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72nd EAGE Conference and Exhibition incorporating SPE EUROPEC 2010
- Conference date: 14 Jun 2010 - 17 Jun 2010
- Location: Barcelona, Spain
- ISBN: 978-90-73781-86-3
- Published: 14 June 2010
81 - 100 of 797 results
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Airborne TDEM by He-filled balloon
More LessPerforming Time Domain Electromagnetic surveys in rugged terrain is a challenging task and as an alternative to mundanely laying out loops or using a helicopter system, a central loop configuration Time Domain EM system has been fitted to a 5m diameter He–filled balloon with a capacity to lift a ~48kg payload. The transmitter and receiver loops have a diameter of 10 and 5m, respectively. The balloon is handled by an operator and 3 assistants and measures while drifting above the topography. The TDEM electronics was custom built by Elta-Geo in Novosibirsk, Russia and records the soundings to a HP IPAQ handheld PC via BlueTooth communication. A maximum transmitter current input of up to 20 Amp is possible using a conventional half sine waveform. One of the areas where the balloon was employed was the N'teisha gold occurrence in Yemen where the gold is situated in narrow shear zones, between 0.5 and 2m wide. Although the shear zones are possibly too narrow to be detected by the TDEM system an interesting conductivity anomaly was detected in the vicinity of the shear zones that still has to be further investigated.
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Shallow Water CSEM Using a Surface-towed Source
Authors D. V. Shantsev, F. Roth, C. Twarz, A. Frisvoll and A. K. NguyenWe address challenges for using controlled-source electromagnetic (CSEM) surveys at small water depths. A novel deployment setup where electrodes of a conventional CSEM source are suspended from two GPS positioned buoys and towed 10 m below the sea surface is proposed. This setup allows better control of the source position and orientation along with improved speed and manoeuvrability as demonstrated by a test survey in the North Sea. The finite-difference 3D modelling code used in data interpretation has been improved by a careful representation of source and receivers allowing an accurate modelling in shallow waters. Modelling results assuming a 2.2 km deep resistor demonstrate that a surface-towed source has essentially the same efficiency in detecting the target as the traditional deep-towed source if the water depth is within a few hundred meters. Additional attenuation of EM fields travelling through the water layer for surface-towing may be compensated by a better knowledge of these fields due to precise control of source position and orientation.
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Case Study – A Towed EM Test at the Peon Discovery in the North Sea
Authors J. M. Mattsson, L. L. Lund, J. L. Lima, F. E. Engelmark and A. M. McKayA newly developed towed EM system has been tested offshore in the North Sea. In this paper we use modeling and inversion to investigate the ability of the towed EM system to detect and characterize a shallow gas discovery. We show that the measured electric field data are of sufficient quality and signal-to-noise ratio for successful detection and inversion of the high resistivity reservoir area including distinction of some of the shallow gas accumulations above the reservoir. 1D inversion in the frequency domain has been performed on individual common mid points (cmps) along a survey line across the reservoir with robust results. The estimated model from the 1D inversion outside the reservoir is used as the 1D background model in 3D modeling. The imposed 3D resistivity model is based on seismic data and interpreted horizons. It is concluded from the 3D modeling that the resistivity values in the reservoir obtained from 1D inversion are lower than those in the refined 3D model which is also supported by the well-log data.
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3D Inversion of Transient EM Data – A Case Study From the Alvheim Field, North Sea
Authors B. A. Hobbs, M. S. Zhdanov, A. Gribenko, A. Paterson, G. Wilson and C. ClarkeWe present a case study leading to the 3D inversion of transient electromagnetic (EM) data for delineating reservoir extent at the Alvheim field in the Norwegian sector of the North Sea. The survey was conducted in July and August 2008 using one method of marine EM surveying, namely a two ship operation and ocean bottom cables. One ship laid a receiver cable with 30 receivers on the sea floor, and the second ship placed a source cable on the sea floor which was used to generate a coded transient signal. The configuration of the source and receiver spread was analogous to 2D seismic acquisition, as the system was rolled along to obtain multi-fold coverage of the subsurface. The survey spanned 20 km, resulting in measurements of 1270 source-receiver locations. The measured electric field for each source-receiver pair was deconvolved for the measured source current to determine the impulse response function. Preliminary inversions were made for each source-receiver pair using a 1D model, and the results were stitched to a 2D image. Having defined a background model, all data were then simultaneously inverted in 3D with focusing regularization. This revealed high resistivity volumes corresponding to the known hydrocarbon-bearing reservoirs of the Alvheim field.
