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74th EAGE Conference and Exhibition incorporating EUROPEC 2012
- Conference date: 04 Jun 2012 - 07 Jun 2012
- Location: Copenhagen, Denmark
- ISBN: 978-90-73834-27-9
- Published: 04 June 2012
301 - 400 of 948 results
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Rock Physics Modelling of Stress Sensitivity in Patchy Cemented Sandstones
More LessWe demonstrate how to characterize and model the effects of heterogenous grain contacts and associated stress sensitivity in reservoir sandstones. We use nested Hashin-Shtrikman modeling, where we first mix loose sands and cemented sandstones to create a patchy cemented high-porosity end-member. Next, we use Hashin-Shtrikman modeling to account for porosity variation between high porosity end-member and mineral point. This approach enable us to honor stress sensitivity in the patchy cemented sandstones, as the loose grain contacts will behave according to Hertzian contact theory. This approach can be used to quantify stress sensitivity in reservoir sandstones where core samples are lacking or inreliable. Future research will focus on validating this approach on real data.
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Impact of Microscopic Heterogeneities of Rock on Elastic Parameters in Unconsolidated Sandstone Reservoir
Authors J. L. Reveron, C. Joseph, B. Doligez and H. BeucherThe impact of microscopic rock heterogeneities on elastic properties in an unconsolidated sandstone reservoir in the Orinoco Heavy Oil Belt in Eastern Venezuela is studied at 3 wells using rock physics models. Differences in grain and pore sizes and the presence of shale/clay and cement, lead to differences in elastic properties values at same porosity. This allows to detect two different consolidation levels in the reservoir: (1) unconsolidated and (2) weakly consolidated due to cement and smaller grains/pores, where the first one has better rock quality. Being able to detect the two consolidation levels proves the interest of rock physics modelling in our case as tool to relate geology and seismic and to perform a quantitative seismic interpretation.
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A New Rock Physics Model Integrating Diagenesis and Pore Shape and its Application
Authors B. Hou, X. H. Chen, J. Y. Li and Z. ChenS-wave velocity plays an important role in seismic modeling, AVO analysis and fluid identification. However, realistic well logging data lacks of S-wave information, so S-wave velocity prediction become a focus in rock physics research. Integrating differential effective medium (DEM) model and Pride model, a new rock physics model which is used to calculate dry rock moduli is proposed in this paper. The new rock physics model integrates effects of pore shape and diagenesis to bulk modulus and shear modulus of dry rock, so it is more rational and accurate. At the same time, combining Gassmann theory, P-wave and S-wave velocities model of fluid saturated rock is established. The model is applied to S-wave velocity prediction of realistic well logging data. The predicted results demonstrate that S-wave velocity prediction based on the new rock physics model proposed in this paper is effective.
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Evaluation of Rock Properties and Rock Structures in the -range with Sub- X-ray Computed Tomography
Authors G. Zacher, M. Halisch and T. MayerIn recent years high resolution X-ray Computed Tomography (CT) for geological purposes contribute increasing value to the quantitative analysis of rock properties. Especially spatial distribution of minerals, pores and fractures are extremely important in the evaluation of reservoir properties. The possibility to visualize a whole plug volume in a non-destructive way and to use the same plug for further analysis is undoubtedly currently the most valuable feature of this new type of rock analysis and will be a new area for routine application of high resolution X-ray CT in the near future. The paper outlines new developments in hard- and software requirements for high resolution CT. It showcases several geological applications which were performed with the phoenix nanotom and recently phoenix nanotom m, the first 180 kV nanofocus CT system tailored specifically for extremely high-resolution scans of samples up to 240 mm in diameter and weighing up to 3 kg with voxel-resolutions down to <300 nm.
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A Compaction Front in North Sea Chalk
Authors P. Japsen, D. K. Dysthe, E. H. Hartz, S. L. S. Stipp, V. M. Yarushina and B. JamtveitNorth Sea chalk shows a pronounced porosity drop, from 20% to <10% over a compaction front of less than 300 m. The position of the compaction front is closely tied to an effective stress of ~17 MPa. These observations require a strongly nonlinear rheology with a marked increase in compaction rate at a specific effective stress. Grain-scale observations demonstrate that the front coincides with grain coarsening and recrystallization of fossils and fossil fragments. We propose that this nonlinear rheology is caused by stress-driven failure of the larger pores and the associated generation of reactive surface area by subcritical crack propagation away from these pores. Before the onset of this instability, compaction by pressure solution is slowed down by the inhibitory effect of organic compounds associated with the fossils. Although the compaction mechanism is mainly by pressure solution, the rheological response to burial may still be dominantly plastic and controlled by the rate of exposure of reactive surface area. The nonlinear compaction of chalk has significant implications for the evolution of petroleum systems in the central North Sea, both with respect to sea-floor subsidence above hydrocarbon–producing chalk reservoirs and for the formation of low-porosity pressure seals within the chalk.
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Mechanical Compaction in Tectonically Active Basins - Theory and Examples
Authors R. Bachrach and P. AvsethPorosity evaluation in depth is a complex mechanical problem where irreversible mechanical compaction drives the initial consolidation phase of the sediment. The ability to quantitatively model this process has many applications in the oil and gas industry, including AVA modeling, pore pressure prediction, and more. We present a method to model compaction in tectonically active basins using large deformation theory. Mechanical compaction is posed as a plastic-poroelastic process and is modelled using natural strain increments. Our method allows us to model porosity evaluation for different pore pressures and lithologies.
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Digital Noise Attenuation of Particle Motion Data in a Multicomponent 4C Towed Streamer
Authors A. K. Ozdemir, B. A. Kjellesvig, A. Ozbek and J. E. MartinA multicomponent (4C) towed streamer acquires both pressure and the full particle velocity vector with inline, crossline, and vertical components. In this paper, we present the characteristics of the noise recorded by the particle motion sensors in a multicomponent (4C) towed streamer that was tested in the North Sea. The particle motion sensors in a streamer exhibit streamer-borne noise such as longitudinal, transversal, and angular vibrations. The amplitudes of the vibrations are typically several orders of magnitude stronger than the corresponding noise recorded by hydrophones at frequencies below about 20 Hz. We introduce a multiscale noise attenuation algorithm that provides strong noise attenuation for particle velocity data at these frequencies. We show that a high-fidelity particle motion measurement can be obtained with this technique at frequencies down to 3 Hz. This enables 3D receiver deghosting and crossline reconstruction without making assumptions about the wavefield or the subsurface.
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The Double Focal Transformation and its Application to Seismic Data Reconstruction
Authors H. Kutscha and D. J. VerschuurWithin the class of transformation-based reconstruction techniques, observed seismic data is decomposed into certain basis functions, such as plane waves, parabolas or curvelets. In the corresponding model space the aliasing noise has different properties than the seismic signal and can be penalised. However, in cases that subsurface information is available, this information cannot be used in most reconstruction methods. Therefore, the focal transform was derived as a way to include knowledge about the subsurface within the data reconstruction algorithm and, thereby, increase its potential reconstruction capabilities. The basic principle of the focal transformation is to focus seismic energy along source and receiver coordinates simultaneously. The seismic data are represented by a number of spikes in the focal domain whereas aliasing noise spreads out. By imposing a sparse solution in the focal domain, aliasing noise is suppressed and data reconstruction beyond aliasing is achieved. To facilitate the process, only a few effective depth levels need to be included, preferably along the major boundaries in the data, from which the propagation operators serve as the basis functions of this data decomposition method. Results on 2D synthetic and field data illustrate the method's virtues.
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Pre-stack Deghosting for Variable-depth Streamer Data
By R. SoubarasVariable-depth streamer acquisition is an acquisition technique aiming at achieving the best possible signal-to-noise ratio at low frequencies by towing the streamer very deeply, but by using a depth profile varying with offset in order not to limit the high frequency bandwidth. Previous papers have shown how a joint deconvolution allows the post-stack deghosting of such variable-depth streamer acquisitions, and the purpose of this paper is to show how this can be generalized to a multichannel joint deconvolution that allows pre-stack though post-imaging deghosting of such acquisitions. After computing migrated gathers as well as mirror migrated gathers, a deghosted gather is computed through joint deconvolution by assuming a parametrical form in offset for the events. This method preserves the AVO behaviour. Synthetic and real data examples are shown.
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Model-based Attenuation for Scattered Dispersive Waves
Authors C. L. Strobbia, A. Zarkhidze and R. MayCoherent noise in land seismic data primarily consists of source-generated surface-wave modes. This ground roll consists of surface-wave modes propagating directly from sources to receivers. Near surface heterogeneities and discontinuities diffract the surface waves and generate secondary events, which can heavily contaminate records. The diffracted and converted surface waves are often called scattered noise and can be a severe problem particularly in areas with shallow or outcropping hard lithological formations. Conventional noise attenuation techniques are not effective with scattering: they usually can address the tails and not the apices of the scattered events. Large source and receiver arrays can attenuate scattering, but only in exchange for compromise to signal fidelity and resolution. We present a model based technique for the scattering attenuation, based on the estimation of the surface-wave properties and on the prediction of the surface waves with a complex path involving diffractions. The properties are estimated first, to produce surface consistent volumes of the propagation properties. Then, for all gathers to filter, we integrate the contributions of all possible diffractors, building a scattering model. The estimated scattered wavefield is then subtracted from the data. The method can work in different domains, and cope with aliased surface waves.
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Discrete Local Basis Functions as Ringing Free Filter as a Trace by Trace Algorithm for Ground Roll Removing
Authors C. Steiner-Luckabauer, P. O‘Leary and M. HarkerThis paper presents the concept of discrete local basis functions and its application to seismic data. This method is introduced by the application of orthogonal projections, the structure and theory of the matrix operators and their relationship to Savitzky-Golay filtering. As the elimination of ground is still a non-trivial task -especially for highly undersampled data - this new technique demonstrates ringing free, wavelength selective filtering as a trace-by-trace algorithm. As an alternative to frequency-wavenumber filter, it allows a quality improvement for quantitative seismic interpretation.
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Enhancing Channel Detection in Seismic Images Using the Redundant Wavelet Transform
More LessNoise removal is important in enhancing the seismic imaging of channels, which are important in exploration. However, the smoother must be edge preserving. There are a variety of different techniques that can be used. In this work, we illustrate the performance of a method called the Redundant Wavelet Transform by applying it to a time slice obtained from 3D seismic data. Several advantages of this approach vs. other methods are discussed.
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Local Linear Events Extraction and Filtering in the Presence of Time-shifts
Authors P. Hugonnet, J. L. Boelle and F. PratStatic shifts are detrimental for any algorithm that is based on the lateral coherency of the seismic events. We present a multidimensional extraction of linear events, where the time shifts are estimated alongside the other attributes of the extracted event (slopes, and wavelets), leading to a much more effective extraction. Different time-shifts are estimated for each extracted event, making the algorithm more generic than an approach based on classical static shifts. We demonstrate the effectiveness of this algorithm on synthetic and field data.
