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78th EAGE Conference and Exhibition 2016
- Conference date: May 30, 2016 - June 2, 2016
- Location: Online
- Published: 30 May 2016
61 - 80 of 1034 results
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Sparse Ocean Bottom Node on the Alwyn Field - From Acquisition to Joint PP-PS Imaging
Authors J. Brunelliere, S. Sioni, A. Mitra, X. Lu and A. KaragulSummaryThis case study presents the first ocean bottom node (OBN) survey acquisition and processing over the Alwyn field. The objectives of this sparse acquisition were the enhancement of lateral and vertical resolution of Triassic reservoir. Vintage seismic data consists of three conventional towed streamer surveys acquired in 1981, 1995, and 2000, suffering from limited illumination and none are repeat surveys. This new OBN survey, full azimuth, long offset seismic provided both P and PS-waves datasets for imaging and pre-stack AVO joint inversion.
The objective of this paper is to present the acquisition and processing of both P and PS-wave. Early P-wave output, obtained three months after start of processing, showed the potential of OBN data. This volume confirmed the step change in imaging over existing vintage data. PS-wave intermediate images could only be compared with P-wave due to lack of legacy PS data; structural imaging using PS data remained a challenge, however results are expected to bring value at the joint pre-stack PP-PS inversion stage.
This project demonstrated the ability of sparse OBN data to provide improved P-wave imaging at reservoir levels. Parallel P & PS-wave processing allowed a reduced turnaround time for an early PP & PS joint AVO inversion.
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Sparse Nodes and Shallow Water - PS Imaging Challenges on the Alwyn North Field
Authors J. Holden, D. Fritz, O. Bukola, J. McLeman, R. Refaat, C. Page, J. Brunelliere, S. Sioni, A. Mitra and X. LuSummaryImaging PS-wave data acquired in the shallow water at Alwyn North with ROV-deployed ocean-bottom nodes presented particular challenges due to the sparsity of the receivers. Having ensured vector fidelity of all recorded wavefields, the processing flow made simultaneous use of the PP and PS wavefields at several junctures, including construction of the imaging velocity-depth model, requiring all wavefields to be processed in parallel and in a consistent manner. Shear-wave splitting corrections and PS demultiple were addressed to improve PS data resolution and achieve data consistency. Constructing an anisotropic velocity model based on residual move-out analysis alone was not feasible as the sparse receiver sampling resulted in poor near-offset coverage. To mitigate this full waveform inversion was used to update the P-leg velocity, and surface wave inversion for the S-leg in the crucial near-seabed interval most affected by the slowest shear velocities. Joint PP-PS non-linear tomography was used to refine this velocity model. PS-wave controlled-beam pre-stack depth migration was used extensively to assess the pre-processing as well as to validate updates to the velocity model. The resultant PS-wave data, imaged with a depth model consistent with that used for the P-wave imaging, were thus suitable for joint PP-PS elastic inversion.
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Benefit of OBS PP and PS Data for Structural Interpretation on Snøhvit and Albatross Fields
Authors B. Osdal, H. M. Zadeh, M. S. Guttormsen, H. A. Aronsen, D. C. Cannavo and G. O. ØxnevadSummaryIn this case study from Snøhvit and Albatross we show how Ocean Bottom Seismic (OBS) PP and PS data is used to improve the structural interpretation of reservoirs below overburden shallow gas anomalies. Comprehensive processing, velocity model building and data integration were crucial to achieve good results. The OBS PS has a good quality below the shallow gas anomaly on both Albatross and Snøhvit. On Snøhvit, OBS PP data shows better quality compare to streamer data. This clearly reduces uncertainties related to structural interpretation compare to streamer data alone.
However, on Albatross the quality of both OBS PP and streamer data are poor, and high quality OBS PS data has given largest impact on structural interpretation. Updated structural interpretation greatly improved our geological understanding and may change the reservoir management of the field.
