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70th EAGE Conference and Exhibition incorporating SPE EUROPEC 2008
- Conference date: 09 Jun 2008 - 12 Jun 2008
- Location: Rome, Italy
- ISBN: 978-90-73781-53-5
- Published: 09 June 2008
301 - 400 of 556 results
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Towards 3D Spatio-temporal Pore Pressure Stress Coupling Prediction in Reservoirs
Authors J. B. Altmann, B. Müller, M. Tingay, O. Heidbach and T. M. MüllerGeomechanical modelling of a reservoir is important to understand the coupling between pore pressure and stress state in a reservoir. Changes of pore pressure due to injection or depletion implies also a spatio-temporal re-distribution of the stress state. Reservoirs are complex underground storage volumes in which the role of pore pressure stress coupling is not fully understood. To quantify the spatio-temporal evolution of pore pressure stress coupling in complex settings we solve the coupled differential equations of quasi-static poroelasticity with the 3D finite element method. Our modelling results indicate that in the early stage of fluid injection experiments pore pressure stress coupling deviates significantly from a description in the static limit.
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Non-double-couple Mechanisms of Microearthquakes Induced by Hydraulic Fracturing
Authors J. Sileny, L. Eisner, D. P. Hill and F. H. CornetWe have inverted polarity and amplitude information of representative microearthquakes to investigate source mechanisms of seismicity induced by hydraulic fracturing in the Carthage Cotton Valley gas field. With vertical arrays of four and eight geophones in two monitoring wells, respectively, we were able to reliably determine source mechanisms of the strongest events with the best signal-to-noise ratio. Our analysis indicates predominantly non-double-couple source mechanisms with positive volumetric component consistent with opening cracks oriented close to expected hydraulic fracture orientation. Our observations suggest the induced events are directly the result of opening cracks by fluid injection, in contrast to many previous studies where the seismicity is interpreted to be primarily shearing caused by elevated pore pressure in the surrounding rock or associated with shear stresses created at the hydraulic fracture tip.
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Microseismic Signatures of Nonlinear Pore Pressure Diffusion
Authors N. Hummel and T. M. MüllerModelling and analysis of microseismic signatures based on the nonlinear pore pressure diffusion equation in 2D is studied. The nonlinear diffusion equation is solved by using a finite element method on an irregular grid. Following the seismicity-based reservoir characterization approach microseismic earthquakes are triggered and analyzed regarding their spatio-temporal distribution. We find that there are significant differences in the microseismic signatures of linear and nonlinear pore pressure diffusion. It is shown that the microseismic event distribution is governed by two event triggering processes. One process interpreted as a fracturing front changing significantly the fluid transport properties and induces a large number of microearthquakes. Another triggering front that induces only a small number of microearthquakes ahead of the fracturing front. For the Fenton Hill hydrofrac experiment we demonstrate the existence of these nonlinear pore pressure diffusion signatures.
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How Long Should We Observe Microseismicity after Finishing a Borehole Fluid Stimulation?
Authors C. Langenbruch and S. A. ShapiroMicroseismic monitoring of fluid injections from boreholes can be expensive. Thus, it is important to know how long a significant microseismicity can be observed after a stop of the injection. Here we present results of theoretical and numerical modelling of the seismicity rate induced by a fluid injection into a poroelastic medium with pre-existing fractures. We obtain an analytical solution for the post-stimulational seismicity rate and show that the Omori law, which is well known in global Seismology, can be applied as a good approximation. We show how parameters of the Omori law depends on the time elapsed after the injection termination. Our analytical results are in a very well agreement with the numerical modelling.
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Microseismic Monitoring of Reservoir Stimulation - Magnitudes of Induced Seismicity
Authors J. Kummerow, S. A. Shapiro and C. DinskeWe investigate the magnitudes of earthquakes stimulated by fluid injections in boreholes. A model is developed which provides a theoretical basis for observed magnitude distributions. We assume that the process of pore pressure diffusion is responsable for the induced seismicity. Both injection-related parameters as well as the statistics of the pre-existing fracture systems influence the number of earthquakes. This increases with injection pressure and injection time and also with the hydraulic diffusivity and the tectonic activity of the injection site. We test our model with well-documented real datasets and find a good agreement between the observations and the predictions from the model. The presented concept should be helpful for the future design of fluid injection experiment, e.g. in hydrocarbon and geothermic reservoirs, in such a way that the probability of stimulating larger earthquakes is minimized.
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Earthquake Triggered Modifications of Microtremor Signals above and nearby a Hydrocarbon Reservoir in Voitsdorf, Austria
Authors T. T. Nguyen, E. H. Saenger, S. M. Schmalholz and B. ArtmanWe analyse the effects of an earthquake on the spectra of ground motion signals measured above and nearby a known oil and gas reservoir in the vicinity of Voitsdorf, Austria. The comparison of spectra of passive seismic data recorded before, during and after the earthquake indicates that a hydrocarbon reservoir can be stimulated by an earthquake. The stimulation results in an increase of power spectral density (PSD) in the low frequency range (around 3Hz) above the reservoir after the earthquake. An attribute quantifying the energy content of a spectrum, termed PSD-IZ, is increased after the earthquake above the reservoir. Also, the difference in the PSD-IZ values before and after the earthquake is largest above the reservoir. On the other hand, the ratio of the vertical to horizontal (V/H) spectral components of the ground motion signals is not sensitive to the earthquake.
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Back-azimuth Relocation of Monitoring Arrays
Authors A. Bouskova, L. Eisner and J. H. Le CalvezMicroseismic monitoring of hydraulic fracture treatment is commonly carried out without accurate deviation surveys of monitoring boreholes. This can lead to significant errors in the observed azimuths and other parameters of the mapped fracture system geometry derived from microseismic event locations. We have developed a new technique to find the horizontal position of the monitoring geophones. This technique uses relative back-azimuths of perforations at known locations. It is particularly suitable for vertical monitoring boreholes and horizontally distributed perforations. It can be combined with travel-time measurements to achieve improved accuracy of the monitoring array positions relative to the perforations (i.e., injection points).
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3D Tomography Imaging Using Micro-seismic Events
Authors A. A. Al Ramadhan and B. M. HartleyActivities within a hydrocarbon reservoir, such as producing oil or injecting fluid, change in-situ stresses and pore-pressure which consequently cause micro-earthquakes. The induced microseismic events are small earthquakes producing high frequency waves which can be used to give a better understanding of the hydrocarbon reservoir. The microseismic events generated within a hydrocarbon reservoir as a result of the production activities are recorded. Then, the recorded first arrival times are used in a model based inversion process to map a detailed velocity model in the vicinity of the reservoir and hence the heterogeneity within such reservoir. The inversion process is based on a fast 3D finite-difference code using the eikonal equation to generate the synthetic travel times of first arriving seismic events of an initial model; the inversion of large 3D model is practical. The synthetic data are compared with observed data. The model is updated using a gradient based optimization approach until an appropriate match between observed and synthetic data is obtained. The methodology could lead to enhanced understanding and hence efficient management of the hydrocarbon reservoir. This in turn would enhance the understanding of fluid movements resulting in improved petroleum recovery from the reservoir.
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From 3D to 4D Passive Seismic Tomography - The Sub-surface Structure Imaging of the Val d’Agri Region, Southern Italy
Authors L. Valoroso, L. Improta, P. De Gori, R. Di Stefano, L. Chiaraluce and C. CLocal earthquakes (passive seismic) tomography (LET) is a well established tool for the imaging of the sub-surface structure. Alternative to active seismics, the main advantages of using natural sources are the better sounding in deeper portions of the upper crust, the relatively low cost, and the direct availability of S-waves. The main drawback is the achievable model resolution, which is limited by the density of the seismic network and the distribution of elastic sources, rather than the elastic wave frequency. Recently, 4D variations (in space and time) of velocity anomalies have been recognized in active volcanoes (Patanè et al., 2006) and normal faulting systems and ascribed to the medium response to transient geological processes, like dyke intrusions or fluid pressure increase on fault planes. In this paper we show how LET contributes to the imaging of the upper crust in a very attractive region like the Val d’Agri in southern Italy, which hosts both significant oil fields and seismogenic structures. We show that LET allows to improve the definition of the crust structure, at depths larger than those sampled by conventional seismic profiles, and detect the space-time dependency of elastic properties in response to local variations of fluid pressure.
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Non-double-couple Seismic Events Induced by Hydraulic Fracturing Observed from a Linear Array of Receivers
Authors Z. Jechumtálová and L. EisnerWe developed a new method to invert source mechanisms of seismic events observed from a single (vertical or near vertical) array of receivers. This method was tested on a synthetic dataset and applied to a seismic dataset acquired during hydraulic fracturing of a sedimentary formation. We show that a crack-opening seismic event recorded on a single vertical array can be inverted as a pure shear seismic event, indicating that the best fitting shear mechanisms inverted from seismic data acquired along a vertical borehole provide misleading information. We found that the majority of the analyzed source mechanisms of the induced events are not consistent with pure shear faulting as previously thought. The non-shear source mechanisms are required even when seismic noise, location uncertainty, and uncertainty in the attenuation structural model are considered.
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Resolution of Non-double-couple Mechanisms in Microseismic Monitoring - Simulation of HDR Experiment at Soultz, Alsace
By J. SilenyMechanism of microearthquakes observed in induced seismicity (related to mining, hydrofracture experiments in industry etc.) may contain - in addition to the traditional shear slip - also non-double-couple components in the moment tensor. Their resolution can however be difficult taking into account errors in location of the hypocenter and modeling the velocity structure - effects which themselves generate spurious non-double-couple components. We simulate a shear-tensile source (slip vector off the fault plane) in the configuration of the seismic monitoring in the Soultz geothermal facility and explore resolution of the tensile component when various types of hypocenter mislocation and/or mismodeling the velocity structure take place. Effects of deteriorating the quality of the focal sphere coverage by seismic stations as well as absence of S data are also investigated.
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Shear-wave Splitting Analysis of Microearthquake Data Observed with Downhole Seismic Network
By V. OyeThis paper presents a semi-automatic method to analyse shear-wave splitting of microearthquakes. The rotation-correlation method, co-variance matrix method, aspect ratio method and the measure of polarization strength are combined to provide a stable estimate of a semi-automatic estimation of the shear-wave splitting parameters, namely rotation angle of the fast direction and the delay time. The method is then applied to microearthquakes that have been recorded with deep borehole seismometers in the San Andreas Fault Observatory at Depth in combination with data from the High Resolution Seismic Network. Both networks are borehole installations and the sampling rates are relatively high (4000 Hz and 250 Hz, respectively). The abundance of microearthquakes in the magnitude ranges from about -2 to 2 and the good coverage of high quality seismic stations makes it possible to map the anisotropy with respect to the ray paths between sources and receivers. Preliminary results already show significant differences of shear-wave splitting delay times in some areas of the network.