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Automatic First-break Picking Using the Instantaneous Traveltime Attribute
Authors C. Saragiotis and T. AlkhalifahPicking the first breaks is an important step in seismic processing. The large volume of the seismic data calls for automatic and objective picking. We introduce a new automatic first-break picker, which uses specifically designed time windows and an iterative procedure based on the instantaneous traveltime attribute. The method is fast as it only uses a few FFT's per trace. We demonstrate the effectiveness of this automatic method by applying it on real test data.
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Residual Static Estimation by Sparsity Maximization
By A. GholamiResidual static estimation in complex areas is one of the main challenging problems in seismic data processing. Inappropriate estimation of residual statics affects the quality of seismic images dramatically. In this paper, I propose a novel method for residual static estimation based on sparsity maximization. If pre-stack data have a sparse representation in a known transform domain (e.g. Fourier or Curvelet), then short-wavelength time shifts are incoherent in that transform domain (i.e. produce noise like artifacts). Thus, the time shifts can be estimated by a sparsity-promoting non-linear problem. A fast algorithm is presented to solve the problem. The performance of the method is illustrated by synthetic and real numerical experiments.
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Automated Seismic-to-well Ties?
Authors R. H. Herrera and M. van der BaanThe quality of seismic-to-well tie is commonly quantified using the classical Pearson's correlation coefficient. However the seismic wavelet is time-variant, well logging and upscaling is only approximate, and the correlation coefficient does not follow this nonlinear behavior. We introduce the Dynamic Time Warping (DTW) to automate the tying process, accounting for frequency and time variance. The Dynamic Time Warping method can follow the nonlinear behavior better than the commonly used correlation coefficient. Furthermore, the quality of the similarity value does not depend on the selected correlating window. We compare the developed method with the manual seismic-to-well tie in a benchmark case study.
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Implementation Aspects of Eigenstructure-based Velocity Spectra
Authors T. Barros, R. Lopes, M. Tygel and J. M. T. RomanoIn this paper we discuss high-resolution coherence functions for the estimation of stacking shape parameters in seismic signal processing. We focus on the MUltiple SIgnal Classification (MUSIC) algorithm, which uses the eigendecomposition of the seismic data to measure the coherence. MUSIC can outperform the traditional semblance in cases of close or interfering reflections. Our main contribution is to propose several simplifications to the implementation of MUSIC. Namely, we propose an iterative way to obtain the MUSIC coherence function, called Power Method MUSIC (PM-MUSIC). We also propose a new way to obtain the MUSIC pseudospectrum, based on the eigendecomposition of the temporal covariance matrix of the seismic data. This is in contrast to the algorithms in the literature, which are based on the spatial covariance. We compared spatial and temporal covariance matrices, implemented with PM-MUSIC, in a simple synthetic example with two reflections corrupted by additive white gaussian noise. Initial simulations indicated that PM-MUSIC outperforms semblance and that the temporal variant of PM-MUSIC is superior to its spatial counterpart. Moreover, temporal PM-MUSIC is particularly useful when dealing with correlated signals.
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Elimination of Seismic Multiples by Anisotropic, Pre Stack Depth Migration and Filtering
More LessGroup and phase velocities are derived such that, when used in constant velocity migration, a set of seismic multiples from the sea surface are focused such that they separate from reflection primaries. The velocities are found to be homogeneous but anisotropic. The focused multiples are then deleted from the data and surrounding data are used to fill in the deleted areas. Unmigration restores the data to it's input stat sans the multiples. A before and after comparison using synthetic data verifies that this is a very clean process in that multiples are removed without loss of primaries.
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Improved Fast Wave-equation Based Multiple Removal
More LessFor marine data from areas with hard and/or irregular sea-floor, water-layer multiples and peg-legs are often the most troublesome part of the free-surface multiples. These multiples can be efficiently removed by wave-equation (WE) based approaches. The WE approaches consist of two main steps – multiple prediction by data extrapolation through the water layer, and then multiple suppression by adaptive subtraction of the predicted multiples from the input data. Our fast WE based multiple removal requires three prediction terms independent of the hardness of the sea-floor or the thickness of the water layer. When sea-floor geometry is simple and water-bottom reflectivity varies slowly along the sea-floor, the main two steps - WE prediction and adaptive subtraction - are consistent, since each prediction term contains multiples, which require the same amplitude correction. In the current work we improve the performance of the fast approach in the cases of dipping sea-floor and fast variation of water-bottom reflectivity. Iterative implementation of the approach allows the removal not just of water-layer multiples, but also of remaining free-surface multiples from the next strongest reflector(s).
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Ground Roll Suppression Using Optimum Weighted Stacking
Authors O. Tiapkina, M. Landr and Y. TyapkinIn land seismic surveys, ground roll is one of the most troublesome types of noise, and its attenuation is a challenging problem. A number of methods use singular value decomposition (SVD) or some related transformations for filtering of this noise. This wide class of filters is very popular because it gives a denser approximation to ground roll before its subtraction from seismic data than many others, due to the use of more appropriate basis functions. However, it is true if and only if the energy of additive random noise on different seismic traces is quite stable. Otherwise, any SVD-based filter can fail and therefore should be replaced by another processing method. We propose optimum weighted stacking, which accounts for variations of the noise energy on different traces. To demonstrate the advantages of this method over SVD-based filtering, we use a dataset from Spitsbergen to some traces of which we have added synthetic random noise.
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Minimum-phase Property of Memory Functions in the Wave Equation
Authors K. N. van Dalen, E. C. Slob and F. C. SchoemakerMemory functions occur in the wave equation as time-convolution operators and generally account for the instantaneous and non-instantaneous responses of a medium. The specific memory function that is causal and stable, and the inverse of which is also causal and stable, is conventionally referred to as minimum phase. In this paper we present "extended minimum-phase relations" between the amplitude and phase spectra of a memory function that has different properties. The considered memory function and its inverse are both causal, but they do not need to be stable. We still address the function as minimum phase because the phase spectrum exhibits minimum group delay, like a conventional minimum-phase function. We have successfully tested the derived relations for the well-known Maxwell and Kelvin-Voigt models. The relations have potential applications in acoustics, seismology, poroelasticity, electromagnetics, electrokinetics and any other effective-medium theory that employs memory functions.
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Flow Properties in Saturated Soils from Differing Behaviour of Dispersive Seismic Velocity and Attenuation
Authors R. Ghose and A. ZhubayevA careful look into the pertinent models of poroelasticity reveals that in water-saturated sediments or soils, the seismic (P and S wave) velocity dispersion and attenuation in the low field-seismic frequency band (20-200 Hz) have a contrasting behaviour in the porosity-permeability domain.Taking advantage of this nearly orthogonal behaviour, a new approach has been proposed, which leads to unique estimates of both porosity and permeability simultaneously. Through realistic numerical tests, the effect of maximum frequency content in data and the integration of P and S waves on the accuracy and robustness of the estimates are demonstrated.
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Flow Properties in Saturated Soils from Dispersive Seismic Velocity and Attenuation - Test on Field Data
Authors A. Zhubayev and R. GhoseIn this paper, we have tested on field data a recently developed approach for in-situ estimation of the flow properties – especially porosity and permeability- in near-surface soil layers, utilizing intrinsic velocity dispersion and attenuation of shear waves and a pertinent theory of poroelasticity. Shallow VSP experiments have been performed using a novel digital array seismic cone penetrometer. The results show that even at the low field-seismic frequency band, we can obtain realistic and stable estimates of porosity and permeability for relatively coarse-grained (higher permeability) soil layers. It is difficult to get these properties when the permeability is very low. This problem is alleviated when the frequency content in the data is increased.
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Do Seismic Waves and Fluid Flow Sense the Same Permeability?
Authors J. G. Rubino, L. B. Monachesi, L. Guarracino, T. M. Mller and K. HolligerWe study the discrepancy between the effective flow permeability and the effective seismic permeability, that is, the effective permeability controlling seismic attenuation due to wave-induced fluid flow, in 2D rock samples having mesoscopic heterogeneities and in the presence of strong permeability fluctuations. In order to do so, we employ a numerical oscillatory compressibility test to determine attenuation and velocity dispersion due to wave-induced fluid flow in these kinds of media and compare the responses with those obtained by replacing the heterogeneous permeability field by constant values, including the average permeability as well as the effective flow permeability of the sample. The latter is estimated in a separate upscaling procedure by solving the steady-state flow equation in the rock sample under study. Numerical experiments let us verify that attenuation levels are less significant and the attenuation peak gets broader in the presence of such strong permeability fluctuations. Moreover, we observe that for very low frequencies the effective seismic permeability is similar to the effective flow permeability, while for very high frequencies it approaches the arithmetic average of the permeability field.
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Bounds for Effective Stress Coefficients in Poroelasticity
Authors T. M. Mueller and P. N. SahayBiot’s theory of poroelasticity implicitely contains a porosity equation that predicts changes in the porosity due to stress loading to be governed by the difference between the macroscopic solid and fluid pressures. This means that the effective stress coefficient for porosity equals 1. However, there is experimental and theoretical evidence that this porosity effective stress coefficient can be smaller and also greater than unity. In order to model such poroelastic rock behavior we make use of a generalized poroelasticity framework. We analyze a generalized porosity equation that depends on an effective pressure and, in particular, we obtain bounds for the porosity effective stress coefficient. These results are then used to obtain new bounds on various other effective stress coefficients.
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Frequency-dependent Effective Hydraulic Conductivity in Randomly Layered Porous Medium - Strong Contrast Approximation
Authors E. Caspari, T. M. Müller and B. GurevichUpscaling of transport properties of heterogeneous porous rocks, such as hydraulic conductivity, is controlled not only by the spatial heterogeneity, but also by the temporal variations of the flow field. Thus effective hydraulic conductivity and permeability of a heterogeneous porous medium is frequency-dependent. Existing upscaling methods for this property are often limited to small spatial variations of permeability, which is a significant limitation. In this study we apply a strong contrast expansion method, originally suggested in the context of the dynamic dielectric constant, to the problem of upscaling hydraulic conductivity in a 1D random medium. A closed-form expression for the effective hydraulic conductivity, which depends on the second order statistics, is derived. Unlike the small contrast approximations, in the low- and high-frequency limits this new expression yields harmonic and arithmetic averages of permeability, respectively, which are known to be exact low- and high-frequency limits in 1D.
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Integrated Solutions to Igneous Rock Developed in the Near Surface in Fushan Sag
Authors B. Q. Zhang, M. X. Zhou, Q. G. Liu, R. H. Zhang and Y. L. FuIgneous rocks developed near the surface in Fushan sag bring great challenges to seismic data acquisition and processing. Due to the shielding effect of igneous rocks, seismic energy is very difficult to propagate downwards and thus makes the seismic data very poor; the velocity and thickness of the near surface changes greatly and statics is complicated. The existence of the igneous rocks results in strong multiples and very small velocity difference between multiple and primary. These problems have been successfully solved with a set of solutions including (1) uphole based accurate near-surface investigation to design the shooting parameters point by point; (2) a long spread, wide azimuth and high fold acquisition technique to enhance the raw data quality; (3) using pre-stack 4D-denoise to improve the signal-to-noise ratio; (4) using a tomographic inversion static correction based on uphole and marked layer, interactive layered and segmented first arrival static correction, global optimization reflection residual and non-surface consistent static correction to solve the static problems; (5) using an adaptive subtraction method with constructing multiple model to remove the multiples. Good results have been achieved by using these methods, suggesting our methods are feasible.
