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4th International Congress of the Brazilian Geophysical Society
- Conference date: 20 Aug 1995 - 24 Aug 1995
- Location: Rio de Janero, Brazil
- Published: 20 August 1995
101 - 150 of 330 results
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Resultados de Sondeos MT en Precordillera Oriental (Matagusanos-La Laja-Angaco) Provincia de San Juan, Argentina
Authors F. Ruiz, M. Marmani, N. Ponti, B. Castiglione, D. Trad and J. VenenciaEn la Privincia de San Juan, sobre una línea de 35 km de extensión, a la latitud de Bańos La Laja (31· 20' sur y 68· 30' de longitud oeste), se efectuaron cinco sondeos magnetotelúricos (MT), apoyados con Sondeos EIéctricos Verticales. Abarca desde el Valle de Matagusanos al oeste, transpone la Sierra de Villicum y Ilega al Valle de Tulum al este, límite entre Precordillera y Sierras Pampeanas. Hay dos sondeos geoeléctricos diferenciados, el del oeste. La Laja 5 (LL 5) es más resistivo que el ubicado al este (LL 1), esta condición permite mayor penetración en el primero, según los sensores utilizados. En los tres sondeos interiores de la linea (LL 2, LL 3 y LL 4). las resistividades de los estratos superiores son menores y con mayores variaciones. Los resultados de su interpretación confirman la complejidad estructural del área dada por la geologla regional, donde Precordillera Oriental puede considerarse la zona de transición entre dos unidades tectonoestratigráficas distintas, Precordillera y Sierras Pampeanas.
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Resultados de MT en Cuenca Sedimentaria Profunda Y Area de Contacto de Provincias Geologicas, Mendoza-Argentina
Authors M. Marmani, D. Trad, C. Moyano and B. CastiglioneSe realizaron tres sondeos magnetotelúricos en la cuenca sedimentaria triásica en el norte de la provincia de Mendoza, Argentina (33"Sur - 69'W). Los resuItados muestran el espesor y la resistividad de estratos del subsuelo donde podria explotarse agua subterranea (cuartario), subyacente a éstos, se encuentra el de edad terciaria, del orden de 2000 mt y de baja resistividad. Un estrato conductor determinado a la profundidad promedio de 9.5 km, indicaría la zona de sobrecorrimiento entre las provincias geológicas de la Precordillera y las Sierras Pampeanas determinada en las cercanias por sismica profunda. Este resultado permite la búsqueda de esta capa conductora intercalada a lo largo del frente orogénico de Precordillera hacia ambos sentidos (Norte y Sur) de los sitios estudiados.
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Three-Dimensional EMAP Modelling
Authors C.R. Teixeira Régis and L. RijoEMAP data is modelled in a 3D environment via integral equation. We simulate a two layer earth with one body in the first layer. The presence of this body creates static shift on the data that distorts the response of the embasement. Using the EMAP filter we eliminate the static distortions getting a better evaluation of the basement depth. Different body sizes and dipole line positions were modelled.
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Multidimensional Inversion of Magnetotelluric Data
Authors J. Gouvęa Luiz and L. RijoOne-dimensional (ID) inversion of two-dimensional (2D) magnetotelluric (MT) data was used to find a priori information for 2D inversion. The result reduced both ambiguity and computation time, since only part of the 2D model needed to be really inverted. The technique can be valuable when doing tri-dimensional (3D) inversion, now considered economically impractical because of the large computation time demanded.
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Magnetotelluric Response of I-D Inhomogeneous Earth
By L. RijoTwo simple strategies for computing the one-dimensional magnetotelluric response due to an inhomogeneous conductive (or resistive) earth is implemented in this work. Both schemes use piecewise linear approximation for the earth conductivity profile. One is numerical, based on the finite element method, the other is an analytical approach. They perform egually well as shown by some examples.
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A MT Regional Profile in the Ponta Grossa Arch, Parana Basin, Brazil
Authors P. de Tarso Luiz Menezes, J. de Menezes Travassos and D. BeamishOne of the most important structure in Paraná Basin is the Ponta Grossa Arch. It has been intensely studied, but very little is known about it crustal structure. Two hypothesis are considered. The first one is crustal thinning, and the second one, crustal thickening with intrusion of mantle derived material into the crust. A MT regional profile was carried out to obtain a geoeletric model to the Arch, in order to acess if the region had undergone crustal thinning or thickening. I-D inversion indicate that the crustal thickness in the region is around 40 km.
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Magnetotelluric evidence for a crustal conductor beneath the Serra do Mar - RJ, Brazil
Authors I. Figueiredo and S.L. FontesIn order to understand the tectonic evolution of the Brazilian Southeastern region, it is certainly relevant to study its deep crustal structure. An important parameter in that case is the poorly understood electrical structure of the region. As the first attempt in that matter, a broadband (0.003 - 300Hz) magnetotelluric (MT) field survey has been carried out with the objective of mapping the geoelectric structure along a profile about 70 Km long crossing transversely the 'Serra do Mar' mountain range in Rio de Janeiro State, Brazil. The decomposition of the magnetotelluric impedance tensor (Groom and Bailey, 1989) was applied to the magnetotelluric data to remove existent local galvanic distortion. A new definition of apparent resistivity introduced by Basokur (1994) was used. Preliminary one-dimensional (1-0) modelling results suggest a conductor around 10 Km deep along the entire profile.
