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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
41 - 60 of 399 results
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Equatorial Spread-F Irregularities As Observed By Three Different Rocket-Borne Plasma Density Probes
Authors P. Muralikrishna, M.A. Abdu, M.G.S. Aquino and M.C. De FariaSome new results obtained from in-situ measurements of the height variation of the ionospheric electron density made
with rocket-borne electron density probes during a campaign conducted from Alcantara (2.31oS; 35.2oW) in Brazil are
presented here. The campaign designated Guará was conducted in collaboration with NASA. In addition to several
plasma diagnostic instruments provided by other participating institutes the Aeronomy Division in the Instiuto Nacional de
Pesquisas Espaciais in Brazil provided a High Frequency Capacitance probe that measured the height profile of the
electron density. During the Guara campaign, a Black Brant X sounding rocket was launched on 14-th October 1994 at
1955hrs (LT) to investigate into the phenomenon of high-altitude equatorial spread-F events. The rocket, as expected
passed through an active topside spread-F event, monitored simultaneously by several ground-based instruments. The
electron density height profile and the amplitude of the electron density fluctuations were measured simultaneously by
three different plasma density probes; a High Frequency Capacitance (HFC) probe, a conventional Langmuir Probe (LP)
and a Plasma Frequency Probe (PFP). While the PFP provided the absolute electron density, the LP gave the relative
variation in the electron density. The electron density profile obtained from the HFC probe measurements is practically
absolute except for a plasma sheath factor. But this technique does not provide the small scale electron density
fluctuation amplitude. The three experiments provided data, which could be used not only to obtain reliable electron
density data, but also to arrive at some of the inherent difficulties associated with each of these techniques. For example
the elctron density profiles estimated from the HFC and PFP experiments are almost identical except for a small factor
varying with altitude. The amplitude of large scale fluctuations provided by the LP measurements is considerably less
than that provided by HFC and PFP. The k-spectra of the plasma irregularities were obtained by the spectral analysis of
the electron density fluctuation data. The electron density irregularites associated with the plasma bubbles were seen to
have rather sharp lines in their k-spectra extending over a wide range of altitude. What one would expect from the
existing theories on the generation of small scale irregularities by the cascading process is a flat k-spectrum. Present
results may be indicative of the presence of preferred wave modes in developing plasma bubbles.
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Obtencion De Parametros Fisicos Y Elasticos Mediante Tomografia De Transmision "Cross-Hole
More LessEn este trabajo presentamos los resultados de emplear una técnica tomográfica de inversión sísmica para la
reconstrucción de imágenes del subsuelo, en términos de las velocidades de las ondas P y S, así como una
técnica para la representación de parámetros sísmicos, tal como la relación de velocidades de la onda P y onda
S, la densidad, las constantes elásticas, y la porosidad, a partir de observaciones del tiempo de tránsito.
La metodología se aplicó a datos reales obtenidos de un experimento realizado en un campo petrolero ubicado
en el Estado de Texas, E.U.A., del cual se tenían algunos datos geológicos. Se obtuvieron los tomogramas de
los parámetros sísmicos, los cuales permitieron corroborar los datos litológicos, y así mismo inferir otros
detalles de la estructura interna del subsuelo.
Finalmente, hacemos énfasis de la importancia de aplicar la Tecnología descrita, en los campos petroleros de
México, ya que nos permitirá conocer las características físicas y elásticas de los yacimientos y analizar su
comportamiento lateral, etapas importantes que se realizan durante la caracterización de yacimientos.
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Disturbances On Magnetotelluric Data Due To Electrified Railway
More LessMagnetotelluric (MT) soundings were made on two transverse profiles to the Campos do Jordão Railway (CJR) in the
period range of 4 to 3000s. The profiles are located in two adjacent regions of the São Paulo State with contrasting
conductivity: the conductive sedimentary region of Taubaté Basin and the resistive crystalline region of Serra da
Mantiqueira. The data were collected during diurnal and nocturnal periods, when the CJR operates with DC current and
when it is turned off, respectively. The objective of this study is to attempt to eliminate the electromagnetic noise
produced by the CJR in the diurnal data. To achieve this goal we plan to use some techniques based on the MT
response function estimation and then to estimate the resistivity profile of the region. This profile will be compared with
the results obtained from nocturnal data, not contaminated with the noise of the CJR, to verify the relative efficacy of the
methods employed to filter out the electromagnetic noise. The preliminary results indicate that the Serra da Mantiqueira
region is 177.8 times more resistive than the Basin and that the skin depth in the Basin is of 2.7 km.