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Determination of S-wave Backazimuth from a Linear Array of Receivers
Authors T. Fischer, L. Eisner, S. Buske and S. ShapiroLocations of seismic events from a linear array of receivers may require determination of slowness vectors of arriving waves. In an isotropic medium, P-waves are polarized along the slowness vector, which enables direct determination of backazimuth (i.e. azimuth of a source at a receiver) from P-wave polarization. In contrast, S-waves are polarized in a plane perpendicular to their slowness vectors, which prevents direct determination of their backazimuth. We have developed a novel technique to determine the slowness vector of S-waves detected in a linear array of receivers in an isotropic medium. We combine the S-wave polarization measurements and the derivative of the S-wave slowness vector along the array to obtain the full slowness vector and backazimuth. The proposed method allows location of seismic events from a single linear array of receivers using only shear waves, which usually have much larger signal-to-noise ratio than P-waves. This technique is not affected by SV-waves, which is shown by a test on a synthetic dataset. We also test the method on two real microseismic datasets from a hydraulic fracturing treatment and show that it outperforms the backazimuth determination from P-waves and from horizontal polarization of S-waves.
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Computation of Kinematic Attributes for Pre-stack Time Migration
Authors G. Lambaré, P. Herrmann, J. P. Touré, E. Suaudeau and D. LecerfIn many areas time imaging still represents the majority of seismic imaging activity in the industry. In this context pre-stack time migration remains a central process. We propose a new approach for estimating various kinematic attributes associated with pre-stack time migrated results. Our approach is based on a kinematic demigration of locally coherent events characterized by their position and dip in common offset time migrated images. The reflection angle, the instantaneous velocity or the geological dip can be estimated with the assumption that time imaging exactly positions laterally the events. In addition, the stretch factor and the "kinematic invariants" (the demigrated facet in the unmigrated domain!) can be recovered accurately independently of the positioning accuracy of time imaging. As opposed to many conventional approaches, the estimation is directly based on the travel time curves used in pre-stack time migration with no assumption regarding the structural dips or the lateral variations of the velocity model. The attributes may be used at various steps of the processing sequence: stretch factor to design de-stretching operators, reflection angle to design partial angle stacks for AVO/AVA studies and kinematic invariants to perform reflection tomography.
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3D Prestack Time Migration by Multipath Summation
Authors T. J. Moser, S. K. Keydar, V. Shtivelman and M. MikenbergA new method for 3D prestack time migration is proposed. The method is a very fast, fully automatic, macromodel-independent technique based on multipath summation of seismic traces. Contrary to the conventional techniques requiring information on migration velocities which are usually estimated by an optimization procedure for traveltime surfaces with maximum coherency, the proposed approach uses a multipath spatial summation over a range of traveltime surfaces. Thus, the multipath summation is implemented as a velocity-independent automatic procedure avoiding the need of time-consuming manual picking of velocities. The method was tested on synthetic and real data. The results of the testing show that the obtained time migration images are usually similar to those obtained using the time-consuming Kirchhoff migration. In some cases of smoothly varying velocities the multipath images may be even superior to the conventional ones.
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Object-oriented Procedures for Imaging of Reservoirs in Complex Environment
Authors V. Tcheverda and V. A. PozdnyakovEast Siberia is one of the very promising areas for oil and gas explorations, but recovery and development of new deposits face essential troubles due to extremely complicated geological environment. For the first of all these troubles are connected with low self-descriptiveness of images computed with the help of regular migration procedures, both post- and prestack. In order to improve their quality and to increase resolving ability we propose and develop so called object-oriented technology of seismic processing. Its main advantage is ability to handle together reflected and scattered/diffracted waves both individually and in combination. This provides detailed knowledge of different features of the target geological objects - reflected waves describe their regular constituents (interfaces), while scattered/diffracted ones are responsible for fine properties like distribution of cracks/fractures/cavities.
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Comparison of Average Methods to Build a Velocity Model from Sonic Log
Authors M. B. C. Silva and A. StovasThe aim of this work is to compare different average methods (with two and three terms) used to build a velocity model from sonic logs and verify which method is more accurate with respect to the phase factor of overall propagator. The traveltime parameters are computed and analyzed for each averaging model. The analysis is illustrated on a well log data from the North Sea.
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Quasi-acoustic Approximations for qSV-waves in a Transversely Isotropic Medium
Authors Y. V. Roganov and A. StovasWe derive "quasi-acoustic" approximations valid for qSV-wave propagation in transversely isotropic and tilted transversely isotropic media by applying approximation extracted from acoustic approximation for qP-wave propagation. One approximation has the same accuracy as qP-wave acoustic approximation for the same range of horizontal slowness, the other approximations are wide-angle approximations.
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Anisotropic 2.5D - 3C Finite-difference Modeling
Authors A. Kostyukevych, N. Marmalevskyi, Y. Roganov and V. TulchinskyAn example of 2.5D modeling for an anisotropic medium, by using a second order accurate central finite-difference scheme throughout the variables of the three distributed grids is demonstrated. We show an example of HTI anisotropic medium modeling for quasi-primary qP, and fast qS1 and slow qS2 quasi- shear waves. The time of calculation with 2.5 modeling is considerably lower than in the case of a 3D modeling for the same medium.
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Large-scale Massively Parallel Finite-difference Time-domain Modelling for Realistic Simulations of 3D Seismic Surveys
Authors R. Streich, A. Osen, S. K. Foss and S. Pannetier-LescoffitWith rapid advances in parallel computing facilities, modelling of field-scale 3D seismic surveys is coming within reach. To this end, we have extended and optimized a finite-difference (FD) time-domain modelling code. The code combines state-of-the-art algorithms for the (visco ) acoustic, (visco ) elastic and anisotropic FD modelling kernels with high-order optimized FD operators for controlling numerical dispersion, and various source models. We have included perfectly-matched layer absorbing boundaries, explicit free surfaces, intrinsic attenuation, and precomputation for limiting the modelling aperture at the initial time steps. Parallelization has been extended to allow for both domain decomposition and simultaneous simulation of numerous shots. Our performance evaluation has shown that this code enables modelling 3D surveys within days on an industrial-size cluster. We present the improvements this code offers and demonstrate its performance for realistic modelling cases.
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Parallel Seismic Modeling Using the Pseudospectral Method on Marmousi2
Authors P. L. Stoffa and C. ChuWe present parallel pseudospectral seismic modeling results for the elastic, variable-density acoustic and constant-density acoustic wave equations on the Marmousi2 model. We focus on the parallel implementation on distributed-memory systems to explore the potentials of the pseudospectral method for large scale seismic simulations. The numerical results demonstrate superior accuracy of the pseudospectral method and good scalability on a Linux cluster, which suggest the promising extensions of the current work to solving 3D seismic modeling and imaging problems on supercomputers.
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An Efficient Parallel Chebyshev Pseudo-spectral Method for Large Scale 3D Seismic Forward Modelling
Authors P. Danecek and G. SerianiIn this work we propose an efficient and scalable parallel Chebyshev pseudo-spectral method for large scale 3D seismic forward modelling. The Multidomain Block Decomposition method divides the whole computational domain into a large number of subdomains. On these subdomains the pseudospectral Chebyshev operator is applied locally. Thereby the global character of a pseudospectral operator is avoided and the method makes efficient and scalable implementations possible. The overlap implicitly ensures continuity of the field variables without complicated coupling schemes. We implement a parallel version of the method in order to obtain sufficient high frequencies. The method and the implementation are tested with various setups and the scalability is demonstrated.
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Finite-difference Simulation of Seismic Waves in Multiscale Media
Authors V. A. Tcheverda, J. Guilbot, V. Khaidukov, E. Landa and G. ReshetovaNew numerical method for large-scale computer simulation of seismic waves in multiscale heterogeneous media is developed. As multiscale hereafter we mean media with heterogeneities of extremely different scales. In the paper some specific multiscale medium is dealt with – karst intrusions within thin layered package. Karsts are produced as a result of interaction of water (surface or ground) with underlying bedrock, often limestone or marble. This causes very complicated, extremely irregular structure of karst intrusions - as a rule they form conjunction of local heterogeneities of different scales and very complicated shape. In order to be able to perform reliable finite-difference simulation of seismic wave’s propagation through these structures we develop finite-difference techniques based on grids with local spatial refinement within karstic areas.
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A 3D Parsimonious Finite-volume Frequency-domain Method for Elastic Wave Modelling
Authors V. Etienne, R. Brossier, J. Virieux and S. OpertoWe present the finite-volume (FV or P0 Galerkin Discontinuous) formulation applied to the 3D visco-elastic wave equation in the frequency domain. This work is motivated within the framework of global offset seismic imaging by full waveform inversion. Concerning the direct problem, the FV formulation leads to the resolution of a large and sparse system of linear equations. This system can be solved with a direct solver particuraly suitable to tomographic applications since only one matrix factorization is performed per frequency for all the right hand terms (i.e. the sources). On the other hand, direct solvers require large amount of RAM and therefore restrict the possible field of realistic applications. The memory complexity of the proposed method implies reduced size models spanning over several wavelengths. In order to push back this limitation, the use of a higher order of interpolation, as Pk Galerkin Discontinuous, should decrease the discretization step allowing coarser meshes leading to a possible managing situation. Furthermore, the use of a domain decomposition method might reduce significantly the memory requirements of the FV frequency domain approach.
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Double Porosity Biot Theory Extension
Authors M. A. Barsottelli-Botelho and F. H. CabreraReservoir stones are generally a porous medium, heterogeneous, fractured and saturated with oil, gas or water. The analysis of wave propagation on these media is carried out using the Biot-Gassman theory for porous media and an extended Biot's theory for fractured porous medium. One way to model fractures is using the concept of double porosity, understanding it as a system of three phases: solid, fluid in the pores and fluid in the fractures. This work shows a macroscopical or phenomena-logical description of seismic wave propagation on fractured porous media. From the movement and constitutive equations of the double porosity system and as a natural generalization of Biot's Theory we obtain the dispersion relations as analytical expressions of the three P waves and an S wave by the way of a six order polynomial for the P waves and a second order polynomial for the S wave. In this research the stress-particle velocity scheme is used, writing it as a first order system of differential equations with the advantage of higher precision in its numerical implementation.
