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53rd EAEG Meeting
- Conference date: 26 May 1991 - 30 May 1991
- Location: Florence, Italy
- ISBN: 978-90-73781-03-0
- Published: 28 May 1991
101 - 120 of 315 results
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Anisotropic tomography: Part I, Background
Authors F. Muir, R. Michelena and J. HarrisHydrocarbon reservoirs may often be found in rocks which have quite variable degrees of velocity anisotropy. For example, marine shales may be much more anisotropic than the porous sands associated with them. It follows that anisotropic inversion methods present opportunities for adding useful information, and for preventing image mispositioning due to an overly simplified velocity model.
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A complete waveform inversion
Authors S. C. Singh, M. Dietrich and F. ChapelSeismic waveform inversion consists of minimizing misfit between observed and calculated wavefields. Since this misfit function could be highly non-linear, the only way to be sure of arriving at the global solution is to use a Monte-Carlo search. This is feasible if the number of unknown parameters is not too large. If it is large, as is the case with the seismic inverse problems, it is extremely expensive to use the Monte-Carlo search to optimize for a single misfit function over the whole model space. However, if one uses a number of misfit functions depending up on the sensitivity of the model parameters on the data, one can implement the Monte-Carlo method efficiently. Fortunately, this is possible as the different wavelengths of the model parameters influence the different parts of the data.
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Nonlinear wave form inversion: Application to real data
Authors N. Roxix and A. TarantolaIn principle, nonlinear inversion that finds the distribution of elastic parameters in the Earth whose synthetic data best matches the observed data is an ideal inversion. All seismic wave phenomena such as reflected waves, mode-conversions, refracted waves, transmitted waves, Rayleigh waves etc. are used to obtain the inversion result.
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Petrophysical characterisation of the tertiary for seismic AVO investigations, UK Central North Sea
By J. M. ReillyThe investigation of the variation in seismic Amplitude Versus Offset (AVO) is an increasingly important geophysical technique in the exploration for, and exploitation of, hydrocarbon bearing reservoirs in the Eocene of the U.K. North Sea.
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Simulated annealing based 2-D inversion of reflection seismic data from a carbonate evaporite sequence
Authors J. M. Pedersen, P. D. Vestergaard, K. G. Maver and J. J. MøllerA recently developed inversion method based on a global model optimization method called simulated annealing has been applied to seismic reflection data from a carbonate evaporite sequence onshore in Denmark. The inversion resulted in detailed information about the acoustic impedance profile of a target zone.
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Null spaces in AVO inversion of single interfaces
Authors A. Mazzotti and G. DrufacaThe AVO Inversion and Interpretation are research topics of great interest since they may yield precious information on the elastic parameters of sub-surface targets. This paper is focused on the problem of AVO Inversion of reflections from a single interface. A real data example shows that, even in the simple case of single-interface reflections, a non-unique estimate of the unknown parameters results from a limited-offset data set. The real data are reflections from a low-velocity gas-sand embedded in shales at about 0.7 s traveltime. They produce a classic bright spot that clearly stands out in the stack section.
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Statistical analysis on AVO data
Authors F. Hindlet, Y. Z. Ma and A. G. HaasAmplitude versus offset (avo) interpretations involve a large volume of data. In order to carry out lithologic predictions, the avo data is reduced and simplified by various methods: avo gradients, zero offset amplitudes, even common mid point (cmp) stacks. All these displays are simplifications of a data set that may be influenced by several amplitude factors, such as lithology, tuning effects, correlated noise etc.
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Transformations of volume reflectivity into seismic classifiers
Authors L. Sonneland, O. Barkved, M. Olsen and J. -H. NittebergThe conventional description of the seismic reflection process is an interaction between the incident wavefield. the reflected and transmitted wavefield at a surface boundary. The surface-boundary is defined by the discontinuity surface of some elastic parameter. Dependent on the wavefield and the contrast in elastic parameters a reflection coefficient can be associated with this discontinuity surface. We will refer to this description of the reflection process as "surface reflectivity".
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Evaluation of the ambiguities of seismic inversion
More LessThe problem of acoustic and elastic inversion has been studied by several authors; however, there is still the need to understand completely the ambiguities of the inversion, i.e. to determine the reliability of the solution. The wavefield obtained insonifying the medium that results from the inversion will match the data, but there is no guarantee that the solution proposed is the realone. Besides, another interesting problem, not yet completely solved, is the determination of the resolution of velocity analyses, in presence of an unknown AVO.
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A study of the orientational variation of induced polarization time domain data
Authors F. E. R. Almeida and M. J. S. MatiasThe Induced Polarization Exploration Method has been used intensively in areas of complex geology. In such areas pronounced orientational effects have been observed in resistivity measurements. However the orientational behaviour of induced polarization data has been overlooked.
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Two-modal cole-cole modelling of lP time domain relaxations
Authors R. di Maio and D. PatellaThe dominant method for detecting and delineating native pyrite mineralization is Induced Polarization (lP). In presence of hydrocarbon reservoirs the lP anomalies are thought to he caused by epigenetic pyritization located in a plume over oil fields in a geochemically active environment. In geothermal areas the observed lP anomalies can be associated with pyrite concentrations deposited where sulfur-rich thermal waters interrnix with meteoric waters.
