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7th International Congress of the Brazilian Geophysical Society
- Conference date: 28 Oct 2001 - 31 Oct 2001
- Location: Salvador, Bahia, Brazil
- Published: 28 October 2001
1 - 50 of 465 results
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Aproximações Quadráticas Para Análise De Avo De Ondas Convertidas
Linearized approximations for reflections coefficients
used for AVO analysis are not suitable to determine
density contrasts. Second order approximations of
converted waves reflected amplitudes are necessary to
estimate density and the lithological indicators associated
to it. We present explicit second order expression
for reflected and transmitted amplitudes of converted
waves in isotropic media which can be used to
extend AVO analysis accurately to converted waves.
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Combining P-P And P-Sv Avo To Improve Estimation Of Elastic Parameters
More LessCompressional wave amplitude-versus-offset analysis
(AVO) is being widely used in different geological
settings as a lithology indicator and also as a direct
hydrocarbon indicator. Ambiguities associated to
elastic parameters estimation from AVO is
commonplace in most inversion procedures used up
today. Basic approaches to determine elastic
parameters directly from the coefficients of the P-P
reflectivity approximation (Rpp) fail because of the
uncertainties associated to the third coefficient, which
depend on larger incidence angles than intercept (A)
and gradient (B) estimates. One way to get around
this problem is to use the reflectivity information
from P-SV AVO (Rps) in addition to the traditional PP
AVO. This procedure can potentially reduce the
uncertainties and improve estimation of elastic parameters.
In this paper I demonstrate how to extract elastic
parameters from P-SV AVO and the way to combine
these parameters with those extracted from P-P AVO.
A simple low-contrast approximation is derived and
is compared with a more accurate, yet more complex,
high-contrast approximation. This procedure makes
possible to access the uncertainties associated with
the parameters estimated from P-SV AVO.
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Determination Of Reservoir Characteristics By Multiple Diffraction Stacking (Mds)
Authors J.C.R. Cruz, G. Garabito and K. ObaidullahAmplitude-versus-offset or (AVO) analysis is usually
carried out by using linear approximations of the
Knott-Zoeppritz equation. For comprehensive
analysis, a detailed knowledge of velocity model is
essential to determine incidence angles using ray
tracing. The angular reflection coefficients and the
incidence angles are determined by means of doublediffraction
stack of common-offset seismic data. For
determining zero-offset reflection coefficients is used
a true amplitude Kirchhoff migration operator. The
two resulting sets are then used in Hilterman's linear
approximation of Zoepritz Equations to determine NI
and PR. As long as we are within the bounds of
assumptions involved, a reasonable estimate of the
zero-offset reflection coefficient or the Poisson
reflectivity can be made. The data sets of the two
parameters are then available for attribute
determination and reservoir characterization.
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Interpretation Of Long Offset Avo Attributes
Authors Douglas Foster, Jeff Malloy, Bill Beck and Marc SbarAVO effects in large offset P-wave data (up to 60°) is
investigated. In addition to conventional P-wave
AVO attributes (A and B) a third term (C) is exa mined.
Background trends for crossplots of the attributes
are related to rock property relationships. Exact
expressions are given for the first 3 terms, (A, B, C),
of the expansion of Knott-Zoeppritz equations of the
reflected P-wave energy. These exact expressions are
compared to the commonly used linearized (first
order contrasts of rock properties) attributes. The
effect of variation in water saturation and porosity in
a sandstone embedded in shale is investigated in a
numerical example, based on rock physics models of
a well log. The AVO attributes show promise to distinguish
so-called fizz-gas (low saturated gas sands)
from commercial gas and differentiate porosity in
reservoir rocks.
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Quantifying Avo Attributes And Their Effectiveness
More LessQuantifying the effectiveness of any AVO technique
has always been a concern for progressive
geoscientists assessing and modeling reservoirs. This
paper compares three major AVO-based attributes
and their applications. Elastic impedance, lambdamu-
rho, and conventional intercept-gradient AVO
techniques are compared for various angular
apertures and well availability using a 3D prestack
seismic model. The effectiveness of each technique
is determined by comparing the apparent reservoir
extent to the known reservoir extent.
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Reflection Impedance
Authors Lúcio T. Santos, Martin Tygel and Antonio C. Buginga RamosAVO is now an established technology and has been
widely deployed as a lithology indicator and also as a
direct hydrocarbon indicator. In recent years this
technology has become a routine processing and its
application to large 3D volumes has relied on the use
of near- and far-offset stack volumes. These volumes
greatly reduce the amount of pre-stack information
that needs to be stored for standard AVO processing.
Additionally, these volumes are easily converted into
usual AVO attributes, like intercept and gradient,
which can then be interpreted in terms of anomalies
and calibrated with well logs. Reservoir
characterization studies make use not only of these
traditional AVO attributes but also impedance
volumes. The near-offset, or the intercept, stack
volume offers a natural way of obtaining acoustic
impedance volume through the use of post-stack
inversion algorithms. However, to invert far-stack
volume one needs an approach capable of estimating
impedances for a variable incidence angle. This
approach has been described in the elastic impedance
function presented by Connolly (1999). In this work
we propose an approach called reflection impedance,
which is based on constant ray parameter and a power
relationship between density and S-wave velocity.
