reality of this brazilian basin. For optimization of investments in seismic acquisition, the suggestion is to perform feasibility studies that take into account petrophysical and seismic aspects as well as operational limitations and costs related to obtaining a high repeatability level.
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Inferência Bayesiana da saturação
Authors Raquel G. Teixeira and Luiz Geraldo LouresThis methodology was developed following the Bayesian methodology of inferece and seeks for an fluid indicator for the pore fluid and an associated quality meassurement. The sources of information are well log data and seismic attribut data. This article presents the theorectical background and results from tests using a set of well log data.
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Assessment of rock quality classification based on Principal Coordinates Analysis of conventional petrophysical and NMR data
More LessThis paper shows how principal coordinates analysis, PCO, can contribute to asses the correspondence between porosity, permeability and the ratio of moveable fluid over bound fluid volumes data and the rock quality classification of water saturated core plug samples. The rock quality characterization of the samples from El Furrial field in eastern Venezuela is based on the determination of the Winland-Pittman equation of the port size. This characterization, taken from the literature, is based on NMR measurements of water saturated core plugs samples. The results show that a plot of the first and second PCO yields a characteristic horse shoe curve (2D), or a more accurate spiral curve (3D) by taking also the third PCO (3D) into account. The trajectory of these curves follows a continuous variation in the rock quality of the samples, from Megaporosity down to Nanoporosity. This procedure represents an easy, non-destructive way to obtain a rock quality classification based on conventional and NMR derived petrophysical variables and PCO.
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Dry Rock Elastic Moduli Behavior with Pressure
More LessThe knowledge of the pressure dependence of dry rock bulk and shear moduli is essential in time-lapse seismic studies, both on the feasibility and on the interpretation of 4D results. This behavior is accessed only through experimental determination, and there are many different regression laws proposed on the literature to represent it. We had compared the effectiveness of a set of different laws on large data sets of dry rock velocities measured on the lab. In this paper we focus our attention in three of these laws that are widely used on time-lapse and also seems more reasonable than other relations often applied.
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Saturation and pressure inference from velocities and density
Authors Fernando Moraes and Colin MacBethWe present a Bayesian formulation for the inference of saturation and pressure from seismic attributes, using Gassmann’s equations in connection with a dry frame pressure sensitivity law. Laboratory data is analyzed for the most representative pressure sensitivity parameters. A 1D inversion example on synthetic data illustrates how the methodology works.
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Effective design for full-wave three-dimensional land and marine seismic surveys
More LessWhile many of the same basic principles apply, designing full-wave 3D seismic surveys is considerably more complicated than designing conventional P-wave 3Ds or even pure S-wave 3Ds. With faster down-going (P-wave) energy and slower up-going (S-wave) reflections, common midpoint analysis fails for converted wave data. These data must be analyzed in the common conversion point (CCP) domain. Since Vp/Vs ratios vary with depth and lithology, estimating converted wave fold coverage depends strongly on knowledge of the subsurface geology. Because of the asymmetry of converted wave ray paths, a survey design that acquires uniform P-wave (or S-wave) fold coverage will result in non-uniform converted wave fold coverage. Filling gaps and reducing variability in the CCP fold coverage can be a complex exercise. Several land and marine (OBC/OBS) survey design types (wide and narrow azimuth, orthogonal and parallel, slant and variable line spacing) are examined using different bulk Vp/Vs ratios for estimating converted wave fold coverage for targets at different depths. Additional comparisons are conducted using converted wave ray tracing through a layered model with variable Vp/Vs ratios. Due to offset mutes, shallow fold coverage is more difficult to acquire in all 3D survey design problems and requires denser source and receiver line spacings. Because of the asymmetry of ray paths for converted wave data and the typically high Vp/Vs ratios encountered in shallow sedimentary sequences, this shallow imaging problem is even more dificult for converted wave data, requiring even higher density receiver line spacings on the surface. Slanted shot lines and variable receiver and source line intervals will help to reduce CCP fold variations. Wide-azimuth survey designs will allow more subsurface overlap and will improve the cross-line fold coverage of converted wave surveys. For the purposes of this paper, full-wave 3D survey design will focus on P-wave and converted-wave (PSwave) acquisition. The main design factors involved in including the pure S-wave ray paths would be the addition of shear wave sources in orthogonal polarizations.
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Desafios, técnicas e equipamentos utilizados em levantamentos sísmicos terrestres recentes na bacia do Espírito Santo
More LessIn the Espirito Santo basin, the seismic data shows very good response, mainly when compared with another Brazilian land basins. In spite of this, many times the stacked data presents a characteristic low quality aspect, because the seismic parameters used in acquisition or the physiographic features that restrict the work in the surface (lakes and swampy areas). Considering the requirement for better seismic data, including that unavailable data like in lakes, the land seismic acquisition activities in the Espirito Santo has incorporated the technologies from the transition zone acquisition crews and used different planning and designs. In the last year (2004), a new seismic acquisition took place in the BT-ES-22 block, where was chosen a new field layout different from the normal one used by Petrobras (the swath field layout is normally applied). In this way, a non-orthogonal field layout was used – with receiver lines in a forty-five degrees angle from the shot lines – and with parameters defining a 20x20 meters square bin. And taking advantage of hydrophones, air guns and transition zone tools, it was possible to accomplish the acquisition in all over the area of BT-ES-22, including lakes and swampy areas. The seismic data displays a better quality than the ones acquired in previous projects, mainly considering the high frequency content, azimuthal distribution, nominal fold and bin shape and size.