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On The Role Of Cosmic Rays In The Atmospheric Processes
Authors Yuri I. Stozhkov, V.I. Ermankov, I.M. Martin, P.E. Pokrevsky and M.G.S. MelloThe role of charged particles in processes in the atmosphere is considered. It is shown that these particles play the
important role in the global electric circuit provided the conductivity of air. The steady state equation describing ion
production rate and their recombination is analyzed on the base of experimental data on cosmic ray fluxes and ions
concentrations. These data were obtained at polar, middle and equatorial latitudes in the atmosphere from the ground
level up to 30-35 km. The influence of charged particle fluxes on rainfall values during Forbush decrease and powerful
solar proton events is demonstrated. Also, the mechanism of thunderstorm electricity production, separation of opposite
sign charges, and lightning production is discussed.
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Participação Da Unicamp Com Experimentos Na Estação Espacial Internacional
A collaborative project to study Solar-Earth relationship using International Space Station (ISS), prepared by Institute of
Space Research (IKI) of Rusian Academy of Science and State University of Campinas (Unicamp) will be described. The
main objective will be identify phenomena relating solar influences in climatology variations on Earth, mainly intensity of
rains in Brazil. Infrared, visible and X-ray detectors will be installed on board of ISS russian segment to realize monitoring
on Brazilian territory.
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Structure And Development Of The Powell Basin (Ne Antarctic Peninsula)
More LessPowell Basin is one of the few present-day examples of a small ocean basin largely surrounded by blocks of
continental crust. The western and eastern margins are conjugate passive margins having a transitional crust in the
continent-ocean boundary. The distinct characteristics of these two margins suggest a degree of asymmetry in their
development. A possible origin of asymmetrical extension during the initial rifting stage may be the occurrence of a
shear zone with an eastwards dipping component. A spreading ridge, located in the central part of the basin, split
into two crests that may be interpreted as two segments of an overlapping spreading centre (OSC). The rifting
episode could have begun about 35-40 Ma. A maximum age of Early Oligocene (29 Ma latest Early Oligocene) is
proposed for the ocean basin spreading, that appears to have ceased during the Early Miocene (23 Ma). The
eastward motion of the South Orkney Microcontinent relative to the Antarctic Peninsula created this basin. A pullapart
origin, related to a dextral fault system, has been proposed for the basin development. However, a possible
alternative is to consider that the transcurrent fault bounding the basin has opposite kinematics (sinistral the
northern one and dextral the southern one). In this model, the continental crust at the north of the Powell Basin
remains fixed in relation to the Antarctic Peninsula.
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Resultados Preliminares De La Campaña De Ozonosondeo En Una Localidad De Altura (La Paz, Bolivia)
Authors Francesco Zaratti, Marcos Andrade and Ricardo FornoSe analizan los datos de once lanzamientos de sondas de ozono realizados en la región de La Paz ( 3400 m
snm) en el mes de Junio de 1998. Se estudia el comportamiento de variables como la temperatura, la altura de la
tropopausa, el contenido troposférico de ozono, la distribución estratosférica del ozono y el valor integral del
mismo. Se comparan los resultados obtenidos en esa campaña con los datos de un trabajo similar realizado en
1963 en la misma localidad con el fin de poner en evidencia eventuales modificaciones en la atmósfera local. Se
discute también un posible origen de la disminución del ozono total sobre las tierras altas tropicales.
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Impactos Sócio-Ambientais Provocados Por Emanações Radiogênicas De Rochas “Graníticas” Utilizadas Em Ambientes Internos
More LessSão avaliados fatos referentes ao estudo das emanações radiogênicas produzidas por rochas “graníticas”
utilizadas em ambientes internos, relativos aos impactos sociais, ambientais, de segurança no trabalho,na
indústria e na comercialização desses materiais, observados durante o desenvolvimento das medições.
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Applicability Of Eudo-Rotation Technique For Land Vsp With Difference In Surface Conditions At The Source Locations In Anisotropy Study
Authors Sergei Gorshkalev and Wladimir KarstenPseudo-rotation technique may be applied in marine VSP study of azìmuthal anisotropy of horizontally layered
media. lt uses multicomponent downhole registration of downgoing converted PS-waves from two symmetrical
sources with small offsets because directed sources of S-waves can not be used on sea. Land symmetrícal
sources are more widely used than directed sources, so we would like to estimate efficiency of this technique
for land VSP. The land survey conditions involve variations of near-surface properties and altitudes, which
cause the travel times of identical waves to differ. The technique of equalization of these travel times before the
pseudo-rotation was suggested. lt was tested on synthetical data of full wavefield. The model used was based
on the VSP surveys in the Yurubchen-Tokhomo oilfield zone in Eastern Siberia.