This new method proved to be of better accuracy for
angular impedance estimation and reflection
coefficient recovery when compared with the elastic
impedance approach.
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Shear Waves Avo Azimuthal Analysis In Vacuum Field, New Mexico
More LessShear wave amplitude versus offset (AVO)
analysis was used in Phase VII Reservoir
Characterization Project in Vacuum Field, New
Mexico, for porosity and permeability estimation and
time-lapse investigation of the Upper San Andres reservoir.
DeVault (1998) pointed out that plane-wave
reflection coefficients for shear waves can be
inverted for density contrast, fracture density
contrast and shear velocity contrast.
The interpretation of the AVO attributes can
help to estimate the porosity and permeability of the
reservoir. The result obtained for porosity agrees with
the data from well logs, especially in the center of the
reservoir area, around the producers and injector
wells. Upper San Andres reservoir is a dolomite
where most of the permeability is provided by
vertical fractures. The fracture density was
particularly efficient in portraying the intense
fracturing between CVU-97 and CVU-200,
confirmed by the oil production increase after CO2 injection.
The time-lapse analysis showed variation in the
density between the pre and the post CO2 injection in
the northern part of the reservoir, with agreement
with the production wells in this area.
AVO attributes inversion proved to be a reliable
method for reservoir attribute determination and
time-lapse analysis, and provides an important link
between seismic data and rock properties.
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The Generation Of A Rock And Fluid Properties Volume Via The Integration Of Multiple Seismic Attributes And Log Data
Authors Matthew Carr, Richard Cooper, Maggie Smith, M. Turhan Taner and Gareth TaylorWe present a systematic seismic reservoir characterization
workflow that integrates log and seismic data using an artificial neural network.
Seismic attributes are examined both qualitatively and
quantitatively to determine the best discriminators of rock
and fluid properties. These attributes are systematically
classified using an artificial neural network, the Kohonen
self-organizing map (K-SOM) algorithm. Ultimately the
classified litho-facies volume is calibrated to available well
control by applying the K-SOM technology to well-derived data.
The product is a seismic-scale rock and fluid properties reservoir model that is consistent with borehole and surface seismic data.
The workflow is applied to the characterization of a Vicksburg-age reservoir in South Texas.
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The Rock Physics Of Seismic Fluid Attributes
Authors Vaughn L. Ball, Kevin Northey and Doug FosterThe tendency for a rock to change its bulk properties
upon fluid substitution is its "Fluid Sensitivity", and
is an intrinsic property of the rock. The magnitude of
the change in bulk rock properties is predictably
related to changes in seismic reflection attributes.
These changes are often called the "fluid effect". We
can speak of a fluid effect in relation to the classic
"A", "B", and "C" coefficients of the Aki & Richards
linearization. Fluid sensitivity and fluid effect are
often qualitatively related to porosity and P-wave
velocity, but can be more precisely related to an
important property of porous rocks called the "Biot
Coefficient". For most seismic attributes, the fluid
effect is independent of the surrounding lithology.
Only the fluid effect for the "B" attribute has a
component that depends on surrounding rocks. The
fluid effect for seismic attributes are separable in
relation to properties of the rock, and properties of
the fluid, and can be expressed as the product of rock
properties and fluid properties.
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3D Wavefield Imaging In The Deepwater Gulf Of Mexico
Authors Paul Valasek, Kay Dautenhahn Wyatt, Dan Whitmore, Yunqing Shen, Zhaobo Meng and Keith L. BranhamWhen complex structures with large velocity contrasts are present, wavefield techniques can provide improved imaging (structure, amplitude, and phase) over the more traditional Kirchhoff integral methods. Wavefield techniques rely on two elements: firstly, an advanced acquisition design with complete azimuthal coverage; and, secondly, wave equation based prestack
imaging methods. Together, these two elements offer improved seismic imaging in complex structural environments by better handling of multipathing, amplitudes and phase. A wavefield
experiment was conducted last year in a complex subsalt area of the deepwater Gulf of Mexico using a full-azimuth vertical cable acquisition strategy. Here, we discuss some of the unique
challenges associated with wavefield imaging such as velocity analysis and residuals correction. Finally, comparisons will be shown between conventional Kirchhoff imaging and wavefield imaging.
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A Deepwater Pore Pressure Risk Management Case In Offshore Brazil
Authors William L. Abriel, Dieter Letsch, Mike Luken, Stan Teerman and Jay BrutonA well was planned as the first in the deepwater
Cumuraxitiba basin offshore Brasil. The well casing
and the mud program were designed for abnormal
pressure based on risk from offset wells on the shelf
and a seismic velocity model. Interval probability
analysis illustrated an opportunity to decrease the
uncertainty of this risk.
Basin modeling and seismic tomography were
performed and uncertainty was reduced, pointing to
the probability of a normally pressured well. The well
design was altered reducing cost, and the subsequent
drilling of the well has confirmed the normal pressure prediction.
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Mantle-Plume-Generated Triple Junction Between Ne Brazil And West Africa?
More LessThis paper has three purposes. First, it offers an overview
of the nature and history of the interior and
offshore basins of Northeast Brazil and West Africa,
and their relationship to the development of the
Southern and Equatorial branches of the Atlantic.