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A Simple Approximate Method of TEM Data Analysis
By M.A. MejuAlthough the field instrumentation for the transient electromagnetic (lEM) method Is rapidly evolving as simple. high resolution systems for recording accurate subsurface signatures. the development of matching simple Interpretation schemes for field data has lagged behind and is one reason why the uninitiated finds it more convenient to fall back on alternative geotechnologlcal tools with robust Infield data analysis capabllities. An effective practical method of lEM apparent resistivity data analysis is described In this paper and leans on a recently developed heuristic lEM inversion scheme that can be implemented easily on a hand-held calculator. However. unlike the previous scheme. it has the advantage of enhancing the resistive as well as the conductive parts of the reconstructed resistivity-depth profile leading to improved model identification. A comparison of the proposed scheme and that in current use is made using hydrogeophysical (TEM and VES) survey data recorded in the vicinity of a validation borehole.
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The Treatment of Noise in Electromagnetic Induction Studies - A Review
By A. JungeConductivity models of the earth derived from observations of electromagnetic field variations require reliable data. Thus the treatment of noise is essential. Various different types and sources of noise exist (e.g. Szarka, 1988; Nicols and Morrison, 1988; Adam et al.: 1986), which sometimes are difficult to identify and to separate. The estimation of relations between simultaneous time series of field components observed at different field sites either yield estimates of the conductivity distribution or serve to test the validity of hypothetical conductivity models. The algorithm for deriving the estimate from a given data set is called the estimator. Estimates are generally characterized by a systematic deviation from the true value (bias) and by a random deviation. The optimum choice of the estimator depends on the assumed physical and statistical properties of the observed quantities. Both properties may vary with time (non-stationarity). The dilemma of all processing schemes is not the intrinsic uncertainty of randomly distributed data but the problem of choosing the right estimator. As the processing of the data starts right after their collection visual inspection and subsequent selection of subsets of data is in a wider sense part of the estimation procedure. Time variations in the data which are obviously not related to the assumed induction process may be of various origin. Some aspects of noise characterization will be given, however, the main emphasis is put on the treatment of noise by numerical processing algorithms.
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Time Domain Electromagnetic Modeling in the Media with Smoothed Boundaries
Authors M.B. Rabinovich, L.A. Tabarovsky and K.-M. StrackThe transient electromagnetic (TEM) method is becoming increasingly important in the surface geophysics as well as in airborne and borehole applications. The required quality, resolution and accuracy of interpretation cannot be achieved without considerable modification of modeling concepts. In particular. the simulation of complex geology is largely based on the assumption that transition between different rocks occurs abruptly. Models with sharp boundaries only roughly represent smooth sedimentation processes. They are also inaccurate in simulating transition zones that appear between a borehole and a virgin formation. . Here we present a new time domain forward modeling algorithm where smooth transition zones substitute sharp boundaries. Since the conductivity becomes a continuous function. no boundary conditions are required for differential equations. This considerably simplifies the algorithm, makes it uniform and more stable. Use of the alternating direction implicit finite-difference scheme allows for solving the equations sequentially. Comparison against the existing 2.5-0 solutions with sharp boundaries in the time and frequency domains validates the approach. Improvement of the algorithm stability allows for modeling high conductivity contrasts. A numerical example illustrates modeling in the medium with a gradient conductivity distribution.
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Business Performance and Value of Exploration 3D Seismic
By B. AylorThis paper supplements a similar paper titled "Business Performance and Value of Exploitation 3D Seismic," which was presented at Workshop S, Highlights of the 1994 SEG Development & Production Forum, at the 1994 International SEG Convention in Los Angeles, CA. In that paper the incremental value that 3D seismic adds to Exploitation Ventures was shown to mainly derive from its ability to detect new development locations, locations with higher than normal Initial Flow Rates and improvements in cycle time. This paper analyzes the very extensive data base Amoco has collected which documents 3D seismic's features and its ability to impact exploration activities.
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Ciclicidade de Alta Frequencia (Escala de Milankovitch) e Halocinese em Sedimentos Albianos de Agua Rasa - Plataforma de Regencia do Espirito Santo
Authors I. Riveron de Gruber, J. Moreno and M. CorderoO Albiano (Grupo Barra Nova) na Plataforma de Regęncia, Bacia do Espírito Santo, é representado pela alternância, de caráter cíclico, entre sedimentos siliciclásticos e carbonáticos, depositados em ambiente que varia de continental transicional a marinho raso (plataforma). As evidęncias obtidas até o presente (em parte selecionadas a partir da interpretaçăo de seçőes sísmicas e perfis de raios gama), favorecem a hipótese de geraçăo alocíclica para esta alternância, causada fundamentaImente por oscileçőes eustáticas de alta frequencia (4a e 5a ordens). Nesta escala de alta frequęncia as variaçőes eustáticas tęm sido consideradas, ultimamente, como as principais geradoras de ciclicidade, pois nem a tectono-eustasia, nem os movimentos tectônicos do assoalho da Bacia, atuam em escala tao pequena de tempo (Plint et al., (1992).