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Inversion of CSEM Data in the Presence of Shallow Resistive Inhomogeneities
Authors L. D. Masnaghetti, M. Watts and L. BornaticiMarine controlled-source EM data can be strongly distorted by shallow conductive (shale diapirs) or resistive (salt) bodies. Interpretation strategies using a priori models and sharp-boundary inversion are discussed.
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2.5D Anisotropic Inversion of Marine CSEM Survey Data – An Example from West Africa
Authors C. J. Ramananjaona and L. M. MacGregorAn example of 2D interpretation of marine CSEM survey data is presented.Following the conclusion of initial 1D modelling of the response measured in line with the source dipole, it is shown that assuming vertical anisotropy of the 2D earth model improves the inversion results signigicantly compared to the isotropic assumption.
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3D Iterative Migration of Marine Controlled–source Electromagnetic Data with Focusing Regularization
Authors M. S. Zhdanov, M. S. Zhdanov, M. Cuma and G. A. WilsonWe present our implementation of an iterative migration algorithm for marine controlled-source electromagnetic (MCSEM) data based on the 3D integral equation method with inhomogeneous background conductivity and focusing regularization with a priori terms. The use of focusing stabilizers makes it possible to recover subsurface models with sharper geoelectric contrasts and boundaries than can be obtained using traditional smooth stabilizers. The method is implemented in a fully parallelized code which makes it practical to run large-scale 3D iterative migration within a day on multi-component, multi-line MCSEM surveys for models with millions of cells. We present a suite of interpretations obtained from different iterative migration scenarios for a synthetic 3D MCSEM survey computed from a very detailed model of stacked anticline structures and reservoir units of the Shtokman gas field in the Russian sector of the Barents Sea.
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Understanding Wave-equation Image-gathers – A Prerequisite for Advanced Post-processing and Velocity Model Building
Authors J. Sirgue, P. Jousselin and F. AudebertWave-Equation Migration has evolved towards a routine tool to generate final stack images but the computation of wave-equation image gathers has not yet spread out in the industry. Nevertheless, provided a good understanding of wave equation gathers’ typology, this prestack information proves to be useful to improve final wave-equation stack images and to build accurate velocity models. In this paper, we explore these new image-gathers, describe their behaviour in terms of true specular events as opposed to artefacts and propose solutions for the applications of these image-gathers in improved imaging and in velocity model building. We first describe what are the image-gathers produced by Wave-Equation Migration. Then we characterize the specular events and the acquisition and processing artefacts they exhibit. On analytical and synthetic image-gathers, we illustrate the artefacts linked with the acquisition truncation. Although feeding tomography with these gathers may require a sophisticated method to recover only the specular part, an adapted mute can prove to be a very efficient in a post-processing workflow. We illustrate on a real 3D subsalt dataset how the use of wave-equation image gathers significantly improves the seismic stack.
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On the Gradient of Wave-equation Migration Velocity Analysis
More LessWave Equation Migration Velocity Analysis (WEMVA) is an image-domain tomography based on a wave-equation propagator (one-way or two-way). Different algorithms for residual calculation, propagation and optimization add varying flavors to WEMVA. The two most popular ways to compute the residual are either using the Differential Semblance Optimization (DSO) or using the Differential Residual Migration (DRM). In this paper, we present relevant theory to understand the difference these two algorithms make in the final gradient calculation. We compare and contrast the two methods with the help of numerical examples. Both methods have some advantages and disadvantages over the other and they should be kept in mind while making the choice. The theory and results presented are based on one-way wave equation migration.
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Differential Semblance Wavefield Tomography Using Extended Images
Authors T. Yang and P. C. SavaWavefield tomography can be formulated based on extended images constructed at discrete locations in the subsurface, i.e. by analysis of extended common-image-point gathers. Such gathers can be built at locations that conform to the geologic structure, thus achieving optimal sampling of the image. The extended CIPs indicate velocity error by defocusing from the origin of the space- and time-lag domain. We can formulate an objective function based on this information by penalizing the image departure from its ideal shape. Such penalty resembles the process used for differential semblance optimization on more conventional common-image-gathers and is characterized by similar robustness and simplicity.