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3D Reflection Seismic Investigation and Scaling Behaviour of Geophysical Logs in the Brunswick No. 6, Canada
Authors S. Cheraghi, A. Malehmir and G. BellefleurWe have studied scaling behavior of velocity, density and acoustic impedance down-hole logs from an approximately 850 m deep hole in the Brunswick No. 6 area. Power spectrum of all the logs indicates that measured properties exhibit scaling behaviour and suggests that lithological variation is a major contributor to observed scaling behaviour. We have also analyzed and processed an about 38-km non-orthogonal 3D surface reflection seismic data from the Brunswick No. 6 area, to provide 3D information about the main geological structures hosting the mineralization. The 3D data was processed using a prestack DMO and poststack migration algorithm with special focus to refraction static corrections, velocity analysis and DMO corrections that are very important for the data recorded in crystalline environment. The processing results indicate that the highly prospective and mineralized Brunswick horizon is part of a continuous reflective package that extends down to at least a depth of about 4-5 km. The reflectivity character can be used to guide future deep mineral exploration in this mining camp.
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New Insights on Large-scale Structures at the Forsmark Site, Central Sweden - Reprocessing of High-resolution Seismic Data
Authors F. Sharifi Brojerdi, C. Juhlin, A. Malehmir and E. LundbergWe present reprocessed data from seven profiles from the Forsmark site, the location where Sweden will store its spent nuclear fuel. The reprocessed data show improved images of the crust below 1 km since lower frequencies were preserved in the new processing sequences. These sections together with data from a longer more regional profile to the south of the area indicate that there may be a previously unidentified larger southwesterly dipping deformation or fracture zone in the southern part of the Forsmark area, well away from the planned repository.
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GCSP Ray Tracing Dynamic Corrections - Coalfield Example, Western China
Authors Y. Li, S. P. Peng, M. C. Liu and X. QiIn the mountainous area of the western China, coalfield seismic exploration makes the static corrections issue caused by complex near surface to be even more prominent. Compared with oil and gas seismic exploration, the depth of coal seam is often much shallower, even less than 100 m. In this case, the raypaths will not be vertical and static shifting will introduce much more errors if we adopt the surface-consistent assumption. In order to solve the serious static corrections, we develop an effective ray tracing dynamic corrections based on ghost common-source points (GCSP) gather, which differ form different reflection times, source-to-receiver offsets and azimuths. After dynamic corrections applied, a non zero-offset CDP gather is converted to a zero-offset CDP gather, and then stack can be implemented without NMO corrections. This avoids velocity analysis without effective velocity semblance contour plots when seismic data with low signal-to-noise ratio in complex near-surface areas. In addition, NMO stretching of coal seam reflection is usually serious since the depth of coal seam is shallow. Luckily, dynamic corrections can avoid to mute distortion of NMO stretching. Application of the method to real data shows the near-surface anomaly is eliminated effectively and the reflector continuity is enhanced greatly.
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High-resolution 2D Landstreamer Vibroseis Applied in Groundwater Mapping of Miocene Deposits
Authors T. Vangkilde-Pedersen, E. S. Rasmussen and M. KristensenThe Miocene succession in central and southern Jylland was formed during four phases of shoreline progradation into the basin that covers present-day Denmark. The deposits comprise several aquifers with potential drinking water resources and have been investigated by drilling and acquisition of seismic data integrated with sedimentology and biostratigraphy. The multi-disciplinary approach and an exclusively large dataset of 1200 km high-resolution 2D onshore seismic data and 150 boreholes, 100-400 m deep, has formed the basis for an extensive mapping of the Miocene succession. One of the cornerstones has been the generally very high data quality of the seismic data acquired in the past 10 years using the landstreamer vibroseis method. A detailed interpretation of the seismic data has been performed, focussing on mapping the extent of sand-rich bodies, by identifying top and base of separate delta lobes and fluvial deposits. The result of the investigations is a dynamic 3D geological model that reflects the basin development and the depositional processes as well as the palaeogeographical development during the Miocene. The model is intended to serve as a geological database of lithological and stratigraphical information and provides a better understanding of the geological architecture of the aquifers than traditional lithofacies models.
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Paleolandscape Seismic Reflection Researches for City Territory
Authors I. A. Sanfirov, A. G. Yroslavtsev, V. V. Nikiforov and V. A. BereznevShallow seismic reflection technology for city territory was presented. Subsurface relief of the loose rocks bottom and next in the depth solid rocks properties were main targets. The equipment, acquisition data, methods of the interpretation and check drilling results were described.
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On the Effects of Geometrical Spreading Corrections for a 2D Full Waveform Inversion of Recorded Shallow Seismic Surface Waves
Authors M. Schäfer, L. Groos, T. Forbriger and T. BohlenElastic full waveform inversion (FWI) of shallow seismic surface waves has the potential to reconstruct lateral variations of the shallow subsurface which is important e.g. for geotechnical site characterization. In order to make a 2D full waveform inversion algorithm applicable, shallow seismic field recordings excited by a point source (usually a hammer blow) must be transformed to mimic equivalent wavefields excited by a line source. Waves excited by a point source differ in geometrical spreading, i.e. amplitude decay and phase delay of Pi/4 in the far field from waves excited by a line source. When using the L2 norm as a measure of misfit a correction for the different decay of amplitudes is mandatory but not sufficient to obtain reliable results. In addition the signal phase must also be corrected which can be done by a convolution with 1/sqrt(t) or a Fourier-Bessel-expansion. Both spreading corrections work quite well for surface waves.
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Development of Vertical Cable Seismic System
Authors E. Asakawa, F. Murakami, Y. Sekino, T. Okamoto, K. Ishikawa, H. Tsukahara and T. ShimuraThe vertical cable seismic is one of the reflection seismic methods. It uses hydrophone arrays vertically moored from the seafloor to record acoustic waves generated by surface, deep-towed or ocean bottom sources. Analyzing the reflections from the sub-seabed, we could look into the subsurface structure. Because the VCS is an efficient high-resolution 3D seismic survey method for a spatially-bounded area, we proposed it for the hydrothermal deposit survey tool development program by Japanese government. Our first experiment of the VCS surveys has been carried out in Lake Biwa, JAPAN in 2009 for a feasibility study. Prestack depth migration is applied to 3D data to obtain good quality 3D depth volume. Based on the results, we have developed four autonomous recording VCS systems. After a trial experiment, we carried out several survey cruises for hydrothermal deposit in Okinawa Trough. Through the data acquisition, we could confirm that our VCS system works well even in the severe circumstances around hydrothermal deposits. We have, however, also confirmed that the uncertainty in the positions of the source and the hydrophones could lower the quality of subsurface image. It is necessary to develop a total survey system that assures an accurate positioning and deployment techniques.
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Frequency-domain Acoustic Modeling and Inversion in the Logarithmic Grid Set
More LessSince seismic modeling and inversion are performed using finite-sized models, boundary conditions are necessary to remove edge reflections arising from boundaries. Although several boundary conditions were developed and have been used for seismic modeling, it is not easy to remove edge reflections perfectly using those boundary conditions for some cases. In this study, we propose using the logarithmic grid set for an edge-reflection-free seismic modeling algorithm and apply it to acoustic modeling and inversion. For modeling and inversion in the logarithmic grid set, wave equation and source position should be converted to the logarithmic grid set and interpolation is required to convert data from the logarithmic grid set to the conventional grid set or reversely. The logarithmic grid set can allow us to achieve computational efficiency when the record length is not too long. Our algorithms are based on the finite-difference method, the gradient method using the new pseudo-Hessian matrix for scaling and the conjugate gradient method. We verify the acoustic modeling and inversion techniques performed in the logarithmic grid set for the Marmousi-2 model. Numerical results demonstrate that the modeling and inversion techniques for the logarithmic grid set give reasonable solutions.
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Full Waveform Inversion Using Blended Acquisition Geometry with Different Frequency Strategies
Authors A. Anagaw and M. D. SacchiIn multiscale full-waveform inversion strategies, a finite set of discrete frequencies are selected and the inversion is carried out sequentially from low to high frequency data components. First, the long wavelength components of the model parameters are recovered from low frequency data, and then more details and features are extracted as the inversion proceeds with higher frequencies. In this paper, we investigate different frequency selection strategies on the solution of a matrix-free Gauss-Newton full waveform algorithm that uses simultaneous sources. We examine five strategies for frequency selection and test the performance of the algorithm with the BP/EAGE data set. Numerical results on the BP/EAGE model show that high fidelity results can be attained by inverting partially overlapped groups of temporal discrete frequencies.
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Heeding the Waveform Inversion Nonlinearity by Unwrapping the Model and Data
Authors T. Alkhalifah and Y. ChoiUnlike traveltime inversion, waveform inversion provides relatively higher-resolution inverted models. This feature, however, comes at the cost of introducing complex nonlinearity to the inversion operator complicating the convergence process. We use unwrapped-phase-based objective functions to reduce such nonlinearity in a domain in which the high-frequency component is given by the traveltime inversion. Such information is packaged in a frequency-dependent attribute (or traveltime) that can be easily manipulated at different frequencies. It unwraps the phase of the wavefield yielding far less nonlinearity in the objective function than those experienced with the conventional misfit objective function, and yet it still holds most of the critical waveform information in its frequency dependency. However, it suffers from nonlinearity introduced by the model (or reflectivity), as events interact with each other (something like cross talk). This stems from the sinusoidal nature of the band-limited reflectivity model. Unwrapping the phase for such a model can mitigate this nonlinearity as well. Specifically, a simple modification to the inverted domain (or model), can reduce the effect of the model-induced nonlinearity and, thus, make the inversion more convergent. Simple examples are used to highlight such features.
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Recent Developments In Preconditioning the FWI Hessian - A Dimensionality Reduction Approach
Authors C. Da Silva and F. HerrmannRecent advances based on the mathematical understanding of the Hessian as, under certain conditions, a pseudo-differential operator have resulted in a new preconditioner by L. Demanet et al. Basing their approach on a suitable basis expansion for the Hessian, by suitably 'probing' the Hessian, i.e. applying the Hessian to a small number of randomized model perturbations, one can obtain an approximation to the inverse Hessian in an efficient manner. Building upon this approach, we consider this preconditioner in the context of least-squares migration and Full Waveform Inversion and specifically dimensionality reduction techniques in these domains. By utilizing previous work in simultaneous sources, we are able to develop an efficient Full Waveform Inversion scheme which recovers higher quality images than simply using simultaneous sources alone. As we will see, the preconditioning technique described above will help to regularize much of the noisy crosstalk introduced by the simultaneous sources technique without having to perform significantly more work in order to do so.
