- Home
- Conferences
- Conference Proceedings
- Conferences
9th International Congress of the Brazilian Geophysical Society
- Conference date: 11 Sep 2005 - 14 Sep 2005
- Location: Salvador, Bahia, Brazil
- Published: 11 September 2005
361 - 380 of 462 results
-
-
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.
-
-
-
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.
-
-
-
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.
-
-
-
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.
-
-
-
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.
-
-
-
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.
-
-
-
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.
-
-
-
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.
-
-
-
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.
-
-
-
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.
-
-
-
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.
-
-
-
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.
-
-
-
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.
-
-
-
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.
-
-
-
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.
-
-
-
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.
-
-
-
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.
-
-
-
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.
-
-
-
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.
-
-
-
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.
-