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Get to the Point – Seismic Data Acquisition for Full-wave Imaging
Authors Paul Brettwood, Marty Williams and Cara KigerSince the earliest use of seismic data in hydrocarbon exploration, geophysicists have primarily concerned themselves with the apparent direction and speed with which the waves travelled through the earth. They effectively focused on the P-waves and treated the shear and near-surface waves as noise that need to be removed.
Full-wave imaging questions this conventional approach, as geophysicists now realise that a rock particle’s behaviour when a seismic wave passes it reveals new information about the subsurface. This is in part due to the fact that the rock particles do not necessarily appear to move neither in the same direction nor with the same velocity as the passing wave.
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Optimized AVO analysis by using an optimal linear approximation
Authors M. Riede, E. Causse, A.J. van Wijngaarden, A. Buland, J.F. Dutzer and R. FillonWe propose a method, namely OptAVO to build enhanced linear AVO approximations. The basis functions of the approximation are orthogonal and their coefficients represent a new set of AVO attributes. These attributes can directly be used for AVO classification or to obtain better estimates of the usual coefficients (e.g., intercept, gradient). The method will be illustrated for class I reflectors using large reflection angles. A real data example shows the applicability of the proposed approach.
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Diagnosis of “fizz-gas” and gas reservoirs in deep-water environment
Authors De-hua Han and Michael Batzle“Fizz-water” or “Fizz-gas” is a rather ill-defined and misused concept. For some, it refers to gas in solution with brine; for others, it is defined as small amounts of free gas phase. This small, uneconomic gas content then gives rise to seismic bright-spots or other Direct Hydrocarbon Indicators (DHIs). Unfortunately, it is often the culprit of choice when no other reason can be found. However, progress has been made in assessing the problem. We have systematically examined physical properties of fluid and rock, and fluid interaction with rock to examine gas saturation effect on acoustic velocities, especially in deep-water sands of the Gulf Mexico . Furthermore, we have reviewed the current AVO and Rock physics interpretation techniques to propose optimum DHIs. Several promising techniques of seismic evaluation of gas saturation are in development.
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On the Inversion of Seismic Amplitudes for Lithology, Fluid and Pressure Analysis
By N. C. DuttaReservoir description using seismic data has two major components: inversion of seismic data to extract attributes such as P- and S-wave velocities and bulk densities, and relate those parameters to reservoir properties, such as rock and fluid types, fluid saturation, porosity and pore pressure through fundamentals of rock physics. Implicit in the process are two key as-sumptions: the seismic data quality must be good enough for inversion, and the inversion algorithm must be robust and fast enough to yield reliable and consistent results economically with acceptable non-uniqueness. We have seen much progress in both areas. In seismic acquisition, we have seen considerable advancement that contributed to the quality of the seismic data, especially in the prestack domain. These are: single sensor recording (a large number of channels/offset), accurate and cali-brated source and receivers and their positions, digital group forming, and towing cables at shallow water depths to minimize swell noise. All of these enable us to access surface seismic data with high S/N and fidelity that often rivals the fidelity of the Vertical Seismic Profiles (VSP). Seismic inversion algorithms also reached new heights. The advent of high-speed digital data processors and cluster technology impacted significantly the performance of various inversion algorithms; we are now inverting routinely seismic data in the full offset domain and just not restricting ourselves to the stacked data.
In this paper we discuss the advances in seismic inversion technology that utilizes the amplitude information along with data consistent velocity analyses. First, we briefly review the basic assumptions behind the AVO, EI and full waveform inversion techniques in the prestack domain. While the users of the AVO and EI technology have focused on carrying out inversions using “large angle” assumption- presumably for extracting bulk density information, we show that this may not be feasible due to the neglect of various physical effects, such as interbed multiples, mode conversions and reflection and transmission losses which are omnipresent.
Full waveform prestack inversion (FWPI) is currently the highest level of inversion technology in the industry. Unlike prestack inversion methods such as AVO, the FWPI technique uses a finite-difference elastic model to compute the entire seismic waveform. This provides an advantage over AVO methods in capturing thin-bed effects in data and increasing the potential resolution of the process. The process is computationally intensive and discussed in detail by Mallick (1999). Rock physics-based constraints are applied to speed convergence as well as to reduce ambiguities. Nonetheless, results from this as well as any non-linear inversion process tend to be sensitive to the quality of the a-priori model. However, given a good rock physics model and a-priori information, the output resolution exceeds that produced by other inversion techniques. We have also found that the ambiguities associated with the inversion process can be further reduced if the prestack data has high S/N.