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Toward Gauss-Newton and Exact Newton Optimization for Full Waveform Inversion
Authors L. Métivier, R. Brossier, J. Virieux and S. OpertoFull Waveform Inversion (FWI) applications classically rely on efficient first-order optimization schemes, as the steepest descent or the nonlinear conjugate gradient optimization. However, second-order information provided by the Hessian matrix is proven to give a useful help in the scaling of the FWI problem and in the speed-up of the optimization. In this study, we propose an efficient matrix-free Hessian-vector formalism, that should allow to tackle Gauss-Newton (GN) and Exact-Newton (EN) optimization for large and realistic FWI targets. Our method relies on general second order adjoint formulas, based on a Lagrangian formalism. These formulas yield the possibility of computing Hessian-vector products at the cost of 2 forward simulations per shot. In this context, the computational cost (per shot) of one GN or one EN nonlinear iteration amounts to the resolution of 2 forward simulations for the computation of the gradient plus 2 forward simulations per inner linear conjugate gradient iteration. A numerical test is provided, emphasizing the possible improvement of the resolution when accounting for the exact Hessian in the inversion algorithm.
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Improved Pseudo-hessian for Frequency-domain Elastic Waveform Inversion
More LessIn seismic waveform inversion using the adjoint operator, using the pseudo-Hessian matrix as a pre-conditioner can allow us to achieve computation efficiency. However, the former pseudo-Hessian matrices (i.e., the original and new pseudo-Hessian matrices) have a limitation to simulate the features of the approximate Hessian matrix for the deeper parts of given models, in particular in elastic waveform inversion. To compensate for this limitation, we propose the improved pseudo-Hessian matrix, which is obtained introducing an auxiliary matrix in the original pseudo-Hessian matrix. Comparing the values of the improved pseudo-Hessian matrix with those of the two former pseudo-Hessian matrices indicates that the improved pseudo-Hessian matrix is much closer to the approximate Hessian matrix. Inversion results for the elastic Marmousi-2 model show that the improved pseudo-Hessian matrix increases the convergence rate and yields better results, in particular for the deeper part.
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Source Estimation for Frequency-domain FWI with Robust Penalties
Authors A. Y. Aravkin, T. van Leeuwen, H. Calandra and F. J. HerrmannSource estimation is an essential component of full waveform inversion. In the standard frequency- domain formulation, there is closed form solution for the the optimal source weights, which can thus be cheaply estimated on the fly. A growing body of work underscores the importance of robust modeling for data with large outliers or artifacts that are not captured by the forward model. Ef- fectively, the least-squares penalty on the residual is replaced by a robust penalty, such as Huber, Hybrid l1-l2 or Student’s t. As we will demonstrate, it is essential to use the same robust penalty for source estimation. In this abstract, we present a general approach to robust waveform inversion with robust source estimation. In this general formulation, there is no closed form solution for the optimal source weights so we need to solve a scalar optimization problem to obtain these weights. We can efficiently solve this optimization problem with a Newton-like method in a few iterations. The computational cost involved is of the same order as the usual least-squares source estimation procedure. We show numerical examples illustrating robust source estimation and robust waveform inversion on synthetic data with outliers.
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Microseismic Surface Monitoring Network Design - Sensitivity and Accuracy
By M. HalloHow to design a seismic network for monitoring seismicity in the vicinity of a reservoir and achieve the highest sensitivity and accuracy? What is the smallest earthquake you can detect with a given seismic network? How to design a seismic network when you want to distinguish between induced and natural earthquake? Numerical simulations are done to answer these questions. Simulation results are presented as depth slices of the minimum detectable earthquake magnitude or as depth slices of the accuracy of an earthquake location. Simulations with various network geometries helped us to draw simple rules for the optimal surface monitoring network.
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Fast and Accurate Traveltime Inversion for Locating Passive Seismic Events
More LessIn this work, a fast and accurate nonlinear traveltime inversion is presented for estimating hypocenter location parameters and origin time of a passive seismic event. To reduce the computational cost, a coarse three-dimensional gridded velocity model is adopted in which traveltimes are calculated using the eikonal equation. First, a systematic grid search algorithm is applied to minimize the objective function constructed by fitting the model response traveltimes to a finite set of observed data using an l_1 norm, as the derivative is not required. The hypocenter location parameters and origin time are obtained as the grid search technique seeks a global solution for the nonlinear and multimodal objective function. Second, the conjugate gradient algorithm is applied to improve the accuracy of the derived location parameters and origin time estimates. The conjugate gradient algorithm is immune to local minima since the starting estimates, obtained by the grid search method, are close enough to the global minimizers. To prove this inversion concept, the algorithm is implemented to a synthetic case study where the model is generated using parameterization that could represent a recording setup used during hydraulic fracture monitoring of a tight hydrocarbon reservoir, or in a CO2 sequestration project.
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Ambient Wave Field Characterization Using Three-component Arrays above a Gas Storage Reservoir
Authors N. Riahi and E. H. SaengerWe present a three-component array processor based on the Multiple Signal Characterization (MUSIC) technique. The method estimates slowness, propagation azimuth, polarization parameters, and seismic power for simultaneously active sources. This work is a step towards evaluating the hypothesis that the physical property contrasts between a partially saturated gas reservoir and the surrounding host medium are sufficient to modify the ambient seismic wave field. The new estimator is compared to the Capon high-resolution method (using the vertical component) by means of synthetic tests and two hours of night-time data from an array above an underground gas storage reservoir in France. We find that the three-component MUSIC processor provides superior sensitivity to horizontally polarized waves and considerably more accurate power estimations. Performance is otherwise similar to the Capon method. Combining propagation and polarization parameters, wave types of body waves and surface waves are identified. We observe suspected oceanic P-waves in the frequency range 1.2-1.7 Hz. As a next step, these waves and their seismic power will be compared to data acquired at the same locations six months later, when significantly more gas was stored in the reservoir.
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Near Real-time Estimation of the Seismic Source Parameters in a Dense Network of Recording Stations
More LessThrough the use of sparse representation theory and compressive sensing a methodology for the simultaneous estimation of the seismic source parameters (origin time, hypocenter location and seismic moment tensor) is presented. The methodology provides continuous, automatic estimations and is suitable for real-time monitoring systems. Furthermore, by working in a compressed domain, the limits of real-time applicability with respect to other existing methodologies are extended to consider larger subsurface monitored volumes with a larger number of recording stations. A numerical example in a dense network of recording stations emphasizes the advantages of the compressive sensing approach by reducing the memory requirements to handle the inversion variables from 52 Terabytes to 32 Megabytes with a consequent reduction in processing time from an estimated value of above 90 days to an average of around 42 seconds. The impact of the compression process in the resolvability and accuracy of the estimated source parameters is also discussed.
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Evaluation of Location Capabilities of Statistically Optimal Algorithms for Microseismic Monitoring
Authors A. F. Kushnir, M. V. Rozhkov, A. Varypaev and I. G. DrickerThe benefits of data processing approach based on the Statistically Optimal (SO) Algorithms for processing of microseismic data observed during hydraulic fracturing jobs is presented in the paper. The presence of correlated noise is a factor that increases the number of false detections in microseismic bulletins created by semblance-based SET techniques. Significant suppression of correlated noise can be achieved by application of the SO algorithms for surface array data processing if statistical characteristics of noise are taken into account; specifically, if spatial and/or temporal correlations of noise are strong enough. The two SO-methods outlined in the paper: Adaptive Microseismic Location Algorithm and Frequency-Phase Microseismic Location Algorithm. Each SO-algorithm has its advantages and limitations, so the best results of detection/location are reachable by a combination of SO-algorithms. Results of SO-algorithms benchmarking for microseismic synthetic events using a double-couple source mixed with the recorded noise during hydraulic fracturing demonstrate that for 141-sensor surface array SO-algorithms allow to accurately locate microseismic events with SNR ~ 0.05, while traditional SET processing fails to even detect events with SNR lower than 0.3.
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Lessons Learned in Evaluating Stimulation Effectiveness after a Decade of Microseismic Monitoring
Authors A. Wuestefeld, T. Urbancic and A. BaigMicroseismic monitoring of hydraulic fracturing has developed from a niche technology a decade ago into an invaluable tool to constrain fracture characteristics. We review the evolution of microseismic monitoring from a viewpoint of data collection and data analysis to the incorporation of microseismic parameters to constrain and validate reservoir models. The use of advanced processing and analysis techniques such as Seismic Moment Tensor Inversion (SMTI) have promoted the accuracy of complex Discreet Fracture Network (DFN) models. This in turn has greatly improved the predictive capabilities of reservoir simulators. Compared to simplistic models using only the event location cloud, the better knowledge of the DFN provides better constrains on the Stimulated Reservoir Volume (SRV) and Stimulated Reservoir Area (SRA).
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Microseismic Monitoring of Source Parameters in Diatomite Steam Injections Characterizes Out-of-zone Growth
Authors A. M. Baig, T. I. Urbancic and M. P. PrinceDiatomite is a highly porous, low permeability rock that can frequently be a reservoir for oil. Steam can be injected after hydraulically fracturing to reduce the viscosity of the oil and generate drainage pathways. Optimizing the production involves careful monitoring of the growth of the steam chambers to ensure containment. Microseismic monitoring offers the ability to investigate the growth of these steam chambers. Deploying properly calibrated arrays of geophones downhole or on the surface can record this energy such that the sources may be located thereby outlining the growth of the steam chamber. However, there is much more information recorded in the waveforms that may be useful for the characterization of stable development of a steam chamber versus out-of-zone growth. In one example, we determine the moment tensors of the events characterizing stable steam chamber growth and contrast those mechanisms with events from a steam cycle showing significant out-of-zone vertical growth. In a second example, we examine the spectral waveform characteristics the in-zone and out-of-zone events generated over a number of steam cycles and culminating in a rupture to the surface.
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Upper Crustal Structure of the West Bohemia/Vogtland Swarm Region Reflected by Local Seismic Signals
Authors A. Bouskova, P. Hrubcova, J. Horalek, P. Kolar and V. VavrycukWe study seismicity of the West Bohemia seismic swarm area. The area is covered by high quality seismic stations of WEBNET network. In seismic records of local events we can identify not only direct P- and S- wave arrivals but also other waves. Seismograms contain many reflected, refracted or split phases, which reflects complex geological as well as tectonic settings of the region. We try to identify these phases in seismograms and their origin at structural interfaces. Results of such investigation will improve our knowledge about upper crustal structure of the swarm area.
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Application of Full-azimuth 3D Seismic Techniques to Ultra-deep Cave-fractured Reservoirs
Authors Y. F. Wang, N. J. Wang, G. C. Gao, M. S. Wang, J. W. Di and X. ZhouIn the Tabei area of the Tarim Basin, it is hard to spatially locate beads-shaped reflection of caved petroleum reservoirs at ultra-deep depths. Additionally, the size of beads-shaped reflection is not consistent with that of the caves themselves, resulting in low precision of quantitative characterization. To overcome these challenges, we propose a full-azimuth, high-density 3D seismic acquisition method, featuring high fold and even CDP binning properties after azimuth division. This method guarantees good S/N ratio of weak reflections from ultra-deep depths by analyzing seismic P-wave theory applied in fracture system prediction. The method strengthens the identification accuracy of cave-fractured reservoirs at ultra-depths, especially improving the imaging accuracy of the connected cave-fractured system and enhancing the ability for river channel and fault system characterization.