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Exploration Drilling in the Madre de Dios Basin, Bolivia: Balancing Potential and Risk in the Amazon Rain Forest
Authors K. T. Conrad and I. SeguraThe Nueva Victoria-Xl wildcat well, located in the Bolivian portion of the Madre de Dios basin, is an excellent example of Mobil's ability to operate successfully in a cost effective manner in the remote, environmentally-sensitive Amazon rain forest. Although commercially unsuccessful, the well was important -- because it 1) tested our applied seismic techniques of seismic inversion and amplitude strength analysis for sand detection and 2) helped accelerate our evaluation of the area. This discussion will review the Nueva Victoria prospect from concept through post-drill analysis with a focus on key technologies that unpacted the interpretation.
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Fast and Stable Two-dimensional Inversion of Magnetotelluric Data
Authors M.S. Zhdanov, P.P de Lugao and O. PortniaguineThe two-dimensional inverse problem in magnetotellurics has been addressed by several authors, the most well-known approaches being developed by de Groot-Hedlin and Constable (1990) and by Smith and Booker (1991) in the search for a smooth model. However, a smooth model can not describe well the real model of the earth, which contains sharp contrasts at the boundaries of structures with different conductivities. In this paper, we present an approach that deals with the model of an arbitrary structure. This approach is based on regularization theory and the quasianalytic calculation of the Frechet derivatives. For the forward solution we use a fast and efficient finite difference formulation to the solution of the Helmholtz equation based on the balance method (Berdichevsky and Zhdanov, 1984). One of the most computationally expensive tasks in the solution of an inverse problem, the calculation of the Frechet derivative matrix is obtained as a solution to simple forward and back substitution of the LU decomposed matrix of coefficients from the forward problem with a different right hand term. The method of steepest descent is utilized to minimize the parametric functional in the search for a stable solution of the two-dimensional magnetotelluric inversion problem. Preliminary results for synthetic data of a conductive body in a resistive host are presented. The method can be applied for the inversion of MT and CSAMT data.
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Magnetotelluric Investigation of the Serra da Cangalha Impact Structure, Brazil
Authors W. Masero, S.L. Fontes and P.A. SchneggIn order to know more about the structural characteristics underneath impact crater a series of magnetotelluric (MT) soundings were carried out in the Serra da Cangalha in Northeast Brazil. The MT data indicate in general a one-dimensional conductivity distribution of the subsurface. Nevertheless, anisotropic MT responses in the period range between 0.01 and 0.1 sec are observed at a few sites, all located in the vicinity of the centre. They reflect a fractured zone, which reaches a depth of a few hundred metres and has an horizontal extension smaller than the total diameter of the structure. Additionally, the identification at all MT sites of a good conducting layer at a depth around 1100 metres, provides an accurate maximum depth for the observed disturbances related to the impact event.
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Geoelectromagnetic investigations in the Central Andes within the Framework of an Integrated Geophysics Project
By H. BrasseDuring the last ten years, electromagnetic investigations have been carried out in the Central Andes to study the conductivity structure of the active subduction zone at the South American western margin. They are part of an integrated geological and geophysical project, which also comprises seismic refraction, reflection and gravity studies, The working area is located north and south of the tropic of Capricorn, between the Chilean town of Antofagasta to the Andean foreland in Bolivia and Argentina.
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Distorçőes em Sondagens Magnetotelúricas na Regiăo da Anomalia Magnétics do Atâintico Sul
Authors A.L. Padilha, S.L.G. Dutra, Í. Vitorello, N.B. Trivedi and J.M. da CostaEfeitos de fonte em estudos MT foram antecipados em latitudes aurorais e equatoriais e tem sido extensivamente discutidos na literatura de forma teórica. Como conclusăo desses estudos foi demostrado que quando esses efeitos săo significativos, a relaçăo entre os campos vetoriais elétrico e magnético é dominada pelas características da fonte.
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Regiăo do Espaço que Mais Influencia em Medidas Eletromagnéticas no Domínio da Frequęncia, Para uma Linha de Corrente Sobre um Semi-Espaço Condutor: Modelo Bidimensional
Authors L. Peixoto de Brito and C.A. DiasQuando se mede uma grandeza física, que corresponde a uma propriedade de um meio, tem-se em geral dificuldade em identificar a porçăo do espaço que é a principal responsavel pelas medidas observadas. Sabe-se que cada porçăo do espaço influęncia diferentemente os resultados observados e que existe uma determinada regiăo, relativamente pequena, responsável pela maior parte do sinal medido.
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Levantamento CSAMT/MT na Regiao do Graben de Baquirivu-Guacu (Aeroporto de Guarulhos)
Authors H. Araújo Franco, J. de Menezes Travassos and J.M.F. MiccolisA regiăo do Aeroporto de Guarulhos está inserida na Bacia Sedimentar de Săo Paulo, no Graben do Baquirivu-Guaçu. Nesta bacia, foi realizado urn perfil CSAMT/MT com o intuito de mapear, em subsuperficie, estruturas associadas ao sistema graben. O Graben do Baquirivu-Guaçu, na regiăo do aeroporto, desenvolveu-se nos xistos do Grupo Săo Roque e Conjunto Paranapiacaba. Além disso, na área, a bacia de Săo Paulo está preenchida pelos sedimentos terciários inconsolidados das formaçőes Resende e Itaquaquecetuba, e por depósitos aluvionares recentes. Os dados obtidos com o método CSAMT/MT demonstram grande potencialidade no mapeamento de estruturas em subsuperficie com fins de prospecçăo de aguas subterrâneas.