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Full-azimuth Ocean Bottom Seismic for Imaging Beneath Complex Overburden at Johan Sverdrup
Authors B. King, S. Winterstø, J. Nilssen, D. Underwood, D. Brager, S. Mitchell and J. AvilesSummaryStructural interpretation of reservoir events at the Johan Sverdrup field in the North Sea can be challenging in areas with complex overburden. Re-mobilised sands (injectites) with very high velocity relative to surrounding sediments cause strongly localized pull-up of deeper reflections. In addition, shallow channels less than 200m below sea bed filled with low velocity sediments cause pull-down effects. Both pull-up and pull-down effects can leave imprints in the interpretation of reservoir events.
We will show how Ocean Bottom Seismic (OBS) data with long offset and rich azimuth can help resolve such problems, by either under-shooting the injectities completely, or by providing high quality data to enable better processing and velocity model building, including FWI.
The survey was acquired at a reduced cost by using a system with multi-component nodes connected by passive cables. We will show that good data quality was achieved using this system, and we will discuss the additional processing steps required.
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Efficient Wave Mode Separation in Anisotropic Media - Part I-Separation Operators
More LessSummaryIn this paper we present efficient wave mode separation operators in anisotropic media. These operators are constructed by local rotation of wave vector to direction where qP-wave is polarized. The deviation angle between wave normal and qP-wave’s polarization direction is spatially estimated using poynting vector. The proposed operators provide comparable results with those generated by classical space domain convolution operators in anisotropic media. In the meantime, the added computational cost by applying new operators is small when compared to divergence and curl operators as used in isotropic case. Synthetic results validate the effectiveness of the present operators.
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Efficient Wave Mode Separation in Anisotropic Media - Part II-Phase and Amplitude Corrections
More LessSummaryIn this paper we present two improved wave mode separation methods in anisotropic media. The proposed methods aim to efficiently implement wave mode separation while preserving properties of decomposed physical quantities from original elastic wavefields. The first approach is using vector decomposition which output decomposed wave modes with same amplitude, phase as the input wavfields. The second method is introducing amplitude and phase corrections to divergence-like and curl-like operators, then wavefields energy can be conserved in wave mode separation. Synthetic results verify the effectiveness of the proposed methods.
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A New Scalar Imaging Condition for Vector-based Elastic Reverse Time Migration
More LessSummaryScalar images (i.e. PP-, PS-, SP- and SS-wave images) of elastic reverse time migration (RTM) can generated by applying an imaging condition to pure wave modes. Decoupled P- and S-wave equation provides an alternative way to separate P- and S-wavefields in elastic RTM. However, the output separated wavefields are both vectorial. To obtain the scalar images, we propose a new imaging condition in which the scalar product of two vector wavefields is exploited to produce scalar images in elastic RTM. For this scalar-product-based source-normalized elastic crosscorrelation condition needs, no additional prior information is required, except for the P- and S-wave velocities. And additional polarity reversal correction methods are not required to correct polarity changes in converted-wave images for the output separated wavefields have consistent amplitude polarity. Numerical example on Marmosui2 is used to test the effectiveness and robustness of our new imaging condition.
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Elastic Reverse Time Migration Based on Decoupled Wave Equation and Inner Product Imaging Condition
More LessSummaryIn conventional elastic reverse time migration, Helmholtz Decomposition is applied for wavefield separation, which leads to polarity reversal in converted wave imaging. In this paper, we introduce a vector wavefields separation method with amplitude preserved based on decoupled elastic wave equation and illuminate the specific physical meaning of the vector P- and S-wave. In addition, the inner product imaging condition is utilized to obtain image results form the vector P- and S-wavefield. Analysis of imaging kernel indicates that the presented method can effectively circumvent polarity reversal in ground seismic exploration. Migration results have demonstrated the feasibility and effectiveness of this imaging method.
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A Single-sided Representation for Virtual Sources and Virtual Receivers
Authors K. Wapenaar, J.W. Thorbecke, J.R. van der Neut, S. Singh, E.C. Slob and R. SniederSummaryVirtual sources can be created in several ways. In seismic interferometry, a virtual source is created by crosscorrelating responses at different receivers, which are illuminated from all directions. Seismic interferometry can be mathematically described by the homogeneous Green’s function representation, which is a closed boundary integral.