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3D Modeling of Acoustic Green's Function in Layered Media with Diffracting Edges
Authors M. Ayzenberg, A. Aizenberg, H. B. Helle, K. Klem-Musatov, J. Pajchel and B. UrsinWe present the results of 3D acoustic Green's function modeling using the multiple tip wave superposition method (MTWSM) in a layered medium with interfaces containing diffracting edges. The MTWSM algorithm is based on an explicit representation of the scattered wavefield as the superposition of events multiply reflected/transmitted in accordance to their wavecodes. Each event is computed using high-frequency uniform asymptotic approximations of the Kirchhoff-type surface integral by superposition of tip-diffracted waves and convolution-type reflection and transmission operators in the form of effective coefficients. We show that the analytical technique is capable of reproducing complex wave phenomena such as caustics, edge diffractions and head waves at curved interfaces. We also compare the wavefield modeled by the MTWSM algorithm with that of asymptotic ray theory, diffraction theory and finite-differences
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Efficient Seismic Modelling Using Finite Differences and Ray Tracing
Authors J. Miksat and T. M. MüllerRay tracing and hybrid modelling techniques such as the combination of dynamic ray tracing and finite difference (FD) solvers are standard tools in reservoir modelling. However, accurate wave amplitudes using dynamic ray tracing are difficult to obtain whereas 3D FD simulations are too computer memory consuming. When the spatial distribution of elastic properties in earth models can be approximated by a 2.5D heterogeneous structure, we propose a modelling strategy that combines 2D FD simulations with kinematic ray tracing. We demonstrate the accuracy of our modelling approach with help of two numerical examples.
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Accelerating Seismic Computations on FPGAs – From the Perspective of Number Representations
Authors H. Fu, W. Osborne, R. G. Clapp and O. PellConventional seismic processing is performed on a CPU with 32 or 64-bit precision for all operations. In certain cases, using a reduced precision produces equivalent result within acceptable tolerances. However, as CPUs do not support configurable bit-widths, reducing precision brings no benefits to performance. In contrast, Field Programmable Gate Arrays (FPGA) enable application specific number representations. With hardware support for reconfigurable number format and bit-width, reduced precision can greatly decrease the area cost and I/O bandwidth of the design, thus multiplying the performance with concurrent processing cores on an FPGA. In this paper, we present a tool to determine the most appropriate number format for a given seismic application. We demonstrate the methodology on a source-receiver downward continuous based migration. The acceleration device is a Maxeler Technologies MAX-1 card containing a Xilinx Virtex-4 FPGA and connects to a PC over a PCI Express bus. Using optimized number representations, we can implement two concurrent processing cores on the FPGA, and achieve a speedup of 14 times compared to a Intel Xeon 1.86GHz CPU. Moreover, with sufficient bandwidth between the CPU and FPGA, we show that a further increase to 48x speedup is possible.
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Parallel Computing Optimization for Processing the Seismic Data
Authors V. G. Tulchinsky, O. K. Iushchenko and R. A. IushchenkoEfficiency of explicitly parallel data processing programs is sometimes restricted by bottlenecks like file storage bandwidth or network capacity. An approach to the calculation time prediction based on few program execution parameter estimates is proposed. It can be used to select the best processor number for a specific computation, to optimize hardware architecture of a parallel computer for a set of typical tasks, to balance choice of hardware components for better real productivity under a budget limitation.
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Design Space Analysis for the Acoustic Wave Equation Implementation on FPGA Circuits
Authors T. Nemeth, O. Pell, J. Stefani and R. ErgasSeismic modeling and data processing operates on gigabyte to terabyte-size datasets and involves large CPU clusters running applications for extended periods of time. Accelerating these applications would make a big impact and the emerging new co-processor hardware architectures need to be carefully tested for this purpose. One of these new architectures is based on the reconfigurable circuit field-programmable gate arrays (FPGA). These circuits offer potentially large speedups of applications, but to succeed, the FPGA implementation of algorithms need to be carefully evaluated to discover the tradeoffs in compute capability, memory capacity and memory bandwidth. In many cases, taking the existing software implementation of an algorithm and mapping it to reconfigurable circuits does not result in the fastest implementation on FPGA. Careful evaluation of the various implementation strategies that we call design space analysis serves as an important tool to successfully implement an algorithm on FPGAs. In this abstract we provide a short design space analysis for a 3D acoustic forward modeling algorithm.
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Elastic Wave Modeling Using the Cell-based Finite Differences in Anisotropic Media
Authors S. C. Lim, H. Y. Lee, D. J. Min, B. D. Kwon and H. S. YooSince seismic anisotropic features are often found in geological structures, we need to describe anisotropic features in seismic modeling and inversion. Although a number of modeling algorithms were developed to address seismic anisotropy, we still need to develop a simple but accurate modeling algorithm to simulate geological-scale models. For such a modeling algorithm for anisotropic media, we extend a time-domain finite-difference method based on the cell-based grid set to anisotropic media. Considering that the cell-based finite-difference method only employs displacements and its accuracy is verified for isotropic Lamb's problem, we can expect that our anisotropic modeling algorithm can also achieve computational efficiency as well as accuracy. Since any interpolations are not needed in our algorithm, we can also simulate a model whose material properties abruptly change. In order to reduce artificial reflections originating from the outer boundaries of a model, we slightly modify Higdon's absorbing boundary conditions. Numerical examples show that our modeling algorithm can properly address anisotropic features. To qualitatively analyze the accuracy of our algorithm, we need to compare numerical solutions with analytic solutions and also apply it to complicated models.
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Prestack Seismic Signal Enhancement by Dip Detection and Dip Selection
Authors N. Gulunay, G. Ajlani, A. Leveque and J. HoldenHistorically, 3D land data has been noisy when compared to 3D marine data. Recently we have witnessed the advent of high density 3D land recordings with folds in excess of 400 that help to reduce noise on the final images. There are many surveys where gathers for such high fold data are so noisy to allow any prestack work. Furthermore noisy, low fold, mostly old vintage 3D data is still a common occurrence on interpreter's desk. To enhance signal on such noisy gathers as well as on their stacks is the task that this paper aims to address.
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Uniform Discrete Curvelet Transform for Seismic Processing
Authors H. Chauris and T. T. NguyenA novel implementation of the discrete curvelet transform is proposed in this work. The transform is based on the Fast Fourier Transform (FFT) and has the same order of complexity as the FFT. The discrete curvelet functions are defined by a parameterized family of smooth windowed functions that are 2-pi periodic and form a partition of unity. The transform is named the Uniform Discrete Curvelet Transform (UDCT) because the centers of the curvelet functions at each resolution are located on a uniform grid. The forward and inverse transforms form a tight frame, in the sense that they are the exact transpose of each other. The novel discrete transform has several advantages over existing transforms, such as lower redundancy, hierarchical data structure, ease of implementation and possible extension to N dimension. Finally, we present a simple initial application of the UDCT in sparseness constraint seismic data interpolation to recover missing traces.
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Spectral Decomposition of Seismic Data via Wigner-ville Distribution and Chirplet Transform
By R. ZhangSpectral decomposition has proven to be a very effective tool for seismic interpretation. Wavelet transform has been used as a new way to do spectral decomposition, but the basic element used to do the transform is stationary. The frequency does not change with time. This is in contrast to the actual seismic data. I introduce chirplet transform and combine it with Wigner-Ville distribution to do spectral decomposition. In a chirplet, the frequency changes with time. With this method, many high resolution results can be achieved, and additional useful information is obtained.
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Recognizing Apparent P-wave Azimuthal Velocity Anisotropy Caused by Isotropic Lateral Velocity Variations
By E. L. JennerAzimuthal variation in traveltimes is becoming increasingly used as a tool for reservoir characterization and fracture detection. One issue in interpreting the results of azimuthal velocity analyses is distinguishing intrinsic azimuthal anisotropy from apparent anisotropy caused by lateral velocity variations. In an endeavor to differentiate between the two cases, I model discrete overburden lateral velocity variations in an isotropic medium. This paper illustrates the impact of those lateral variations on azimuthal velocity analyses through several models and a field data example. In general I conclude that the spatial patterns observed in the apparent velocity anisotropy are very distinctive with obvious correlations between ellipse parameters (fast and slow velocities and azimuth of the fast velocity). These patterns can also be expected to correlate between RMS and interval attributes, something that is unlikely to occur within truly azimuthally anisotropic media.
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Time-lapse Tomographic Inversion Using a Gaussian Parameterization of the Velocity Changes
Authors A. K. Evensen and M. LandrøChanges in acoustic velocity due to production of hydrocarbons may occur in reservoir or overburden layers. These changes may be detected as time shifts in time lapse seismic data. To map these velocity changes we propose using a least squares tomographic inversion algorithm utilizing pre stack time shifts as input. The velocity changes are modeled as Gaussian functions in two dimensions, thus limiting the amount of parameters to six per Gaussian. Ray tracing is used for the forward modeling. The inversion is carried out using a iterative re-weighted least squares algorithm, and several synthetic data experiments have successfully been conducted on both simple cases with one Gaussian anomaly and more complex cases with and without noise added to the data.
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Gather Flattening Based on Event Tracking for Each Time Sample
Authors N. Gulunay, M. Magesan and H. RoendeThere are times seismic gathers show abnormal Residual Move Out (RMO) which is difficult to correct by curve based RMO scan methods. This paper illustrates a general gather flattening method that is based on event tracking to correct such gathers.
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Robust Residual Gather Flattening
Authors F. Gamar-Sadat, N. Gulunay, H. Hoeber, M. Dyce, C. McKenzie and D. WhitcombeWe present a robust residual gather flattening technique based on trace correlations, which time-aligns coherent events across offsets or angles. We show how gathers to which we apply this method following migration and residual moveout corrections yield more coherent stacks, cleaner AVO results as well as more reliable velocity estimates.
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Improved Time Delay Estimation
Authors E. Zabihi Naeini and H. HoeberWe compare cross-correlation based time delay estimation to two alternative algorithms: a time-domain least squares Taylor method and a time-domain based bi-coherence analysis. For short-window seismic analysis and uncorrelated noise the Taylor method performs best. This is demonstrated on synthetics and with a North Sea 4D data example for which we find a significant reduction in NRMS (13%) with the improved algorithm, when compared to cross-correlation time-alignment.