The FWPI technique is computer intensive. Currently this is used in the 1D mode to create pseudo-logs of Vp, Vs and density at selected points in a 3D data volume. These pseudo-logs are then used in the hybrid inversion technique (Mallick (1999) to propagate the benefits of FWPI inversion over 3D volumes. In its current form, hybrid inversion involves computation of pseudologs from full-offset seismic data at selected pilot points in the 3D volume using the FWPI technique, as well as AVO p-intercept and pseudo-shear data over the entire volume. Shear-wave output from FWPI pilot points are used to calibrate the AVO pseudo-shear data. Then both p-wave and pseudo-shear data are independently inverted using a poststack algorithm. Poisson's ratio and other elastic attributes for lithology discrimination are computed from the poststack inversion results. The hybrid inversion process requires input of a 3D a-priori model and a geologically consistent rock physics model, calibration of 3D attribute volumes, and interactive 3D visualization.
We illustrate the entire procedure with examples from several basins.
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Current status and future trends in seismic lithology and fluid prediction
More LessWe present cutting edge techniques for lithology and pore fluid prediction from pre-stack seismic amplitudes, by combining statistical techniques, geological constraints and rock physics models. A promising tool for early exploration detection of reservoir lithologies and hydrocarbons is to do AVO classification constrained by depth trends. We show a blind-test from an offshore Brazil discovery where the method successfully predict the presence of oil.
Another rock physics tool for seismic reservoir prediction, useful for late stage exploration and production stage oil fields, is the rock physics template (RPT) technology. This technique can be used to classify elastic seismic inversion results.
Finally, we summerize what we foresee as the future trends in rock phyisics and seismic lithology and fluid prediction. There is a clear trend of more sophisticated inversion routines (3-term AVO and full-waveform inversions) as the computer intensivity increases. Also, there is a trend of more integration of disciplines like geology, statistics and physics both in modelling and interpretation. Attenuation and frequency attributes will be increasingly important especially in gas fields.
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Kirchhoff modeling and migration in weakly anisotropic medium using the pWFC (perturbed WaveFront Construction method)
Authors Alcides Aggio and Rodrigo Portugal and Eduardo FilpoThe presence of velocity anisotropy in the layers or in the overburden leads to mispositioning of reflector structures and degradation the image quality, when an isotropic migration processing is performed on seismic data. These distortions occur even for weakly anisotropic layers, which are the most common type of rocks in geologic models for seismic applications. In this paper we present a method where a first-order perturbation is applied to the wavefront construction algorithm which generates a traveltime table that fits the traveltime of a weakly anisotropic medium with arbitrary of symmetry. We call this method the perturbed WaveFront Construction (pWFC). These traveltime tables are used in traditional Kirchhoff modeling and migration algorithm. The pWFC allows an approximated computation of traveltime tables for weakly anisotropic medium efficiently, avoiding a more intensive computation by a full anisotropic ray-tracing algorithm. To illustrate the difference when considering or not weak anisotropy in the seismic processing, two common-offset (100m and 1100m) data were modeled using WFC isotropic and using pWFC method. The 1100m dataset was migrated twice using a Kirchhoff algorithm once using an isotropic traveltime tables and the other a traveltime tables computed by the pWFC method.
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Variações da metodologia de micronivelamento de Minty em dados aerogeofísicos no domínio do espaço
Authors Mariana V. Maziviero, Rodrigo S. Portugal and Adalene M. SilvaO método de Minty (1991) vem sendo utilizado extensivamente para micronivelamento de dados aerogeofísicos no domínio do espaço, 1-D. No entanto, este método possui algumas limitações, que motivou a remodelagem desta rotina utilizando diferentes filtros. Estes filtros foram testado exaustivamente em um levantamento aerogeofísico a fim de verificar as características dos resultados finais (dado micronivelado) e possibilitando uma comparação entre eles.
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Estimativa de parâmetros elásticos anisotrópicos efetivos a partir de perfis geofísicos de poços: Aplicação ao reservatório Namorado, Bacia de Campos
More LessThe sedimentary package associated to the Namorado reservoir, in Campos Basin, can be represented by a layer-induced anisotropic interval. In this contribuition, we determined, for the corresponding reservoir, the elastic matrix associated to a transversely isotropic medium with vertical axis of symmetry. We used the so-called Backus averaging to calculate the density-normalized stiffnesses from bulk density and shear and compressional sonic well logs. To assess the strength of anisotropy in the Namorado sedimentary interval, we also calculated Thomsen’s parameters ε, γ and δ. The calculation of these measures of anisotropy confirm the expected values for sedimentary rocks to which weak anisotropy is associated.
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Propagação de ondas SH em meios monoclínicos
A monoclinic media, inside of the limits of the linear elasticity, has a specular plan of symmetry. The propagation in this plan is the case most general of propagation of elastic wave in an anisotropic media for which, a purely cisalhante deformation and normal to the plane of propagation can occur in all the directions. When this plan is vertical, these purely cisalhantes waves are waves SH and so that its propagation is possible, the media must have a vertical plan of specular symmetry and the propagation must be given in this plan. This work investigates the effect of the anisotropy on scale SH waves and that information on the anisotropy can be extracted of these effect. The analysis made here, for more general anisotropic environment where these waves exist, is sufficiently simple and produces some results surprising.
