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77th EAGE Conference and Exhibition - Workshops
- Conference date: June 1-4, 2015
- Location: Madrid, Spain
- Published: 01 June 2015
51 - 100 of 128 results
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Diffraction Imaging - A Tool for Detection Small Scale Subsurface Heterogeneities
By E. LandaSummarySeismic diffraction is a carrier of information coming from subsurface objects of sub-wavelength scale. Isolating diffraction from the full wavefield and imaging them separately is a first step in establishing super-resolution of structural details. Although the importance of diffracted waved has been long time recognized in seismic exploration, in practice they are ignored in standard data processing and imaging. Naturally fractured reservoirs are an important target for the oil and gas industry. Usually information about the fractures comes from coherency cube interpretation or azimuthal analysis of the effective media. It is only seismic diffraction can directly indicate sharp structural and lithological changes in the subsurface.
Separation diffraction from specular reflection in the data domain is the first step of diffraction imaging. It can be efficiently done using differences in kinematic and dynamic properties of reflected and diffracted events. At second step, focusing diffractive component allows us reliably visualize small and medium scale elements of the subsurface such as faults, pinchouts, karsts, fractures etc.
I will demonstrate diffraction imaging on synthetic and real data examples.
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Super-resolution by Moveout Correction and Migration of Surface-related Resonant Multiples
Authors B. Guo, Y. Huang and G.T. SchusterSummarySurface-related resonant multiples can be migrated to achieve better resolution than primary reflection data. However, such multiples are only recorded by near-offset traces. As a result, the migration image using resonant multiples suffers from poor signal-to-noise ratio. In this report, I mitigate this problem by aligning the first-order surface-related multiples across different offsets with the zero-offset resonant multiples. Such an alignment is based on the moveout characteristics of the first-order surface-related multiples. Stacking the aligned multiples improves the signal-to-noise ratio of the zero-offset resonant multiples as well as the resulting migration image. Results with both synthetic and field data results validate this method and show migration images with super-resolution characteristics.
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Handling the Conflicting Dip Problem in the CRS/i-CRS Methods
Authors J. Walda and D. GajewskiSummaryMost current implementations of the CRS operator suffer from the occurrence of conflicting dip situations in the acquired data. To address this properly we apply the idea of the CDS. We use the i-CRS operator that can be related to the CRS operator, and show, that conflicting dips can be resolved well in multiparameter processing. The results are promising and reveal a lot of potential for further applications. This is shown by a diffraction separation technique applied to field data obtained in the Levantine Basin.
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3D CRS-based Prestack Diffraction Separation and Imaging
Authors P. Bakhtiari Rad, C. Vanelle and D. GajewskiSummaryImaging of seismic diffractions is a challenge since it is inherently a 3D problem. Diffractions carry useful information about the subsurface and allow to identify the presence of small-scale heterogeneities and structures e.g. fractures, pinch-outs, thin lenses etc. Thus, diffraction separation and imaging can lead to higher resolution, which is of particular interest for reservoir characterisation and exploration.
In this work, we suggest a 3D workflow based on common-reflection-surface (CRS) method for prestack diffraction separation and imaging in time domain. The workflow combines the ideas developed for diffraction separation with the partial CRS stack technique. It comprises not only the diffraction separation facility but also includes a prestack data enhancement, i.e., an improved SNR in diffraction-only data. Application to a 3D synthetic model confirms its effectiveness in prestack diffraction separation. It also demonstrates potential for time migration velocity analysis using diffraction-only data.
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Zero-offset Based Prediction of Common-offset Diffraction Traveltimes
Authors A. Bauer, B. Schwarz and D. GajewskiSummaryThe imaging of diffracted waves is a crucial challenge in seismic processing, because they carry important information about small-scale subsurface structures. A key step of diffraction imaging is their separation and enhancement in the pre-stack data volume, which requires common-offset processing. However, due to the higher dimensionality of the problem, common-offset stacking is computationally more expensive than the stable and commonly used zero-offset processing. In this work, we motivate a straightforward decomposition principle for diffractions, which establishes a direct connection between zero-offset and common-offset diffraction wavefield attributes based on the decoupling of diffraction raypaths. We show, theoretically and on simple waveform data, that each common-offset diffraction operator can be decomposed exactly into two zero-offset operators. This allows the direct prediction of common-offset diffraction attributes solely based on their zero-offset counterparts. Application of the new method to complex data reveals its ability to reliably image diffractions in the common-offset domain using only results from zero-offset processing as input. The promising results in terms of both image and attributes reveal a high potential for improved pre-stack diffraction separation and diffraction-stereotomography.