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Single-sensor Technology - A Story of Success from Kuwait
Authors A. H. El-Emam, A. Al-Eidan and K. Shams Al-DeenIn 2002 a step change in seismic industry took place with the introduction of the single sensor technology. In early 2004, KOC conducted the first single-sensor (point-source/receiver) onshore seismic survey in the Gulf area. The detailed qualitative and quantitative analysis of the 3D pilot study proven that the technique is capable of delivering an accurate images and detailed reservoir properties. Since then, KOC adopted the single-sensor technology and successfully acquired and processed more than 4,500 Km² of Single Sensor 3D seismic surveys over nine different oilfields and prospect areas in Kuwait. Over a dozen of technical papers were published in international conferences which contributed to the understanding of the technology. This paper will navigate through this successful journey and will summarize KOC’s story of success over the past decade.
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Determination of Safety Distances for Acquiring High Resolution Seismic Data in a Populated Area of Kuwait
Authors Z. G. Jeha, G. Rached, K. Shams Al Deen and A. Al EidanFor the first time a single sensor 3D seismic survey was acquired inside a very densely populated city of Kuwait, the city of Jahra. The main challenge of this survey was to establish optimum vibrator safety distances for the infrastructures of the city as available reference safety distances charts define too conservative safety distances. A Peak Particle Velocity Survey (PPV) was conducted using the DIN 4150 German Standard as a reference. Based on the PPV results, a set of optimum safety distances were concluded to ensure that the quality of the final seismic imaging is optimized. The survey was successfully completed without any damages and the data quality achieved met the survey objectives.
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Cracking Very LF Land Seismic Acquisition – A Step Change
Authors S. Mahrooqi, S. Rawahi, S. Yarubi, A. Yahyai, K. Hunt, F. Clow and I. VincentThe emission and recording of very low frequency land seismic data provided greater potential for imaging the deep target, and for enhancing potential of applications such as Full Waveform Inversion. Nevertheless, there were several recognised or perceived limitations to producing low frequency using conventional vibrators and conventional 10 Hz geophones. Additional challenges in producing low frequency data included vibrator phase and distortion control, and the impact of similar amplitude low frequency noise associated with the acquisition environment. In March 2011, Petroleum Development Oman (PDO) commenced acquiring seismic data using conventional vibrators and 10Hz geophones with a non linear, customised 9 sec sweep with bandwidth 1.5-86 Hz. Very low frequency seismic acquisition is now ongoing on two high production DS3 land seismic crews in the Sultanate of Oman. Prior to embarking into low frequency acquisition, PDO and its contractors carried out a number of source tests, keeping in mind quality and productivity requirements. In this paper we will be presenting tests results to include harmonics appearance and to demonstrate the presence of the low frequency land data. We also discuss the impact and mitigation of external noise sources of similar low frequency.
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Broadband Seismic Data with Vibrator Harmonic Signals
Authors J. T. Hu and H. Z. WangTraditionally, the harmonic signal generated by the vibrator is recognized as noise because it degrades the shot gather after correlating the raw shot record with the sweeping signal. In fact, from the blended acquisition point of view, the vibrator harmonic signal is also a signal like the sweeping one, which is of different sweep rates. Therefore, these harmonic signals can be used to broaden the bandwidth of the correlated shot gather and enhance their quality. We first separate the different orders of harmonic record from the raw shot record, and then use all these signals to correlate with their corresponding harmonic sweeping signal to extract different correlated components, and lastly stack them together to produce the final correlated shot gather which is of broader bandwidth and higher quality. The numerical examples on field data demonstrate the feasibility of our idea.
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An Improved Swell Noise Attenuation Method Using Statistics
More LessUsing the characteristics of low frequency and high-amplitude, conventional swell noise attenuation methods basically identify the swell noise in the f-x domain according to a user-defined amplitude threshold. As it is difficult for the user to estimate the trace-and-frequency-dependent amplitude thresholds, this method cannot fully distinguish the variable swell noise from the signal. Based on a statistical technique, Bekara et al. gives a new swell noise attenuation method. Although this method can automatically estimate the trace-and-frequency-dependent amplitude thresholds, it doesn't work well when the condition of high amplitude swell noise is not satisfied. In this paper, we introduce the correlation of the signal and the randomness of the swell noise into swell noise identification. Using the correlation and randomness as weighting coefficients, our method can accurately attenuate swell noise without compromising the signal through a weighted expectation-maximization algorithm.
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Iterative Seismic Data Interpolation beyond Aliasing Using Seislet Transform
More LessThe regular and fine sampling along the time axis is common, whereas good spatial sampling is often more expensive or prohibitive and therefore is the main bottleneck for seismic resolution. We generalize missing data interpolation, a special case of data regularization, as a basis pursuit problem and use a Bregman iteration with seislet transform for recovering missing data. The seislet transform employs antialiasing dip pattern to handle aliasing information, which utilizes the scale-invariance property of prediction-error filters (PEFs). Bregman iteration provides the practical interpolation characteristics such as fast iteration convergence and reasonable interpolating result. Benchmark synthetic and field data tests confirm the effectiveness of the proposed iterative algorithm.
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An Adaptive Over / Under Data Combination Method
More LessIn this paper, we propose an adaptive over/under data combination method, which is one of the “dephase and sum” algorithms. The traditional “dephase and sum” algorithms adopt simple modeling of the ghost by assuming calm sea-surface reflection coefficients, which is invalid in rough sea conditions. In the proposed method, the amplitude spectrum of the ghost operator corresponding to each trace is estimated from the data adaptively. After that, the data combinations are implemented using the estimated ghost operators. A real dataset is used to demonstrate the performance of our method.
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Matched Filtering with Orthogonal Constraints
Authors S. Costagliola, P. Mazzucchelli and N. BienatiIn this paper we outline a new method to improve the solution of the filter set estimation problem applied to the separation of two interfering wave-fields. The problem is well described in [Guitton, 2005] where the two data components are assumed to be non-Gaussian and the cost function is minimized by using a Hybrid L1/L2 norm. Another kind of hybrid-norm solver has been derived in [Costagliola et al, 2011]. Here we want to extend the latter by introducing an uncorrelation assumption. Our aim is to use this tool to better separate the full-wavefield starting from a preliminary estimation of its components. The problem can be casted as the minimization over the set of Stiefel matrices and it can be solved by SVD decomposition (plus additional iterations in a gradient based algorithm if the set is orthogonal but not necessary orthonormal). However, this procedure is not only non-linear and non-convex, but also numerically expensive. Thus, we reformulate the problem as a constrained optimization solved by a gradient based method that searches for the solution in the neighborhood of the unconstrained optimization solution. Results on synthetic and field data show that introducing orthogonality constraints can improve the wavefield component separation.
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A Time-domain High-resolution Radon Transform Based on Iterative Model Shrinkage
By W. K. LuIn this paper, I propose a novel high-resolution time-invariant RT in the time domain based on iterative Radon model shrinkage, denoted as THRTIMS. The proposed method is realized in both time and frequency domains. The sparsity of the Radon model is promoted by simple shrinkage operation in the time domain, and the forward and inverse RTs are implemented in the frequency domain. The synthetic and real data demultiple examples demonstrate the better performance of the THRTIMS, compared with the RT and FHRT.
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Waveform Signatures in Micro-fractured Media - The Role of Effective Orthotropy
Authors H. Bernth and I. VasconcelosFracture characterization from full-waveform elastic seismic data is a key technology for detecting sweet spots and monitoring production in reservoirs such as carbonates, shale oil, tight and shale gas. We analyze the effects of small-scale fractures on elastic waveform signatures using i) the Kachanov effective medium theory to describe effective stiffness tensors and ii) an efficient Lebedev-grid finite-difference modeling scheme for monoclinic and orthorhombic symmetries. To address the waveform sensitivity to crack infill together with the accuracy of the effective orthotropy approximation, we analyze elastic waveform signatures with multiple, obliquely-oriented crack sets with varying crack densities. The waveforms show that the effective orthotropy approximation based on the second-rank crack compliance tensor is not sufficiently accurate to describe elastic waveforms. This approximation can be acceptable for dry cracks and small crack densities, but waveform errors associated with neglecting the fourth-rank crack compliance tensors increase rapidly as crack density and crack infill stiffness increase. Also, waveforms are substantially different for dry versus fluid-filled cracks. Our findings suggest that waveforms may provide sufficient sensitivity to invert for components of both second- and fourth-rank crack compliance tensors. This could aid in seismic-based discrimination of multiple fractures and fracture infill parameters.
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Controlled-source Electromagnetic Inversion Using Truncated Wavelet Representations
Authors Y. Lin, M. Li, A. Abubakar and T. M. HabashyWe present a model compression scheme for improving the efficiency of the regularized Gauss-Newton inversion algorithm for marine controlled-source electromagnetic applications. In this scheme the unknown resistivity distribution is represented in terms of a basis such as Fourier, cosine, or wavelet. By applying a proper truncation criterion, the model may then be approximated by a reduced number of basis functions, which is usually much less than the number of the model parameters. Furthermore, since the controlled-source electromagnetic measurements have low-resolution, we will show that for inversion it is sufficient to only keep the low-spatial frequency part of the image. This model compression scheme accelerates the computational time as well as reduces the memory usage of the Gauss-Newton method. For demonstration purposes, we show both synthetic and field data inversions. The results show that we are able to significantly reduce the algorithm computational complexity without compromising the quality of the inverted models.
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3D Inversion of Towed Streamer EM Data Using a Moving Sensitivity Domain
Authors M. S. Zhdanov, M. Endo, L. H. Cox, M. Cuma, G. A. Wilson and C. AndersonA towed streamer electromagnetic (EM) system capable of simultaneous seismic and EM data acquisition has recently been developed and tested in the North Sea. We introduce a 3D inversion methodology for towed streamer EM data that includes a moving sensitivity domain. Our implementation is based on the 3D integral equation method for computing the responses and Frchet derivatives, and uses the re-weighted regularized conjugate gradient method for minimizing the objective functional with focusing regularization. We present a model study relevant to hydrocarbon exploration in the North Sea. We demonstrate the ability of a towed streamer EM system to detect and characterize the Peon discovery, which is representative of an infrastructure-led shallow gas play in the North Sea. We conclude that 3D inversion of towed streamer EM can adequately recover hydrocarbon-bearing formations to depths of approximately 2 km. Obviating the need for ocean bottom receivers, the towed-streamer EM system enables EM data can be acquired over very large areas in frontier and mature basins for higher production rates and relatively lower cost than conventional CSEM methods.
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3D Inversion of Helicopter Time-domain Electromagnetic Data for Oil Sands Exploration near Fort McMurray, Alberta
Authors L. H. Cox, G. A. Wilson, M. S. Zhdanov and J. RuddHelicopter time-domain electromagnetic (HTEM) systems have dominated the airborne electromagnetic (AEM) industry for mineral exploration and environmental studies over the past decade. For oil exploration and related environmental due diligence near Fort McMurray, Alberta, Canada, recent interest in HTEM has been driven by the requirement for a cost-effective method for characterizing surface mineable oil sands and shallow steam-assisted gravity drainage (SAGD) prospects. The primary advantage of HTEM is that high-resolution data can be easily and safely acquired over large areas with zero surface disturbances at a fraction of the cost of seismic reflection. We recently demonstrated that HTEM data can be interpreted with full 3D inversion with a moving sensitivity domain, obviating reliance on various 1D methods. In this paper, we present a case study for the 3D inversion of HTEM data for oil sands exploration near Fort McMurray.