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SPAM MK-III Sistema de Aquisiçăo, Processamento e Análise de Dados para Aplicaçăo em Geofisica
Authors D. Nascimento and G. DawesO Departamento de Geologia e Geofísica da Universidade de Edinburgh, Escócia, desenvolve instrumentaçăo eletrônica para aplicaçăo em Metodos Magnetoteluricos desde 1980. Desde a sua última versăo, novos componentes eletrônicos foram desenvolvidos e outros tiveram suas características de ruído, consumo de energia e dimensőes físicas bastante implementadas. Alem disto, novos conceitos em aplicaçőes computacionais se tornaram possíveis. Em funçăo disto, em Outubro/94 uma nova geraçăo desta série de equipamentos foi entregue ŕ comunidade científica e seu empréstimo vem sendo requisitado continuamente através do pool de equipamentos do NERC-National Environment Research Council.
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Principles of Magnetotelluric Interpretation:The World Between the Uniform Half-Space and the Anisotropic 3-D Conductor - A Tutorial
By P. WeideltThe talk will briefly review sucessful concepts and methods of magneto-telluric interpretation, which have been devised for conductivity models of increasing complexity. After introducing the basic assumptions of magnetotellurics, it starts with the induction in a uniform isotropic half-space, which gives rise to the notion of penetration depth and apparent resistivity. Then approximate and exact methods of 1-0 inversion are presented in some detail [including problems of existence and uniqueness and extremal models (best fit, minimum structure, minimal and maximal average conductivity in a given depth range)].
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MT Survey in Canoinhas, Parana Basin, Brazil
Authors M.A. Rodrigues de Ceia, J. Menezes Travassos, F. Morrison and E. NicholsA Magnetotelluric survey was conducted in the Parana Basin near Canoinhas, SC. The survey was designed in such a way that the MT line cut a gravimetric feature characterized by a high to a low Bouguer anomaly. The frequency range spans from 0.001 to 100 Hz. With the results it was possile to produce a clear picture of both the sedimentary cover and the basement.
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A Magnetotelluric Survey Over a Reported Geothermal Anomaly in Southern Portugal: Some Results and Interpretation
Authors A. Correia and F.W. JonesA magnetotelluric (MT) survey was carried out in southern Portugal in a region where a geothermal anomaly with heat flow density (HFD) values of about 200 mW/m2 near its center has been reported to exist. The objectives of the MT survey were to delineate the anomaly in electrical terms and to understand its origin. The area where it occurs is geologically complex and is crossed by two major tectonic features: the Messejana fault and the Ferreira-Ficalho overthrust. To measure the electric and magnetic fields, thirty-four sites were occupied over an area of about 2,500 km2 and, after processing the MT data, electrical resistivity maps for several depths were constructed. Interpretation of the processed data in conjunction with other geophysical information indicates that the HFD values reported for the area of the MT survey do not represent the thermal state of the crust in the region. Low electrical resistivity values are, however, found in zones that coincide at the surface with the Messejana fault and the Ferreira-Ficalho overthrust. This may be an indication that fluid flow occurs within these two tectonic features and may generate local high HFD values near the surface.
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STRUCTUrAL STYLES IN THE SUKESTE BASINS, MEXICO,
Authors G. Garcia-Molina and A. W. BallyThe Yucatan Platform bisects the NW-SE Sierra de Chiapas fold belt of SE Mexico at right angle. The outcropping Sierra de Chiapas involves Mesozoic platform carbonates, but its northwestern subsurface continuation involves mostly Mesozoic basinal and lope facies sediments in the Villahermosa folds and their offshore continuation, the Sonda de Campeche folds. The main decollement level for the folds is a middle Jurassic evaporite sequence. The pre-salt "basement" of the area is poorly defined but estimated o dip from about a depth of 6 km to the north (Campeche offshore) to 13km in the south (Sierra de Chiapas). The fold belt was formed during upper Miocene time and Is characterized by bivergent NW-SE striking folds. The amount of shortening is estimated to e in the . order of 45 km to 65 km. In the onshore and offshore subsurface the folded belt is orthogonally superposed by a late Neogene growth fault system which soles out near the base of the Neogene. This growth fault system developed on the continental slope and intercepted salt diapirs that probably emanated from the core of deep-seated folds. Much of the salt accumulated farther north in the large allochthonous mass of the Campeche salt domes.
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Continuous Magnetotellurics in a Section of the Folded Belt, Neuquen Basin, Argentina
Authors C.E. Cruz, E.E. Kozlowski and R. Fernandez-TomeIn the folded belt northwest of the Neuquen Basin in Argentina, one finds an extensive and continuous volcanic cover where the gathering of even moderately good quality seismic data is difficult. As part of an ongoing exploration program in this area, we have applied a continuous magnetotelluric (MT) profiling technique adding more than 100 km of data, spread over several profiles mostly oriented eastwest. The subsurface resistivity information obtained was associated to known geological information in the area. In this manner we were able to determine the spatial distribution and the thickness variability of the volcanic cover. In addition, very low resistivity units were associated to the fluvial sediments of the Neuquen Group and to the pelitic-evaporitic Rayoso Group. Intermediate values of resistivity are found in areas where the Mendoza Group (largely limestones, black shales and marls) are suspected to have been intruded by Tertiary volcanics. Finally, deeper sectors of the resultant geoelectric image are in close agreement with the overall tectonic behaviour of the basement, at least in this section of the Neuquen Basin. Two dimensional forward model responses generated from geological sections have yielded important similarities to the data obtained.