Second, it places emphasis on the differences of the
rift systems and the control exercised by basement
structures on the conjugate margins of West Africa
and Northeast Brazil. Third, as the most important, it
challenges the idea of a mantle-plume-generated
triple junction between Northeast Brazil and West
Africa.The Benue Trough is very often cited as one of
the best examples of a failed arm of an RRR triple
junction (Burke and Wilson, 1976), formed through
the activation of a mantle plume. This would imply
in: (1) early crustal doming, warping and flexure over
a plume with crestal alkaline volcanoes; (2) formation
of three simultaneous rift arms, symmetrically orientated
at 1200 to each other; (3) if one rift arm is
aborted, the other two develop into a single ridge
(new margin) and drift begin; and (4) growth of delta
at the mouth of a failed arm. Among them, just the
last one is corroborated by the data set. Voluminous
magmatism clearly post-dated the opening of the
Equatorial Atlantic. Rifting and drifting were diachronous
among the African - Brazilian basins and the
rift arms are not symmetrically orientated, and finally,
instead of one, there are two failed arms: Benue
Trough and the onshore Potiguar basin.
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Sequence Stratigraphy, Facies Analysis And Paleoceanographic Events Of The Neogene Deep-Water Section In The Campos Basin, Offshore Brazil
More LessThe Campos Basin, situated offshore Rio de
Janeiro State, around 23 degrees latitude south,
has a total area around 100,000 Km2. It is the
most prolific basin in Brazil, with more than five
giant oil fields, of which at least four produce in
Neogene reservoirs.
This work was carried out in a thick deep-water
sedimentary section developed on the basin slope
during the Neogene and Upper Oligocene, using
3D seismic sections, well log correlations, cored
facies analyses, nanofossil biozones and
paleoecology (fig. 1).
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Offshore Brazil Santos Basin Exploration Potential From Recently Acquired Seismic Data
Authors Roberto Fainstein, George Jamieson, Andrew Hannan, Norman Biles, Ana Krueger and Dianna ShelanderThe deep and ultradeep water regions offshore east Brazil is
increasingly attracting attention from exploration and
production companies. Brazil hydrocarbon production is
derived mainly from the giant deepwater fields of the
Campos Basin. By contrast, the deepwater regions of the,
adjacent to the south, Santos Basin have remained largely
under-explored. In actuality, Santos Basin is one of the
largest under-explored basins of the world. In the last two
decades, there have been several significant discoveries in the
southern portion of Santos Basin mainly on carbonate
reservoirs. Recently in 1999 a significant discovery occurred
on Upper Cretaceous turbidite reservoirs in deepwater. This
paper outlines the encouraging hydrocarbon potential of the
deepwater realm of Santos Basin by utilizing a recently
acquired, processed and interpreted 2D dataset which has
revealed numerous, largely untested, play types. These plays
include both structural and stratigraphic components. Saltrelated
large 4-way closures, including faulted and unfaulted
turtle structures and drapes over salt have been identified
along with sub-unconformity traps. All these identified play
types have examples with associated amplitude anomalies,
which are widespread throughout the basin, at varying
stratigraphic and structural levels.
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A Multi-Frequency Inductive 5Th Generation Em System For Geophysical Exploration
The multi-frequency inductive electromagnetic systems had their origin in the work by Dias (1968; 1973) and five generations of these equipments appeared in the market since then. The system discussed here belongs to the last generation. It consists of a loop-loop minimum coupling system operating at 54 values of frequency in the interval 1.125 Hz to 10.473 KHz, with transmitterreceiver separation spreading up to 1 km on a terrain of 100 Ωm resistivity background. A scheme is also presented here for processing the field data and constructing
2D pseudo-sections for resistivity and polarization parameter. The area test for this experiment is situated on a gnaissic basement covered by intemperized basement rock and quaternary beach sediments on the Farm São João in Rio das Ostras/RJ. This system was capable to discriminate and map a zone of washed sand, probably more indicated for groundwater taking, and a zone where clay minerals are present.
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An Easy Windows Graphic Interface For 3-D Electrical Geophysics Modeling
Authors Humberto Deodato M. Monteiro and Luiz RijoThe finite element method is a very powerful tool
for modeling complex problems in physical science,
engineering and geophysics. However, for three
dimensional analysis, the arduous pre-processing
stage which consists of supplying the data for the
mesh and the physical properties for each element is,
certainly, a weakness of the method. In order to
handle this disadvantage, we developed a simple
Windows graphic interface for entering the
information of the physical properties across the mesh
at no great pains. We use the 3-D Resistivity and IP
modeling problem to show how the proposed graphic
interface facilitates the access of finite element data.
Apparent resistivy and PFE pseudo-sections at 10
parallel survey lines, one dipole unit apart, are
displayed graphically to exhibit the output graphic
interface that was also developed for presentation of
the modeling results.
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Analysis Of Georadar Reflection Responses
Authors Jacob Fokkema, Emile Fokkema, Sicco Beekman and Evert SlobUnderstanding the nature of the georadar reflection
response in terms of the dielectric permittivity and
conductivity contrast is important in the realm of
interpretation. This motivates the present study,
which has the aim to carry out an experiment in a
controlled fashion such that the relation can be established
between the measurement and the relevant
physical parameters. In the present paper we present
the results of such a measurement and an interpretation
based upon a 1D model, which is realistic enough
to accommodate sophisticated physical parameter models.