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3D Mathematical Modeling of Electromagnetic Fields in the Presence of Hydrocarbon Pool
Authors P. A. Dubinin, N. B. Volkova, A. S. Goriunov, E. S. Kiseliov and T. A. MylnikovaThe task of both IP effect and electromagnetic phenomena related to media conductivity cojoint study by means of the time-and-frequency high resolution electrical survey (HRES-IP) technology has been solved.On the basis of 3D modeling, substantiation has been obtained of hydrocarbon deposits appearence within the range of frequency-time electrical survey parameters. A geoelectrical model of the oil deposit has been suggested and comparison of modeling calculations with the observed practical results has been performed. It has been demonstrated that increase of the induced polarization in the OWC vicinity and resistivity anisotropy of the layer which contains the deposit is a direct search property of oil reservoir
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Integral Equation Modeling for 3D Seabed Logging Applications
Authors A. Moradi Tehrani and E. SlobA fast iterative approach to full vector three-dimensional forward scattering problems is discussed, where the unknown object is an assumed reservoir in the layered earth where conductivity and permittivity are different from the known background medium. Since this problem involves a large number of unknowns, it has to be solved effectively and efficiently so that the results can be obtained in a timely manner. To achieve this goal, approximations can give us reasonable results. Therefore Born approximation and Extended Born approximation are implemented and compared with the full solution of the CG-FFT method. The forward modeling is based on a domain integral equation formulated in terms of the electric contrast source. Integral Equation (IE) solver has several advantages such as separation of incident and scattered field which gives us a more accurate result, furthermore IE method requires the discretization of the domain of the reservoir only and not the entire of modeling domain, in addition because of the unique solution to the domain equation, the method is suitable for inverse modeling.
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An Approximate Solution to the Electromagnetic Problem for Two Half Spaces
Authors D. Vázquez Anzola, W. A. Mulder and E. C. SlobWe present analytical approximations for the computation of the diffusive electromagnetic field generated by a current source at the interface between two homogeneous half spaces. We obtain different expressions for the near and the far field. The method is applicable to arbitrarily large resistivity contrasts and is very fast. It can be useful for the validation of computer modelling codes and for the computation of the primary background field when a primary-secondary formulation is used.
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Applying Essentially Non-oscillatory Interpolation to CSEM Modelling
Authors M. Wirianto, W. A. Mulder and E. C. SlobModelling and inversion of Controlled-Source Electro-Magnetics requires accurate interpolation near the resistivity contrasts, where the field component perpendicular to the interface that separates the two resistivities is discontinuous. Simple linear interpolation may produce large errors in that case. Here, we propose to use the essentially non-oscillatory piecewise polynomial interpolation that was designed for piecewise smooth functions that contain discontinuities or have discontinuous first or higher derivatives. The scheme uses a non-linear adaptive algorithm to choose the smoothest stencil. Some examples are presented to illustrate the behaviour of this scheme.
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Processing of Full Azimuth, High-density Land Acquisition - 3D Irharen, Algeria
By L. BovetUsing a square spread of closely-spaced receiver stations, a full azimuth dense 3D survey was recorded in 2005-06 over the Irharen gas discovery in Algeria. A basic square-spread geometry was chosen for reasons of azimuthal coverage. The processing had to deal with a huge amount of data and encountered the classical land processing issues of statics and amplitudes. An innovative processing sequence was developed to take into account the multi-azimuth distribution. The high density acquisition provided a good spatial sampling of the subsurface and allowed high density velocity picking to determine 2nd and 4th order velocity fields. The parallel processing and interpretation saved considerable time by producing several fast-track products and intermediate inversion cubes, necessary to answer operational constraints. This approach gave very encouraging qualitative results in terms of fractured reservoir definition.
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Imaging and Mapping of Hydrocarbon Traps in Areas of Complex Salt Tectonics in Ukraine - Case Studies
Authors O. M. Tiapkina, Z. Y. Voitsytski, G. D. Sydorenko and T. V. ParkhomenkoSeismic data interpretation in areas of complex salt tectonics in Ukraine suffers from the shortcomings of 2D data acquisition technique and out-of-date time-domain imaging. With numerous examples from onshore Ukraine, we demonstrate how high-quality depth processing in combination with 3D seismic acquisition techniques can improve both imaging of prospective features in the vicinity of salt domes and exploration decision making.
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Depth Imaging Sequence on the Volve Node Dataset
Authors B. M. Traub, M. Alerini, C. Ravaut and E. DuveneckData quality is an important issue in seismic data processing. In this context ocean bottom nodes are particularly interesting. 4-component nodes are plugged in the sea bottom by a remote operating vehicle and record the seismic vector wave field. As depth imaging and in particular the background velocity estimation is a difficult task, the processing of such data is usually performed by conventional time sequences based on crude approximations. Therefore the full potential of these data is not used. We propose and apply here a prestack depth imaging sequence which aims at exploiting fully the high quality of these data. This sequence combines multiple attenuation, background macro-model estimation and prestack depth migration in the angle domain. In addition, it requires very little human interaction. The feasibility of the work flow is shown on a 2D line from the Volve Node data set (North Sea).
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Depth Imaging of OBS Reflection Data with Wavefield Separation
Authors E. Asakawa, S. Mizohata, S. Tsujimoto, H. Mikada and A. NishizawaWe propose a depth imaging approach for OBS (Ocean Bottom Seismometer) reflection data in active-source structural surveys using wavefield separation and PSDM (Prestack Depth Migration). Wide-angle and long-offset OBS data includes a lot of valuable signals, not only reflection but refraction. The data will contain upgoing and downgoing waves because it is recorded on sea bottom. Therefore, even considering just reflections, the wavefields is contaminated with various modes of waves that degrade the quality of the depth image. In our OBS survey, the spacing of OBS locations is 3-5 km and typical water depth is more than 2km. The sparse spacing and ultra deep sea water means that signal-to-noise (S/N) ratio enhancement by signal stacking is not expected. We investigate the characteristics of OBS wavefield and separate it into four categories which include primary and multiple reflections. Each of them has similar features in the signal, and this approach can enhance the S/N ratio. Applying adequate PSDM to the four separated wavefields, we obtain the four depth images without contamination of the different type of wavefields. We apply the proposed approach to field data and demonstrate its effectiveness to enhance the final depth image quality with high reliability.
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Analysis of GPR Data for Snow Water Equivalent
By A. GodioGPR permits to map over large the thickness and density of snow pack: these parameters are required when the snow water equivalent has to be estimated. The results of GPR , conducted on a slope in the Alps in Northern Italy, are discussed. The data acquisition along several profiles was repeated in different period of the winter season. The GPR data were calibrated with direct observations of density and thickness of the snow cover: this led to estimate the average wave velocity of the snow pack. When the snow is in dry conditions, the wave velocity values can be converted in density values using simple mixing rules. The good accuracy in the estimate of the average density and snow pack thickness by combining electromagnetic data permits to assess the effectiveness of the approach for the snow water equivalent analysis.
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Shallow Geophysical Techniques During Underwater Gas Pipeline Inspection
Authors V. P. Lisin, Y. P. Bezrodnykh and I. A. DolgovThe methods and results of the monitoring of the trunk gas pipeline underwater transitions, connecting the largest West Siberian gas deposits with the European part of Russia and Western Europe, are considered. The conditions of the pipeline position under the bottom of a water barrier and their impact on the bottom soil mass movements on the pipeline conditions were investigated, and failure areas were identified. Based on the obtained data, a short-term forecast of the bottom soil displacement dynamics was made and recommendations for preventive maintenance and repairs were prepared.
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Evaluation of the Influence of Vinasse Diposal on Soil by DC-resistivity Data Analysis
The production of each liter of ethanol generates thirteen liters of a brown liquid waste, known as vinasse, which inappropriate disposal represents one of the biggest problems for the alcohol industry due to its elevated pollutant potential. Its chemical composition and its large available amount turns the vinasse into an alternative nutrients supplier for soils in sugarcane crops. However, this practice has great potential of pollution in both groundwater and superficial water resources, and excessive use of vinasse could change the soil behavior. The need of viable methods of detecting and evaluating such environmental contamination by vinasse has made geophysical methods to be valuable tools, thanks to their non invasive characteristics and low cost investigation. This paper reports the use of the DC-resistivity method for the evaluation of the influence of the disposal of large amounts of vinasse on soil in a storage tank. Four resistivity profiles were carried out across the study area, using the dipole-dipole array with 10m of electrode spacing. The geophysical surveys allowed us to conclude that there are lower resistivity values inside the tank compared to uncontaminated areas, showing that the method is a appropriate tool to map the soil influence caused by vinasse.
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Influences of Moisture Content in Geoelectric Monitoring of Underground Oil Contamination
Authors F. Chitea, P. Georgescu, D. Ioane and V. NiculescuVertical Electrical Soundings (VES) were carried out on a long profile close to the Petrobrazi Refinery (Romania) in order to evaluate geophysical possibilities of monitoring the oil-contaminated plume. The profile was located almost perpendicular to the strike direction of the contaminated plume (NW-SE). The first geoelectric campaign was made in a dry period (June 2007) and the second one in a wet period (November 2007). The interpretation of apparent resistivity sections was made considering several 1D models. In spite of the influences on the field measurements of the climatic conditions (rapid changes from hot & dry to wet conditions) the geoelectrical monitoring of the oil contamination plume was still possible.
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Geoelectrical Detection of Oil-contaminated Plumes in the Vicinity of Ploiesti Refineries, Romania
Authors D. Ioane, P. Georgescu, F. Chitea, A. Diaconu and B. NiculescuThe contamination of the environment (topsoil, shallow sediments and aquifers) with oil contaminants resulted during more then seven decades in areas of the refineries located at the outskirts of Ploiesti city, Romania. Geoelectrical resistivity methods (VES and ERT) performed on profiles located in the vicinity of Petrobrazi and Petrotel refineries proved to be the most effective in locating the underground contamination by high resistivity anomalies. The high resistivity anomalies associated with the oil-contaminated plumes are locally interrupted and also characterised by variations in intensity. Such aspects are interpreted to be associated with inhomogeneities in compaction of the geological formations that host the aquifer and the floating oil contaminant. Local changes in the shallow geological structure affect the degree of underground contamination and induce supplementary difficulties in the interpretation of geoelectric results.
