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12th International Congress of the Brazilian Geophysical Society
- Conference date: 15 Aug 2011 - 18 Aug 2011
- Location: Rio de Janeiro, Brazil
- Published: 15 August 2011
21 - 40 of 465 results
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Magnetotelluric Response Estimates under the Equatorial Electrojet in Brazil
Authors E.U. Ulugergerli, S.L. Fontes, R.M Carvalho, C.R. Germano and A. CarrasquillaMagnetotelluric responses of soundings under the equatorial electrojet in Brazil have been estimated for four distinct locations at the north and south of the Magnetic Equator. It has been observed that the effect of the concentrated ionospheric currents (electrojet) in equatorial zones is added to the electromagnetic data and act as source. The difference between daytime and nighttime data indicates that source boosted by the electrojet, mainly observed at daytime, provides better estimation of the earth response functions even for lower frequencies (<0.01Hz).
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Aerogammaspectrometry for Lode-Au Deposits Exploration in Western Pampean Ranges-Argentine
Authors Gerardo Fanton, Patricia M. Martinez and Mario E. GimenezGiven the importance and economic potential of the region was held data processing geophysics obtained gammaspectrometrics maps relationship Th / K, Kd (potassium anomalous) and parameter F. Finally a ternary map data gammaspectrometrics where anomalous potassium integrate data, parameter F and K / Th. From the interpretation of these maps could define hydrothermal alteration zones associated with the presence of gold mineralization.
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Analysis of Atmospheric Variables Influence on the Surface Global Solar Irradiation
The current national energy scenery indicates the need of clean and renewable energy sources applications. Then, it is necessary to study the renewable energy resources and their variability. Therefore, the article aims at studying the influence of atmospheric aerosols and precipitable water in the global solar irradiation at the surface. Stochastic models were developed to compar the ratio betweenthe global solar irradiation at the surface and at the top of the atmosphere (Kt) with the atmospheric variables by using data acquired at the Southern Space Observatory, located in the Brazilian Southern region. Only clear sky days were used. After data validation, it was observed that the developed models have presented low statistical deviations, however, there was significant improvement in the estimates provided by the one that used as input variables the transmittances associated with atmospheric aerosols and Rayleigh scattering, and vapor water absorptivity.
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Efeito de Diferentes Sistemas de Manejo na Resistividade Elétrica de Latossolos
More LessIn this work, DC electrical resistivity measurements were used for to evaluate agricultural areas where no-tillage and conventional management systems have been established for 20 years. The study area is located at Distrito Federal, in the central region of Brazil. It was observed that conventional system area is less resistive than no-tillage system area. This result can be related to the fact that management systems influenced soil micro and macro aggregates distribution, what is related with permeability. Conventional system causes disruption of soil macro aggregates, increasing soil porosity and water retention and decreasing permeability and resistivity.
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Combining Wavelet Transform and Neural Network to Differentiate the Stratigraphy from Logs of Namorado Oilfield in Campos Basin
More LessOn well logging, there is a great interest to improve the vertical resolution of the logs, aiming the identification of different layers or geological formations along the borehole and the construction of a reservoir model. Generally, the identification of hydrocarbon formation lithology from geophysical logs employs several approaches as lithology crossplots (such as ‘‘M–N lithology plot’’ which requires a sonic log, density log, and neutron log) or the combination gamma-ray neutron-density log method. Also, numerous
mathematical approaches have been proposed to perform this task computationally, between them, artificial intelligence techniques. In this sense, the purpose of this study was to identify the formation interfaces from geophysical well logs using a combination of wavelet transform and neural network methods. The first technique was applied to smooth the logs, while the second was utilized to fit them to a selected lithological model. The input variables were gamma-ray, resistivity, density, neutron porosity and
sonic logs from Namorado Oilfield in Campos Basin. This method is easy to implement in a computer with MATLAB platform and it showed a good performance in the discrimination of main layers.