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Coast Effects On Magnetotelluric Data From The Brazilian Southeastern Region
Authors Antonio L. Padilha, Ícaro Vitorello and Marcelo B. PáduaThe coast effect on magnetotelluric (MT) data acquired
near the Brazilian SE coast in the state of São
Paulo is investigated using two-dimensional (2D)
modeling. The Atlantic ocean was modeled as a layer
of constant resistivity (0.3 ohm.m) and thicknesses up
to 2.0 km at 160 km from the coast, according to
bathymetry. Offshore Santos Basin was also included
in the model using parameters derived from boreholes
and seismic lines. The continental resistivity model
was constructed based on the electrical structure inferred
from previous MT studies. The modeling exercise
shows that the ocean-coast effects are very important
for periods greater than some seconds at distances
of tens of kilometers from the coast. Synthetic
data from the model are compared with a MT sounding
carried out in the Serra da Mantiqueira plateau.
Splitting between TE and TM modes observed at a
period of 30 s in the MT data can be explained as
being generated by electric currents induced in the
highly conductive Atlantic ocean (coast effect), about 90 km away.
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Crosswell Electromagnetic Tomography: Some Results Using Born Approximation
Authors Licurgo Peixoto de Brito and José Felipe Souza de AlmeidaA method of crosswell electromagnetic imaging is
presented and used to analyze the relevant parameters
for electromagnetic tomography. The influence of a
punctual inhomogeneity on the magnetic field is
calculated through the Sensitivity function based on
Born approximation. The analysis of the extremals of
that function is applied to generate crosswell images.
The method showed that the resolution is very good
vertically and very poor horizontally. Images are not
affected by increasing the number of receivers and
their horizontal alignment with the scatter, but local
maxima occur if transmitter and scatter are not
horizontally aligned. Furthermore, vertical resolution
is improved for increasing number of transmitters and
the aperture. These results are in agreement with
those well known in electromagnetic tomography literature.
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Desenvolvimento De Um Sistema Hlem De Campo Magnético Induzido, No Domínio Da Freqüência, Para Prospecção Mineral
More LessAn electromagnetic system was developed
using some parts of an old Slingram ABEM EM35/88
as transmitter and receiver coils and matching
transformers. Modern circuits with low noise lock-in
amplifiers and analog multiplier were used to detect
signal from a high magnetic permeability receiver
coil. In phase and quadrature voltage of lock-in
amplifiers outputs were achieved using square
waveform reference from the transmitter coil circuit.
Comparison of these voltage by a precise analog
multiplier give us the electrical apparent conductive
of the overburden layer (conductor). One third lock-in
circuit was used to adjust the spacing intercoil by
nulling the total field component.
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Dual-Mode Mt Static Shift Correction Using In-Loop And Single-Loop Tem Data
By Max A. MejuAccurate removal of static shifts caused by nearsurface
heterogeneities in dual-mode magnetotelluric (MT) apparent resistivity
sounding curves in complex geological terrains is
an outstanding problem in MT data analysis.
Using field examples, it is shown that the in-loop
and single-loop TEM curves vary in the same
manner as transverse electric and transverse
magnetic mode MT apparent resistivity curves
over 2D geological terrains. This suggests that
dual-mode TEM data can be used to effectively
remove static shift in MT sounding curves
leaving only those signatures due to
electromagnetic induction in larger local and
regional structures that may then be reliably
modelled using existing multi-dimensional
interpretation schemes.
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Electromagnetic Tomography With Absolute Constrains
Authors Victor Cezar T. de Souza, Luiz Rijo and João J. BaptistaAn iterative inversion process is employed to analyze
the incorporation of absolute constrains to obtain high
resolution image of cross-well electromagnetic (EM)
tomography. The image scheme assumes a cylindrical
symmetry about vertical magnetic dipole sources and
employs the finite element method to solve the
scattered EM fields by anomalous conductivity
imbedded in an homogeneous background. Images of
illustrative models show that to obtain relevant results
of inversion process, we have to incorporate absolute
constrains for the first guess and also on the borders
of model, to guarantee the convergence to
experimental model. The resolution of method is
examined through changing of frequency and
conductivity contrast between the targets and the
background. Reconstructed models show that, at low
conductivity contrasts, the resolution improves with
increasing frequency. On the other hand, for one
given frequency, large contrasts make the resolution
deteriorates. However when the frequency is
decreased, the quality of image improves.
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Electromagnetic Tomography With Derivative Constraints
Authors João J. Baptista, Luiz Rijo and João J. BaptistaIn this paper about electromagnetic tomography we
test the use of smoothings constraints implemented by
derivative operators of first, second and third order
calculated by the regularization method proposed by
Tihkonov. To get this, we use the same model
published by Alumbaugh & Morrison (1995). During
the inversion of the data we adopted the Maquardt
algorithm. The finite elements method was used to
calculated de forward model The knowledge of the
true conductivity of host is sufficient to guarantee the
recuperation of the image at any frequency between
10 to 1000 kHz. In this case no other constraint is
necessary. When the conductivity of the host is
partially known, within 20%, the use of derivative
constraints permits the achievement of good results in
range of 50 and 500 kHz.