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Heavy Metal Treatment Using for Agents for Acid Mine Drainage
By J. C. ChoiThis study investigates As, Ni, Zn, Cd, and Pb removal onto minerals, waste materials, iron coated sand from abandined acid mine drainage. Natural materials or waste products with a high capacity for heavy metals can be obtained and employed of with little cost. For investigating the neutralization capacity, the change in pH, Eh, and EC as a function of time was quantified. Of the adsorbing agents, concrete showed that the high neutralization efficiency for AMD and maintained the pH value above 11. The adsorption of Fe and Mn by the samples was influenced by compositions of adsorbing agents. The experimental results of desorption revealed that when the adsorption or precipitation occurs, there was no significant releasing from all adsorbing agents. In this study, the results suggest that concrete can be used successfully in the treatment of acid mine drainages with mixed metal-contaminated wastes
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Urban Geophysical Investigations in Ukraine
Authors O. Menshov and A. SukhoradaEconomic power and world social political significance of modern countries interengage with developed industry and production complex. The powerful megapolises are often the centers of heavy industry. The necessity to conduct ecological investigations in these centers is evident.Urban geophysics is a part of ecological geophysics, which deals with the ecological situation at the territories of big industrial cities and megapolises, as well as with spatial distributions of dangerous chemical compounds, determining the sources of these pollutions. A number of geophysical methods and approaches are aimed at investigating the ecosystem of megapolis and the forms of its pollution as well as at studying physical and chemical fields, created due to natural and artificial sources in the process of human multifunctional activity. Magnetic investigations play the basic role in the urban geophysical study. Currently, we have received only primary results of the urban geophysical investigation. Among the four basic components of megapolis ecological system, there are optimally useful objects, which absorb basic forms of the city pollution. They are: soils, biota, snow, air. We will present examples of the soil investigations of different parts of Ukrainian capital - Kyiv and the main industrial center of Ukraine - Donetsk.
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Newer Results of Environmental Analysis Using TAU-trasformation of Time-domain IP Data
Authors E. Turai, M. Dobroka, E. Takacs and T. OrmosThe paper shortly presents the TAU-Transform of the Time-Domain IP curves and the approximate solution of the TAU-Transform using inverse theory. A method for estimation of the type and the value of soil contaminations will be introduced. The calculation of contamination type comes from the time constant value of the time constant spectra. The value estimation of the soil contamination is based on the waited amplitude values of the time constant spectra In this paper we show two new interpretation results measured over a slag ash site (Tiszapalkonya - 2004) and near a communal waste site (Berekböszörmény - 2007). The analysis of the IP data measured over the ash sludge site shows that relatively higher specific resistivity is connected with relatively higher polarizability in slag ash body, and the types of the polarization are membrane and redox.
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Regional Mapping of the Average Shear Wave Velocity for Geotechnical Applications - An Example in North-eastern Italy
Authors R. G. Francese, A. Bondesan, S. Busoni and N. WardellWe utilised the Refraction Microtremors (ReMi) seismic technique to conduct a regional survey targeting the average shear wave velocity in the uppermost 30 m in the Province of Treviso in north-eastern Italy. The ReMi technique was chosen because of its cost effectiveness as the field operations can be easily conducted by single person The data acquisition grid was optimised using a multidisciplinary approach. Each VS recording station was precisely located based on a high-resolution depositional model of the entire venetian plain. A total of 250 recording stations, scattered over an area of 2000 square km, were collected in a time window of a month in the early 2006. Data processing faced some problems as in some cases spectral quality was not satisfying and the dispersion curve could not be identified with an adequate degree of confidence. A percentage of 10% of the collected were discarded. The final VS map showed a surprisingly correlation with the alluvial and fluvio-glacial depositional model acting during late Pleistocene and Holocene. The ReMi technique and more generally the surface wave based techniques proved to be a valuable tool for the regional mapping of average values of VS in the near surface strata.
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Seismic Prediction Ahead of Tunnel Construction Using Rayleigh-waves
Authors S. J. Jetschny, T. B. Bohlen and D. D. N. De NilTo increase safety and efficiency of tunnel constructions, online seismic exploration ahead of a tunnel can become a valuable tool. We developed a new forward looking seismic imaging technique e.g. to determine weak and water bearing zones ahead of the constructions. Our approach is based on the excitation and registration of tunnel surface-waves. These waves are excited at the tunnel face behind the cutter head of a tunnel boring machine and travel into drilling direction. Arriving at the front face they generate body-waves propagating further ahead. Reflected S-waves are back-converted into tunnel surface-waves.
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Approximate Computation of the 3D Seismic Response of Small-size Buried Objects
Authors M. Dietrich, O. Magnin, P. Franek and P. MoczoWe present an approximate method to compute the three-dimensional seismic response of small-size solid objects of arbitrary shape buried in the subsurface. The computation method relies on a representation of the objects by a set of closely spaced point scatterers simulating weak material perturbations in terms of density and P- and S-wave velocities. The method is illustrated with a parallelepiped structure embedded in an elastic half space, whose theoretical seismic response is compared to finite difference computations. The results obtained with the two computation techniques show that the proposed approach can accurately reproduce the main features of the seismic response of buried objects, provided that the following conditions are fulfilled: i) 5 to 10 scatterers are required per wavelength, ii) the contrasts in density and P- and S-wave velocities between the buried objects and surrounding medium must be moderate, and iii) the size of the objects considered must be small enough to ensure that the propagation time through the objects remains accurate.
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Integrated Geophysical Approach for the Detection of Underground Voids in a Construction Site
Authors C. Orfanos, G. Apostolopoulos, G. Amolochitis and K. LeontarakisDuring the excavation for the construction of a multi-store building in the centre of Athens, underground voids were encountered in a compact limestone with karstic zones. Some voids are full of loose material. Three geophysical surveys (GPR, ERT and microgravity) applied for a quick detection of the underground. Analyzing the results of each method, for the detection of voids, GPR is a quick method to start locating areas of interest, with the resistivity tomography defining all features of the underground with the correct positioning if a 3D approach is applied. The identification of a void is better applied with a microgravity survey correlating with the results of the previous surveys. In order to diminish the equivalence in the geophysical models, to eliminate the artifacts and generally to improve the reliability and the accuracy of geophysical interpretation, an overall geophysical approach was followed using the three different geophysical methods. Karstic zones with loose material and possibly filled or small opened voids inside the compact rock were outlined. Specific areas were outlined, where all three geophysical methods indicate that opened voids possibly exist.
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Monitoring of Engineering-geological Conditions Along Area of the Surface Bedding Underground
Authors V. V. Prylukov, S. P. Levashov, I. N. Korchagin, N. A. Yakymchuk and D. N. BozhezhaThe results of the integrated geophysical investigations of engineering-geological conditions along the area on acting branch of the surface bedding underground are given. It is shown that the complex of geophysical express-methods of the forming short-pulsed electromagnetic field (FSPEF), vertical electric-resonance sounding (VERS), as well as seismic-acoustic and georadar sounding can effectively and operatively to solve specific engineering-geological problems during new line of surface bedding underground construction. The regular monitoring of engineering-geological condition of the environment can be realized also along area of already acting metro line. The method of seismic-acoustic sounding is effective at the solving of a problem of an estimation of quality of tunnels fastening by cementing solutions in a vertical cross-section. The character of cementing solutions penetration in a ground on the area is determined confidently according to data of survey by a FSPEF method. The sites of the increased moistening of soil and the zones of a filtration of underground water streams are allocated and mapped reliably by a FSPEF method survey. The depths of bedding of moistening sites of a ground and zones of a filtration of water streams are confidently fixed by VERS method.
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Influence of Ensemble Geostatistics on Production History Matching and Prediction at New Wells
Authors E. Peters, O. Leeuwenburgh and R. G. HaneaAutomatic history matching techniques such as the Ensemble Kalman Filter (EnKF) have been shown to provide reliable results for matching and prediction at existing wells. It is much less clear if the prediction outside of existing wells improves as a result of history matching. The amount of information in production data on the spatial distribution of properties is limited, thus the prior information derived from well logs and seismic data is likely to be important for the predictions outside of existing wells. To test this, a twin experiment was done in a fluvial system dominated by channel belts. The facies distribution of the synthetic truth was simulated with a process based model. Three different cases were tested: the position of the channel belts is perfectly known, is approximately known from seismic data and is not known except in the wells. The EnKF was used to estimate the distribution of the permeability and porosity. The resulting models were used to predict production at existing wells and at new wells not used in the history match. The results showed that the EnKF always improved the predictions at existing wells, but was often not able to improve the prediction at the new wells.
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Time-lapse Seismic Reservoir Characterisation to Aid History Matching
Authors L. M. Barens, P. Berthet and P. D. PratA seismic reservoir characterisation is method is explained and used to build a 3D facies model for the deep offshore Girassol field, Angola. This 3D seismic facies model is updated using time-lapse attributes derived from 2 repeat seismic surveys. During the update, non-reservoir cells that proved to have shown production induced impedance changes are reassigned to a most probable reservoir facies. Using the updated facies model in history matching, the breakthrough time of a injector/producer pair is significantly improved when compared to the initial reservoir model.
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Incorporating Crossing Conjugate Faults in Reservoir Models
Authors K. S. Hoffman, J. W. Neave and E. H. NilsenCrossing conjugate faults are difficult to incorporate into an earth model due to complex fault interactions and the limitations of fault modeling methods to handle these fault relationships. Y or lambda faults often require special handling, not only in the fault modeling stage but also when creating the horizon model and the reservoir grid. Correctly modeling half-Y or half-lambda faults - created by overlapping convergent or divergent conjugate faults - is even more problematic. These faults have truncation characteristics of Y or lambda faults along part of their length, but are not truncated in other sections. Previous methods of fault modeling, based on pillars or on binary trees, do not provide the flexibility necessary for modeling the complex truncations present in these situations. Our method not only provides a simple solution to modeling Y and lambda faults, but also to the more complex problem of the half-Y or half-lambda, including fault modeling, horizon modeling, and reservoir gridding. The inclusion of these faults in the framework not only results in a more accurate representation of the structure, but also improves the subsequent workflow. Volume calculations, well planning, reservoir gridding, and property modeling all benefit from the improved structural framework.