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2011-Inctet Progress Report on Magnetotelluric and Geomagnetic Depth Soundings in the Borborema Province and Adjacent Terrains, Ne Brazil
In order to enhance our knowledge about the tectonic processes responsible for the geological evolution of the Borborema Province and adjacent regions, the National Institute of Space Research (INPE) and the National Observatory (ON) are conducting
Electromagnetic Induction studies in northeastern Brazil, which are being supported by the National Institute of Science & Technology for Tectonic Studies (INCTET) project during the past three years. Such studies contemplate the deep geophysical
probing of this region with Magnetotelluric surveys (MT) and Transient Electromagnetic soundings (TEM) along several geoelectric transects and an array of Geomagnetic Depth Soundings (GDS), which complement other geophysical methods. The focus of
this study is directed to the identification of geoelectric variations in electric conductivity at distinct lithospheric depths and associations of geoelectric strikes and anisotropy with structural grain and stress patterns, particularly in the long ubiquitous structural features that cross the province and that constitute major geoelectric heterogeneities, at the near surface crust and also in the lower crust and upper mantle. The preliminary results indicate a lithosphere with a very variable electrical resistivity. The upper-mid crust shows up as a very resistive layer, overlying a less resistive lower crust and upper mantle. In some areas the mantle is very resistive, characteristic of a cratonic origin. Exceptions to a resistive crust are found under the Seridó belt, Jatobá rift basin and the Araripe intracratonic basin, marked by an underlying conductive crust. Laterally, the deeper portion of the crust and upper mantle is highly segmented in blocks with alternating juxtaposed higher and lower resistivities. This pattern is suggestive of highly deformed regions by transform dominated tectonic regimes, with the presence of contrasting mechanically strong cratonic-like keels against weaker zones likely subjected to delamination processes that might have produced magmatic related lithospheric re-fertilization by metasomatism.
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Geoelectric Dimensionality of a Magnetotelluric (MT) Survey in Parecis
Authors Jean Marie Flexor and Hans Schmidt SantosA survey of eighty-eight broadband (0001-1000s) magnetotelluric (MT) sounding stations was deployed in a transect from Parecis Basin, Brazil, in a direction al-most perpendicular to the dominant structural lineament. An image of the subsurface conductivity distribution was obtained from the 2-D inversion of MT data, linked to well logging resistivity data. In a first time, we assumed previ-ously a 2-D dimensionality for the inversion procedure, using as geoelectric "strike" an angle of 1400 (± 15 0) clockwise with respect to the geographic north. This is the direction of the dominant regional structural lineaments shown by the elongated geometry of the basin and by the lineaments revealed in the aeromagnetic data. In a second time, we went to check the consistency between the direction previously adopted for the 2D inversion and the intrinsic directionality deduced from rotational properties of the tensor impedance through a procedure of decomposition (McNeice and Jones, 2001), where we retrieve the geoelectric regional direction for a set of 36 MT soundings in the profile. The results show that the distribution of angles is not uniform as it would be expected but that the direction previously assumed is perfectly compatible with the trend of the mean directions calculated for the "strike. " This result shows that in the presence of indications of the directions of structural lineaments, it is not necessary to proceed the decomposition of tensor impedance.
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Aplicações da Filtragem Svd na Análise de Velocidades e no Empilhamento Cmp
Authors Oscar F. Mojica, Milton J. Porsani and e Michelangelo G. da SilvaNo presente trabalho aplicamos o m´etodo de filtragem adaptativa baseada no m´etodo SVD (Singular Value Decomposition) para a melhoria da an´ alise de velocidades e do empilhamento CMP. Antes da aplicac¸ ˜ao do m´etodo ´e feita a correc¸ ˜ao
normal moveout (NMO) dos sismogramas, tendo como finalidade deixar as reflex˜oes de interesse aproximadamente horizontais. A filtragem SVD trabalha na forma multicanal e pode ser aplicada seguindo um procedimento padr˜ao que consiste na
selec¸ ˜ao de um conjunto de trac¸os vizinhos tomados ao redor do trac¸o alvo da filtragem. Desta forma trac¸os de diferentes tiros podem ser utilizados na filtragem SVD. A coleta de trac¸os pertencentes a diferentes tiros, no mapa de cobertura, pode ser representada por um operador espacial. No presente trabalho utilizamos um operador de cinco pontos que denominamos operador em cruz que opera sobre todos os trac¸os do mapa de cobertura do levantamento s´ısmico. A cada posic¸ ˜ao do operador, o trac¸o filtrado (centro do operador) ´e obtido tomando-se a primeira ou somando-se a(s) primeira(s) autoimagem(ns) do painel de 5 trac¸os selecionados. Desta forma reforc¸amos a coerˆencia correspondente `as reflex ˜oes prim´ arias, em detrimento do sinal restante (ground-roll, m´ ultiplas e demais eventos n˜ao correlacionados), localizado nas demais autoimagens. A filtragem SVD pode ser vista como um m´etodo de filtragem adaptativa multicanal onde cada trac¸o filtrado guarda certo grau de coerˆencia
com os trac¸os imediatamente vizinhos. O m´etodo foi testado sobre uma linha s´ısmica terrestre da Bacia do Tacutu. Os resultados obtidos mostram espectros de velocidades com melhor definic¸ ˜ao, como tamb´em sec¸ ˜oes empilhadas exibindo melhor continuidade das reflex˜oes e menor ru´ıdo ground-roll, comparado com os resultados do processamento bruto (sem a filtragem SVD).