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Geoeletroestratigrafia Bi-Dimensional Da Parte Central Da Bacia Do São Francisco (Mg)
Authors Ana Lucia Rodrigues and Sergio Luiz FontesNo âmbito de um programa de exploração realizado
no início da década de 90, o Observatório Nacional
realizou 43 sondagens magnetotelúricas (MT),
perfazendo um total de 7 perfis geoelétricos, ao
longo de linhas sísmicas previamente levantadas,
cobrindo uma área de 11.000 km2, ao norte do
estado de MG, na Bacia Proterozóica do São
Francisco (ou Bacia do Bambuí). A variação de
freqüência utilizada foi de 0,001 a 400 Hz. A
região localiza-se entre as latitudes 46º e 45º e
longitudes 16º e 17º, próximo da zona de
confluência dos rios S. Francisco e Paracatu,
estendendo-se até as margens do Rio Urucuia
(região transversal ao Rio S. Francisco). As
cidades mais próximas são Santa Fé de Minas e São
Romão. Os dados MT obtidos no levantamento
geofísico foram interpretados anteriormente a partir
de inversão 1D (Porsani e Fontes, 1992; Porsani,
1993), que supõe a variação da resistividade
elétrica apenas com a profundidade, não se
admitindo variação lateral ao longo dos perfis.
Tendo em vista o potencial para hidrocarbonetos da
bacia, objetivou-se, neste trabalho, reinterpretar os
perfis MT obtidos anteriormente, utilizando-se um
algoritmo de inversão bi-dimensional (Mackie et
al., 1997), que permite observar as mudanças
laterais da resistividade elétrica, fornecendo
informações que possibilitam a obtenção de
interpretações mais precisas em sub-superfície.
Os resultados preliminares das inversões 2-D de
quatro perfis MT apresentam estruturas de falhas
não observadas nos estudos de inversão 1D.
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Geração De Campo Magnético Direcional E Focalização Por Combinações De Dipolos Magnéticos
Authors Antonio V.C. Barbosa and Om P. VermaAn enhanced resolution is required in
electromagnetic investigation of the “low Induction
Number – LIN” targets. This could be obtained by
creating a directional magnetic field flux and focusing
it at in a predetermine depth zone. This is achieved by
combining the field of two magnetic dipoles in
different configurations to obtain vertically and
horizontally directed fields in the central portion of
the dipole configuration. Focalization of the field in
depth is controlled by the separation between the
dipoles. Results in the forms of directional magnetic
fields maps and the curves of the variations in the
intensity of these magnetic field components with
depth are presented. This study suggests an
innovative way to create directional and focalized
magnetic fields; consequently opens a new horizon in
electromagnetic exploration methods.
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Imageamento Geoelétrico De Colunas E Secções De Modelos 1–D Utilizando Técnicas Geoestatísticas
More LessO método do imageamento geoelétrico é uma importante
ferramenta no mapeamento de estruturas 2-D,
tendo sido propostas muitas técnicas de inversão com
esta finalidade. No entanto, quando a estrutura em
sub-superficie é próxima de camadas horizontais, a
hipótese 1-D deve ser considerada como a mais efetiva
para uma solução precisa. A combinação da técnica
da sondagem elétrica convencional com técnicas
geoestatísticas de imageamento, para investigação em
áreas de geologia não complexa, foi aqui desenvolvida.
As técnicas geoestatísticas, que incluem a krigagem
associada a uma forte “anisotropia” horizontal,
foram usadas primeiramente na elaboração de colunas
geoelétricas de SEVs individualmente. Posteriormente,
foram estendidas para o imageamento de secções
eoelétricas. As secções imageadas em dois levantamentos
são consistentes com as informações
geológicas/hidrogeológicas disponíveis e com um
levantamento anterior de eletrorresistividade realizado
pela Universidade Federal do Ceará. A metodologia
Geoestatística aqui estabelecida pode ser utilizada
na representação de variáveis da perfilagem geofísica,
assim como no imageamento bidimensional de propriedades
físicas com distribuição em camadas horizontais.
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Inversão Bidimentsional De Sondagens Elétricas Verticais Com Algoritmos Genéticos
Authors Niraldo R. Ferreira, Milton J. Porsani and Olivar A.L. LimaThis work investigates the two-dimensional (2D) resistivity inversion using genetic algorithm combined with Gauss-Newton method.
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Investigações Eletromagnéticas (Mt/Tem) Na Porção Continental Da Bacia Do Espírito Santo: Primeiros Resultados
A 200 Km geophysical transect crossing both on
shore terrains of the Espirito Santo basin and the
contiguous crystalline region has recently been
established. It includes magnetotellurics - MT,
transient electromagnetics – TEM, potential field -
magnetic and gravimetric mesurements as part of a
multi-institutional project involving UENF, UnB,
Petrobras and Observatório Nacional aiming at the
regional characterization of the basin. The objectives
of the study comprehend better knowledge of the
basement structure, continental crust – oceanic crust
contact, crust- mantle contact, lateral variations of
crustal structures, etc. This paper is concerned with
the very first results obtained with the MT and TEM
methods along the transect. Main results to date are
the geoelectrical signatures of the on shore basin
limits and the Colatina Vitoria fault.