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Amplitudes e padrões de polarização do espalhamento de pulsos em meios anisotrópicos
More LessTo extract lithological information from subsurface by means of a survey of seismic data has become a great challenging in relation to a seismic research because of the lack of insufficient hypotheses of stratification made by isotropic layers, which should represent through surveys the behavior of elastic field with great offset between source and receiver, geophones multicomponent data, measures of three-dimensional vertical seismic profile (VSP) and so forth. For this reason, a pattern of subsurface has been demanded in order to be adequated to this new reality. As a result, the seismic prospecting starts considering the presence of anisotropy, so the anisotropic pattern of subsurface is the one which is closer in of supra named surveys. In special, for the seismic survey in relation to the reservoirs characterization, the anisotropy that is induced by fractures is the one that is more similar to the needs and it is widely referred in the literature. This paper aims at showing formalism to modeling the scattered of pulses from incident plane waves occurred in horizontal plain interface which separates anisotropic layers. This scatter is obtained through clear formulation of traction deformation fields as function of propagator, polarization and impedance matrixes of media, this formalism will be later used to obtain the of transmission and reflection coefficients matrixes through a horizontal plain interface and, finally, it is generalized to a scattering through multiple layers. Doing that, it is possible to calculate the pulse scattering through layers after we insert the amplitude of analytical pulse to an incident wavefield.
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Aumentando o limite de visibilidade de dados sísmicos 3D em reservatórios de águas profundas com detalhes estruturais e estratigráficos
Authors Rogério de A. Santos, Márcio Spínola and Ricardo R. P. AlvesThis work attempts to show some aspects of 3D-seismic reservoir characterization using seismic attributes in a presence of channel-lobes turbidite play located in Brazilian passive Margin. Some attributes were selected and combined to extract qualitative information on subtle faults and its relationship with stratigraphic and depositional systems. Some latest industrial algorithms for attribute calculation, 3D image processing and detection of subtle structural features were applied. Recommendations on workflows and selection of 3Dseismic attributes during a seismic reservoir characterization study were suggested, which the choice of the kind of seismic attribute to be used will intrinsically depend on the quality of the data, initial seismic processing to recover the signal quality , the geological knowledge of the particular area, the kind of geological features and associated geometry as well as the knowledge of the attribute itself to be used for enhancement and detection. All these aspects have impact on the economic
uncertainty for field development because it can produce deviation on the predicted production curves and NPV of some deep water exploitation projects.
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APLICAÇÃO DE TÉCNICAS DE TEMPO FREQÜÊNCIA PARA A DETECÇÃO DE CAMADAS DELGADAS
More LessA new alternative method to characterize thin beds is presented here. We propose to use the ridges of the joint time-frequency analysis, as a new way to detect the instantaneous frequencies and their associated amplitudes, in each trace, and then to use them as a tool to detect thin beds. The technique was applied to a synthesized wedge model and also to a thin bed offshore turbidite reservoir in Campos Basin, Brazil.
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Using Seismic Attributes in Facies Distribution for Reservoir Models
More LessWe describe a workflow that was successfully applied on two case studies in the Campos basin, southeast Brazil, to allow (1) the extraction of reliable geological information from multivariate seismic data, and (2) to incorporate this information as a constraint into highresolution geological models. This paper focuses on the first part of this workflow (extraction of a meaningfull seismic constraint).
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Application of S Transform in the Spectral Decomposition of Seismic Data
Authors Mauren Paola Ruthner and Adelson S. de Oliveiraidentification of stratigraphic features, especially in the characterization of thin reservoir. However, Spectral Decomposition is based on the windowed Discrete Fourier Transform, which provides a less optimized time-frequency analysis.
The S Transform differs from the windowed Fourier Transform in the adoption of a window model whose width is adjusted to the frequency that is analyzed. This confers the S Transform properties similar to those of wavelets Transforms in terms of time-frequency resolution.
This paper presents results of the application of the S Transform to the spectral decomposition of synthetic and real seismic data.
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Automatic Fault Extraction Using Ant Tracking Algorithm in the Marlim South Field, Campos Basin
More LessThe main objective of this paper is to present a comparison between manually interpreted faults and automatically generated fault surfaces using an innovative workflow recently introduced in the geophysical community. The manual fault interpretation performed in an area of Marlim South Field at Campos Basin, located offshore the southeastern Brazilian coast, took 10 days of work, during which a geophysicist delineated the main regional faults. On the other hand, the automated fault extraction approach allowed the interpreter to perform the same task in 3 days. The results showed a good match between the manually and the automatically interpreted faults. The Ant Tracking attribute was significantly helpful for the identification of the fault framework and the extraction of the fault surfaces for later editing, analysis and filtering.
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Princípio da SismoCamada Elementar e sua aplicação à Técnica Volume de Amplitudes (tecVA)
Authors Élvio M. Bulhões and Wander Nogueira de AmorimA SismoCamada Elementar (SCE) ou camada de rocha de menor espessura que o dado sísmico consegue resolver é aqui definida como o elemento chave de ponderação para o cálculo e obtenção do dado sísmico com a técnica Volume de Amplitudes (tecVA), com processamento sísmico pós empilhamento, com e sem rotação de fase.