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Inversão de Impedância Acústica Usando Regularização Mista
More LessA inversão sísmica de impedância acústica é uma etapa fundamental para caracterização de reservatório, pois além de fornecer informação quantitativa permite que o dado seja interpretado em termos de propriedades intervalares. Para se atingir um bom resultado, esta inversão tem que caracterizar bem as camadas, ou seja, marcar e posicionar bem as interfaces que as separam e recuperar o valor correto de suas impedâncias. No entanto diferentes modelos de impedância podem atender igualmente bem aos dados. Para solucionar este problema, neste trabalho é proposta o uso simultâneo de dois critérios para regularizar a inversão sísmica de impedância acústica: a minimização da norma L2 do vetor de diferença entre o modelo invertido e o modelo de referência e a minimização da variação total do vetor de parâmetros.
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Aplicação do Método de Polarização Induzida em Área De Disposição de Resíduos Sólidos no Município de Caçapava do Sul - Rs
Authors Mônica Teixeira de Oliveira and César Augusto MoreiraThe geophysics is an effective method for investigation of contaminated areas for the most several types of pollutants, mainly in areas of disposition of residues solid, used in the characterization of contrasts of physical properties in the presence of pollutant substances. This work applies the Polarization Induced method in waste landfill, located in Caçapava do Sul city (RS). The landfill this seated on fractured metamorphic rocks, belonging to the Vacacaí Metamorphic Complex (Neoproterozoic). Were accomplished 8 lines of electric profiling, with spacing of 5m between electrodes and 10 investigation levels. Were also accomplished 83 measures of strike and dip fractures. The geophysical result allows characterize the layer of residues for low cargabilidade
values. Vertical anomalies below the layer of residues are interpreted as fracture zones with possible leachate accumulation, characterized by low cargabilidade values.
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Interferometric Microseism Localization for Multistage Fracture Monitoring
Authors Oleg V. Poliannikov and Alison MalcolmWe show how interferometric methods can be used to improve the location of microseismic events when those events come from several different fractures and are observed from a single well. This is the standard setup for a multi-stage hydraulic fracturing
experiment. Traditionally, in such experiments each event is located separately. Here, we adapt the interferometric approach to the problem of locating events relative to one another and show that this reduces the uncertainty in location estimates. To
completely recover the Green’s function between two events with interferometry requires a 2D array of receivers. When only a single observation well is available, we do not attempt to recover the full Green’s function, but instead perform a partial redatuming of
the data allowing us to reduce the uncertainty in two of the three components of the event location.
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2D Acoustic Full Waveform Tomography of Marine Streamer Data – Data Preparation and Choice of Inversion Strategies
Authors Anna Przebindowska and Andre Kurzmann and Thomas BohlenIn recent years, many synthetic studies have shown the great resolution potential of full waveform tomography. Nevertheless, application to field data is not a common standard yet. This study discusses some of the problems related to the inversion of conventional single sensor marine streamer data in the 2D acoustic approximation. To reconstruct realistic velocity models from the field data additional effort is required to overcome the problem of the local minima and to improve the convergence of the waveform inversion. This mainly concerns the field data preprocessing and the choice of adequate inversion strategies. For a marine field data set we experienced that the application of a 3D-2D correction, a data denoising algorithm, and amplitude corrections are essential preprocessing steps. Furthermore, we investigate synthetic case studies which mimic real acquisition geometries and source signals to evaluate the benefits of different preconditioning approaches and model assumptions.
