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6th International Congress of the Brazilian Geophysical Society
- Conference date: 15 Aug 1999 - 19 Aug 1999
- Location: Rio de Janeiro, Brazil
- Published: 15 August 1999
81 - 100 of 399 results
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South Shetland Block - Antarctic Plate Boundary: A Present-Day Example Of Progressive Transition From Extensional To Transcurrent Boudary
Authors Luiz Gamboa, Jesús Galindo-Zaldivar, Andrés Maldonado, Saizo Nakao and Yao BochuThe South Shetland Block is a fragment of continental crust that extends between the South Shetland Trench
and the Bransfield Basin. Its southern boundary shows present-day tectonic activity and its nature changes
along strike from a spreading center in the area between the South Shetland Islands and the Antarctic Peninsula
to a transtensional boundary at the northern part of the block. Eastwards, along the South Scotia Ridge, the fault
system finally intersects the transcurrent boundary between the Antarctic and Scotia Plate. The variability in the
features of the southern boundary of the block is probably a consequence of the variable orientation of active
faults in a nearly constant stress field. The study of several transects of the boundary indicates that extensional
structures are generally asymmetrical. The Bransfield Strait has been formed by an asymmetrical extension,
probably related to a low-angle normal fault dipping towards the NW.
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4D Seismics - Principles And Application
More LessThe aspect of monitoring reservoirs clearly comprises all aspects of data acquisition, processing and
interpretation. Certain rules apply in order to keep the monitoring process stable, where the most important
ones are:
- to maintain the same 3D-acquisition geometry: the same geometry then guarantees that the same S/N-ratio in
terms of noise-suppression is achieved, one example being here e.g. the aspect of multiple-suppression for the
entire data-volume.
- to maintain the same processing flow: nevertheless of course e.g. residual statics have to be computed based
on the individual volumes of data. Another example here is the process of deconvolution.
- to use the first 3D-volume (usually called the base-line survey) to establish the basic processing flow as well
as to establish the underlying macromodel (velocities V(x,y,z) and velocity interfaces) and (if needed), by
sequence stratigraphic interpretation the 3D-litho model of the reservoir unit.
- to decide upon the monitoring quantity, i.e. which quantity to use for 4D-evaluation. The default here is to use
Acoustic Impedance (AI) (which means that after proper imaging the data is subject to a suitable inversion
algorithm, usually at post-migration stage).
Clearly the type of reservoir to be monitored has impact on the acquisition effort, since we have to distinguish
between two basic types of reservoirs:
- High-impedance reservoirs, i.e. carbonates. As a „rule of thumb“ the maximum changes here are up to 5% (in
acoustic impedance) and
- Low-impedance reservoirs, i.e. sandstones. The „rule of thumb“-Figure here is up to 10% of AI-change.
In any case for the actual reservoir under consideration its sensitivity to fluid substitution has to be evaluated,
usually based on the Biot-Gassmann-formalism for the Zero-Offset case. This part of a 4D-evaluation is called
the calibration part. If done properly, the outcome is a reliable estimate of the dynamic range of the changes in
acoustic impedance due to ongoing production. In addition one needs to estimate the lateral „area of change“ to
be expected by producing the reservoir
during the timespan in between the two seismic surveys.
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Thermobaric Dependences Of Physical Properties Of Rocks Discavered By The Krivoy Rog Ultradeep Borehole
By T.S. LebedevInformation about results of studies of thermobaric changes of physical properties of rocks discovered in
drilling the Krivoy Rog ultradeep borehole (Ukraine) are summarized. Pethrophysical models of the Earth’s crust
of the area of its location are set up.
The main tasks of drilling this borehole located in an iron province which is one of the largest in the world were
to elucidate the formation conditions, composition, properties and evolution of the Earth’s crust of continental
type within the area of the development of precambrian iron-silicon and granite-greenstone formations of the
Central Ukrainian Shield. Also specified was the interpretation of anomalies of the observed geophysical fields
and the nature of seismic boundaries in crust.
For this purpose and to set up a regional petrophysical model of the interior we have obtained a new unique
information on PT-changes of physical characteristics of rocks discovered by the borehole. The drifted part of
the section is mainly represented by metaconglomerates, various schists, quartzites, amphibolites,
plagiogranites and some other lower-proterozoic and upper-archean mineral formations of different
composition. Almost all rocks encountered in drilling show differently expressed banding, schistosity and linear
arrangement of minerals.