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Uncertainty Analysis of Velocity to Resistivity Transforms for Near Field Exploration
Authors D. Werthmüller, A. M. Ziolkowski and D. A. WrightJoint analysis of seismic and electromagnetic data is difficult because the data sets lack a common physical parameter, and rock physics is usually applied to link the two methods via porosity. However, rock physics parameters are not well known in near field exploration, and estimates are likely to have large errors associated with them. In this work, we use the Gassmann equation to link velocity to porosity, and the self-similar model to link porosity to resistivity. We calculate a simple depth-trend from the data, and estimate the uncertainty of our model. We apply our methodology to well logs from the North Sea. We show that the background resistivities of a field can be modelled by (1) calibrating the rock physics model on a well log from an adjacent field (including a depth-trend), and (2) calculating the corresponding uncertainty. This method is a useful tool for joint analysis of seismic and electromagnetic data.
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Combining Bandwidth Extension Techniques and Pre-stack Q Inversion for Enhanced Imaging of Intra-basalt Plays
Authors A. J. Hardwick, J. Whittaker and N. WoodburnExtensive sequences of flood basalts dominate the northwest European Atlantic margin. The basalt flows absorb and scatter the higher frequencies present in any source wavelet through intrinsic and apparent seismic attenuation. Successful sub-basalt imaging therefore tends to focus on the generation of low frequencies in acquisition and retention in processing. Conversely the Rosebank discovery in the Faroe-Shetland Basin of oil bearing sequences interbedded within the Basalt illustrates that high frequencies need to be recovered in order to delineate subtle stratigraphic plays. For this we require broad bandwidth, a robust approach to signal to noise improvement and accurate knowledge of the seismic quality factor (Q) to compensate for locally strong attenuation effects. We show that the required bandwidth can be obtained from conventionally acquired seismic data through the use of low and high frequency boosting operators and application of a horizon consistent Q field. The latter is derived using the pre-stack Q-inversion (PSQI) method to determine effective Q in defined intervals from the data itself. Through a combination of both techniques a substantial uplift in resolution is demonstrated for the highly prospective intra-basalt units with useable frequencies up to 50Hz.
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Wavepath-consistent Effective Q Estimation for Q-compensated Reverse-time Migration
Authors R. P. Fletcher, D. Nichols and M. CavalcaWe propose a method for compensating for Q effects in reverse-time migration. The new method relies upon running acoustic propagators twice to estimate attenuated traveltimes along wavepaths that are then used to filter the conventional source and receiver wavefields to compensate for amplitude and phase effects prior to imaging. As this method does not rely upon two-way time-domain viscoacoustic propagators, we avoid the difficulty of stabilizing backward propagation of the receiver wavefield when amplitudes are amplified. By separately applying phase and amplitude filters post modelling we can efficiently tune the stabilization of the amplitude amplification filter without altering the phase compensation. If required for model building, we can produce a suite of images using different percentages of the original Q model at little extra cost.
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Orthorhombic Reverse-time Migration
Authors P. J. Fowler, R. King and C. LapilliWe derive coupled partial differential equations that allow accurate and efficient modeling of P-waves in orthorhombic media but do not require explicit knowledge of shear velocities. These allow extension to orthorhombic media of methods commonly used for reverse-time migration of P-waves in transversely isotropic media.
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Generalized Pseudospectral Methods for Modeling and Reverse-time Migration in Orthorhombic Media
Authors P. J. Fowler and C. LapilliWe derive separable approximations to orthorhombic dispersion relations that allow accurate and efficient modeling of P-waves. These allow extension to orthorhombic media of generalized pseudospectral methods previously used for modeling and reverse-time migration of P-waves in transversely isotropic media.
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Raytracing and Traveltime Calculations for Orthorhombic Anisotropic Media
More LessTilted orthorhombic anisotropy exists in sediments with fractures that are aligned because of the presence of geological stress. Accurate seismic depth imaging requires coping with this form of anisotropy. In this paper, we investigate anisotropic raytracing in tilted orthorhombic media for quasi-P wave simulation and migration. First, we apply an acoustic approximation to the elastic wave equation to obtain the quasi-P wave dispersion equation, and then we propose a Hamiltonian function to derive the ray equations. We have implemented our orthorhombic raytracing in the wavefront reconstruction algorithm to compute traveltimes for Kirchhoff depth migration.
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A 3D Anisotropic Ray-tracing Methodology to Accurately Calculate 3D Salt Face Exit Points for VSP Salt Proximity Surveys
Authors M. Lou, D. Cheng and F. DohertyHighly irregular and nearly vertical salt flanks are difficult to image with surface seismic methods. Borehole seismic salt proximity surveys (SPS) have been widely used to delineate and map salt flanks adjacent to a borehole. The accuracy and reliability of the estimated salt flank from SPS data depends on the sedimentary velocity model around the borehole and above the salt, as well as the 3D ray-tracing method used to calculate seismic ray paths and travel times from seismic source to borehole receivers. Presently most of salt-exit–point calculations from SPS data are based on 3D isotropic ray-tracing algorithms. It has been recognized however that many sedimentary rocks exhibit vertical transverse isotropy (or VTI anisotropy) to seismic waves. Neglecting anisotropy could significantly affect the accuracy of the computed salt exit positions determined from SPS data. This paper presents a 3D anisotropic ray-tracing methodology to determine salt face exit points from VSP salt proximity survey data. We derive an analytic 3D vector ray equation across any curved anisotropic interface in sedimentary model. Through numerical tests imitating typical SPS case examples we demonstrate the effectiveness of our methodology and significant impact of anisotropy in determining 3D salt face exit points from SPS data.
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New Moveout Approximation for VSP Geometry in a 2D Model with Anisotropic Layers
By E. BliasI introduce a new explicit form of VSP traveltime approximation for a 2D model with non-horizontal boundaries and anisotropic layers. The goal of the new approximation is to dramatically decrease cost of time calculations by reducing the number of calculated rays in a complex multi-layered anisotropic model for VSP walkaway data with many sources. This traveltime approximation extends the generalized moveout approximation, proposed by Fomel and Stovas. The new equation is designed for borehole seismic geometry where the receivers are placed in the well while the sources are on the surface. For this, the time-offset function is presented as a sum of odd and even functions. The coefficients in this approximation are determined by calculating the travel-time and its first and second order derivations at several specific rays. Once these coefficients determined, the travel-times at other rays are calculated by this approximation. Testing of this new approximation for 2D anisotropic model with dipping boundaries shows its very high accuracy for offsets up to three depths. The new approximation can be used for 2D anisotropic models with tilted symmetry axis models for practical VSP geometry calculations.
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Combination of Multi-component Streamer Pressure and Vertical Particle Velocity - Theory and Application to Data
In this paper, we generalize the optimal deghosting (ODG) method used for deghosting over/under data to combine pressure (P) and vertical velocity (Z) data recorded with a multi-component streamer to minimize the impact of the noise on the deghosted data. The ODG approach uses pressure and velocity ghost models and the statistics of the residual noise to minimize, in a least-squares sense, the noise on the up-going/deghosted wavefield. ODG and the standard PZ summation (PZSUM) combinations are applied to pressure and velocity data recorded in the North Sea. We show that both methods attenuate the receiver ghost, fill in information at the pressure notch frequencies and that ODG has the least post-combination noise level. We also show pre- and post-stack vertical velocity data with encouraging signal-to-noise ratios. Finally, in order to further improve the PZ deghosted data, we suggest a toolbox approach that takes advantage of both ODG and PZSUM combinations and accounts for the varying signal-to-noise ratios observed on multi-component streamer data.
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Improved Imaging at Clair Field with Mirror Migration of OBC Data
Authors J. C. Mathewson, M. Ibram, N. Zimmerman, D. Underwood, A. Dawson and G. AlexanderIn conventional processing of ocean-bottom cable (OBC) seismic, after separation of upgoing and downgoing wavefields, the upgoing data are processed while downgoing data are discarded. However, the illumination of the shallow section is much better for downgoing data. During depth imaging of the Clair 3D OBC survey there was evidence of a shallow high-velocity layer that was invisible on conventionally processed upgoing data. Mirror imaging of downgoing data was applied, producing an improved image of the shallow sediments. The mirror-imaged downgoing data were input along with conventionally-imaged upgoing data to ray-traced common-image-point (CIP) tomography. Imaging of offset-vector tiles (OVTs) was used, to incorporate offset and azimuth information as accurately as possible. The inclusion of mirror-imaged downgoing data with upgoing data in CIP tomography has greatly improved the reliability and resolution of the velocity model. Depth-imaged results are also improved, for deep as well as shallow events, showing simpler structure and better event continuity.
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Piceance 3C 3D Survey - Processing of Converted-wave Data in an Area with Azimuthal Anisotropy
Authors A. P. Shatilo, R. Bansal, J. Hefti and C. RochetteThe impact of the data processing flow on converted-wave imaging and azimuthal anisotropy (HTI) analysis is investigated based on a high fold 3D 3C survey recently acquired in Colorado, USA. The processing flows with HTI compensation lead to substantial image improvement. In addition, pre-stack time migration results in higher PS image quality than a post-stack migration approach. Early pre-stack PS1 and PS2 mode separation enables more accurate velocity model building and more effective signal processing. The HTI models built using PoSTM and PreSTM data have similar trends, but the magnitudes and lateral details of the anomalies are sufficiently different to potentially result in different interpretations.
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Pure-mode SS-wave Reflections from P-wave Sources in Azimuthally Anisotropic Media
More LessEarly theoretical and field studies showed that P-wave sources generate a substantial amount of shear-wave energy for pure-mode (SS-wave) exploration seismology. SV-waves are created in isotropic or VTI (vertical transversely isotropic) media by either buried explosives or surface vibrator and weight drop sources. These methods have not been embraced by our industry; however, benefits could include shallow S-wave velocity model building to help PS-wave processing (CMP processing is easier), and S-wave surface-consistent residual statics. The purpose of this study is to re-examine the feasibility of recovering SS-waves generated by P-wave sources in azimuthally anisotropic media. We analyze 3D synthetic seismograms for VTI and HTI (horizontal transversely isotropic) media to demonstrate the analysis and retrieval of SS-waves. Using conventional PS-wave azimuth processing and analysis techniques, it is possible to recover both the fast and slow SV1- and SV2-waves related to vertically fractured media. Although fast and slow SH1- and SH2-waves are not excited by conventional P-wave sources in the anisotropy symmetry planes, these waves can also be recovered from paraxial azimuths up to 45 degrees. A field 3D-3C dataset over the Marcellus shale is an ideal test case due to the presence of SS-waves and S-wave splitting.
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Source Separation by Iterative Rank Reduction - Theory and Applications
Authors M. Maraschini, R. Dyer, K. Stevens and D. BirdBlended acquisition is quickly becoming an important topic in exploration geophysics because of the economical benefits it promises. The acquisition technique involves firing two or more sources almost simultaneously, effectively increasing spatial sampling per unit of time, but at the expense of cross-contamination from all the individual source energies overlapping one another on the blended shot record. In this expanded abstract we present an innovative method for separating a blended dataset into its constituent source components. The algorithm is an iterative scheme based on matrix rank reduction, during which the individual source components are estimated. The effectiveness of this deblending algorithm is demonstrated using first a synthetically blended dataset and then a real wide-azimuth blended dataset.