Virtual sources can also be created with the Marchenko method. For the Marchenko method it is sufficient that the position of the virtual source is illuminated from one side. We derive a single-sided homogeneous Green’s function representation, which is an open boundary integral along reflection measurements at the surface. Applying this representation, we obtain virtual sources and virtual receivers in the subsurface from real sources and receivers at the surface (note that in our earlier work on the Marchenko method the response to the virtual source was only obtained for receivers at the surface). The retrieved virtual data show the entire evolution of the response to a virtual source in the subsurface, including primary and multiple scattering at unknown interfaces.
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Full-wavefield Redatuming of Perturbed Fields with the Marchenko Method
Authors I. Vasconcelos and J. van der NeutSummaryWavefield extrapolation, or redatuming, is a critical step for imaging. It is particularly challenging in areas such as subsalt or under complex overburdens. The framework of Marchenko redatuming allows for the retrieval of up- and downgoing fields at chosen locations in the subsurface that contain primary arrivals and internal multiples, while requiring relatively little knowledge of the subsurface model. In this paper, we present a new form of the Marchenko system for perturbed fields. Based on this system, we present a new iterative scheme that explicitly reconstructs only the unknown perturbations to the Marchenko focusing functions, and by consequence only the perturbed/scattered up- and downgoing Green’s functions. This new scheme departs from previous versions of the method in that it requires additional inputs, which include an extra initial focusing operator and perturbations to the surface reflection data. We validate our method with numerical tests, showing that it is particularly well-suited to properly handle complex models with large/sharp constrasts such as salt boundaries. We foresee this new approach to be of use not only in general imaging applications, but also for time-lapse studies as it can directly redatum time-lapse changes.
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Constructing Only the Primary Reflections in Seismic Data - Without Multiple Removal
Authors G.A. Meles, K. Wapenaar, A. Curtis and C. da Costa FilhoSummaryState of the art methods to image the Earth’s subsurface using active-source seismic reflection data involve reverse-time migration (RTM). This, and other standard seismic processing methods such as velocity analysis, provide best results only when all waves in the data set are primaries (waves reflected only once). A variety of methods are therefore deployed as pre-processing to predict multiples (waves reflected several times); however, accurate removal of those predicted multiples from recorded data using adaptive subtraction techniques proves challenging, even in cases where they can be predicted with reasonable accuracy. We describe a new, alternative strategy: we construct a parallel data set consisting of only primaries, which is calculated directly from recorded data. This obviates the need for both multiple prediction and removal methods. Primaries are constructed using convolutional interferometry to combine first arriving events of up-going and direct-wave down-going Green’s functions to virtual receivers in the subsurface. The required up-going wavefields to virtual receivers are constructed by Marchenko redatuming. Crucially, this is possible without detailed models of the Earth’s subsurface velocity structure. The method is shown both to be particularly robust against errors in the reference velocity model used, and to improve migrated images substantially.
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Coupled Seismo-Electromagnetic Interferometry for 2D Homogeneous SH-TE Scenarios
Authors N. Grobbe, E.C. Slob and C.P.A. WapenaarSummaryWe here explore the application of interferometric principles to the coupled seismo-electromagnetic system. We consider 2D homogeneous space scenarios, and focus on one of the two seismo-electromagnetic propagation modes, the SH-TE mode, where horizontally polarized shear-waves are coupled to the transverse electric mode.
We start by presenting the theory for retrieving seismo-electromagnetic Green’s function responses via interferometry by cross-correlation.
Using explicit homogeneous space solutions, we numerically investigate the interferometric retrieval of an electric field response and a magnetic field response due to a seismic source. We first study the theoretically desirable circular source configuration, providing illumination from all sides, followed by a more realistic line source configuration, exploiting the interferometric far-field approximation. These two numerical examples prove for the selected source-receiver type combinations, that we can indeed retrieve correct dynamic seismo-electromagnetic 2D SH-TE responses using seismic boundary sources only. This is a promising result for the next step: applying 3D seismo-electromagnetic interferometry in the field.