The method worked out earlier in which specialized chambers are used was applied in a set of laboratory
petrophysical PT-experiments on samples of a representative collection of cores. We carefully studied elastic,
thermal, electrical and magnetic parameters of rocks in characteristic strictly oriented structural and textural
with simulating respective deep thermobaric conditions in PT-experiments. We are founding the nature of the
anisotropy of the distribution of the studied petrophysical parameters found within the section and which is
most probably preserved even at greater depth (below the hole bottom).
The features of thermobaric changes of specific electrical resistance and dielectrical constant of the rocks
studied are due to their saturation with fluids, mineralization of the latter, dissociation and dehydration of
carbonates, micas and other minerals. With increasing pressure the thermal conductivity and thermal diffusivity
coefficients of rocks are increased by almost one third. They evently decrease with increasing temperature. We
have established a relatively early stage of metamorphism of mineral formations. At the simultaneous
programmed PT-effect the remanent magnetization of the most samples containing ferromagnetics is featured
by anomalous changes in respective directions. Based on experimental PT data obtained by the method
proposed, changes of elastic anisotropy of core samples were studied and criteria-precursors of the destruction
of the near-shaft space during borehole drifting were found. They are mainly due to violation of the elastic
lithostatic balance of the geologic medium.
An analysis of the results of the whole set of petrophysical PT-experiments and the geologic-geophysical
information enabled us to assume possible changes of properties, composition and state of rocks at depth so
far not reached by drilling. We have also set up prognosed deep petrovelocity, geoelectrical and lithological
models of the Earth’s crust of the area of the Krivoy Rog over- and underthrust structure.
I sincerely thank all my researchers that participated in the investigations.
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Migration And Modeling Of Pick Data With Traveltimes
Authors Cole C. Harris and Michael O. MarcouxKirchhoff migration and modeling algorithms are integral methods in which the dominant contribution to the
integral is from the stationary phase regions of the operator-mapped input data. We have developed an analogy
to such Kirchhoff methods for application to event picks derived from seismic data. In contrast to the Kirchhoff
algorithm, our method explicitly detects the stationary phase regions in the operator-mapped input pick dataset
and associates with each an output pick. Unlike map-migration type algorithms, this method does not require
horizon dip information, or even that the picks be assembled into distinct horizons. Thus, relatively simple
automated seismic data picking algorithms may be employed to generate the input pick dataset.
Two applications of these algorithms are depth model verification using reflection time pick migration and the
generation of reflection time picks from time-migrated data for use in traveltime tomography.
Both migration and modeling algorithms have been verified on a pick dataset derived from a relatively complex
model incorporating steep dips and non-smooth reflectors.
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Estudos De Cavidades Em Arenito Utilizando Gravimetria E Caminhamento Elétrico
Authors J.C. Dourado, W.M. Filho, A.C. de O. Braga, V.R. Elis and N. NavaEste trabalho mostra o resultado dos ensaios geofísicos realizados na Mineração Jundu, com o objetivo de
detectar cavidades no interior do arenito. Os levantamentos de campo foram realizados em uma bancada onde,
devido ao desmoronamento do teto, uma cavidade foi encontrada. No total foram levantadas 269 pontos
gravimétricos em uma malha aproximada de 5 por 5 metros e aproximadamente 1400 metros lineares de perfis
de resitividade elétrica utilizando-se da técnica de caminhamento elétrico com arranjo dipolo-dipolo, sendo que
o espaçamento entre os dipolos foi de 5metros.
No local onde se encontrava a cavidade os ensaios responderam coerentemente, isto é, o levantamento
gravimétrico mostrou um baixo na anomalia Bouguer, enquanto que a resistividade elétrica mostrava um
aumento de valores. Outra anomalia foi confirmada posteriormente onde anomalias geofísicas foram
detectadas.