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Local Plane Wave Tomography
Authors D. Lokshtanov, J. K. Lotsberg and E. KurinWe propose a local plane wave tomography where we use the output of a wave equation migration in a consistent manner as input to the tomography. The tomography is based on an efficient layer-stripping inversion of residual moveout of offset slowness gathers after double-square-root migration. The method does not require closeness of the initial model to the true model and it does not involve ray tracing through complex overburden. The results of testing on 2D synthetic FD data from the SEG-EAGE salt model are very promising. In the current work we use split-step-Fourier migration, but it is straightforward to extend the suggested approach to any other migration which provides information on incident and reflected angles at the image point. The considered tomography itself is very fast, but the whole layer-stripping process can be quite time consuming. The method requires at least one new migration for each layer included in the velocity model. Therefore, optimally, it should be used for target oriented velocity inversion in combination with faster approaches for simple overburden. Also, fast and reliable auto-picking of residual moveout is of crucial importance.
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Acoustic Wave Equations for a Virtual Source Shift
More LessThe direct relation between the shape of the wavefield and the source location can provide insights useful for velocity estimation and interpolation. As a result, I derive partial differential equations that relate changes in the wavefield shape to perturbations in the source location, especially along the Earth's surface. These partial differential equations have the same structure as the wave equation with a source function that depends on the background (original source) wavefield. The similarity in form implies that we can use familiar numerical methods to solve the perturbation equations, including finite difference and downward continuation. The solutions of the perturbation equations represent the coefficients of a Taylor's series type expansion for the wavefield. As a result, we can speed up the wavefield calculation as we can approximate the wavefield shape in the vicinity of the original source. The new formula introduces changes to the background wavefield only in the presence of lateral velocity variation or in general terms velocity variations in the perturbation direction. The accuracy of the representation, as demonstrated on the Marmousi model.
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Velocity-independent "tau"-p Moveout in a Layered VTI Medium
Authors L. Casasanta and S. FomelLocal seismic event slopes carry complete information about the structure of the subsurface. This information is enough to accomplish all time-domain imaging steps, without knowing the seismic velocity model. In this paper, we develop a velocity-independent tau-p imaging approach to perform moveout correction in 2D layered VTI media. The effective and interval anisotropic parameters turn into data attributes through the use of slopes and they are directly mappable to the zero-slope traveltime. The tau-p transform is the natural domain for anisotropy parameter estimation in layered media because it simplifies data processing and allows more accurate traveltime modeling and inversion than traditional t-x methods. Synthetic and field data tests show the practical effectiveness of our method.
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Quantifying Structural Uncertainty in Anisotropic Depth Imaging – Gulf of Mexico Case Study
Authors K. Osypov, D. Nichols, Y. Yang, F. Qiao, M. O‘Briain and O. ZdravevaQuantifying structural uncertainty in the process of anisotropic model building is of paramount importance, especially in volumetrics estimation and drilling risk analysis. The modern concept of geophysical model building is based on integration of various data types and constraining by knowledge databases and targeted numerical modeling. Tomographic inversion of common-image-point gathers is the main engine for building the earth model. However, this inversion alone is very ambiguous, especially in the presence of anisotropy. Therefore, it is necessary to add well information and other measurements like electromagnetics and gravity in a simultaneous joint inversion process. Furthermore, geological knowledge, basin and geomechanical modeling, and lithoclassification are important constraints for model building. This novel methodology using uncertainty analysis delivers an entire suite of models that fit all available data equally well, allowing the user to select the most geologically plausible solution. In other words, uncertainty analysis has the capability to provide a new paradigm for model building. A case study for the Walker Ridge area of the Gulf of Mexico is presented in this paper.
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Rapid Salt Model Scenario Testing with Fast Pre-stack Beam Migration
Authors A. Osen, S. K. Foss, M. Rhodes, D. Michel, C. Lojewski and N. EttrichWe develop a system that enable the interpreter and processor to quickly test different salt models and their impact on the seismic image. The migration algorithm is a Parsimonious Kirchhoff-type method.