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Spatial resolution of 3-D downward extrapolation of incomplete data
Authors C.P.A. Wapenaar and W.E.A. RietveldThree-dimensional prestack downward extrapolation of a complete areal survey over a point diffractor yields a circular symmetric resolution function with the main lobe width being approximately 6>/5. In practice, however, the sampling distance of the sources and detectors never fulfils the anti-aliasing condition in the x- as well as in the y-direction. In this paper we analyze an extreme case of spatial undersampling, i.e., we consider a single line of sources along the x-axis and a single line of detectors along the y-axis. For this situation the "matched filter approach" cannot be used. We propose a "true amplitude" operator that is adapted for this acquisition configuration. Although its resolution function is no longer circular symmetric and side lobes occur, its main lobe is only approximately V2 times wider than in the case of full areal acquisition. Hence, even for this extreme case of spatial undersampling, reasonable images can be obtained. This is illustrated with a prestack migration example of physical model data.
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Lessons In 3·D Prestack Depth Migration
By S.A. Levin3-D prestack depth migration was applied by Mobil to data from offshore Nigeria (Figure 1). The use of parallel supercomputers enabled completion of the computations in a bit more than a week, where a high-performance modem workstation would have taken a full year. Ipresent the improvements this processing made for these data and discuss lessons learned from the project. A key observation is that far more time was spent building and validating each 3-D migration velocity than was devoted to the actual imaging computations. Lastly, Iproject industry trends and areas for future improvements and extensions in 3-D prestack imaging and inversion.
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Putting Prestack 3-D Depth Imaging into Production
Authors U. Albertin, W. Wiggins, P. Fowler, S. MacKay, C. Beasley and C. Koeniger3-D prestack depth migration is quickly coming of age. New plays beneath salt-injection features have received much recent attention. However, accurately defining reservoir geometries in more mature areas, such as the North Sea or the Gulf of Suez, also requires 3-D depth imaging technologies. Although prestack 3-D depth imaging is a field of active development, basic elements such as velocity definition and the migration algorithm itself are developed enough to discuss their applications on a field data set.
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A Unified Approach to Seismic Reflection Imaging
Authors M. Tygel, P. Hubral and J. SchleicherGiven a three-dimensional (3-D) seismic record in a fairly arbitrary measurement configuration and assuming a laterally inhomogeneous, isotropic macrovelocity model, a complete theory of amplitudepreserving seismic reflection imaging is formulated. It consists of (a) a Kirchhoff-type weighted diffraction stack to transform (migrate) the seismic data from the record space into the image space, and of (b) a weighted isochrone stack to transform (demigrate) the migrated seismic image back into the record space. Two integrals describe the amplitude aspects of both stacks. They are the key operations upon which the complete amplitude-preserving imaging theory is described.
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Prestack Imaging of 3-D Marine Data Sets
Authors B. Biondi, S. Fornel and G. PalacharlaThe routine application of 3-D prestack migration to the imaging of large-scale 3-D marine surveys requires the development of migration methods that are more computationally efficient than conventional Kirchhoff migration. To prestack image marine data we propose an efficient two-step procedure. The first step transforms the recorded data into effective common-azimuth data by applying a pres tack partial- , migration operator (AMO). The common-azimuth data synthesized by AMO is then imaged in depth using a new commonazimuth downward continuation operator. The main advantage of common-azimuth downward continuation is that the computations are carried out in the original 4-D space, as opposed to the 5-D space that would be required by straightforward application of full-prestack downward continuation. We applied common-azimuth migration to two synthetic data sets: the first data set was generated assuming a vertical velocity gradient, the second one assuming a velocity gradient with components along both the vertical and horizontal axes. Common-azimuth migration correctly imaged the reflectors in both cases.
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A Perspective on 3-D PSDM in Compressive Tectonics
Authors A. Banna, H. Calandra, P. Esquier, J.P. Goetschy, C. Lafond, T. Lecoq, Y. Kerdraon, G. Noual, X. Payre and E. Robein3-D Prestack Depth Migration (3-D PSDM) is becoming the ultimate tool for seismic imaging in complex areas. Now generally available, this technique is becoming more efficient and cheaper to use, thanks to the ever growing power of today's massively parallel supercomputers, the availability of large memory storage and very fast VO capabilities. However, one should bear in mind that 3-D PSDM cannot be used as a black box; in fact, its success depends on three procedures : 1) deriving and updating an appropriate velocity model, 2) performing careful QC at every step during the imaging process, and 3) using a robust method for computing traveltime tables and other Green function attributes. In areas such as the Gulf of Mexico, where the velocities are well-known and varying smoothly, 3-D PSDM does not always need to follow these three steps and can be applied with success in almost a blind way with most algorithms. In more structured areas such as the North Sea, where large horizontal heterogeneities exist in the velocity field, one should be more cautious: the main difficulty there is to derive an appropriate layer-based velocity model. Finally, in very complex areas such as compressive zones, the challenge is still ahead. In those areas, if not used carefully, 3-D PSDM can yield very poor results and is not cost -effective. In this case, we propose to use first an alternative, cheaper 3-D imaging method to evaluate the difficulties involved. If 3-D PSDM is then justified, we recommend following very carefully the three steps mentioned above.