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Separating Simultaneous Source Data Using Weighted Tau-P Transform
Authors C. Zhang and B. OlofssonIn order to improve seismic acquisition productivity, simultaneous source (SiS) shooting on-shore and offshore, has drawn more and more attention recently. For marine acquisition, SiS means two or more vessels shooting at the same time along different source lines without waiting for the signals from the other to die out. The cross-talk of signals from different sources poses a great challenge to seismic data processing. This abstract introduces a methodology to separate SiS data collected in an ocean bottom node (OBN) survey. The method enhances signals alternately in different shot lines using weighted tau-p transform to extract coherent signals.
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Distance Separated Simultaneous Sweeping, Providing Record Breaking Productivity on BP's Risha 3D Seismic Survey
By J. StoneIn January 2010, BP Exploration Jordan Ltd signed an agreement with the Hashemite Kingdom of Jordan to explore and appraise the 7,200 sq km Risha Concession, located in Eastern Jordan, bordering Syria, Iraq and Saudi Arabia. In this concession agreement, a short Exploration and Appraisal period required that BP needed to acquire 5000 sq.km.of 3D data, one of the world’s largest onshore seismic surveys, faster and more cost effectively, than ever before. These goals would mean that BP Jordan would have to achieve production rates and costs (per sq km) typical of marine seismic surveys, therefore mandating another step change in vibroseis efficiency. To do so, the survey utilised the Distance Separated Simultaneous Sweeping (DS3 or DSSS) simultaneous source technology first developed and implemented by BP in 2008 (Bouska, 2009). DS3 is one of a number of high productivity technologies developed by BP (see also Howe 2008). Armed with this technology, the crew acquisition rate increased to 1,500 sq.km. per month with a peak of 59 sq. km. per day, resulting in a state of the art, high fold, wide azimuth, dataset
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Accelerating Seismic Imaging Using Novel Boundary Condition Handling
Authors M. P. Perrone and M. A. PerroneIn this paper, we present a load balancing method that attempts to avoid typical load balancing problems by removing the need for absorbing boundary condition calculations. We accomplish this by introducing an invertible, nonlinear transformation that maps the entire infinite model space into a finite space. This transform creates a finite halo of points around the modeling area of interest that models the infinite region of space beyond the limits of the model. The nonlinearity results in a slowing of wave propagation in the halo region and thereby provides a buffer zone that shields the model from unphysical reflections. Since the calculation is the same at every point of the model, load balancing is guaranteed.
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Least Squares Reverse Time Migration on GPUs - Balancing IO and Computation
More LessInverse imaging in 3D creates huge computational challenges, particularly in terms of data handling (IO) and the number of floating point operations required. Adapting inverse imaging, such as Least Squares Reverse Time Migration, to a GPU system can accelerate certain aspects of the computation however it inevitably leads to an IO dominated scheme with serious data movement bottlenecks. There are several methods, such as restricting disk accesses, asynchronously moving wavefields, saturating CPU memories and using random boundaries that can reduce the amount of compute time spent on IO. We present, through the context of LSRTM in 3D, how it is possible to map the algorithm and adapt certain aspects to create a processing scheme that is balanced between wavefield computation, IO and imaging.
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Sonic Log-derived Pore Pressure Prediction in a West Kazakhstan Dolomite Field
Authors T. Kadyrov and A. N. TutuncuNormal-trend methods and explicit methods have been used to predict pore pressure in a West Kazakhstan Dolomite field to use in the integrated wellbore stability analysis . Eaton method, Holbrook method and Bowers method were applied with modifications using sonic log data in the study. The modified Eaton method provided the best fit to MDT field data with Eaton’s exponent in the range of 0.1-0.3. Holbrook and Bower methods did not provide good agreement to field data in spite of additional modifications attempted. It was shown that the compaction exponents of the normal-trends methods and explicit methods could vary significantly from the original coefficients and from one basin to another.
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A Summary of the Positioning Formats Update undertaken by OGP
Authors T. M. H. Owen, G. S. Svik, T. G. Blackburn, L. Boer and R. J. WyldeSince the early 1990s, all marine seismic navigation data recorded in the field has been recorded using the ‘P’ formats. These formats were originally developed by UKOOA and then adopted by the industry for use worldwide, and have been in use ever since. The International Association of Oil & Gas Producers has been undertaking an update to these formats and is now aiming to release the updates to the industry. This paper details the changes made and provides an introduction to the new formats.
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On the Role of Priors in Generalized Matching Pursuit to Reconstruct Wavefields from Multicomponent Streamer Data
Authors A. Ozbek, M. Vassallo, K. Eggenberger, D. J. van Manen, K. Ozdemir and T. CurtisIn conventional single-component data acquisition, a common way to solve the problem of reconstructing aliased seismic data is to use priors that are computed at low frequencies and applied at high frequencies. In contrast, parametric matching pursuit methods such as Generalized Matching Pursuit applied on multicomponent data do not need priors in most conditions to achieve accurate reconstruction under aliasing. In this paper, we examine how and when soft priors can provide further robustness to multichannel matching pursuit algorithms. We illustrate our concepts on synthetic data generated by finite-difference modelling and on data acquired by a 3D-4C towed cable array. We find that multicomponent data allow matching pursuit algorithms to compute and use the priors in ways that are not possible with single-component data. For instance, the priors can be estimated by matching pursuit within an intermediate temporal frequency band, where the signal-to-noise ratios of all the components are high, while the data are already subject to spatial aliasing. The priors generated at these intermediate frequencies can then be used at higher frequencies where the aliasing is stronger, and also at lower frequencies, still aliased and affected by stronger noise.
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5D Semblance Based Interpolator in Exploration - Theory and Practice
Authors R. Wojslaw, J. A. Stein and T. LangstonA generalization of the standard semblance equation is used to develop a 5D interpolation algorithm. After explaining the methodology, we demonstrate its effectiveness by applying it to two onshore data sets. Both datasets have numerous acquisition skips and irregularities due to cultural obstructions or permitting issues and, as acquired, are full of 5D holes. The first is a structurally complex supra salt survey and has additional coverage and regularity issues resulting from the merging of two 3D surveys with different acquisition parameters and vintages. The second survey is a project on which azimuthal orthorhombic migration will be performed requiring appropriate offset/azimuth coverage and distribution. We will show how the technology described in this paper can address complex coverage issues by regularizing and interpolating the input data in all five dimensions to optimize velocity modeling and migration. This approach pushes toward the ultimate goal of yielding the best possible results to satisfy the exploration requirements envisioned with the acquisition of these 3D surveys.
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3-D Seismic Operational Optimization in the Lusitanian Basin, Portugal
Authors R. McWhorter, A. Clark, G. Schultz, T. Branch and R. M. LansleyThis paper will discuss the successful completion of a 3-D seismic survey in a very difficult area in Western Portugal. The survey area has many operational problems caused by small farms and vineyards, hilly terrain and many small roads and houses. A survey had been attempted previously using a cabled recording system but was abandoned before completion. The geologic background leading to the survey acquisition will be discussed, together with the careful planning and execution that were necessary to enable the successful completion of the survey. Two of the key solutions to the operational difficulties were the use of multiple different source types, two different types of vibrators and explosives, together with a cable-free recording system.
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Multi-dataset Analysis of the Tectonic Development and Framework of the Afghan-Tajik Basin
Authors D. Jackson, A. Protacio, M. Jameson and R. Johnson SabineIn 2010 ARKeX Ltd was contracted by Tethys Petroleum to perform a gravity gradiometry and magnetic survey in the Afghan-Tajik Basin. The survey involved the acquisition, processing and interpretation of airborne high resolution full tensor gravity gradiometry (FTG) and magnetic data. A multi-dataset integrated interpretation of the potential fields data with the selected 2D seismic, well, and geological data was carried out. The main aims were to define the basement depth and structure, the presence and relative amounts of salt in the geological section, and the overlying structures in the sedimentary cover. The full tensor gravity data allows more detail to be gleaned in terms of the compartmentalization of the thrustbelt geometries of the overlying suprasalt section. The quantitative interpretation involved the use of 2D Modelling where seismic data is interpreted, and densities along with rock magnetic susceptibilities are assigned to the full suite of rock bodies within the section. An iterative approach, constantly using geological rationale to change inputs in the model, is then carried out to ensure reasonable matches between observed and calculated gravity, gravity gradiometry, and magnetic profiles. The information from this study has provided valuable insights into the structural development and hydrocarbon potential of the area.
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Broadband Seismic Reviewed for the End-user Benefits in Interpretation and Reservoir Geophysics
Authors C. Reiser, F. Engelmark and E. AndersonOver the last few years, starting in 2007 with the introduction of the dual-sensor towed streamer technology, new acquisition methods and technologies have been made available with the aim of providing broader seismic bandwidth without any compromise in data quality or tradeoffs in efficiency. In this paper, we review the overall added value benefits offered by broadband seismic from the end users point of view: seismic interpreters as well as reservoir geophysicists knowing that both benefit from extended bandwidth and improved signal to noise (S/N). The benefits are presented by means of case studies in different geological settings as well as different development phase (more exploration and more appraisal/development).
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Advanced Imaging and Inversion for Unconventional Resource Plays
Authors G. M. Johnson and P. J. MillerWith a typical 3350m lateral well with more than 30 completion stages costing $8 million and upwards (upstreamonline.com 2011), the infill plan dramatically impacts long-term economics of the program. For very little relative additional cost, 3D seismic technology can be effectively utilized to reduce the risk of drilling costs overrun and maximise ultimate recovery from the field. The key is processing the seismic data specifically for these types of plays without taking shortcuts due to perceived time and cost constraints. We present a case study from the Bakken shale play in North Dakota, U.S., where advanced imaging and inversion techniques unlock the true predictive power of 3D seismic methodology for optimal development of unconventional resource plays.
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Subsalt Optimal Survey Design with Dipping Scatterers
Authors H. Djikpesse, M. Prange, M. Khodja, S. Duchenne and N. MoldoveanuWe describe a Bayesian framework for designing seismic surveys that takes into account the anisotropic nature of the radiation patterns associated with scattering from locally dipping interfaces. These interfaces are modeled as a discrete set of finite-size planar scattering elements, with the size of each element controlling the width of its radiation pattern. We show that the size of these dipping scatterers may be used to characterize the uncertainty on the dip angles that is observed on many subsalt reservoir illumination problems, thus complementing the information provided by the model-parameter uncertainties and ultimately leading to better survey designs. The benefits of the proposed method are further demonstrated with a coil survey design that aims at illuminating a complex subsalt reservoir in deep water Gulf of Mexico.