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Factorial Kriging Analysis - A Geostatistical Approach To Improve Reservoir Characterization With Seismic Data
More LessIn this work the Factorial Kriging analysis for the filtering of seismic attributes applied to reservoir
characterization is considered. Factorial Kriging works in the spatial domain in a similar way to the Spectral
Analysis in the frequency domain. The incorporation of filtered attributes as a secondary variable in kriging
system is discussed. Results prove that Factorial Kriging is an efficient technique for the filtering of seismic
attributes images, of which geologic features are enhanced. The attribute filtering improves the correlation
between the attributes and the well data and the estimates of the reservoir properties. The differences between
the estimates obtained by External Drift Kriging and Collocated Cokriging are also reduced.
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Estudio Geofisico De Los Depositos De Grafito, Area Osumita, Estado Cojedes, Venezuela
Authors Nuris Orihuela Guevara and Laura PiñeroEn el contenido de este trabajo se presentan los resultados de la evaluación geofísica preliminar desarrollada en la zona del Cerro Osumita, Estado Cojedes. A partir del objetivo de la evaluación: Mineralización de Grafito Cristalizado, se selecciona la técnica electromagnética (EM) descrita como VLF, esperando respuestas EM suficientemente significativa como para distinguir en profundidad la mineralización de su roca caja descrita esencialmente como rocas metamórficas. Para cumplir con este objetivo fueron levantados dos perfiles de orientación NW-SE en los cuales se midió la componente en fase (%F) y la componente en cuadratura (%Q) del campo resultante. A partir de esta información se genera la interpretación cualitativa y cuantitativa de resultados de la cual destaca la presencia de 2 cuerpos de grafito distribuidos en 1250 m a lo largo del perfil I, de estos se considera de interés comercial sólo uno de ellos. Adicionalmente se debe indicar que existen dificultades en la interpretación de resultados en la franja norte del perfil antes indicado motivado en la presencia de cambios en las propiedades electromagnéticas de la roca caja.
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Salt-Tectonics Provinces Across The Continental-Oceanic Boundary In The Brazilian And West African Margins
Authors W.U. Mohriak, M.P.A. Jackson and C. CramezRecently acquired seismic data extends from the platform toward the continental-oceanic crustal boundary in
ultradeep water along the South Atlantic conjugate margins. Preliminary interpretation, particularly along a
regional transect in the Eastern Brazilian margin, reveals four main salt tectonics provinces that correspond to
tectonic domains: the proximal (shelf to upper slope); the intermediate (mid to lower slope), the salt diapir
province (lower slope to deep basin), and the deep basin to oceanic crust domain. Extensional tectonics
predominate in the proximal and intermediate domains, whereas the deep-water domain is characterized by
compressional features. The transition from rifted continental to oceanic crust is characterized by volcanic
features, igneous intrusions and wedges of seaward-dipping reflecto.
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Method Of Transformation Between Reflector Dip Angle And Stratigraphic Dip Angle
By Niantian LinThe calibration of stratigraphic horizon usually depends on outcrop data when interpreting seismic data in frontier area
where there is lack of well. The outcrop data is often expressed as the stratigraphic dip angle, how can we directly and
easily transform the stratigraphic dip angle from depth domain to time domain. The paper derives a depth-time
transformation equation of the stratigraphic dip angle with various scale parameters of seismic section and outcrop data
by analytic geometry, which may be suitable for the depth-time transformation under any geological structural feature
and other field.
Keywords: Stratigraphic dip angle, Reflector dip angle, Depth-time transformation.
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The Magnetotail During Geomagnetic Perturbed Periods
More LessBy studying geomagnetic indices and interplanetary magnetic field records, two geomagnetic storm characterized by
Dst peak less than -100 nT are researched. Both intervals are characterized by substorm events with important
maximum amplitude values (AE index greater than 800 nT) which don't contribute in the same manner, to build the ring current.
The lack of the satellite data during both geomagnetic disturbances do not allow to estimate the plasmoid energy. Some
hours without data could indicate the disappearance of plasma and magnetic field into the plasmasheet.
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Utilização De Dados Gamaespectrométricos No Auxílio Ao Mapeamento Geológico Da Região De Barra Bonita, Cerro Azul-Pr
Authors M. Liki, B. Trzaskos and F.F. VeselyThe present paper discuss a geological–geophysical study at Cerro Azul district, State of Paraná, Brazil, in an
area of 42 Km2. The district is part of the Proterozoic Ribeira fold belt, and it contains metamorphic rocks, two
syncollisional granitic intrusions (Três Córregos and Morro Grande) as well as Mesozoic mafic dykes.