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Interdisciplinary Approach To Maximize Benefits of Prestack Depth Migration
Authors R. Gonzalez and D.E. WillenThe advent of massively parallel computers has made the use of fast, efficient, accurate, 3D prestack depth migration (PSDM) a viable option for the exploration and production of hydrocarbons. However, it is still a significant investment in terms of computer resources, time, money, and personnel to carry out this process. Therefore, we need a methodology to intelligently utilize this new tool and to manage the associated technical risks and expectations. In this paper, we suggest an integrated, staged approach to both predict where it's beneficial, and to anticipate its results within an acceptable confidence level. Thus we avoid the temptation to abuse PSDM as a brute force tool on all imaging problems and instead maximize our potential benefits by knowing how to apply it judiciously.
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The Pace of Change in the Latin American Oil Patch
Authors G.E. Jones and R.L. ConroyThe social, political and economic forces impacting the traditional worldwide energy companies are now, by necessity, being addressed by the Latin American state owned oil companies. The changes brought about by the restructuring have been examined to determine how they may affect the general interdependence within the Western Hemisphere. Available competitively priced energy is, and will continue to be a prime ingredient in the rate of development of the Western Hemisphere trading partners. A country by country evaluation of available production and proven reserves indicates that adequate energy exists to meet the projected Western Hemisphere demands. The defming factor is how well the required restructuring of the oil industry in Latin America is managed by the principal stakeholders in matching the needed technology, fmancial resources and talent to the resource development needs. The current status of change in each country is discussed. State oil monopolies are fmding ways to attract private capital where needed, international oil companies are assuming service roles in some areas and service companies are being required to take new responsibilities and risks. All these changes must be accomplished in a rapidly evolving social, political and business environment.
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Modelaje de Densidad a Partir del Sonico en Litologia Complejas, Subcuenca de Barinas, Venezuela
Authors P. Calderon, D. Flores and Y.M. JordanLa correlación de registros de densidad y sónico de cinco campos de la Subcuenca de Barinas permitió derivar una relación que da resultados consistentemente mejores que los obtenidos con la relación de Gardner. Su aplicación permite derivar valores de densidad mas confiables a partir de la velocidad del sónico para aplicaciones sísmicas y gravimetricas, empleando coeficientes empíricos obtenidos de la correlación de datos de campo con un sistema petrofísico interactivo, que permita discriminar lítologícas a partir del registro de rayos gamma e información de micIeos. Multiplicando por un factor de 1.42 el coeficiente de reflexión a partir de los contrastes de velocidad en la aproximación de Peterson, se toma en cuenta los efectos de la densidad sobre la velocidad. El estudio demuestra la necesidad de modificar las relaciones empíricas de aplicación general, como la de Gardner, cuando se desea obtener resultados mas confiables en cuencas con litologías complejas, como la mencionada.
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Abrupt Onset of Geopressure: An Artifact of Sampling in Multi-Well Composites of Pore Pressure vs. Depth Data
Authors M.L. Greenberg, Z. He, T.K. Kan, L.Y. Kuo and G.J. MitchTwo-dimensional basin models were constructed to study geopressure variations in the Gulf of Mexico. Two transects, 51.7 and 100 km long, were calibrated to pore pressures estimated from sonic logs. 97% and 90% of variance in pressure data from the two transects, respectively, was accounted for with a compaction disequilibrium mechanism for geopressure. Simulated multi-well composites of pressure measurements could be interpreted as indicating vertically abrupt onset of geopressure, however, this is shown to be an artifact of sampling. Multi-well composites of measured pore pressures may best be understood with families of predicted pressure vs. depth profiles that are based on a compaction disequilibrium mechanism.
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Reservoir Characterisation Using Nuclear Geochemical Logs
Authors J.F. Bristow, J.C. Lofts, P.K. Harvey and M.A. LovellAdvances in the technique of nuclear spectroscopy logging have resulted in the ability to determine continuous. in situ abundances of most major rock-forming elements. If the use of log-derived geochemistry is to be applied, widely quality assessment must be improved to cover a wider variety of environments. A case study of a producing well from the Gulf of Mexico exposes some of the problems in the processing of the geochemical data quantitatively assesses the accuracy of that data in comparison to core measurements and examines the validity of an elemental uncertainty estimate. Neutron transport simulation studies show the measurement of Al to be particularly sensitive to borehole environmental effects.
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LWD and Wireline Resistivity Logs: Correction for Time and Temperature Dependent Resistivity
Authors A.Q. Howard, K. Cozzolino, E. Decoster, D. Allen and J. SeydouxThis paper quantifies transient temperature effects on resistivity logs caused by large differences in mud and formation temperature. The differential temperature between formation and circulating borehole fluid can be large, particularly in offshore drilIing for example when several hundreds of meters of casing is exposed to the ocean and acts as a heat sink. The temperature transient gives rise to a thermal invasion profile and translates to a time-dependent monotonic radial resistivity profile, independent of whether the rock is permeable or not. The transient problem includes the heat diffusion transient with charactersitic time of a day or so depending upon the ratio K/(pC) where K (W/(moK)) is the thermal conductivity, p (kg/m3) is the density, and C (J/(kgOK) is the heat capacity of the mineral. The changes in the distribution of water saturation driven by invasion is also time dependent. A Buckley-Leverett model for multi-phase fluid· flow in porous media is used to predict time dependent water saturation Sw(p, t). The resulting radial resistivity profiles are then input into electromagnetic modeling codes to predict the transient apparent resistivity as measured by both LWD and wireline resistivity tools. Numerical results for wellbore environments within normal operating conditions show transient temperature effects alone can cause noticable LWD and wireline curve separation.