Sendo assim, apesar da robustez dessa técnica, quanto maior for o grau de fidelidade na tomada dos valores dessa SCE para determinado dado sísmico, maior será a resposta geológica, litológica e estrutural, advinda do dado sísmico tratado.
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Análise de sensibilidade das aproximações parabólica e hiperbólica do tempo de trânsito do método de reflexão sísmica
Authors C. C. C. and C.C.C. LimaIn this paper we present a sensitivity analysis of the parabolic and hyperbolic double traveltime surfaces for a model formed by homogeneous and isotropic layers with curved interfaces. The model is described by the paraxial ray theory, where the normal ray is taken as the zerooffset trajectory. The sensitivity analysis is performed on first order derivatives of the parabolic and hyperbolic traveltimes with respect to each one of the wavefront parameters: (1) the emergence angle of the normal ray (b0); (2) the radius of curvature of the normal-incidencepoint wave (NIP wave) (RNIP); and (3) the radius of curvature of the N wave (RN). In this analysis we look for to indicate a relationship between linear and non-linear parts of the optimization, the relative participation of the close-in to the distant data in the inversion problem, as well as to show that to reach the semblance minimal (or maximal) needs few iterations in the optimization process.
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Combined seismic attributes to recognize the architecture and evolution of turbidities- Marlim Sul field, Campos Basin, Brazil
More LessThis method was employed on studies of reservoirs in Marlim Sul field located in Campos Basin, Brazil. It takes account two simple seismic attributes, minimum amplitude of one surface and acoustic impedance of one time interval between two surfaces. However, both attributes are combined to allow volumetric visualization of sand bodies and recognize the geometries e genetic relationships besides the evolution of reservoir system of turbidities .
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Identificação de reflexões múltiplas utilizando os parâmetros das frentes de onda PIN e Normal
Authors João Carlos R. Cruz, Pedro Chira-Oliva and Francisco J. M. de SouzaThe multiple reflections included in the seismograms hide informations important about the reflectors in subsurface and can become completely invisible. Especially in marine data acquisition, the water layer often behaves as a wave trap and the waves are repeatedly reflected at the sea surface and sea bottom without significant amplitude loss. In order to identify and locate a target reflectors, these multiple must be eliminated or, at least, attenuated. In this work, the interbed symmetric multiple reflections were identified in synthetic datasets. We compare the parameters of hypothetical wavefronts Normal-Incidence- Point (NIP) and Normal (N) obtained by forward modeling and Kirchhoff migration. This comparison was extended to consider the Normal-Moveout (NMO) velocity. These comparisons let us to identify and differentiate between multiple and primary reflections.
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Using band-tridiagonal preconditioners for 1D seismic waveform inversion
Authors Milton J. Porsani and Saulo Pomponet de OliveiraGeophysical inversion is quite a challenge. Many different methods and strategies have been developed in the past decades. Global methods like simulated annealing and genetic algorithms do not depend on an initial model, although they may be time consuming using a huge number of fitness evaluations. Local methods such Gauss-Newton require a ’good’ start model and may be not convergent to the global minimum of the objective function. In this paper we evaluate the use a band-tridiagonal preconditioner in the multiple reweighted least square inversion method (MRLS) to the seismic waveform inversion. The MRLS inversion method is a type-gradient method that generates many candidate models for each sensitivity matrix, exploring the model space in a very effective way. A numeric example shows that the inverted model has better fitness and may be obtained with less computational CPU time, when compared to the results given by the Gauss-Newton approach.
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Deconvoluçao de volume sısmico 3D utilizando norma Lp
Authors Paulo E. M. de Melo and Milton J. PorsaniWe present some results obtained with a new method of iteractive deconvolution applied to 3D seismic volumes. A sub-set of seismic traces of the original volume is deconvolved with the iteractive method and the result is used in the estimation of a non-causal filter which is applied to the whole remaining volume. This procedure was applied to a maritime 3D pos-stacking seismic volume (demo VoxelGeo PARADIGM) and it showed itself to be quite effective and efficient in computational terms. Events associated to the deconvolved 3D volume translate the increase in temporal resolution of the XT panels.
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Determination of reservoir features by using the Prony filtration and Log-parameter ratios
The Prony filtration method provides a high resolution in study of frequency variations for media response. It allows us to analyze the production horizons features for short time intervals. The analysis is based on the general dynamics Prony parameters, which characterize the frequency variations for each of the time intervals. The parameters can be used for the determination of wells the parameters when a ratio between these parameters and Log-data is available. We discuss two examples of application of the dynamics Prony parameters for prediction of target horizons features. The obtained results show high potential of this technique for the analysis of thin layer objects.
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Calibração do Fluxo de Processamento: Um Enfoque na Análise Espectral
More LessHaving in mind the increasing claim for high-resolution images from subsurface and knowing that new methods and algorithms are being developed to get around this problem, we present and discuss in this paper a processing flow created by the free package data processing. By this processing flow, which is addressed to the application of pos-stack migration and the method Wave Analogue of The Common Depth Point (WCDP), comparisons were made about the images from subsurface obtained, coming to conclusions about the application of the processing flow to the quality of the results obtained. Besides that, it has been trying to provide a processing routine that can relieve futures researches, especially in scientific community, where the application of the seismic processing tool is needed.