Comparing geological map and gamaespectrometrical data (K, U, Th) aerial and terrestrial, it was observed that:
1) the thorium values and profiles seem to reflect the distinct litologies of the area; 2) potassium maps and
profiles show the contacts between granites and host rocks, as well as, the different facies at the Três Córregos
granite; 3) comparing the U, K and Th values of the two granite bodies, Morro Grande is more radioactive.
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Caracterización Sísmica Del Corte Geológico En El Cinturón Sobrecorrido Cubano. Nuevas Estructuras Para La Búsqueda De Hidrocarburos
Authors Sofia López, Rafael Socorro, P. Vedado and C. HabanaEl archipiélago cubano es un cinturón plegado y sobrecorrido desde el Cretácico Superior
hasta el Reciente, derivado de la colisión del arco volcánico cretácico con el margen
continental de América del Norte, por 10 que presenta una geología muy compleja dentro
del área del Caribe.
Desde 1972 hasta nuestros días se han adquirido más de 10 000 Km. de líneas sísmicas 2D
y 2 levantamientos 3D, se han perforado más de 1500 pozos a más de 1500m y existen
levantamientos gravimagnetométricos en varias escalas. Todos estos datos muestran la
complejidad y variabilidad de las condiciones sismogeológicas, particularmente acentuadas
en la Provincia Gasopetrolífera Norte Cubana, donde se localizan los dos yacimientos más
importantes de Cuba “Varadero” y “Puerto Escondido”, asociados a las rocas carbonatadas
del margen continental.
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Upper Mantle Seismic Velocity Structure Beneath Se Brazil From P- And S-Wave Travel Time Inversions
Authors Martin Schimmel, Marcelo Assumpção and John VanDecarWe present preliminary results from teleseismic travel time inversions for P- and S-wave data mainly recorded at portable broadband stations in SE Brazil. The stations have been employed at more than 27 sites within an area of 500 km x 1000 km during the years 1992 - 1998. More than 3800 relative P- and 3900 relative S-wave arrival times, including core phases, have been obtained from the waveforms simultaneously recorded at 4 or more stations. The model is parameterized by splines under tensions and the P and S relative phase times are independently inverted for slowness perturbations, earthquake relocations, and station corrections. The final models represent the least amount of structure required to explain the residuals within a defined standard error. The results confirm the existence of a fossil plume conduit, permit a more detailed separation of geological structures and velocity perturbations down to about 1000 km depth.
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Ray Tracing By Simulated Annealing In Complicated 3-D Models
Authors Danilo R. Velis and Tadeusz J. UlrychWe present a novel ray-tracing method for obtaining the minimum traveltime raypath connecting any two fixed points in complicated 3-D geological models.
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Determinação Da Velocidade De Sedimentação Da Baia De Todos Os Santos Usando Pb-210
More LessCinco testemunhos de sedimento da parte norte da baía de Todos os Santos foram analisados para Pb210. Os
testemunhos eram compostos majoritariamente por areia fina, seguida de areia muito fina e lama. No testemunho BTS2,
a atividade do excesso de Pb210 em fatias de 1 cm esteve entre 0,475 e 0,073 dpm g-1, com uma atividade do Pb210
suportado de 0,379 dpm g-1, os quais estão dentro dos valores esperados para sedimentos marinhos ricos em areia.
Para este testemunho, o fluxo anual de massa foi de 545 ± 61 mg cm–2 a–1, ao qual corresponde uma taxa de
sedimentação de 0,218 ± 0,024 cm a-1. Nos testemunhos BTS1, BTS3, BTS4 e BTS5, a atividade do Pb210 foi
praticamente constante com a profundidade sugerindo que o material foi remexido recentemente.
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Construção De Um Espectro-Fotômetro Para Monitoramento Da Temperatura Da Mesopausa Na Região Polar Antártica
Authors Cristiano Max Wrasse, Hisao Takahashi and Geancarlos Guedes RettoriUm espectro-fotômetro, para observar a emissão de luminescência da hidroxila está sendo construido
no laboratório LUME do INPE. O fotômetro mede o espectro do ramo P da banda OH(8,3), determinando a
temperatura rotacional da molécula. Esta temperatura rotacional indica a temperatura cinética da atmosfera
ambiente. Portanto a emissão do OH pode ser utilizada para monitorar a temperatura da atmosfera na região da
mesopausa. O presente trabalho apresenta o objetivo, metodologia e o diagrama de blocos do fotômetro em
construção.