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Anisotropic Seismic Noise Gradiometry by Elliptically-anisotropic Wave Equation Inversion - An Example Ekofisk
Authors S.A.L. de Ridder and A. CurtisSummaryWe propose an anisotropic wavefield gradiometry technique to extract azimuthally anisotropic phase velocities from seismic noise that is dominated by a single surface wave mode. The method relies on a two-dimensional elliptical-anisotropic wave equation. This wave equation equates the spatial derivatives of the wavefield amplitudes with the temporal derivatives through the elements of a two-by-two matrix characterizing the medium parameters. The derivatives are evaluated using finite differences, and the system is inverted with a smoothness constraint. We test the procedure on ambient seismic noise recorded in a large and dense array installed over Ekofisk field. Because the station spacing is much larger cross-line then inline, the approximation error of the spatial finite difference results in an apparent anisotropy. From an experiment with synthetic isotropic plane waves, we define a Jacobian to correct the finite difference stencils. With the corrected finite difference stencils, we extracted anisotropic phase velocities at Ekofisk from as little as 10 minutes of seismic noise recordings. The azimuthal anisotropy forms a circular geometry around the production induced subsidence bowl. The methodology is a promising technique for studying changes in the subsurface geomechanical stress-state resulting from time-dependent phenomena operating at a short time-scales.
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Use of Ambient Noise to Enhance Low Frequencies Seismic Migration Images
Authors B. De Cacqueray, J. Cotton, F. Duret, C. Berron and E. ForguesSummaryIn this study, we propose an example of body wave retrieval using ambient noise correlation. The correlated data are migrated using a Kirchhoff pre-stack time migration (PSTM) and then with the migration obtained with conventional active data. It allows to considerably broaden the final image spectrum toward the low frequencies.
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Shallow Rayleigh-wave Tomography Using Traffic Noise From Long Beach, California, USA
Authors J.P. Chang, S.A.L. de Ridder and B.L. BiondiSummaryUsing recordings from a dense seismic array in Long Beach, California, USA, we demonstrate the effectiveness of using traffic noise for shallow subsurface imaging. Using the ambient-noise cross-correlation technique at frequencies greater than 3 Hz, we extract fundamental- and first-order-mode Rayleigh waves generated by Interstate 405 and local roads. We use group travel times associated with the fundamental mode in a weighted straight-ray tomography procedure to produce group velocity maps at 3.0 Hz and 3.5 Hz. The velocity trends in our results correspond to shallow depths and coincide well with lithologies outlined in a geologic map of the survey area. The most prominent features resolved in our velocity maps are the low velocities in the north corresponding to less-consolidated materials, high velocities in the south corresponding to more-consolidated materials, a low-velocity zone corresponding to artificial fill in Alamitos Bay, and a low-velocity linear feature in the Newport-Inglewood Fault Zone.
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Locating Scatterers Ahead of a Tunnel Boring Machine Using Noise Correlation
Authors U. Harmankaya, A. Kaslilar, K. Wapenaar and D. DraganovSummaryThe investigation and detection of faults, fractures, karst zones, cavities, etc., is important to reduce hazardous risks, in particular during excavation with tunnel boring machines. To locate such scatterers, we propose a method based on seismic interferometry that uses the noise signals generated by a TBM. Using finite-difference modelling of TBM noise in a homogeneous half-space, we model noise recordings. We then correlate the noise to obtain non-physical scattered arrivals for various scatterer locations. We use both non-physical scattered P- and S-wave arrival times to successfully estimate the location of scatterers.