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Investigation Up-scaled Petrophysical Measurements Using Several Micro-level FOV Petrographic Images on Burgan 3rd Sand
Authors Y. W. Al-Mehanna, W. H. A. Al-Bazzaz and A. R. Al-FarhanThis paper will present all results and discussion of Burgan 3rd sandstone reservoir, part of its pore level micro-analyses and characterizations. One rock fragment, situated at a depth of 4785 feet, is selected for the analysis. After sample preparation, several images are captured with a Scanning Electron Microscope (SEM) at different depths of magnification. According to Al-Bazzaz and Al-Mehanna (2007), a process, where measuring petrographic images into petrophysical parameters such as porosity, permeability, MHR and measuring sand-grain sizes at their native selected spaces, is documented. For the purpose of this study, the pore networks as well as the sand-grain particles are studied in space-domains between a lower bound of 50 µm and an upper bound of 2 mm. The ultimate goal for this study, where consistent data is measured and delineated from different field-of-view (FOV), is to investigate up-scaling porosity, matrix permeability, MHR and grain-size at micro-levels. Another objective of this study is to develop a simple statistical model of these measured petrophysical parameters obtained from up-scaling different field-of-view (FOV) scale sizes. The proposed sample size scale starts with 600X-(FOV) (adequate for recognizing features at 50 µm-scale size), then gradually escalate to 450X-(FOV) (recognizing features at 100 µm-scale size), 300X-(FOV) (recognizing features at 200 µm-scale size), 100X-(FOV) (recognizing features at 500 µm-scale size), and finally, 40X-(FOV) (recognizing features at 2 mm-scale size).
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Seismic Measurements of Fluid Viscosity in HTI Media - A Numerical Modeling Analysis
Authors Z. Qian, M. Chapman and X. LiWe study the effects of fluids saturated HTI medium on elastic properties and seismic measurements and the possibility of discriminating oil-water saturations with them though numerical modeling. The results based on rock physics modelling demonstrate Thomsen anisotropy parameters and the seismic measurements for P- and PS-wave data are sensitive to fluid viscosity and can be used to monitor fluid substitutions in producing reservoirs.
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A Local Linear Neurofuzzy Model for the Prediction of Permeability from Well-log Data in Carbonate Reservoirs
Authors Y. Beiraghdar, M. Nabi-Bidhendi and C. LucasIn this paper a new approach for the prediction of permeability based on recently developed neurofuzzy interpretation of locally linear models, which have led to the introduction of intuitive incremental learning algorithm called locally linear model tree (LOLIMOT) is presented. The incremental learning algorithm initializes the model with an optimal linear least squares estimation and automatically increases the number of neurons in each epoch. The model is optimized for the number of neurons to avoid overfitting and to provide maximum generalization by considering the error index of validation sets during training. The effectiveness of the methodology is demonstrated with a case study in one of the carbonate reservoirs of Iran. Special core analysis from one well that located in the center of the field provide the data for the learning task. Core permeability and well log data from second well provide the basis for model validation. Numerical simulation results show that the neurofuzzy model is more accurate than the conventional multilinear regression analyses (MRA) for the prediction of permeability.
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The Porosity Estimation by Using an Adaptive Neuro-fuzzy Model from Well-log Data
Authors Z. O. K. Khaxar, M. N. B. Nabi-Bidhendi and M. K. H. HafiziPorosity is one of fundamental rock properties which relates to amount of fluid contained in reservoirs. In uncored intervals and wells to heterogeneous formations, porosity estimation from conventional well logs will be a difficult and complex task by statistical approaches. Intelligent computing approaches have been successfully used for rock characterization. It has been proved that the combined approaches such as neuro-fuzzy lead to better results. We used an Adaptive Neuro-Fuzzy Inference System (ANFIS) approach for estimating porosity. ANFIS is a neuro-fuzzy model that implements a Takagi-Sugeno fuzzy inference system and has a five layered architecture. The first hidden layer is for fuzzification of the input variables while T-norm operators are deployed in the second hidden layer to compute the rule antecedent part. The third hidden layer normalizes the rule strengths and is followed by the fourth hidden layer where the consequent parameters of the rule are determined. The output layer computes the overall output as the summation of all incoming signals. ANFIS uses backpropagation learning to determine premise parameters, and least-mean square estimation to determine the consequent parameters. In the study, one well was used for training and checking and another well for testing.
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Electrokinetic Reflection and Transmission at a Fluid/Porous-medium Interface
Authors M. D. Schakel and D. M. J. SmeuldersWhen a seismic or electromagnetic wave propagates through layered, fluid saturated porous media, conversion to electromagnetic or seismic energy takes place. Its existence can be attributed to the electric double layer near the grain surfaces of porous media saturated by an electrolyte. The conversion mechanism involves reflection- and transmission coefficients which have been formulated by several authors for porous-porous and air-porous interfaces. Here we present electrokinetic reflection- and transmission coefficients at a fluid/porous-medium boundary for an incident P-wave from the fluid. Results for seismic scattering is consistent with previous literature. The seismoelectric conversion is on the order 10E(-10).
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A Calculation Procedure of Shear-wave Velocity Estimation
Authors L. Zhang, X. Y. Yin and Y. H. WangShear wave velocity is an important parameter in seismic exploration. In this paper, we try to find a new procedure to calculate shear-wave velocity. (1) Using the well-log and laboratory data, we predict wave velocity based on Xu-White model which combines Gassmann equation with Kuster-Toksoz theory. It is not only considers the effects of rock matrix and fluid inclusions, but also the porosity and pores' shapes. (2) In consideration of the uncertainties which come with input model parameters, measurements of well-logging data and laboratory data and those from the model predication itself, this paper proposed to use Vp and density log curves as the control curves inverse the truethfulness modulus curves, for optimizing the predict result of shear wave velocity. (3) Finally, in view of prestack seismic data including most geological information, such as prestack CDP gather, the more authoritative shear wave velocity can be obtained by prestack elastic wave inversing. The application of the evaluation flow mentioned above was used in a project of Shengli oilfield which has achieved a good result in predicting both the reservoir and the fluid in it.
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Seismic Attribute-based Evaluation of Anisotropic Piezosensitivity Theory
Authors R. Ciz, T. Lokajicek, V. Rudajev and S. A. ShapiroUnderstanding the effect of stress and pore pressure on seismic velocities is important for overpressure prediction and for 4D reflection seismic interpretation. Shapiro and Kaselow (2005) developed an anisotropic piezosensitivity theory describing elastic moduli of rocks and their anisotropy as a nonlinear function of confining and pore stresses. Here, this approach is applied to the rocks being transversely isotropic (TI) in unloaded state. In the case of hydrostatic loading of the TI rock sample there are only two independent shear components of a compliance tensor related to the shear deformation. Developed theory and expression for the TI rocks was tested on the experimental data. The theory developed for seismic velocities is compared with a response of amplitude- and frequency-dependent seismic attributes derived from the experimental waveforms.
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Prediction of Geological Heterogeneities in Coal Seams Using Seismic Technique
Authors M. G. Tirkel, V. A. Antsiferov and A. I. KompanetsThe results of underground seismics application to predict geological heterogeneities in coal seams in order to increase the rates of coal production and to improve mine safety are presented. Developed original seismic procedures and processing of the acquired data enable to identify and trace zones of increased fracturing and faulting of coal-rock mass as well as changes in lithology of the bottom and top of coal seams.
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Full Waveform Seismic Modelling for Gas Hydrate Studies
Authors S. Rajput, N. K. Thakur and A. JoshiWe simulate the seismic response for different models with varying elastic properties to produce a set of plausible seismic signatures for Bottom Simulating Reflectors (BSRs) in different conditions. These simulations are driven by a set of elastic wave equations. We find that the seismic amplitude is very sensitive to the gas hydrates in the host rock. The spatial structure of the reservoir affects the seismic reflection: A thinly-layered reservoir produces noticeably different amplitude strength than a massive reservoir with the same hydrate volume. A model for the occurrence of Double Bottom Simulating Reflector (DBSR) is proposed and validated by real data.
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Geostatistical Integration of Gravity and Magnetotelluric Data
Authors J. S. Park, G. Park, D. O. Kim, S. Oh, H. S. Lee and B. D. KwonWe have studied the feasibility of using the geostatistical approach to enhance the analysis of sparsely obtained MT (magnetotelluric) data by combining them with gravity data. The use of geostatistics to integrate MT data with gravity data was attempted. To evaluate the feasibility of this approach, the interrelationship between the geological boundary and the density distribution was studied. The conversion of the corrected density distribution to a more sensitive geological boundary by maximizing the correlation coefficient between the distributions of resistivity yielded MT inversion and density distributions under the assumption that boundary locations have a high correlation. Next, the correlation between the resistivity structure from MT data and the density structure from gravity inversion was analyzed. Afterwards, the density distribution was converted into the resistivity distribution to yield local mean values for the residual analysis of the resistivity structure. Then, using this method, models and field data from a real intrusive igneous area were tested. The results of the model test agreed well with those of the real model. Moreover, with the real field data, the geostatistical approach used herein demonstrated relatively satisfactory results.
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Integration of Seismic and Electromagnetic Methods to Define Regional Faults
Authors A. V. Antsiferov and V. V. TumanovOn the basis of integration of seismic by CMP reflection method and electromagnetic method for recording the Earth’s natural pulsed magnetic field (NPEMF) we could characterize a regional fault.
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Detrital and Diagenetic Illite Identified from Crystallite Thickness Measurements - A Tool for Paleo-temperature Estimate
Authors L. Aldega and S. CorradoIllite crystals in siliciclastic sediments are heterogeneous assemblages of detrital material coming from various source rocks and, at paleotemperatures >70°C, of superimposed diagenetic modification in the parent sediment. Only diagenetic rather than detrital phases should be used to estimate paleo-temperatures and to constrain the burial history of sedimentary successions that range in alteration intensity from diagenesis to low-grade metamorphism. The relative proportions of 2M1 detrital illite and diagenetic 1Md + 1M illite were distinguished by a combined analysis of crystal-size distribution and illite polytype quantification. It was found that the proportions of 1Md + 1M and 2M1 illite could be determined from crystallite thickness measurements by unmixing measured crystallite thickness distributions using theoretical and calculated lognormal and/or asymptotic distributions. In addition, quantitative polytype analysis was carried out using RockJock software for comparison. The two techniques gave comparable results which indicate that the unmixing method permits one to calculate the proportion of illite polytypes and, therefore, the proportion of 2M1 detrital illite, from crystallite thickness measurements that should not be used for paleotemperature evaluation.