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Noise attenuation for Common Hydrophone Gathers
Authors Gary C. Aldunate and Flor A. VivasThis work shows how the noise present in 2-D prestack marine data sorted in Common Hydrophone Gathers (CHG) can be attenuated via FX deconvolution. Three ways to solve the Hermitian Toeplitz matrix in the FX deconvolution codes were tested and compared as well. The unitary complex prediction Wiener filter FX, is calculated directly from LU decomposition and iteratively by Levinson and Gradient Conjugated recursion, in frequencyspace domain. FX filtering in the CHG domain can avoid the muting of noisy traces in the edition process.
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Empilhamento SRC-2D: Comparação entre as estratégias de busca SRC pragmáticaextendida e global-local
Authors Paschoal Jr, W. G. and G. GarabitoThe seismic imaging method called Common-Reflection- Surface (CRS) stack simulates a zero-offset (ZO) section from multi-coverage seismic data. For the 2-D case, the hyperbolic traveltime approximation used by the CRS stack depends on three kinematics attributes, which defines a stacking surface for each sampling point in the ZO section to be simulated. The main task of CRS stack method is the estimation of these three optimal attributes by means of automatic search strategies, based on coherence analysis evaluated in the pre-stack data. Currently, based on two different parameter search strategies, called extended-pragmatic CRS search strategy and localglobal CRS search strategy, there are two CRS stack implementations, which were validated by simulating ZO sections from synthetic and real data sets. Their results had been compared to other similar results, obtained by the conventional NMO/DMO stack method. In this paper, we present a short description of these two CRS strategies and compare both implementations in their application to the wellknown Marmousi data set.
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Uniform Resampling Using the Sinc Function
Authors Ana Carolina Camargo and Lúcio Tunes SantosIt is common to nd problems in handling data that falls on a nonequally spaced grid. Then, we say that is necessary making an uniform resampling, i.e., interpolating the nonuniform samples of a sign in a set of equally spaced points.
In this work, it is rst shown that the resampling problem can be formulated as a problem of solving a set of linear equations Ax = b, where x and b are vectors of the uniform and nonuniformsamples, respectively, and A is a matrix of the sinc interpolation coef cients [Rosenfeld (1998)]. The solution for this system is given by the pseudoinverse matrix which is computed using singular value decomposition (SVD) in a process that is called Uniform ReSampling (URS). In large problems, the computation of the pseudoinverse is impractical. Using the fact that the contribution of the b(i)'s in the computation of the x(j)'s, when they are distant, is very small, Rosenfeld created an algorithm that was called Block Uniform ReSampling (BURS). Such algorithm uses only a limited number of points around t(j), point of the uniform grid, to calculate each uniform sample x(j), decomposing thus the problem into solving a small set of linear equations for each uniform grid point. These equations are a subset of the original equations Ax = b and are once again solved using SVD. The nal result is both optimal and computationally ef cient. A result is presented to illustrate.
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Phase spectrum applied to pinch out zones analysis
More LessWe are going to discuss the possibility of using the phase spectrum of seismic signals for analysis of pinch out zones in thin layer reservoirs. The results presented show by analysis of the phase spectrums help to determine more accurately the pinch out point, reducing the domain of uncertainty. These results were obtained by using synthetic and real data. The synthetic data was obtained for signals having frequencies similar to real ones.
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Noise suppression in seismic data with sparse coding
Authors Sam T. Kaplan and Tadeusz J. UlrychIndependent and principal component analysis are used to find a series representation of data. Principal component analysis finds a compact representation of the data, while independent component analysis finds a sparse representation. The data are corrupted with additive random noise, and the coefficients in the series expansion are filtered, removing the portion of the series that is more indicative of noise than signal. For the series representation of the data found using principal component analysis, the filter consists of a simple truncation of the series. For independent component analysis, a Bayesian filter is applied to the coefficients of the series expansion; thus, allowing for prior knowledge to be applied to the distribution of the coefficients.
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Compensation for Q-losses revisited - A more stable approach using SVD
More LessThis paper discusses an improved approach for Qcompensation. The approach is based on a singular value decomposition of a matrix that represents earth’s attenuation and dispersion processes according to Futterman’s model. It is shown that the proposed approach may provide a much broader frequency band recover of signals under severe attenuation than methods supported by usual stabilizing techniques. It also briefly discusses a very common misleading thought of timevarying processes as a succession of stationary filters. Synthetic and real data examples are shown.
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Supressão do ground roll utilizando o m´etodo do balanceamento espectral
More LessGround roll is a coherent noise of high amplitude and low frequency that appears in land seismograms. This type of seismic event overlap primary reflexions creating difficulties in the processing and interpretation of the seismic data. In the past years many different filtering techniques has been proposed to attenuate these noise. In this paper, we test a new approach which is based on spectral decomposition of the seismograms coupled with the use of automatic gain control (AGC) applied on each decomposed panel associated to each band of frequency used in the decomposition. A similar approach is used into the software FOCUS (Paradigm). These approach was tested on synthetic and real seismograms. The numerical examples demonstrates that the ground roll may be considerably attenuated suggesting the use of the new approach as a efficient tool useful to increase the ratio signal to noise in land seismograms.