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Estudo Das Estruturas Coerentes Na Cla Clara Por Radar Doppler
More LessThe evaluation of the respective role played by the coherent structures and the turbulence in the vertical transport within
the atmospheric boundary layer was the scientific objective of the TRAC experiment, the experimental framework of this
study, that made use in June 1993 of coordinated aircraft-radar measurements. The analyzed cases of ABL showed that
the radar was able to provide a coherent and continuous representation of the meteorological fields over a horizontal
range of several tens of kilometers and vertically up to 2.5 km, i.e., well beyond the top of ABL. Coherent structures
appeared as a common feature of the ABL under study and not as an exception. From radar measurements we found in
particular that turbulent scales of the inertial sub-range were modulated by the coherent structures. The capability of the
detection of weather cumulis located at the top of the ABL show that the radar is able to provide a description of the
entire ABL including the processes in the entrainement zone.
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High Resolution Aeromagnetics As And Aid To Geologic Mapping Of Rio Das Velhas Greenstone Belt, Quadrilátero Ferrívero, Minas Gerais, Brasil
Authors Adelene M. Silva, Anne McCafferty, Augusto C.B.Pires, Hengrem Xia and Issamu EndoA high resolution geophysical survey was acquired in 1992 as a part of Rio das Velhas Project and was used to investigate the regional structural setting of the area and better define lithological boundaries. High resolution geophysical surveying is fundamental to modern geoscientific mapping, providing cost-effective definition of structure and lithology in a diverse range of geological terrains. The airborne magnetic data are particularly valuable for defining northsouth Archean structures and the continuity inside the study area. This paper illustrates the usefulness of the aeromagnetic data in the support of geological mapping in the Rio das Velhas Greenstone Belt.
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Anisotropy In A Vicinity Of A Borehole From The Qp Wave Slowness And Polarization
Authors Ivan Psencik and Xuyao ZhengWe propose an algorithm for evaluating weak anisotropy parameters at receivers in a borehole in a multi-
azimuthal multiple-source o set VSP experiment. The parameters can be determined from measurements of
slowness vectors and/or deviations of measured qP wave particle motions from the slowness vectors at indi-
vidual receivers. The formulae presented are applicable under assumption of arbitrary but weak anisotropy
and vertical inhomogeneity of the medium. The formulae represent, in several respects, generalization of an
algorithm proposed by Gaiser (1990) and an alternative to the algorithm proposed by Horne et al. (1998).
Basic formulae and a synthetic example of their application are presented.
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Prestack Depht Migration In The Presence Of Low Relief,High Velocity Layers
Authors R. Gareth Williams, Peter Whiting and Jerry YoungDepth migration is more accurate than time migration in that it images seismic data correctly in the presence of
lateral velocity changes. However, for mild lateral velocity gradients we often use time migration for reasons of
cost and stability; the time spent deriving an accurate velocity field and the increased sensitivity of depth
migration to the velocity field make depth migration more difficult to apply. For these reasons, depth migration
has often only been used when time migration is perceived to fail to image the data properly. Very often, this
means that depth migration is only used in particularly difficult and complex geological environments such as
overthrust belts and beneath salt diapirs. Unfortunately, in these environments, depth migration often does not
provide a clear image where none existed without depth migration because either the ray paths diverge causing
the subsurface not to be illuminated by the recording, or current model building techniques are inadequate for
such complicated cases. For example, many model updating techniques assume the starting model is either
close to the correct answer, slowly varying or that flat horizons exist.
Experience of imaging beneath the basalts of the Shetland-Faroes basin, the tabular salt of the Gulf of
Mexico, the shallow carbonates of N. W Australia and the chalk layer of the North Sea has shown that prestack
depth migration can be used to improve imaging substantially beneath these low relief but high velocity layers.
In the Shetland-Faroes basin, volcanic flows form a barrier to imaging of deeper sedimentary rocks. PSDM has
improved imaging both within and below the multiple lava flows. In the Gulf of Mexico, tabular salt causes
significant ray path bending and subsequent distortion of the images of the underlying geology. In the Browse
Basin, rugged seabed topography can cause imaging problems that can be addressed with prestack depth
migration. In many parts of the North Sea a chalk layer with gentle dips lies above the oil and gas bearing
targets. All of these geological environments are characterised by fast velocity layers (for example, the chalk is
typically twice the velocity of the overburden) and even low dips at the top and base of these layers cause
problems for imaging deeper targets. The ray path bending can either prevent an image being formed at all,
distortion of any image that is obtained or loss of resolution. PSDM has been found to be beneficial in all these
cases. Prestack depth migration should not be viewed as just a tool for extreme geological cases.