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Borehole Seismic Applications Using a Slim Well Seismic Receiver in a Production Enviroment
More LessAs the costs for searching for new oil fields continues to escalate, many companies are evaluating the economics of enhanced recovery methods for their older fields. Therefore, there exists a greater need for high resolution borehole seismic surveys to correlate existing field information with the surface seismic. Using a slim well seismic receiver, it is now possible to acquire these surveys without a drilling rig and without killing the well, saving time and money. The data is acquired inside tubing, inside casing - an environment which has not previously been utilized for borehole seismic surveys. A velocity survey and three different vSP surveys using both impulsive and vibrator sources are shown. These types of surveys acquired inside production tubing, inside casing in producing wells provide enhanced well correlation to surface seismic in areas which had little or no well velocity control. This allows important geophysical information to be acquired were it is needed and when it is needed.
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Well Log Correlation Using Backpropagation Neural Networks
Authors S.M. Luthi, I.D. Bryant and H. Gamero de VillaroelOne of the principal tasks of a production geologist is to correlate well logs in order to construct a reservoir model. This task is not only time consuming, but also difficult at times because of geological complications such as fault cutouts - where entire sections of the geological column are missing in a well due to the offset caused by a fault - or depositional complications such as facies changes, layer pinch-outs etc. Thus this task still poses a great challenge in the field ofpattem recognition. We propose in this paper an approach based on neural networks which learn from the actual patterns observed in one or more key wells, and then apply this "knowledge" to other wells. An implicit assumption to geological correlation of markers is the relative position of these marker to each other, which obey stratigraphic rules, and which we built into the neural network as a depth reference function that penalizes an attempted correlation if the distance to other markers is beyond the expected value.
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Consistent Interpretation of Galvanic and Induction Measurements by Joint Inversion Methods
Authors A.G. Mezzatesta, M.H. Eckard and K.-M. StrackAccurate estimation of residual and movable hydrocarbon requires precise estimation of the formation resistivities in the flushed and uncontaminated zones. The estimation of formation resistivities is commonly done by interpreting the data provided by galvanic and induction devices. Different physical principles govern these devices. In galvanic tools, the voltages being measured are related to the surface charge densities at the interfaces between adjacent media of different resistivities, whereas in induction tools, the voltage induced in a measuring coil results from the magnetic fields due to alternating currents flowing in the surrounding formation.
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Interpretation of a Dipole Shear Anisotropy Log in a Thrust Belt Setting
By M.C. MuellerA dipole shear anisotropy log taken in the region of the northern Oman thrust belt reveals complexity in the stress state and natural fracturing. This complexity would not be predicted from a simple assumption of lithology or formation bounded behavior. Three anisotropic signatures are contained in this dataset: high magnitude and consistent azimuth due to natural fracturing, low magnitude and inconsistent azimuth due to isotropy, and medium to high magnitude and consistent azimuth due to borehole ovality caused by tectonic stress. These three signatures, however, do not correspond to formational or lithologic boundaries in the logged interval.
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Numerical Simulation of Electrical Imaging Tools
Authors C.G. Williams, P.D. Jackson, M.A. Lovell, P.K. Harvey and G. ReeceDownhole electrical resistance images are used routinely to visualise in situ subsurface formations. Their relevance has been demonstrated in studies of lithology, fractures, and sedimentary structures. Interpretation of electrical images is generally based on comparisons with optical images; inferences are made assuming a degree of similarity between the two. This enables the qualitative interpretation of textures and the quantitative mapping of fractures and locally planar features. Numerical modeling of the downhole imaging measurement can provide one route to evaluating both the measurement technique and the models on which interpretation is based. Using relatively simple geometric models we investigate the effects of a selection of resistive and conductive anomalies on the tool response. This enables geological features to be appraised through the careful design of suitable resistivity distributions. Examples are presented which compare a real and synthetic tool response to a turbidite sequence, and demonstrate the simulated response of downhole imaging tools to conductive fractures. The results illustrate possible artefacts that can be generated in the images, but equally support their interpretation at different scales, and in a wide range of environments.
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Using a Multilayer Neural Network and Statistical Techniques to Characterise Oceanic Crust
Authors C.A. Gonçalves, P.K. Harvey and M.A. LovellFormation characterisation requires a multidisciplinary study of geophysical and geological data including a rigorous evaluation of the data precision and accuracy. Another essential aspect of the appraisal of any measurement is the quality assessment and quality control of that data. In this work multivariate statistical techniques and an artificial neural network are used to identify heterogeneities in complex formations and to evaluate the boundaries they generate. The precision and accuracy of the data from different sources are considered here using sample support in the integration of measurements at different scales. We use examples from two wells to show the differences in characterisation obtained with each technique. Multivariate statistical analysis is initially used to group the petrophysical, geophysical and geological parameters extracted from the downhole measurements into distinct geological zones. This technique has the advantage of being quasi-independent of any pre-determined ideas we have about the whole data set, and has proved very reliable. Thus the result obtained here is used as a basis for comparison with that obtained from the neural network. An artificial neural network is used to characterise the different lithological sequences present in each well. Neural networks are relatively new tools and have proved very useful in applications where conventional computing methods are inadequate. We present results for two different environments drilled by the Ocean Drilling Program. Both techniques produce encouraging results with minimal discrepancies between the computed stratigraphies and those observed in recovered core.