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Reaction Transport Modelling - Simulation of Reflux and Convection Dolomitization Models in an Isolated Carbonate Platfom
Authors O. Borromeo, P. Ronchi, A. Consonni, A. Battistelli and C. GeloniThe new approach in diagenetic studies is to integrate the classical investigative techniques with the reactive transport modelling (RTM) in order to better define the conceptual models in terms of mass balance, kinetic and thermodynamic constrains. Two synsedimentary dolomitization processes (thermal convection and mesohaline reflux) in an isolated carbonate bank (Paleozoic) were tested applying the software ‘Toughreact’ Thermal convection was able to produce a dolomitized wedge in the bank margin area, the final geometry of which was greatly affected by the permeability assumptions and particularly by the Kv/Kh ratio. A mesoaline reflux circulation was established in the bank lagoon with salinity slightly higher than normal sea water (4%) producing a poorly efficient replacement. Sensitivities on the lagoon salinity suggest that when salinity is lower than about 4,5 % the thermal convection is still active in the bank margin and a more complex dolomitization pattern develops influenced by both hydrological patterns. The tests highlight that the main control factors change depending on the specific dolomitization process. The limiting factor for the convection model is the duration of the process while the brine salinity controls the reflux. However, the most effective control is the permeability distribution that drives the hydrology.
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Controls on Reservoir Stress Path
Authors J. M. Segura, S. Skachkov and Q. J. FisherA reduction in fluid pressure within a reservoir due to hydrocarbon production promotes changes in the effective and total stress distribution, not only within the reservoir but also in the surrounding strata. This stress evolution is the responsible of many problems encountered during production (e.g. surface subsidence, casing deformation, fault reactivation etc.). This work presents the results of an extensive series of 3D numerical hydro-mechanical coupled analyses that study the influence of reservoir geometry and material (reservoir and surrounding rock) properties on the stress path parameter values. It is shown how stress arching is more important when the reservoir Young’s modulus (Er) is lower than that of the bounding material (Eb), especially if the reservoir is small or elongated in shape. In such cases, the stresses will not significantly evolve in the reservoir, and stress evolution occurs in the over and side-burden. The higher the ratio Er/Eb, the less important the development of stress anisotropy is, although for extensive geometries the behaviour of the system tends to the theoretical oedometric case governed by the reservoir Poisson ratio independently of the ratio Er/Eb.
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Exploration Potential in Area 4 - Offshore Malta
Authors L. Lipparini, P. Marsili, M. Moscatelli and D. ScroccaMalta Oil Ltd (a Mediterranean Oil & Gas Group Company) currently holds an exploration license over most of offshore Malta Area 4 on the Mediterranean Pelagian shelf located about 130 km south of Malta, in water depths of 200 m to 900 m. The area is split into four blocks, Blocks 4, 5, 6 and 7. The objective of this note is to summarize the results of our recent interpretation, describe the geological evolution of the area, and give our prospective on the exploration potential of this almost unexplored area. The work done is mainly a re-assessment of the prospectivity of the area, with particular attention to the Block 7, following a recent campaign of seismic acquisition / reprocessing.
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Structural Evolution of the Apenninic-maghrebian Orogen in Eastern Sicily - Thermal and Thermochronological Constraints
Authors L. Di Paolo, L. Aldega and S. CorradoTemperature-dependent clay mineral assemblages and vitrinite reflectance data have been used to investigate levels of diagenesis from the Sicilian-Maghrebian fold-and-thrust belt in Eastern Sicily at the footwall of the Kabilian-Peloritan-Calabrian Arc. Data are from units sampled between the Nebrodi Mountains to the north and Mt. Judica to the south. The integration of organic and inorganic thermal indicators allowed us to distinguish portions of the accretionary wedge with different thermal maturity and thermo-chronological evolution and to define the maximum temperature that thrust-top, foredeep basin deposits and accreted passive margin units experienced. VRo% values between 0.60-0.96% and illite content in mixed-layer I-S (60-85%) are found in trench-involved and in accreted passive margin units. Lower VRo% values (0.20-0.50%), and percentage of illite layers in I-S (30-60%) are found in thrust-top and foredeep basin deposits and in units back-thrust toward the hinterland and mainly gravity-mobilized toward the Hyblean Plateau. Apatite fission-track data from the trench-involved deep structural units were partially to totally annealed during wedge accretion and indicate that subsequent exhumation have occurred mainly in Burdigalian times. Shallow structural units of the wedge have been removed by tectonics and erosion.
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Organic Matter Optical Analysis and Clay Mineralogy to Unravel the Tectonic/Sedimentary Burial in the Laga Basin, Italy
Authors V. Cantarelli, L. Aldega, S. Bigi, P. Casero and S. CorradoVitrinite reflectance data and clay mineral assemblages were used to investigate levels of diagenesis from the Messinian Laga Basin (Central Apennines) developed at the footwall of the Sibillini Mts. and the Gran Sasso Massif. Data derive from stratigraphic units forming the siliciclastic basin fill up to Middle Messinian gypsum-arenites and its pre-orogenic substratum. Highest Ro% values and percentages of illite layers in I/S are found in the basin depocentre and at the footwall of the carbonate thrust sheets. Lower Ro% values and percentage of illite layers in I/S characterize less subsided sectors. I/S records the maximum paleo-temperature the Laga Fm. experienced related to its burial history whereas KI data suggest higher temperatures related to detrital K-micas inherited from the uplift of the Alpine-Apennines chain. A calculation of paleo-temperatures from selected data of organic and inorganic parameters is proposed and compared with recent stratigraphic and structural data. This allowed proposing that the Laga basin fill never experienced temperatures >100-110°C, due to variable sedimentary loading, while localised anomalous heating is due to the effect of the tectonic emplacement and subsequent local erosion of the Sibillini and Gran Sasso thrust sheets.
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Geological Framework of PO Plain and Adriatic Foreland System
Authors R. Fantoni and R. FranciosiThe Po Plain and Adriatic foreland is shared by the converging Southalpine, Apennine and Dinaric/Albanian chains. As well as in the bordering chain sectors, the area compressional architecture is overprinted on the polyphasic framework produced by thr Mesozoic extensional cycles. The Cenozoic compression involved the area with different times and directions of tectonic transport: the Dinaric/Albanian system since Paleocene to Pleistocene, the Southern Alps one during Oligo-Miocene and the Apennines system during Pliocene-Pleistocene. The effect of opposite chain segments interference was a multiple system of differently evolving foredeeps not exclusively played by the chains at their back. Time and amount of flexuring were controlled also by the competition of the opposite chain activity with formation of transversal positive belts that played the role of flexure transfer zones.
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The Constraints to the Subsidence and Exhumation of Alpine Units and the Tertiary Piedmont Basin, NW Italy
Authors P. Mosca, G. Bertotti and J. Juez-LarréThe vertical movements of the orogenic Ligurian Alps and their overlying Tertiary Piedmont Basin (NW Italy) are defined in this work by low-temperature thermochronometers, i.e. apatite (U-Th)/He and fission track. Thermochronological analyses are integrated by seismic and field geological data. Since earliest Oligocene, metamorphic Alpine rocks were exposed, eroded and covered by continental to marginal-marine deposits of the Tertiary Piedmont Basin. Then, the orogenic rocks and the sedimentary succession suffered a km-scale subsidence on the order of 4km before 26 Ma and began their exhumation thereafter: inversion took place in the southernmost part of the Tertiary Piedmont Basin and a regional northwards syn-depositional tilting began. Since Late Oligocene, the evolution of the investigated area is characterized by the coexistence of exhumation/uplift to the south and subsidence to the north. This couple of exhumation and subsidence migrated northwards through time.
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Albanides, a Typical Part of the Alpine Mediterranean Folded Belt, in the Light of the Geophysical Studies
Authors A. Frasheri and S. BushatiDistribution of gravity, magentic and geothermal fields, and their anomalous features in Albanides onshore and in the Albanian Adriatic Shelf in this paper are presented. Results of th reflection seismic surveys are interpreted in the framework of the integrated geological-geophysical data. There are analyzed also a hydrographical phenomenon in the Albanian Adriatic Sea area, which are correlated with Albanides geological setting and climate change.
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Deep Structures of South Caspian Basin from 2D Gravity Modelling
Authors A. H. Gadirov and F. A. KadirovIn the light of new data of geophysical investigations, gravity model of the earth crust along the profile, crossing the South Caspian Basin (SCB) north-southwards has been considered in this paper. Gravity model of a deep structure of the earth crust and the upper mantle along the investigated profile has been created by trial-and-error procedure. To compensate the observed gravity field there has been conducted interpretation of density discontinuities of the upper boundary of the oceanic crust and Moho discontinuity. In the model in the conjugation zones of the oceanic and continental crust, in the oceanic crust one can observe local bending which is likely to be linked with the process of subduction. In the middle of the profile the oceanic crust concave up.
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Advanced Detector of Ground Roll for Single-station SVD-based Polarization Filtering
By O. TiapkinaI present an update of the single-station polarization filter proposed by Jin and Ronen (2005) for ground-roll elimination. This process is carried out by treating a sliding time window of a single-station triaxial recording as a matrix and doing singular value decomposition of this matrix. The efficiency of this filter depends on how reliably the ground roll is detected. For this reason, I derive an attribute to be minimally sensitive to the corruptive impact of additive random noise and, on the other hand, maximally sensitive to the presence of the ground roll. The advantage of using this attribute is demonstrated on 3C field data.
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An Effective Algorithm for Tri-phone Reorientation
Authors D. Maillet and B. GratacosFor multi-components data processing, a critical initial stage is the determination of the orientation of the geophones in the field. A number of solutions have been proposed to achieve this goal, some of them are based on the first break window, others use instead the reflected and converted energy part of the records. In this paper we propose a very simple modeling of the recorded first breaks that allows the estimation of the tri-phone orientation as a 3D rotation matrix. Issues such as vector fidelity or instrument coupling are not considered here. The result of the algorithm is demonstrated using real data.
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Midget Seismic in Sandbox Models - Emission Properties of Piezo-electric Transducers
Authors M. L. Buddensiek, C. M. Krawczyk, N. Kukowski and O. OnckenAnalog sandbox simulation has been applied to study geological processes to provide qualitative and quantitative insights into specific geological problems. In nature, the structures, which are simulated in those sandbox models, are often inferred from seismic data. With the study introduced here, we want to combine the analog sandbox simulation techniques with seismic physical modeling of those sandbox models. The long-term objective of this approach is to image seismic and seismological events of static and actively deforming 3D analog models. To achieve this objective, a new midget-seismic facility for laboratory use was designed and developed, comprising a seismic tank, a PC control unit including piezo-electric transducers, and a positioning system. The first experiments are aimed at studying the wave field properties of the piezo-transducers in order to investigate their feasibility for seismic profiling. The properties investigated are their directionality and the change of waveform due to their size (5-12 mm) compared to the wavelengths (>1.5 mm). The best quality signals and least directionality and waveform change is achieved when the center source frequency is between 350-500 kHz, and the offset is less than 8 cm for a reflector depth of 10 cm.