Post stack depth migration may provide better positioning and imaging than post stack time migration.
Therefore, in low relief areas, it is tempting to use post stack depth migration instead of prestack. However, the
ray path distortion caused by the low relief, fast velocity layers we are considering is significantly offset
dependent. This causes image distortion and loss of resolution that cannot be recovered stack. Also, model
building via post stack depth migration is subjective – the only test is whether the final image agrees with the
interpreter’s expectation. Prestack depth migration affords the qualitative test that if the velocity model is
correct then the primary events within depth migrated image gathers are flattened and will stack coherently.
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Co-Location Of Surveys In Space And Time For Improved Accuracy And Sensitivity In 4D Marine Surveys
Authors R. Gareth Williams, Richard Leggot, Mark Skinner and Steve JesticoTime lapse, or 4D, seismic surveys are designed to monitor changes in the seismic response of hydrocarbon
reservoirs caused by production. In many reservoirs, the acoustic impedance changes induced by production
are small and therefore we are looking for weak 4D signals. This means that 4D surveying requires a high
degree of accuracy in order to detect a reliable 4D signal. In marine 4D surveys, variations in geometry caused
by currents, variations in wave height and variations within the water column all combine to reduce the
repeatability of the recording. Successful 4D processing must take account of these factors
Interpretation of 4D results has often been done on difference sections obtained by simply taking the difference
between two vintages of data previously processed for 3D purposes. Variations in the individual survey results
will be caused by variations in recording equipment and geometry, variations in feathering and infill, variations
within the water column and variations in processing. In order to compensate for these differences, matching
filters are applied to the datasets. In this method, it is assumed that matching the overburden will compensate
for all of the above factors in a single filter (perhaps a single filter per trace) and thus reveal the true change at
the reservoir level. Although this approach implies that the biggest difference between the two surveys will be
outside the matching design window, it does not mean that these changes are only caused by reservoir
production. Indeed, the difference between the datasets is often found to increase everywhere outside the
design window. For example, data shot over the Oseberg field in 1989 and 1991 were normalised by applying
scalars derived in a 150ms window around the water bottom for each trace. Equalising the amplitudes around
the water bottom INCREASED the difference energy over a larger window from 0s to 3s, illustrating the dangers
inherent in simply matching different datasets to optimise the 4D signal.
Since the differences between two datasets are caused by a number of factors, it is likely to be better to address
each problem separately rather than attempting to do so with a single matching filter. The first, obvious step is
to process both datasets simultaneously for 4D purposes. This eliminates differences in processing methods
and algorithms and also testing of whether or not particular processing steps improve the 4D difference. Within
this processing, variations in positioning (in space and time) should be addressed separately from, say, wavelet
changes. Finally, matching filters may still be appropriate, but now they are addressing only residual differences
in the seismic system.
To examine how sensitive taking the difference between two datasets can be, consider two 40 Hz Ricker
wavelets separated by a 1ms static shift. Figure 1 shows the two wavelets and the difference between them. The
peak difference is about 25% of the peak value of each wavelet. Increasing the static shift to 4ms causes the
peak value and the RMS of the difference to approximately equal the equivalent values for the individual
wavelets. Static shifts greater than this cause the difference to have peak and RMS values up to twice that of the
individual wavelets. This demonstrates that we will have to account for small timing and positioning shifts
between vintages if we are to reduce the background difference ‘noise’ and detect true changes caused by
hydrocarbon production. Moreover, many reservoirs require sensitivities of less than 25% in order for the
production induced seismic changes to be detectable i.e accuracy to better than 1ms for the above synthetic
wavelets.
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Deep Crustal Structure Of The Area Of Intersection Between The Shackleton Fracture Zone And The West Scotia Ridge (Drake Passage, Antarctica)
The Shackleton Fracture Zone represents the present-day boundary between the Antarctic and Scotia plates in the
Drake Passage. The West Scotia Ridge is an extinct spreading center, which formed the oceanic crust of the Scotia
Plate in this region and that intersects with the Shackleton Fracture Zone. New multichannel seismic, gravimetric,
magnetometric and multibeam swath bathymetry data were acquired during the ANTPAC 97/98 cruise with the Spanish
vessel B/O HESPERIDES in the area and reveal that these structures are asymmetric and developed as a consequence
of the overprinting of extensional and contractional deformation events. Thrusting of these young oceanic crusts
produces crustal thickening and shallow oceanic reliefs and ridges.