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Modelagem Tridimensional Preliminar de Perfis Eletromagneticos Superficie-Poco
Authors A.A.G. Carrasquilla and M.M.M. MartinezNeste trabalho, utilizamos a formulaçăo matemática da equaçăo integral para simular a resposta eletromagnética em perfis de poços de estruturas tridimensionais simples e confinadas. Como fonte indutora usamos uma linha de corrente infinita localizada na superfície, formando assim um arranjo geofísico conhecido como esquema superfície-poço. Nossos resultados mostram que para poços localizados em diferentes posiçőes em relaçăo ŕ estrutura tridimensional, a componente vertical do campo magnético secundário (H) é a que se apresenta com a melhor definiçăo no conjunto de freqüęncias estudado (0.01 até 30 Hz). Dentro deste intervalo, a freqüęncia de 1 Hz focaliza com maior nitidez uma estrutura localizada numa profundidade de 1000 m. Por outro lado, detectamos que as magnitudes do campo foram sempre maiores naqueles poços localizados fora da heterogeneidade, por causa de urn maier efeito galvânico presente neste setor. Finalmente, a presença de um outro corpo tridimensional, com as mesmas dimensőes e afastado de 200 m do primeiro, provoca a adiçăo das respostas no caso de um contraste corpo/rocha encaixante de 0.1 e a subtraçăo das mesmas no caso de um contraste de 10, quando o poço esta localizado equidistantemente a 100 m de ambas as estruturas.
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Utilizaçăo da Geofisica de Poço na Caracterizaçăo de Derrames Basálticos da Formaçăo Serra Geral em Nuporanga - SP
Authors C.A. Mendonça, T. Higashi Yamabe and M. de Souza BolognaDesde a sua origem na decada de 20, com os trabalhos pioneiros dos irmăos Schlumberger, a geofisica de poço tem encontrado na exploraçăo do petróleo seu maior campo de aplicaçăo. Entretanto, outros setores da atividade exploratória nem sempre tęm se beneficiado da tecnoIogia desenvolvida na indústria do petróleo já que utilizam pocos com especificaçőes diferentes e movimentam recursos insuficientes para adquirir tais serviços. Outra dificuldade decorre das investigaçőes rasas serem feitas quase sempre acima do nivel estático (Daniels & Keys, 1990) dificultando a transferencia de tecnologias e de métodos de interpretaçăo desenvolvidos em outros ambientes.
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SEISMIC IMAGING AND ANALYSIS OF SOURCE AND MIGRATION WITHIN AN INTEGRATED HYDROCARBON System Study: Northern Gulf of Mexico Basin
Authors O.P. Gross, K. C. Hood, L. M. Wenger and S. C. HarrisonThe sources for hydrocarbons in young Tertiary reservoirs of the offshore Gulf of Mexico have been enigmatic in the past due to the lack of source rock penetration in offshore drilling. Exxon formed a multidisciplinary team to address source, maturation, and migration in the northern Gulf of Mexico. The study was initiated in a pilot area east of the MissisSippi River Delta where the complete hydrocarbon system can be seismically imaged, then expanded to the west across much of the shelf.and slope.
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Simulaçăo da Detecçăo de Radiaçăo Gama pelo Metodo Monte Carlo
More LessO presente trabalho simula passo a passo, através do método Monte Carlo, a história de fótons e életrons dentro do cristal cintilador NaI(TI), utilizado na composiçăo dos detectores das sondas NGS e LDT, onde ele pode contribuir ou năo para a formaçăo de um pulso.
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Simulaçăo do Processo de Invasăo I-D
Authors K. Cozzolino and A.Q. Howard Jr.O processo de invasăo ocorre em formaçőes permeáveis onde a pressăo dentro do poço e superior a pressăo do fluido da formaçăo. A medida que a invasăo prossegue surge o aparecimento de duas zonas distintas denominadas de zona invadida e zona virgem. A primeira é composta por zona lavada, zona de transiçăo e anulus (Singer & Barber, 1988) e a segunda é denominada de virgem por prezervar as características originals da formaçăo.
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Caracterizaçăo de Reservaterios em Camadas Finas com Sondas de Induçăo Eletromagnética com Arranjo Coplanar de Bobinas (Dipolo Horizontal)
Authors P.R. de Carvalho and O. Prakash VermaApresentaremos um estudo comparativo entre os perfis de induçăo obtidos em um poço vertical com sondas de apenas um par de bobinas (Fig. 01) com arranjo convencional (coaxial: dipolo vertical) e modificado (coplanar: dipolo horizontal) para seqüęncias sedimentares cada vez mais freqüentes em ambientes acumuladores de hidrocarbonetos, que săo os reservatórios em camadas finas (thin beds reservoirs), devido ao progressivo esgotamento dos reservatórios de maiores espessuras. Utilizaremos a técnica de modelagem física em escala reduzida, simulando com a maior fidelidade possível as condiçőes de campo em laboratório.
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Determinacao de Interfaces em Pedis atraves da Rede Neuronal de Hopfield
Authors A.J. Neves Andrade, S. Luthi and C.E. GuerraA análise dos perfis petrofísicos de poço aberto possui um papel de fundamental importância para os estudos geológicos e geofísicos, no que se refere ŕ obtençăo de um maior conhecimento de subsuperfície, bem como para a identificaçă e exploraçăo de depósitos minerais e petrolíferos.
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