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Metamorphoses Of Maxwell’S Equations
By Luiz RijoMaxwell’s equations are the starting-point of
electrical geophysics. From Magnetotelluric method
(low frequencies) to GPR (high frequencies) they are
behind of every electric geophysics techniques.
Since their origin, more than one hundred years
ago, Maxwell’s equations have been redressed in
many different manners. Maxwell started with a set of
twenty equations. Heaviside purified them to four
vector equations. With the contribution of Einstein’s
special theory of relativity the four vector equations
are recast into two tensor equations. Now, with the
aid of Clifford geometric algebra Cl3 they have been
reduced to just one single equation.
The saga of Maxwell’s equations helps to clarify
the physics of electromagnetism and simultaneously
gives us more confidence to work with them.
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Modelagem Bidimentsional De Resistividade Por Diferenças Finitas Para Um Caso De Modelo Primário Heterogêneo
Authors Frederico A.F. de Oliveira and Hédison K. SatoThe Choise of suitable boundary conditions plays an important role on two-dimensional resistivity modelling by finite differences.
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Modelagem Matemática E Numérica Da Propagação De Ondas Eletromagnéticas Acopladas A Ondas Elásticas, Considerando-Se O Efeito Sismomagnético
More LessInteraction of electromagnetic fields with elastic bodies is
the subject of many theoretical investigations in
mechanics of continua for the last decades. Some variants
of direct and inverse problems have been studied leading
into the determination of some characteristics of medium.
It is very interesting to study this phenomenon due to the
possibility of applying this theory to geophysical
prospecting and study of earthquake sources. Olsen [6]
has represented some experimental results of interactions
of power-line electromagnetic field with the Earth crust.
He has showed that high harmonic fields are produced by
basic harmonic field of frequency ωo = 60 Hz. One
possible explanation of this effect is a nonlinear
interaction of electromagnetic field with seismic field.
Our work is dedicated to the investigation of this
interaction. We form the basic equations for the
description of some variant of this coupling and consider
a simple mathematical model of this process. It is possible
to construct in linear approximation the solution of
corresponding problem, showing that higher harmonic
fields’ generation process can be described for that model.
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Modeling Of Geomagnetic B Field Under The Electrojet
Authors Marcos W.C. Silva and Luiz RijoOne of the traditional geophysical methods employed
in the determination of conductivity distribution of
subsurface in regional scale is the Magnetotelluric
Method (MT). It uses both electric and magnetic
fields at ground surface and is based on the
hypothesis that these fields propagate as plane waves.
In the equatorial region this hypothesis is not always
valid for low frequencies in resistive terrain due to
the presence of the equatorial electrojet.
Instead of using the electric and magnetic fields,
as MT, here we employ the vertical component of the
magnetic field normalized by the horizontal
component to analyse the influence of a 2-D structure
which is parallel to the electrojet upon the
geomagnetic field scattered by a 2-D structure. The
finite elements method was used for numerical
modeling. The electrojet was modeled as infinite
current line and as a planar gaussian distribution of
current. The effects of the two electrojets on the
response of the 2-D heterogeneity were compared
against the plane waves results.
We concluded that at frequencies higher than 10-2
Hz there is no observable discrepancy between the
plane wave and electrojet responses. Between 10-2
and 10-3 Hz the difference is very small. At these
frequencies the lateral edges of the 2-D heterogeneity
is easily detected. At 10-4 Hz the signature of the
buried structure is no longer delineated, showing thus
the influence of geometry of the equatorial electrojet
source on the low frequency response.
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Multicomponent Induction Logging Response In A 3-D Borehole Environment
Authors Tsili Wang, Liming Yu, Otto Fanini and Berthold KriegshäuserA multicomponent induction tool has the capability of resolving formation's resistivity anisotropy properties.
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Processing And Inversion Techniques For Multicomponent Induction Log Data
Authors Berthold F. Kriegshäuser, Otto N. Fanini and L. YuIn this paper we present advanced processing and
inversion techniques to derive horizontal and vertical
formation resistivities from multicomponent
induction log data. This newly developed induction
logging tool (3DEXSM) is designed to identify and to
delineate productive low-resistivity reservoirs
frequently encountered in hydrocarbon exploration.
An example of such a reservoir is a finely laminated
sand/shale sequence of which the sand laminae can
contain hydrocarbons found in deep water
environments. These reservoirs exhibit electrical
anisotropy, i.e., the resistivities parallel and
perpendicular to bedding are different and cannot be
accurately delineated by conventional induction
instruments with their transmitter and receiver
orientation parallel to the borehole axis. However, the
new multicomponent transmitter-receiver
configuration provides direct measurements to derive
both horizontal and vertical resistivity that allows
evaluation of the hydrocarbon bearing sand laminae.
The tool is comprised of three mutually orthogonal
induction transmitter-receiver configurations yielding
all necessary data to derive the horizontal and vertical
resistivities of the formation.
In this paper, we introduce various processing and
inversion techniques such as real-time data
processing, shoulder bed and borehole correction
schemes, an automatic ‘thin-layer’ inversion scheme,
and an efficient ‘pseudo 2-D’ inversion algorithm.