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Métodos Geoelétricos no Estudo da Contaminação Subterrânea na Área do Cemitério Municipal de Rio Claro - Sp
Authors Walter Malagutti Filho and César Augusto MoreiraThis paper presents and discusses the applying results of electrical resistivity method, using the techniques of vertical electric sounding (VES) and 2D electrical profiling (EP), in the groundwater contamination surveying in the municipal cemetery in Rio Claro - SP, by the necrochorume. This effluent can cause soil and groundwater contamination due to the presence of elements such as nitrate, dissolved metals, viruses and bacteria, with inherent risks to human health. Such contaminant presence in the geological environment has often resulted in lower resistivity values when compared to a non-impacted area. The given characteristic allowed the application of the geophysical techniques proposed for the area, whose results allow the definition of three bands which could be seen in the resistivity profiling sections, leading to the identification of uncontaminated areas - high resistivity - areas with residual contamination - intermediate values - and probably contaminated areas - low resistivity. The groundwater flow direction definition through VES has demonstrated the concomitant migration (necrochorume) to E / NE. The geophysical result has shown correlation with the spatial and temporal cemetery occupation evolution throughout its history, once the low and high resistivity values have been respectively associated to the most ancient and most recent built regions in the cemetery area.
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Hierarchical Strategy for Full Waveform Inversion in the Frequency Domain
Authors R. Brossier, C. Castellanos, B. Dupuy, V. Etienne, Y. Gholami, G. Hu, S. Operto, D. Pageot, V. Prieux, A. Ribodetti and A. Roques and J. VirieuxFull waveform inversion (FWI) of seismic traces recorded at the free surface allows the reconstruction of the physical parameters structure on the underlying medium. Our two main objectives are the reconstruction of multiple classes of parameters on one side and the formulation of both the acoustic and elastic FWI for 3D geometries. A quasi-Newtonian method with a preconditioned L-BFGS algorithm provides scaled gradients of the misfit function for each class of parameter. For onshore applications where body waves and surface waves are jointly inverted, P- and S-wave velocities (VP and VS) must be reconstructed simultaneously using a hierarchical inversion algorithm with two nested levels of data preconditioning. The first one concerns the frequency sampling from low to high frequencies during the inversion procedure. The second one performs a data preconditioning by an exponential decay after the first arrival time. Simultaneous inversion of multiple frequencies rather than successive inversions of single frequencies significantly increases the S/N ratio of the models. For offshore applications where VS can have a minor footprint in the data, a hierarchical approach which first reconstructs VP in the acoustic approximation from the hydrophone component followed by the joint reconstruction of VP and VS from the geophone components can be the approach of choice. Among all the possible minimization criteria, we found that the L1 norm provides the most robust and easy-to-tune criterion as expected for this norm. In particular, successfull reconstruction of VP and VS on a realistic synthetic offshore case study is possible when white noise with outliers has been added to the data. The feasibility of 3D FWI is highly dependent on the efficiency of the seismic modelling. Frequency domain modelling based on direct solver allows one to tackle small-scale problems involving few millions of unknowns at low frequencies. If the seismic modelling engine embeds expensive sourcedependent tasks, source encoding can be used to mitigate the computational burden of multiple-source modelling. However, the source encoding is very sensitive to noise in the framework of efficient frequency-domain FWI where a limited number of frequencies is inverted sequentially. Time-domain modelling for the estimation of harmonic components of the solution is an alternative of choice even for 3D frequency-domain FWI because it allows one to extract an arbitrary number of frequencies at a minimum extra cost, a procedure useful when multiple frequencies are inverted together.