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Amplitude Preserving Kirchhoff Pre-stack Time Migration for Time Lapse Processing on Troll West
Authors David Bannister, Momotchil Roussanov and Charles JonesA method of applying weighted scalars in preserved amplitude pre-stack depth migration (Jousset, 1999) has been adapted for time processing and included in a time lapse (4D) processing scheme incorporating Kirchhoff pre-stack time migration. The method, known as ADA (Acquisition Dependent Amplitude) compensation removes the requirement for a pre-migration regularization stage, honours the true source and receiver locations of the data and reduces the acquisition footprint. The method is applicable to 3D acquisition and is particularly well suited to 4D studies, where removal of acquisition effects to improve repeatability is a key requirement. The application of ADA on multiple vintages of 3D data acquired over the same area provides a testing ground to demonstrate the effectiveness of this technique. Presented in this case study is an example of the application of the ADA method to time lapse processing of data from the Troll West gas province.
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Correçao dip moveout em dados s´ısmicos da Bacia sedimentar do Tacutu
Authors Michelângelo G. da Silva and Milton J. PorsaniWe present the results obtained in the processing of reflection seismic data of the Basin Sedimentary of the Tacutu applying the method dip moveout (DMO) in common shot gather using the logarithmic transformation of both the time and the space coordinates. The main objective was obtain a velocity field and to generate the zero-offset stacked section. The processing flowchart involved the residual velocity analysis of the seismic data. We obtain estimation the velocity field of the subsurface most real and to increase the reason signal-noise of the stacked section with the correction dip moveout.
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A practical approach to OBC summation and geophone calibration in areas of shallow water and hard seafloor
Authors Greg Beresford and Gaël JanexA methodology for processing two-component (2C) ocean bottom cable (OBC) seismic data based on the principle of cross-ghosting (Soubaras, 1996) is applicable to some of the most difficult field situations where water depths are in the 20m to 60m range and where there may be a hard variable seafloor giving rise to strong guided waves and geophone coupling “resonances”. Effective summation of hydrophone(H) and geophone(G) components depends on geophone calibration to compensate for this seafloor coupling. The methodology breaks naturally into three steps: data analysis and pre-filtering of common receiver records; LS estimation of calibration filters from pre-filtered data; and finally summation. Pre-filtering to attenuate guided and other horizontally propagating waves is a critical step because such waves “in-fill” the ghost notches especially on the geophone records and reduce the effectiveness of cross-ghosting. The analysis of common receiver records is also critical in order that the best data window can be found which contains primarily reflected P-waves.
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Interpolação e regularização de dados sísmicos usando a transformada de Radon linear (τ-p) 2D e 3D
More LessSeismic surveys are designed on the purpose that all samples collected during the seismic experiment fall on a specific regular grid. Nevertheless, this data regularity is almost impossible to achieve due to different constrains during seismic acquisition. A very important non-cultural irregularity which occurs during marine seismic surveys is hydrophone cable drift caused by ocean currents (cable feathering).
These irregularities must be treated in the first steps of seismic data processing and data regularization can be the right tool to be used. Regularization may improve the overall performance of important steps in seismic processing like velocity analysis and migration.
Data interpolation and regularization are performed using the τ-p transform, with slant stack 2D and 3D algorithms. We discuss different issues in 2D and 3D data regularization using τ-p transforms, such as ideal parameterization to avoid artifacts and the 2D experiments related to interpolation and regularization of gaps in seismic information and cable feathering.
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Pesquisa e desenvolvimento em geof´ısica atrav´es do reprocessamento de dados sísmicos: A experiência do Laboratório de Geofísica Computacional da Unicamp
Desde a significativa mudança do cenário exploratório de petróleo e gás com a criaçöo em 1997 da Agência Nacional de Petróleo (ANP), foram estabelecidas no Brasil especiais condiçöes e oportunidades de pesquisa, desenvolvimento, formaçäo de recursos humanos e prestaçäo de serviços em variadas áreas do setor. Se adequadamente exploradas, estas oportunidades devem promover o estabelecimento de conteúdo local às diferentes atividades, conteúdo este imprescindíıvel para a independência científica e tecnológica do País. O importante acervo de dados s´ısmicos existente no Banco de Dados de Exploraçäo e Produçäo (BDEP), também vinculado à ANP, oferece oportunidades importantes para as atividades de processamento sísmico, sobretudo no reprocessamento de dados mais antigos com a utilizaçäo de m´etodos e tecnologias mais atualizadas. É nesse pano de fundo que se insere o projeto “Aplicaçäo de técnicas especiais em reprocessamento sísmico” celebrado entre a ANP e o Laboratório de Geof´ısica Computacional (LGC) da Universidade Estadual de Campinas (Unicamp) e desenvolvido no período de janeiro de 2002 a julho de 2005. No per´ıodo foram processadas e reprocessadas linhas sísmicas representativas de muitas das bacias sedimentares brasileiras, com resultados importantes de melhoria de imageamento. Os resultados obtidos confirmam o valor do reprocessamento sísmico, mesmo adquiridos com tecnologias antigas e de baixa qualidade, no sentido de extraçäo de conhecimento e informaçäo em várias áreas e localidades, muitas vezes inacessíveis hoje a novos levantamentos.