We demonstrate the performance of these
interpretation techniques on synthetic and field data sets.
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Projeto De Um Sistema Semi-Automatizado De Medidas De Potencial Elétrico Natural Ou Induzido Do Solo Para Aplicações Geoelétricas
More LessThe devices were developed to realize
spontaneous and induce voltage measurement on the
ground. In both case we used a multiwire cable with
multiple voltage inputs along the length that becomes
possible to conect it in six stainless electrodes fixed in
the ground at same time. An automatic sequence
digital system controlled by pushbutton touch allows
to couple electrodes sequentially to a electronic DC
voltmeter with autozeroing circuit based in PID
servocontrol circuit. A 60 Hz notch filter and a roll-off
low pass filter with time constant of 1s are used to
minimize line and spheric noises. Electronic control
system becomes too possible to couple cable with
receiver datalogger of SYSCAL R-2 in dipole- dipole
sounding array.
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Resistivity Imaging From Mt And Seismic Data In The Tucuman Plain, Argentina
Authors A. Favetto, M.C. Pomposiello and E. RosselloMT data along two traverses in the Tucumán Plain
were analyzed and it was found that basin sediments
are formed mainly by two layers. Then, seismic
reflection data coincident to the traverses have been
used to constrain the geometry between the basement
and the sedimentary rocks. The models obtained
from data inversion show a sharp resistivity contrast at the interfaces.
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Scattering Of Electromagnetic Plane Waves By A Vertical Dike Under A Conductive Overburden: Implications Fo Gpr And Tdem
Authors Lurimar Smera Batista and Edson Emanoel Starteri SampaioThe expressions for the exact solution of the scattering of a TE mode electromagnetic plane wave in the time domain, by a vertical dike under a conductive horizontal layer, have been established.
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Tem Survey At Sergipe-Alagoas Basin
Authors Paulo de Tar so L. Menezes, Eutair R. De Morais and Alan S. CunhaCarbonates make up about 20% of the
sedimentary rocks record, but they account for more
than 50% of the world’s proven oil reserves (Palaz &
Marfut, 1997). Carbonates differ from siliciclastics in
generation, geomorphology, and diagenesis, all of
which modify the mineralogy, porosity and
permeability so important to reservoir quality.
Recent discoveries of high porosity’s layers
in deeply seated carbonate sediments have pointed
out that deep hydrocarbon carbonate reservoirs are an
important Brazilian exploration frontier (Brunh,
1990). Among the most impressive deep reservoirs in
Brazil are the fields of Badejo, Pampo e Linguado at
Campos Basin (reservoir is located 2500-3100 m
depth - Lagoa Feia Formation), and Tubarªo at
Santos Basin (reservoir at 4600-4850 m depth -
GuarujÆ Formation).
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The Effect Of Air In The Numerical Solution Of The Te Mode Of The Magnetotelluric Method
Authors Michael Heimer and Luiz RijoWe present the numerical solution of the model of
an infinite fault for the TE mode using the finite
elements method to investigate the effect of the air in
the TE solution. We compare our solution for the
normalized electric field with one approximated
analytic solution, which neglects the effect of the air,
and with another exact semi-analytic solution. The
latter comparison shows a difference that cannot be
explained by numerical errors. Examining the effect
of the air on apparent resistivity, we compare three
profiles of apparent resistivity obtained with our FE
solution with the profiles obtained using the
approximated solution. The discrepancy between the
apparent resistivities shows to be small, figuring in
the range of the instrumental field error.
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The Use Of Telluric And Magnetotelluric Methods In Graben Studies
In past years telluric measurements have been used on
sedimentary basins to explore lateral variation of
electrical resistivity. Magnetotellurics is currently
used in investigation both, vertical and lateral
changes in electrical conductivity of earth subsurface.
This paper concerns the application of both methods
to the study of a graben in NE Portugal and shows
that the complementary of the methods can be used in
order to infer information concerning the threedimensionality
of geological structures.
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Two-Step Inversion Scheme For Complex Magnetotelluric Apparent Resistivity Data
Authors Atahebson B. Santos, Edson E.S. Sampaio and Milton J. PorsaniInterpretation of magnetotelluric data under the assumption of isotropic and one-dimentsional structures is a valuable procedure for exploration and solid earth geophysics investigation.
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Estimativa Das Direções Principais De Tensão In Situ Por Anisotropia De Velocidades
Authors Raquel Velloso, Guilherme Vasquez and Lucia Duarte DillonNeste trabalho estimam-se as direções principais de tensão in situ pelo método denominado VELAN (Velocity Anisotropy), que consiste na medida da velocidade de propagação de ondas ultra-sônicas em diversas direções radiais numa amostra proveniente de um testemunho orientado. Estas medidas permitem determinar as direções de máxima e mínima velocidades
que por sua vez se relacionam com as direções principais de tensões in situ. Realizaram-se ensaios de VELAN com duas amostras que apresentaram resultados bastantes coerentes
entre si. A observação das amplitudes das ondas registradas e a análise de imagens de tomografia computadorizada foram importantes no mapeamento de heterogeneidades, permitindo uma melhor definição das direções de máxima e mínima velocidade. A utilização do método VELAN para a estimativa de direções das tensões in situ mostrou-se bastante vantajosa por se tratar de um método simples, rápido e eficiente.