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Estimation of Ground Stiffness, Ground Viscosity and Captured Ground Mass Using Vibrator Field Measurements
By Z. WeiNear-surface velocity measurements are very valuable for making static corrections in data processing to improve the quality of subsurface images in land seismic surveys. Ali et al (2003) show that the near-surface velocity can be estimated by using ground elastic properties produced from modern vibrator control systems. Direct measurements via uphole surveys can be cost prohibitive. The modern seismic vibrator driven by the feedback control system produces a force to the ground as it vibrates. As the vibrator pushes against the ground, it gets a counter force from the ground. The motion of the ground responding to the applied force is detected through the movements of the baseplate. The dynamic of the vibrator normally is described by the accelerations of the reaction mass and the baseplate, which can be used to estimate underlying ground properties. Accurate estimates for ground properties can only be achieved when key elements are in place including a valid reaction mass-baseplate-ground model and a properly implemented good estimation algorithm. This paper describes a useful method for estimation of ground properties using the reaction mass and baseplate accelerations as a vibrator shakes.
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Next Generation QC Techniques Today - A Case Study
More LessThe quality control (QC) of seismic data processing can be time consuming and therefore adds a significant contribution to the turn around time of the project. A streamlined approach to the QC workflow is presented where the time taken is significantly reduced by intelligent use of attribute information. An increased variety of attributes are generated, analyzed and visualized to make it possible to find problems (either acquisition or geophysical related) in an easy and efficient manner. It is shown that the new workflow is capable of finding the same problems as the conventional workflow and in addition provides a better understanding of the data in both the acquisition and geophysical sense.
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Single Sensors Versus Hard-wire Arrays in Pre-stack Stereotomography
Authors C. Iacob, I. Panea, G. G. Drijkoningen and V. MocanuThe surface waves are considered the main type of noise present on the land seismic reflection data. For decades, the hard-wired arrays are successfully used for their attenuation. Unfortunately, according to some modeling results, the hard-wired arrays can destroy the seismic signal (primary reflected waves). This is the case of the use of arrays in zones with rough topography where, locally, the reflections seen on the array responses are stretched and show very low amplitude; as a result, the signal-to-noise ratio of the responses is lower. And this is only one example of sources of amplitude variations which can decrease the signal-to-noise ratio of seismic records. The processing of the seismic data characterized by low signal-to-noise ratio can be difficult to perform; for example, the velocity analysis depends on the presence of clear reflections. A very efficient technique used for performing a velocity analysis is the pre-stack stereotomography, but we will show that its results are a function of the quality of input data. If we analyze array responses from an area with rough topography as input data, the responses based on the single-sensor recordings can be successfully used in the pre-stack stereotomography compared to those obtained using hard-wired arrays.
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Reconstructing Missing Near Offset Data and Primaries from Multiples
More LessTraditionally, in surface-related multiple elimination (SRME), multiples are predicted from primaries by a spatial convolution process. However, the same relationship between primaries and multiples can be used to describe a deconvolution process, where multiples are transformed into primaries or data. This method is called a focal transform. The method that is presented in this paper is based on the focal transform and reconstructs the primaries or data in the missing near offsets by minimising the estimated primaries. The result of this method is improved by choosing a different subtraction norm and using the fact that multiples predicted along different ways should be the same.
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Optimizing SRME Multiple Prediction
Authors D. Donno, P. Mazzucchelli, F. Rocca, G. Del Molino and E. Loinger3D surface-multiple elimination (SRME) is a powerful method for complex 3D multiple attenuation. However, since standard 3D marine acquisition geometries are very coarsely sampled across line, interpolation of the pre-stack input data is often required. To reduce the high costs associated to this operation, we propose an analysis tool useful to determine the optimal SRME processing parameters. By using multiple ray-paths modeling, we extracted two attributes which both help to select areas where 2D SRME would likely fail and an application of 3D SRME is preferable for good prediction of multiples. Moreover, when 3D SRME is selected, the locations of downward reflection points (DRP) of the multiples and the aperture of the Fresnel zones around the DRP for the multiple contributions summation allow to reduce the survey area to be interpolated, achieving also a better control of aliasing. The reliability and accuracy of the proposed analysis tool is tested through application to a real marine data set.
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Why Should We Use Probability Models for Seismic Data Processing? A Case Study Comparing Wiener and Kriging Filters
Authors L. Sandjivy, T. Faucon and J. L. MariWhy should we use probability models for processing seismic amplitude data? How spatial filters such as factorial kriging compare to their deterministic counter parts? It is not easy to answer such questions and to be understood by operational seismic processing geophysicists. This paper starts from a very basic example taken from a shot point processing that enables to derive a straightforward comparison between standard Wiener amplitude filtering and its geostatistical counterpart, a specific factorial kriging model [3] [4]. ]. A more comprehensive well VSP case is being processed that will also be presented. Comparing both filters from theoretical and practical points of view enables to highlight how they both make use of the same trace autocorrelation function known by geophysicists, but not within the same conceptual framework. It is shown that it is possible to reproduce the geophysical Wiener filters using a factorial kriging probability model and that this type of modelling opens the way to quantification of seismic processing uncertainties.
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F-x Noise Attenuation by Empirical Mode Decomposition
Authors M. Bekara and M. van der BaanThis paper proposes a new frequency-offset (f-x) domain filtering technique for noise attenuation. The novelty is in the use of a nonlinear decomposition (EMD), instead of linear autoregressive prediction filtering. The motivation is to properly account for the nonlinear and nonstationary spatial properties of real data, and to handle irregular trace spacing. The so-called FX-EMD is equivalent to an adaptive, data-driven frequency-wavenumber (f-k) filter with a frequency-dependent, high-cut wavenumber filtering property. It is effective in removing both random and dipping noise, and displays good performance for irregularly spaced land data. It is also a parameter-free filtering technique in its simplest implementation.
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Delft - Inverse Scattering Surface-related Multiple Attenuation in Three Lines
By S. A. LevinIn this abstract I compare theoretical and practical aspects of the Delft and the Inverse Scattering surface-related multiple attenuation. I first show the essential theoretical equivalence of the two using a simple three line mathematical argument. After that I look at similarities and differences in the implementation of the two methods, from deghosting, obliquity and antialiasing, to memory requirements and out-of-core pipelining, parallel processing and finally comparing their relative computational cost. Of particular note, I show that (1) no spatial FFT padding is required to suppress wraparound artifacts and (2) Fourier spatial interpolation does nothing whatsoever to reduce aliasing artifacts.
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A New Median Filter for Suppressing Seismic Random Noise
More LessRandom noise in seismic data is a major factor that affects signal-to-noise ratio, obscures details, and complicates identification of useful information. We present a new method for reducing random noise in seismic data. Called 1D time-varying median filter (TVMF), it is based on the 1D classical median filter (MF). We designed a threshold value that could control filter-window length according to characteristics of random noise. Using the relationship between seismic data and threshold value, we chose median filters with different time-varying filter-window lengths to eliminate random noise. We found that through processing the model and comparing it against two median filter methods, the time-varying median filter could strike a balance between eliminating random noise and protecting useful information. We used synthetic data to demonstrate the feasibility of our method in reducing seismic random noise. An example of synthetic data shows that TVMF is more effective in random noise suppression than the 2D multistage median filter (MLM) or MF. Results using the method on seismic data from the Songliao Basin in China demonstrate its effectiveness.
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Hybrid Inversion of First-arrivals Times
By M. S. MendesA hybrid algorithm, combining Monte-Carlo (MC) optimization with Simultaneous Iterative Reconstructive Technique (SIRT) tomography, is used to invert the first-arrival travel times from seismic data for building velocity models. Stochastic algorithms may localize a point around the global minimum of the misfit function, but are not suitable to identify the precise solution. On the other hand, tomographic model reconstruction, based on a local linearization, will be successful only if an initial model already close to the best solution is available. To overcome these problems, in the new method a model obtained using a classical Monte-Carlo optimization is proposed as a good initial guess for starting the local search with the SIRT tomography. The performance of this two-step optimization scheme has been tested on synthetic data. After that, the first-arrival times picked from two orthogonal lines recorded at the Caldeira of S. Sebastião in the Azores Islands are used. The velocities in the region undergo lateral and vertical changes, and the new technique images successfully these variations with reduced a priori information.
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Moveout Analysis and Velocity Determination for Angle-domain Common-image Gathers
More LessFor the case that a prestack depth migration has been performed with inaccurate velocities, it is shown that second order approximations can be used to predict the residual moveout of angle domain common image gathers. A residual depth correction is proposed; the relevant corrections can be determined from the migrated data. The residual moveout determined can be used both to improve the signal to noise quality of the migrated data and the velocity model for the migration. Although developed for small reflection angles, several realistic examples demonstrate that satisfactory approximations to the residual moveout curve can be obtained over a relatively wide range of reflection angles. A zero offset inversion is applied to the estimated residual radii of curvature. The inversion is applied layerwise; for each layer the velocity is assumed to vary according to v(x,z)=f(z)+w(x) with f(z) known and w(x) to be determined. Iterative applications of the suggested approach have rendered satisfactory results for several representative velocity models.
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3-D Refraction Traveltime Tomography Algorithm Based on Adjoint State Techniques
Authors C. Taillandier, M. Noble, H. Calandra, H. Chauris and P. PodvinClassical 3-D refraction traveltime tomography algorithms may suffer from computational limitations due to the large datasets that come from current seismic acquisition surveys. To overcome these issues, we suggest a 3-D refraction tomography algorithm based on adjoint state techniques to derive the gradient of the traveltime misfit function. We use the Eikonal equation for the forward modelling, and iterate with a conjugate gradient method. A 3-D synthetic example with a realistic size acquisition demonstrates the efficiency and the great potential of the adjoint state method for 3-D applications of refraction tomography.
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Tomographic Velocity Model Building Using Iterative Eigendecomposition
Authors K. Osypov, D. Nichols, M. Woodward and C. E. YarmanTomographic velocity model building has become an industry standard for depth migration. However, regularizing tomography still remains a subjective virtue, if not black magic. Singular value decomposition (SVD) of a tomographic operator or, similarly, eigendecomposition of corresponding normal equations, are well known as a useful framework for analysis of most significant dependencies between model and data. However, application of this approach in velocity model building has been limited, primarily because of the perception that it is computationally prohibitively expensive for the contemporary actual tomographic problems. In this paper, we demonstrate that iterative decomposition for such problems is practical with the computer clusters available today, and as a result, this allows us to efficiently optimize regularization and conduct uncertainty and resolution analysis.
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