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Estudo do impacto do uso de geofones com ressonância de 10 Hz e 60 Hz na aquisição 3D de alta resolução
Authors Selma S. Sacramento and Marco C. SchinelliPara monitoramento da injeção de vapor no campo de Fazenda Alvorada, na Bacia do Recôncavo foi projetada uma aquisição 4D de alta resolução, com o uso de geofones de 60 Hz, cargas e geofones enterrados e cela de 2,5 x 2,5 metros, dentre outros parâmetros não convencionais, com o objetivo de maximizar a resolução sísmica ao nível dos reservatórios a serem monitorados. Simultaneamente, para estudar o impacto do uso dos geofones de 60 Hz, foram usados redes convencionais de geofones de 10 Hz agrupados, cujo resultado serviria para avaliar a possibilidade de em novas campanhas de monitoramento, serem usados geofones convencionais no lugar dos de alta freqüência. O resultado do experimental mostrou que os geofones de 60 Hz não foram determinantes para o resultado obtido pois este também foi alcançado com o uso dos geofones convencionais. Serão descritos neste trabalho os resultados obtidos com o uso dos geofones de 60hz e 10hz no mesmo levantamento 3D, avaliando alguns aspectos operacionais da aquisição e o procedimento usado para processamento, alem das diferenças entre cada conjunto de dados.
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Aplicação dos Métodos PMC e SRC em Dados Sísmicos Sintéticos de um Modelo Representativo da Bacia do Solimões, Região Amazônica, Brasil
Among the several difficulties found in the exploration for oil and gas in the Amazon area, the existence of diabase sills, although of its importance to the process of maturation of organic matter in all Brazilian Paleozoic basins, difficults the seismic exploration, since the diabase disturbs the seismic signal, generating multiple reflections and spherical divergence. With the purpose of understanding the seismic wave propagation and, principally, the difficulties found by the seismic imaging methods in these complex geological settings, in this paper we present the first results of acoustic seismic modeling in a synthetic model of the Solimões Basin- Brazil, using finite-differences techniques, as well as the seismic processing of this data set by the Common Midpoint (CMP) and Common Reflection Surface (CRS) stacking methods.
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Segmentation of well-log data
More LessIn this paper we present a method to automatically detect stationary segments in well-log data sequences. This process is carried out by searching for change points which correspond to abrupt changes in the statistical nature of the underlying process. For this purpose the we analyze the behavior of the probability density functions (pdf) of two adjacent sub-samples as they move along the original data sequence. A statistical test is used to set a significance level of the probability that the two distributions are the same, thus providing a means to decide how many segments comprise the data by keeping those change points that yield low probabilities. Examples using simulated and real well–log data show that the results are in good correspondence with what seems to be a reasonable segmentation.
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Processamento CRS de dados deconvolvidos usando o metodo de Wiener-Levinson
Authors Rosangela Maciel and Milton J. PorsaniThe removal or attenuation of multiple reflections represents a problem for seismic data processing. Such multiple reflections interfere with the primary ones and may generate large difficulties to the interpreter and to the seismic migration methods. The Wiener-Levinson (WL) predictive deconvolution is a technique which has been in use for quite some time in the attenuation of multiples, having its application limited to a periodical occurrence of the event. This work shows some partial results related to the use of the
WL multichannel predictive deconvolution, combined with the method of zero-offset generation section through the stacking of the commom reflection surface (CRS). The CRS method is very recent and it shows itself as a good alternative when compared to the traditional stacking methods (DMO/NMO). It does not require prior information on the velocity model. In the present stage of the work we are introducing NMO correction by using the stacking velocity calculated in function of the parameters trio obtained through the CRS method with the objective of generating common off-set sections to apply the filter. The obtained results were statisfactory and quite promising.
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Imageamento WCDP 2D em profundidade: uma versão para meios lateralmente homogêneos
More LessThis paper presents some results of implementation of a pre-stack migration procedure based on the Wave Analogue of the Common Depth Point (WCDP) method. The method is based on the exact solution of an inverse acoustic scattering problem, considered in the Born approximation. We analyze the performance of the WCDP migration considering a depth variant of the algorithm.
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True-amplitude CRS-based Kirchhoff time migration for AVO analysis
Authors Miriam Spinner and Jürgen MannThe achievable image quality and the reliability of amplitudes in Kirchhoff migration strongly depend on the selection of the migration aperture. Too small an aperture leads to underestimated amplitudes and the loss of steep events. On the other hand, too large apertures tend to cause operator aliasing and include unnecessary noise and contributions from unwanted events. Our aim is to use CRS-based minimum apertures in Kirchhoff prestack time migration to obtain the best possible input for AVO/AVA analyses.
The basic idea is demonstrated for a synthetic data set which contains events from a common sequence of gas/water/oil contacts. We discuss the determination and extrapolation of stationary points and projected Fresnel zones based on CRS wavefield attributes, as well as a simple and efficient way to set up a migration velocity model. The first results show a significant reduction of amplitude dispersion in common-image gathers as well as in the zerooffset section, thus providing superior input to AVO/AVA analyses.
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