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3D Modeling of the Regional Basement Structure Off- and Onshore Congo Based on Inversion of Satellite-derived Gravity Da
More LessSummaryThe regional geology of the African equatorial west coast is complex and not well understood since the seismic coverage of deeper crustal features is sparse. The intricate structural settings are also reflected in potential field anomalies, and in particular in gravity data. The presented study primarily aims at estimating basement depths from satellite-derived gravity anomalies in a 400 km by 300 km wide area off-and onshore the territories of Congo, Cabinda, and the Democratic Republic of Congo. Where available, well and seismic data are used to constrain the inversion. The final 3D density model indicates a lateral division in 3 zones (oceanic crust, transition zone, continental crust) and maximum sediment thicknesses of 2.5 / 7.5 km on- / offshore. Although non-uniqueness and ambiguity of potential field data still pose challenges, it is concluded that public-domain gravity data can add useful information for regional modeling.
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Applying Magnetic Magnitude Transforms to Aid Structural Mapping in Areas Where the RTP Calculation Is Unreliable
Authors S. Cheyney, C.M. Green, S.J. Campbell and D. de LermaSummaryA common use for magnetic data is to locate the edges of subsurface bodies. Numerous transformations have been developed in order to obtain more easily interpretable maps, however many initially require the data to be reduced to the pole (RTP) if they are to be used for edge detection. The RTP operation is unstable at low magnetic latitudes, which affects significant areas of equatorial regions currently of interest to the oil and gas industry. In addition, any remanent magnetisation will often lead to anomalous results.
We demonstrate the advantages and disadvantages of using magnitude transformations, focusing on the scenarios where they are likely to be the most useful: at low latitudes and in the presence of remanent magnetisation. Based on synthetic modelling, magnitude transforms equivalent to 1st or 2nd order derivatives of the magnetic field appear most suitable, however when applied to real data the effects of enhancing the noise level limits the usefulness. The Total Amplitude, which is of the same order as the observed TMI data, has higher centricity than the TMI allowing lineament interpretation, without biasing the result to shorter-wavelength features and accentuating the noise level in the data.
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Noise Rejection in Processing of Magnetic Data - 2D Fourier Transformation Treated as an Inverse Problem
Authors H. Szegedi, A. Kiss, M. Dobroka and Á. GyulaiSummaryIn this paper, a new, robust and resistant, inversion based 2D Fourier transformation is presented where the spectrum is discretized by series expansion (S-IRLS-FT). The series expansion coefficients as model parameters are given by the solution of the inverse problem. Since it is advantageous to use squared-integrable, full, orthogonal and normed basis functions, Hermite-functions are chosen as basis functions of the inversion based Fourier transformation. Taking advantage of the beneficial properties of Hermite polynomials, that they are the eigenfunctions of the inverse Fourier transformation, the elements of the Jacobian matrix can be calculated fast and easily, without integration. The procedure can be robustified using Iteratively Reweighted Least Squares (IRLS) method with Steiner weights. The advantage of the Steiner weights is that the scale parameters ( 2) can be determined from the statistic of the measured data set in an inner iteration process. Thus, a very effective robust and resistant inversion procedure can be defined. Its applicability using magnetic data calculated above a square and “L”-shape object is proved.
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Approximate CRUST1.0 Model-based and Space-domain Calculated Gravitational Effect of the Earth Crust
Authors J. Mikuška, I. Marušiak, P. Zahorec, J. Papčo and R. PaštekaSummaryRecent deep seismic studies support the existing isostatic systems only to a limited extent and, in some areas, considerable disagreements have been pointed at. This implies that we should attempt to replace the classic isostatic corrections by quantities calculated from a-priori information which should be as independent of the gravity data themselves as possible. We believe that CRUST1.0 model provides applicable material for such an attempt. In our contribution we demonstrate that to calculate gravitational effects of the CRUST1.0 model layers taking the model densities into consideration is in fact a viable option. We have used two different methods of calculation, more or less independent of each other, both of them working in space domain. We believe that space-domain calculations are suitable for employing the results in gravimetrical practice since they not only are synoptic, i.e. offering a general view of the whole, but also enable us to analyze their elementary and differential properties.
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