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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
21 - 40 of 465 results
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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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