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Contaminação Do Subsolo Monitorada Por Resistividade Elétrica
Authors Carlos T.C. Nascimento, Sérgio Koide and Augusto C.B. PiresO método geofísico de perfilagem elétrica horizontal é uma técnica que pode ser utilizada para a detecção de
contaminantes no subsolo. Neste trabalho, este método foi empregado para visualizar a migração de efluentes
de fossa séptica dispostos no subsolo, através de valas de infiltração. O contraste de resistividade elétrica
entre o solo seco e o solo com presença de efluente permitiu delimitar as áreas contaminadas. Os resultados
obtidos possibilitaram identificar um padrão de movimentação tanto horizontal, como vertical do contaminante.
Dados complementares, obtidos através de coleta de amostras da água do subsolo, mostram que a
contaminação já atingiu um aqüífero freático que situa-se a cerca de 8 metros abaixo das valas de infiltração.
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Coplanar Coils Response In A Borehole
Authors Paulo Roberto de Carvalho and Om Prakash VermaNone of the traditional resistivity borehole devices possesses azimuthal investigation properties whereas the
unconventional coplanar coil array has, by design, this attribute. Theoretical electromagnetic radial and vertical
responses have been obtained for the conventional two coil coaxial system and the unconventional coplanar array.
From the comparative studies of the responses of these two coil system we can conclude:
1) The skin effects for the coplanar coil array are stronger than coaxial, this disadvantage almost disappear after skin
effects correction; 2) Polarization “horns” occur in coplanar profiles against the bed boundaries indicating their presence;
and 3) The azimuthal attributes of the coplanar system can be explored in the investigations of the axially asymmetrical
geological situations such as vugular or fracture zones, inclinded beds and invasion zones in horizontal wells.
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Redução E Aplicação Da Bte À Geofísica De Poços
More LessA eficiência na modelagem numérica de perfis nucleares de poços esbarra na obtenção de uma equação ideal aos objetivos propostos. A equação de difusão mais simples empregada origina-se da aplicação da Lei de Fick na redução da equação de transporte de Boltzmann (BTE).
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Parâmetros Físicos Em Perfilagem Nuclear De Poços -Raios Gama
More LessNeste trabalho os parâmetros físicos das formçõaes, tais como o coeficiente de difusão e as seções de
choque total e de espalhamento, são determinados através da distribuição de Kleín-Nishina e de regressão
linear múltipla de dados experimentais. A adequação destes parâmetros à modelagem numérica de sistemas
de multigrupos de energia são pressupostos básicos para a compreensão das medidas das diversas sondas
de poço que utilizam como fontes, raios gama artificiais e/ou naturais.
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Inversão Gravimétrica Do Relevo Descontínuo De Uma Bacia Sedimentar
More LessWe present a new stable gravity inversion method applied to the mapping of an interface separating two homogeneous media. In contrast with previous similar methods, it does not impose an overall smoothness on the estimated interface to stabilize the solution. The density contrast between the media is assumed to be known. The interpretation model for the upper medium consists of rectangular juxtaposed prisms whose thicknesses represent the depths to the interface and are the parameters to be estimated. The true interface is assumed to be flat everywhere except at faults. To incorporate this attribute into the estimated relief, we developed an iterative process where three kinds of constraints are imposed on parameters: (a) proximity between values of adjacent parameters, (b) lower and upper bounds to parameters, and (c) proximity between the values of parameters and fixed numerical values. Starting with an initial solution which presents an overall smooth relief, the method enhances initially estimated geometric features of the interface; that is, flat areas will tend to become flatter and steep areas will tend to become steeper. This is accomplished by weighting the constraints, which requires proximity between adjacent parameters. The weights are initialized with values equal to unity and are automatically updated to enhance any discrepancy between adjacent depths that have been detected at the initial solution. Constraints (b) and (c) are used both to compensate for the decrease in solution stability due to the introduction of small weights, and to reinforce flatness at the basin bottom. Constraint (b) imposes that any depth be nonnegative and smaller than an a priori known maximum depth value whereas constraint (c) imposes that all depths be closest to a value greater than the maximum depth. The tradeoff between these conflicting constraints is attained with a final relief presenting flat bottom and steep borders. The method was tested with synthetic data produced by a simulated sedimentary cratonic extensional basin having a smooth floor, steep edges and known depth to the bottom. The results showed an improvement in the resolution of the relief, leading to a reliable mapping both of the sharp discontinuities at the borders and of the lateral extent of the base of the basin. The method was also applied to the Bouguer anomaly from the northern portion of Steptoe Valley, Nevada, delineating an isolated basin with a wider, flat base and more straight borders as compared with the estimate imposing overall smoothness on the relief.
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