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Mapping the Depths Uncertainty by Comparison Between Multibeam Regular and Crosscheck Lines
Authors Adriano Vieira de Souza and Cláudia Pereira KruegerMaps and graphics are useful to approach the total vertical uncertainty and to evaluate the quality of the hydrographic survey. They are created supported by the Standards for Hydrographic Surveys, from International Hydrographic Organization (2008), and according to the analysis of comparison between multibeam regular and crosscheck lines. Complementary, one will be able to identify some biases that were not removed during the sensors’ calibration. The methodology is based on the results of the analysis of two distinct
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Effective Utilization of a Priori Information in Waveform Decomposition
Authors Georgy Mitrofanov and Viatcheslav PriimenkoWe propose an effective algorithm of using a priori information in the problems of decomposition of seismic signals. The algorithm is realized in two steps. In the first stage there is defined a solution using pseudo a priori information, ensuring stability and uniqueness of the decomposition problem solution. In the second step we define a minimum quantity of available a priori information, which allows obtaining the exact solution of the corresponding problem. The latter is one of the principle moments for the waveform inversion using the real seismic signals.
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O Conceito de Profundidade Efetiva Para a Propagação Acústica em Águas Rasas
More LessIn this work we deal with the problem of the sound propagation in shallow ocean waters. More precisely, we show that the classical ocean acoustic sound propagation models, namely, the Ideal and the Pekeris wave guide models have many similar features. This analysis can be developed if we take in to account the fundamental effect of the evanescent waves propagation into the seabed. The present analogy between the two models is based on the concept of the ocean effective depth related to excitation of the
resonant sound propagation modes in these media.
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How to Resolve Hti Effects from Marine Narrow-Azimuth Data?
More LessMedia exhibiting horizontal transverse isotropy (HTI) are usually associated with an oriented stress field or open fractures. Seismic reflection traveltimes beneath such media vary with the source-receiver azimuth. Firstly, we present a P-wave seismic processing method which simultaneously scans for three characteristic HTI parameters to optimize the moveout correction of narrow azimuth (NAZ) CMP gathers, where multi-azimuth (MAZ) techniques like sectoring are unsuitable. Secondly, we extend the method to detect the fracture direction and to run the automatic analysis on 3D super-gathers. Finally, we apply to a marine multi-source/streamer NAZ data demonstrates the effectiveness of this method.
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Inversion of Marine Controlled Source Electromagnetic Data Using a “Structure”-Based Approach
More LessWe present a fast “structure”-based inversion workflow for interpreting marine Controlled Source Electromagnetic (mCSEM) data, which reconstructs the resistivities and shapes of regions of interest using an inversion algorithm where the model is parameterized using both open and closed “structures”. These a priori information may come from independent measurements (e.g., seismic) or from the results of an image-based inversion workflow. We show that the “structure”-based inversion is capable of quantitatively reconstruct the resistivities and shapes of the regions of interest such as prospective reservoirs to obtain models that are consistent with both the mCSEM and the seismic data and that considerable improvement in resolution is achieved compared to the current state-of-the-art. We illustrate the advantages and drawbacks of the “structure” inversion using both synthetic and real data acquired in the Santos Basin, offshore Brazil.
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Efeito da Amostragem Sobre a Distribuição Espacial de Contaminantes no Solo
Authors Adriana Lemos Porto and José Agnelo SoaresThis work aims a sampling mesh optimization to detect a possible contamination by volatile organic compounds (VOCs) in an area of storage of petroleum products, of around 20 hectares, where 2.361 soil samples were collected, 0.5 m, 1.0 m and 1.5 m deep, at each sampling point. The technique used was the ordinary kriging. The soil samples were analyzed by PID technique (Photon Ionization Detection). The strategy adopted was the analysis of krigged maps obtained from sampling with increasing reduction in the number of samples. Random and sequential sampling were adopted. Krigged maps were constructed with 100%, 75%, 50% and 25% of all available data, at each depth level. Mesh optimization was judged from three parameters: 1) the variography of each krigged map; 2) the RMS difference (Root Mean Square) between each krigged map with data reduction and its corresponding krigged map with 100% of data and; 3) the impacted area, according to each krigged map, considering each contaminated point as that with VOCs concentrations greater than 20 ppm. The sampling reduction tends to increase the RMS difference while the impacted area almost do not alters, being this effect higher for the 1,5 m depth level. Considering that the averaged range, the RMS difference and the impacted area do not change substantially with the reduction in the number of samples, until the limit of 50%, it can be recommended, for economic reasons, the reduction in the number of samples up to this limit.
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