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Fracture System Analysis Aided By Gpr: Applications On Dimension Stone Quarries Modelling
Authors Adelir José Strieder and Sérgio Alfredo BuffonFracture system analysis is the subject of structural
geology and its methods can be applied in dimension
stone quarries. The dimension stone modelling
through fracture system access can be aided by GPR
(ground penetrating radar) technique in different
ways. The fracture map of a dimension stone massif
demands lateral interpolation of a discontinuous
vector and the extrapolation (projection) of each
fracture set into depth. Controlled GPR scanning
profiles can enable fracture continuation into depth,
even under soil cover, as well as determination of a
large number of fracture features. Modelling
dimension stone deposits requires a well-defined
analysis of disjunctive structures and an adequate
GPR profiling program based on fracture set
characterization. An appropriate GPR data processing
program can be valuable for engineering purposes
(rock massif characterization), placing aside again
structural geology and rock mechanics disciplines.
Key words: dimension stone modelling, structural
analysis, GPR exploration
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Geophysical Tomography In Engineering Geology: An Overview
More LessAn overview of the tomographic interpretation
method in engineering geophysics is presented,
considering the two approaches of the deterministic
tomography inversion, developed for rock elasticity
analysis, and the probabilistic tomography imaging
developed in the potential fields domain. The
theoretical basis of both approaches is shortly
outlined before showing a laboratory and a field
application.
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Magnetômetro Fluxgate, De Circuito Aberto, Para Análise De Ruídos Em Sensores
More LessDescription of the development of a system to
evaluate noises and alignment for fluxgate sensors
of different sizes and geometries is presented. The
first circuit was built for 7 KHz sensors cores
excitation and a second one is being developed for
16.625 KHz sensors core excitation.
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Mapeamento Dos Dutos Na Baía De Guanabara
Com o grande vazamento de óleo ocorrido em
Janeiro de 2000 na Baía de Guanabara, a Petrobrás
foi solicitada em curto prazo, a apresentar perante
aos órgãos governamentais, um mapeamento
detalhado e atualizado de todos os dutos que
compõem sua malha de transporte de derivados entre
a Refinaria Duque de Caxias (REDUC) e os
terminais da Ilha D’Água. Isto implicou em
determinar a posição espacial inicialmente de 9
dutos ao longo de 13 km de trajeto, confirmando
através de inspeção por mergulho, pontos onde o
assentamento não estivesse em posição ideal. Para
tal fim, foram contratadas empresas de
levantamentos geofísicos, geotécnicos e de
mergulho. Os dados geofísicos oriundos dos vários
métodos utilizados perfizeram 2400 km de dados e
para absorver este grande fluxo respondendo
imediatamente aos vários questionamentos surgidos,
foram montadas equipes de processamento e
interpretação que possibilitaram executar
intervenções para detalhamento de interesses
específicos (inspeções submarinas, coletas de
testemunhos, etc.). Como resultado foram
produzidas plantas com a posição espacial de cada
um dos dutos, desde sua partida da refinaria até a
chegada aos terminais marítimos de escoamento
situados na Baia de Guanabara.
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Monitoração Sismográfica De Edifícos Adjacentes Aos Túneis Da Av. Luiz Eduardo Magalhães
More LessIn this paper I present the result of studies with respect to vibrations in surrounding building due to blasting carried out to build large tunnels (400m long) in urban areas.
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O Emprego Das Técnicas Sísmica E De Radar Entre Furos (Crosshole) Em Estudo Para Fundação De Obra Subterrânea Na Cidade De São Paulo-Sp
Authors Renato Luiz Prado, Rubens Paschoal Cordeiro and Vicente Luiz GalliAiming for the use of the geophysical crosshole
techniques for studies of the deformability and
resistance of soil mass and its geological
characterization, it was carried out a seismic and radar
crosshole tests in na underground work in São Paulo city, Brazil.
It is discussed the main acquisition procedures of the
both techniques and some empirical relationships
between the elastic and the electromagnetic waves
relating to the mechanic properties and the
sedimentologic characteristics of the soil mass. It is
also presented a tomographic section derived from the radar data.
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O Uso Do Potencial Espontâneo (Sp) Em Caso De Infiltração Em Barragem
Authors José Domingos Faraco Gallas and Walter Malagutti FilhoThis paper presents the application of the self
potential method (SP) for leaking in an earth’s dam.
The tests were carried out in part of the embankment
which was submitted to a partial failure originated by
a leakage. It was employed the gradients technique in
this survey. The results showed the efficiency of the
SP method to detect the anomalous flow from the
reservoir and to indicate the leaking area.
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Utilização Do Gpr No Projeto Executivo Da Barragem De Pindobaçu, Rio Itapicuru-Açu (Ba)
This study shows the results of a geophysical survey
conducted with GPR to support the study of a dam in
Itapicuru-Açu River. The objective of the study was
to identify geologic structures found in the
underground surface (rock mass discontinuity, geologic contacts).
The information available from rotative drillings and
the geologic knowledge of the area enabled a good
correlation between data during result interpretation,
and three different types of lithologies were identified in the area.
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