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Imageamento Do Airglow Em Cachoeira Paulista (23 S, 45 W)
More LessUm sistema de imageamento Instalado em Cachoeira Paulista (22.70 S, 45.00) usando uma câmera CCD com
quatro filtros de interferência realiza medidas rotineiramente durante 15 dias de cada mês. São imageados o OI
557,7 nm, o OI 630,0 nm, a banda de Meinel OH(6,2) na região do infravermelho próximo (<930 nm) e o
background contínuo em 578 nm. Foram observadas estruturas de ondas na emissão do OH e em alguns
períodos bolhas de plasma no OI 557,7 nm e OI 630.0 nm. Apresentaremos e discutiremos algumas destas
imagens.
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Interpretación Del Levantamiento Aerogeofísico Complejo 1:50000 Del Nordeste De La Provincia De Hlguín (Cuba)
More LessEl área de estudio está ocupada en más de un 85 % por rocas típicas de corteza oceánica pertenecientes al complejo ofiolítico Moa \endash Baracoa. Los datos aeroespectrométrico y magnéticos que cubren dicha área pertenecen al levantamiento aerogeofísico complejo 1:50 000 del extremo oriental de Cuba. Su interpretación compleja enriqueció el conocimiento geológico del área aportando nuevos elementos litoestructurales. Inicialmente a los datos aeromagnéticos se le realizaron varias ransformaciones del campo: reducción al polo, cálculo de gradientes, mapas de relieve sombreados y Continuación Analítica Ascendente (CAA), a partir de las cuales se generaron mapas que fueron analizados de manera conjunta durante la interpretación.
Los datos aeroespectrométrico de contenidos de eU (ppm), eTh (ppm), K (%) y sus radios (U/Th, U/K y Th/K) fueron representados en forma de imágenes en las cuales se analizaron sus distribuciones espaciales y correlación con las litologías presentes en el área. El análisis combinado de la información geológica y geofísica permitió: definir las áreas de predominio de las principales litologías, zonas de mayores y menores espesores de las serpentinitas, delimitar áreas de alteraciones hidrotermales, definir la forma, extensión y el carácter supuesto y comprobado de las principales estructuras disyuntivas, ubicar posibles frentes de escamas decabalgamientos, corroborar el carácter alóctono del complejo ofiolítico en esta región, delimitar las áreas de desarrollo de las cortezas lateríticas ferroniquelíferas y valorar el grado de meteorización de las ofiolitas serpentinizadas, así como las zonas de mayor enriquecimientos de Vanadio dentro de los yacimientos de lateritas ferroniquelíferas.
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Two-Pass 3D Prestack Time Migration
Authors Andre R. Rosa, Carlos Cunha, Ivan Pedrosa, Jairo Panetta, Silvio Sinedino and Vera BragaWe describe a method to perform two-pass 3-D Kirchho prestack time migration whose com-
putation time is nearly the square-root of that required for the one-pass algorithm. To better
understand the conditions and restrictions associated with the application of the method we
discuss the kinematical approximations involved in its derivation. A real data example shows
that this approach can be applied with success in areas with moderate structural complexity.
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A Wave-Equation Modeling And Migration Algorithm For Anisotropics Media
Authors Ricardo A.R. Fernandes and Irshad Raman MuftiThe interpretation of seismic data acquired in areas characterized by velocity anisotropy is of particular
interest in reservoir geophysics. Foward modeling and adequate seismic migration algorithms can
enhance the dependability of interpretation. We attack this problem by exploiting the most fundamental
property of the anisotropic media which makes them different from the isotropic media, viz., the variation
of velocity with direction. This is accomplished by splitting the wave equation and replacing it by
two or more simpler equations which involve only one spatial derivative but a different velocity in each
case. The evaluation of the wavefield is carried out by setting up a finite-difference model in which a set
of difference relations derived from the simpler equations is used in an alternate fashion. We present
two examples of application, one in 2D and another in 3D.
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