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Simultaneous Joint Inversion as a Salt Detector in South Gabon Land Exploration
Authors M. Mantovani and T. DugoujardIn the special case of non-Gardner bodies (i.e., salts) inside a standard Gardner background setting, simultaneous joint inversion offers a quantitative approach to QC on interpreted layer boundaries for such a formation. Misplacements or boundary errors in salt horizons result in simultaneous joint inversion velocity and density outputs as inconsistencies that may, therefore, indicate layer geometry corrections to otherwise regular solutions. In the presented case, a boundary of validity between a velocity and resistivity positive and negative correlation is detected by simultaneous joint inversion, and used as salt top/bottom edge indicator. For hydrocarbon exploration in the South Gabon sub-basin, because the seismic method alone was failing in salt and subsalt imaging, Perenco hypothesized that gravity data could be utilized to enhance the imaging of pre-salt seismic reflectors by providing additional input for prestack depth migration. Two vintage 2D seismic lines were reprocessed utilizing the technology simultaneous joint inversion technology.
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Gulf of Mexico Case Study of Localized Anisotropic Tomography with Checkshot
Authors A. V. Bakulin, Y. Liu and O. ZdravevaReliably deriving parameters for anisotropic depth models requires use of borehole information. Localized tomographic inversion attempts to streamline and automate this process by directly incorporating the available well data into conventional reflection tomography. We present a case study from Gulf of Mexico where we conduct local VTI anisotropic tomography using a joint dataset consisting of seismic and checkshot data. Because this area has low structural dip, the results can be compared with more traditional manual 1D layer-stripping inversion. Tomographic inversion for three VTI parameters produces a smooth velocity model that both fits the checkshot traveltimes and flattens all seismic gathers. To regularize tomographic inversion, we apply smoothing operators that are oriented along predominant dips of seismic event and have large lateral extent. The anisotropic profiles derived by tomography and 1D inversion have similar low-frequency components, but differ in finer details. Borehole data require careful conditioning before joint inversion because of potential difference in water velocity between seismic and well surveys. The workflow we present can be applied to calibrating anisotropic parameters in the more general case of 3D models with structural dip and borehole data from deviated wells.
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Introducing A Priori Information in Non-linear Slope Tomography as Applied to the Minagish Survey in Kuwait
Authors P. Guillaume, D. Carotti, A. L. Al-Kandari, A. El-Emam and G. LambaréIntroducing a priori information in velocity model building is certainly one of the most challenging problems in seismic imaging. In this context, non-linear slope tomography, which expresses the velocity model-building problem as a multi-data multi-parameter optimization problem, appears particularly adapted for dealing with various types of constraints. With an application to the land Minagish dataset in Kuwait, we address here the problem of introducing information from wells into the velocity model-building problem. We show how short vertical wavelengths of the velocity at wells, which cannot be estimated by the inversion process, can be incorporated in the tomography update, resulting in a very accurate depth velocity model insuring both fitting at wells and focusing.
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Application of Isotropic PP/PS-stereotomography to a Gas Cloud Context – The Snøhvit Gas Field
Authors M. Alerini, P. Eliasson, S. Nag, M. Buddensiek and C. RavautGas clouds may considerably distort the energy of P-waves travelling through them. This complicates imaging, and particularly velocity estimation. When seismic data cannot be used a priori information may help to estimate a satisfying model. Stereotomography is a flexible tool, but the parameterization of the estimated model by cubic B-splines makes the introduction of a priori information difficult. Here, we use a hybrid model, consisting of a smooth layered model and some smooth variations on top of it. This model parameterization is smooth, allowing the use of paraxial ray tracing, and is at the same time connected to some layers, allowing the introduction of a priori information. By using such a model parameterization, we show that we can significantly improve the estimated results on a 2D dataset, which exhibits a low signal-to-noise ratio of the PP-data underneath a gas cloud at the Snøhvit gas field.
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Multi Directional Residual Curvature Analysis to Improve Seismic Anisotropy Estimations
More Lessom dipping horizons and fault planes (multi directional residual curvature analysis – MDRCA). Real 3D seismic data examples are included.
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