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Transformation of transient decays to continuous apparent resistivity curves, a generalized apparent resistivity definition
By L. SöresIn the interpretation of electromagnetic methods a very simple way of getting rid of the geometric and primary field effects is transforming measured data to apparent resistivities. The main principle for this transformation is generally to determine the resistivity of the substituting homogeneous halfspace.
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Sampo, a new wide-band electromagnetic system
Authors H. Soininen and T. JokinenThe SAMPO(Gefinex 400 S) wide-band electromagnetic (EM) system was designed and constructed during 1986-1988 by Outokumpu Electronics in cooperation with the Geological Survey of Finland.
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Application of an electromagnetic borehole induction logger to groundwater contamination mapping
Authors J. D. McNeill and M. BosnarAlthough electromagnetic induction loggers have been used by the oil industry for many years their sondes are generally unsuitable for groundwater applications, for which the requirements are 1 . Slim probes (monitoring Wells are often only 5 cm in diameter) 2 . Good vertical resolution, so as to resolve thin contaminant plumes, combined with a moderate radial depth of investigation (of the order of 1m) 3 . Low noise and good stability so as to measure background conductivities as low as 1 mS/m (1000 ohmm ) 4 . Linear response with conductivity to about 3000 mS/m (0 .3 ohmm ) 5 . Modest depth of exploration, seldom in excess of 250 m, and lastly 6 . The overall system (including winch and console) must be compact, lightweight and relatively inexpensive .
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Geoelectrical methods in geological mapping - Case histories from the Balaton Highland, Hungary
Authors I. Szilagyi, B. Csatho, G. Csillag and L. KoloszarVarious geoelectrical measurements have been widely used for bauxite and coal prospecting in the Hungarian Transdanubian Central Range for decades. These mountain form a syncline structure, bauxite deposits can be found in their central part in tectonically preformed karstic depressions of resistive Mesozoic -mainly Upper Triassic- carbonates. Conductive Cenozoic formations including coal beds cover the bauxite lenses and the basement. This resistivity contrast and the approximately 1-D model allow to determine the depth to the basement and to distinguish the main structural units in the study area using geoelectrical methods.
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Application of a surface-to-borehole frequency EM method for the detection of deep conductive orebodies: A test study in the Iberian Pyrite belt
Authors B. Bourgeois and J. BernardThrough recent advances in instrumentation, procedures and interpretation, borehole electromagnetic (EM) methods have become of great interest in the exploration of conductive orebodies lying at several hundreds of meters below the surface. In particular, when a borehole has missed a suspected mineralised target, these methods can indicate whether or not the expected conductive body does in fact exist in the vicinity of the borehole, and in certain cases, in which direction it lies. Consequently, these techniques are valuable for mining companies, as they can save a great deal of expensive drilling.
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Fixed source EM two-dimensional modelling using finite elements with quadrature base functions
By P. VallaAlthough 2-D modelling has been widely applied to the magnetotelluric method, few results have been published for frequency EM controlled source methods, besides the work of J. Doherty (1988) using surface integral equations. Because it allows to deal with more complex modeis, the finite element techniques (J.H. Coggon, 1971, L. Rijo, 1977) is the most popular, but its application to controlled source EM requires specific improvements of the algorithm, as shown in the following.
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Two dimensional forward seismic modelling: A case study of lithology prediction of the Zechstein interval, Southern Jylland, Denmark
By K. G. MaverFrom a number of wells and a closely spaced grid of seismic sections, it is possible by two dimensional forward seismic modelling, to predict the lithological framework below what is normally thought of as the seismic resolution limit. It is thus possible to build a geological model with sequence thicknesses of for instance 5 meters at a depth of 1600 meters. The modelling is executed by generation of a synthetic section imaging the seismic section and results in a geological model with a high degree of detail.
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An integrated approach to characterization of Pennsylvanian carbonates
Authors B. J. Rafison and G. A. LivesayA multi-disciplinary integration of seismic techniques was used in this work to characterize carbonate mounds in the Midland Basin. Conventional seismic data and computed seismic attributes were studied for four lines from a 3D seismic survey. The analyses were carried out in six phases (Figure 1): I) Pre-stack AVO Analysis; 2) Geological Analysis ; 3) Petrophysical Analysis; 4) Post-stack Attribute Analysis; 5) Seismic Lithologic/Stratigraphic Modeling ; and 6) Interpretation and documentation of the results.
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Integrated study of an unconsolidated reservoir
Authors J. P. Blangy and S. StrandenesThe following two objectives were sought in order to improve volumetric analyses and to enhance the production of a field from the North Sea: map the gas/oil contact, and its evolution through time as the field is produced, and identify areas of high permeability. By integrating petrophysical data such as empirically derived porosity-permeability relationships for each facies, an attempt is made at characterizing the seismic response of high permeability sands and to identify them seismically.
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