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Seismic Attenuation of Flood Basalts in the Brugdan and William Wells and Stratigraphic Correlation on the Faroes Shelf
Authors J. Schuler, P. A. F. Christie and R. S. WhiteThe Brugdan (6104/21-1) and William (6005/13-1A) wells in the Faroe-Shetland Trough penetrated stacked basalt flows for sub-basalt exploration. We examined seismic attenuation of compressional (P-) and shear (S-) waves through the basalts using short-offset vertical seismic profile (VSP) and well-log data. Seismic quality factors (Q) were calculated from spectral ratios and the root-mean-square amplitude technique in the time-domain. We found the latter method gave more robust results especially when signals were analysed from two-receiver pairs within the stacked flows. For the Brugdan basalts, we obtained an effective Q of 19‒24 and 12‒18 for P- and S-waves, respectively, while for the William prospect we estimated an effective Q of 22‒28 for P-waves. Elastic modelling was used to quantify the scattering loss using well-log-derived synthetic seismograms. We found that while P-wave attenuation could be explained by scattering mechanisms, intrinsic attenuation appeared necessary to explain the measured S-wave attenuation. Finally, a well-tie was constructed for the Brugdan well to compare migrated surface seismic data with the VSP and synthetic seismograms generated using the well-log data. While top basalt, some thick inter-basalt flows and the dolerite intrusions below base basalt showed strong reflections, enabling a confident tie, the hyaloclastite sequence shows weak reflections.
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Reconstruction of the Subsurface Reflected Wavefield on a Dense Grid from Multicomponent Streamer Data
Authors M. Vassallo, K. Eggenberger, D. J. van Manen, K. Özdemir, J. O. A. Robertsson and A. ÖzbekMultichannel reconstruction and 3D deghosting techniques based on multicomponent streamer measurements of the pressure wavefield and its associated gradients were recently introduced in literature. In particular, the Generalized Matching Pursuit (GMP) technique was applied to 3D-4C synthetic data bringing significant improvements to address the aliasing arising from sparse crossline sampling. In this abstract, we present an example of real data acquired by an experimental 3D-4C towed cable array and show the performance of GMP applied to the multicomponent measurements. The real data examples illustrate that GMP reconstructs and deghosts the pressure wavefield onto a 2D receiver grid uniformly sampled at 6.25 m in both, the inline and the crossline directions, starting from a very limited number of crossline samples at realistic spacings (i.e., 75 m). We describe the main technical challenges due to the nature of the signal and the limitations of the experiment. Despite these challenges, we show that GMP has produced a very effective reconstruction of the three-dimensional wavefield back-scattered by the subsurface for each recorded seismic shot.
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Assessment of a Marcellus Prospect Using Seismic, Microseismic and Completions Data
Authors S. W. Singleton and M. SudaMuch attention has recently been focused on modifying the traditional conventional geophysical reservoir characterization workflow in order to provide outputs that are of use to integrated asset teams in unconventional resource plays. Geophysicists are consolidating their efforts in four primary areas: prediction of anisotropy from full-azimuth data, prediction of rock properties along the Vfast azimuth (which are the ‘true’ rock properties, having minimal distortion due to vertical fractures), prediction of the three principal stresses, and fracture characterization. We have adopted this philosophy by developing a comprehensive and integrated unconventional reservoir characterization workflow. The result is a fully integrated suite of deliverables that enables the entire asset team to make informed, intelligent decisions on the best course of action in prospect development. This case study showed that petrophysics, rock physics, geophysics, and geology can successfully be integrated with reservoir and production engineering to characterize shale reservoirs. Landing zone rock brittleness as well as pre-existing fractures was shown to significantly impact well production. Therefore, a comprehensive suite of fracture characterization methods (including advanced techniques such as anisotropy and principal stress prediction) are needed to properly determine whether a pre-existing fracture zone will reopen or remain closed when hydrofracked.
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Analysis of Water Column Complexity in OBN Data
Authors C. R. Udengaard and K. CraftOcean Bottom Node (OBN) acquisition requires a very detailed understanding of the shot and receiver positions, receiver timing, and water column velocities. Knowledge of the water column velocities is necessary for processing and accurate imaging of OBN data. Processes that refine the shot positions, receiver positions, receiver timing, and water column velocities individually are subject to leakage of errors in the other domains. A process must be used that refines all values simultaneously. A traveltime inversion process is shown on an example deep water dataset. The lessons learned for preparing the data and conditioning the variables in the traveltime inversion process are shown. The result of the inversion is a detailed time and depth dependent water column velocity model, correct shot and receiver positions, and accurate receiver timing. These results are necessary for regularization, designature, and imaging of OBN data, and vital for full exploitation of 4D analysis.
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Microfabrics Related to Porosity Development, Sedimentary and Diagenetic Processes, and Composition of Unconventional Reservoir
Authors R. Slatt, N. R. O‘Brien and A. F. CadenaThis presentation illustrates and describes features of shales and mudstones observed at the nano-to micro-meter scale from freshly broken surfaces using standard scanning electronmicroscopy (SEM) and field emission scanning electron microscopy (FESEM) techniques. Microfabric observations at this scale provide insights into not only sedimentary and post-depositional processes, but also offer evidence useful in understanding hydrocarbon storage and primary migration patterns in unconventional shale reservoirs. Examples are provided from a diverse set of shale reservoirs (Barnett, Woodford, Eagle Ford, Fayetteville, Marcellus, La Luna, and Longmaxi). Microfabric and pore features illustrated include open-network floccules, clay-aligned fabric, and features associated with non-clay minerals. Types of organic matter include those produced by zooplankton and algae (e.g. coccolithophores, Tasmanites), and which may bind clay flakes into biosediment aggregates. Organic matter is also common within fecal pellets. Coccolithophores and sponge spicules may contain hollow internal chambers which provide porosity and probably permeability. We conclude that conventional scanning electron microscopy provides a rapid and relatively inexpensive way of evaluating pores and microfabrics in unconventional shale reservoirs.
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3D Simultaneous Joint PP-PS AVO Seismic Inversion in Schiehallion Field, UKCS
Authors A. S. Barnola and M. IbramA workflow for the simultaneous joint PP-PS AVO inversion of seismic data from the Schiehallion field (UKCS) is discussed. First a process to address the main challenge of getting reasonable PS to PP data registration, before any prestack inversion, is presented. Then the comparison of the estimated elastic properties from poststack and AVO inversion approaches showed the benefits of including PS along with PP AVO seismic data in a joint inversion process. The acoustic impedance quality estimated from the PP poststack, the PP AVO and the joint PP-PS AVO inversions was equivalent. However the joint PP-PS AVO inversion, compared with the PP AVO inversion, gave improved shear-related property estimation and proved that the density estimation was feasible and with an overall good quality. This inversion study confirmes that acquiring PS data in addition to PP data is justified where density is a good reservoir discriminator, as long as the PP and PS datasets are carefully processed and registered.
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Regional Rock Physics For Exploration in the West of Shetlands
By K. D. WatersOil and gas has been found in clastic reservoirs of Eocene, Palaeocene, Cretaceous and Jurassic age and in fractured basement of Devonian/Carboniferous age in the West of Shetland area. The majority of discoveries have been in Palaeocene age reservoirs. Successful prospects tend to be in combination structural/stratigraphic plays which rely on pinch-out or facies changes up dip to the east as seen at Laggan. Most of the wells targeting Tertiary prospects were drilled on amplitude or AVO anomalies. Roughly three quarters of these wells failed to find hydrocarbons. A rock physics analysis of 35 wells in the WOS was performed on a well by well and subsequently regional basis. The analysis identifies the possibility for additional mechanisms which may help to explain the amplitudes encountered during seismic interpretation in terms of not only rock and fluid properties, but of these properties within the context of complex burial and uplift histories and changing pressure regimes. The study utilises geological reports, digital well logs, pressure data, core data, biostratigraphy, AFTA analysis and temperature data along with rock physics techniques to enhance our understanding of the WOS petroleum system.
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Dynamic Statics - Improving Seismic Image on a 3D-9C Survey
Authors T. E. Galikeev, T. Davis, A. P. Zhukov, I. P. Korotkov and A. BurlakovApplication of innovative Dynamic Statics approach to solving and decoupling statics and velocity allowed for improved seismic image of PP component (pure mode P-wave), SS component (pure mode S-wave). Dynamic Statics algorithm computes statics corrections as pure time shifts on reflected waves and does not require velocity model and first break picking. Converted PS image (C-wave) was obtained automatically and was used as a QC by taking P-wave statics and velocity solution for the source component and S-wave statics and velocity for upgoing (receiver) component. Conventional velocity and first break based statics solution delivered lower quality results on this 3D-9C dataset, characterized by complex near-surface conditions, and C-wave image has never been obtained.
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Comparison of Shear-wave Velocity from Rayleigh Waves Inversion Using Different 3C Receivers - Blackfoot Oilfield Alberta
Authors E. D. Obune and R. StewartIn order to illustrate the pratical usefulness of frequency and multi-component dependency of shear-wave velocity from Rayleigh-wave inversion, we examined an industry scale acquired data. After estimating the near-surface shear wave velocities of the radial and the vertical component at different frequency, we determined the frequency and the receiver component that can see a deeper investigation depth.
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Near Surface Characterization through Simultaneous Joint Inversion of Surface Waves and Refracted Waves
Authors A. Glushchenko, A. El-Emam, W. Zahran, S. Re, C. Strobbia and M. De StefanoNear-surface characterization is an important part of seismic data processing, especially for land data. Conventional approaches rely on refracted waves and use of inversion techniques to estimate compression-wave velocity models from the first-break traveltimes. However, in a complex geologic setting in the shallow part of the subsurface, velocity inversions and hidden layers may reduce the reliability of the refraction solution, even with high-quality first breaks. Surface-wave inversion techniques provide shear-wave velocity models that can be integrated in the near-surface characterization workflow. The VP and VS models inferred from first-break traveltimes and Rayleigh waves have several synergies. Their integration can provide an effective solution for challenging geological situations. A robust way of integrating the two methods is simultaneous joint inversion: within this inversion scheme, the two measurements, together with some geological and/or petrophysical relationships linking the subsurface properties, are used in the computation of a unique cost function, which is to be minimized. The solution of the inverse problem results in a multiproperty model fitting data of both measurements and the linking relationship. Different portions of the model are resolved by the interacting contributions of complementary measurements. A field example from Kuwait is discussed to demonstrate the method.
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Physical Insight into the Elastic Brewster's Angle
Authors R. H. Tatham and P. M. KrailLight (EM) waves are transversely polarized and thus share properties with elastic shear waves. In particular, optical waves reflecting from an interface experience changes in their polarization. Brewster’s angle occurs for light reflecting from an interface such as air over a glass plate. This angle is the angle—for a particular polarization—where energy is entirely transmitted, and thus has zero reflectivity. The polarization where this occurs, in terms of the seismic nomenclature, is the SV polarization—or transverse polarization in a plane normal to the reflecting interface. In the elastic case of a seismic SV wave interacting with a solid/solid boundary, there are four resultant waves, and thus the situation is far more complex. This result occurs because of mode conversion from SV waves upon both reflection and transmission to P-waves. One of the motivations in addressing a physical insight of the Brewster’s angle phenomena is to expand our understanding of that the value of angle jB for an incident SV wave is, for most contrasts in P- and S-impedances, nearly constant. The sensitivity of this angle to contrasts in physical properties, as small as the variations may be, could prove useful in the interpretation of many rock parameters, including anisotropy.
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