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Imaging Spatio-temporal Changes of the Earth with Coda Waves
Authors L. Margerin, T. Planès, J. Mayor, M. Calvet, E. Larose and V. RossettoSummaryCoda-wave interferometry is a powerful technique which exploits waveform perturbations observed in the coda to monitor changes of the propagation medium. In this work, we use a radiative transfer approach to model two relevant observables which may be employed for monitoring purposes: travel-time shifts and de-correlation of waveforms. These observables are sensitive to weak changes of the background velocity or to the addition of mechanical defects, respectively. We develop specific sensitivity functions for each type of observable and evaluate them numerically using analytical solutions of the radiative transfer equation. Our theory can model arbitrarily anisotropic wave fields and is not limited by a diffusion approximation. We show that the coherent wave plays a crucial role in the sensitivity of coda waves in the weak scattering regime. In particular, the coherent wave is responsible for the algebraic divergence of both the travel time and de-correlation sensitivity kernels at the source and receiver. The de-correlation kernel shows an additional zone of high sensitivity in the vicinity of the single-scattering ellipse. These sensitivity functions may be employed to develop a linearized tomographic approach to the monitoring of medium changes in the weak scattering regime.
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Imaging in Random Media
By L. BorceaSummaryI will present an introduction to the mathematical theory of wave propagation in heterogeneous media with uncertain micro-scale, modelled as random media. I will summarize the scattering effects in such media and how to mitigate them in order to obtain high fidelity imaging results.
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Focusing Prestack Depth Imaging Approaches
Authors F. Hlousek, O. Hellwig and S. BuskeSummaryThree focusing imaging approaches are presented which are based on Kirchhoff prestack depth migration. All approaches can be formulated as a weighted diffraction stack with different weighting functions. In any case the smearing along the two-wave-traveltime isochrone is limited to the physically relevant part at the specular reflection or diffraction point. The focusing characteristics of these approaches can be very valuable for increasing the image quality, both for hydrocarbon exploration data sets as well as for imaging in complex crystalline crust.
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Can We Image Everything with Primaries?
Authors D.J. Verschuur, A.J. Berkhout, M. Davydenko and S.R. StaalSummaryFor true amplitude imaging with reduced artifacts a closed-loop approach for seismic migration needs to be employed (‘least-squares migration’). However, today’s least-squares migration algorithms do not take into account multiple reflections and transmission effects. The use of multiples will enlarge the illumination of the subsurface and can be of crucial importance when primaries are not properly measured due to acquisition constraints and background noise. In this paper it is shown that the full wavefield migration (FWM) process can image subsurface structures via internal multiples. This is successfully demonstrated for imaging near-surface structures for land data when their primaries are not properly measured.
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A Practical Approach to Vector-acoustic Imaging of Primaries and Free-surface Multiples
Authors M. Ravasi, I. Vasconcelos, A. Curtis and A. KritskiSummaryFree-surface multiples travel different paths and illuminate different volumes of the subsurface than primaries. When used jointly with primaries to image the subsurface by means of forward and backward extrapolation of separated down- and up-going wave components respectively, free-surface multiples have been shown to improve the continuity of shallow parts of the subsurface image by suppressing acquisition related footprints.
We show that by carefully combining the full pressure and particle velocity data by means of newly developed, vector-acoustic boundary conditions, wavefronts can be forward and backward propagated without ambiguity in their propagation direction. Wavefield decomposition is thus naturally incorporated within the extrapolation procedure.
Moreover, ocean-bottom acquisition geometries generally present source coverage that is wider than the receiver array. A strategy is proposed to incorporate in our imaging scheme energy of primary events whose direct source illumination lies outside of the receiver aperture. This is achieved by combining a directly modelled source illumination with the recorded (down-going) data.
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Complete Wavefield Imaging Using Broadband Dual-sensor Streamer Data
SummaryWhen considering shallow water towed streamer data, developing an accurate velocity model using primary reflections alone is challenging and often only allows an average velocity of the near surface interval to be defined. For robust reservoir imaging, it is essential to resolve near surface velocity heterogeneity, thus the conventional VMB approach, which uses data acquired with standard streamers, is compromised.
A methodology for VMB utilising the complete wavefield has been presented. The foundation of the method is dual-sensor towed streamer data which allows separation of the recorded wavefield and is thus the technology enabler for an advanced imaging workflow. Complete wavefield imaging delivers a highresolution velocity model of the near surface which in a conventional approach typically remains unresolved. The more accurate near surface velocity model and subsequent imaging improves the accuracy and confidence of the imaging at reservoir levels.
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Causality Aspects of the Elastodynamic Marchenko Method
By K. WapenaarSummaryWith the acoustic single-sided Marchenko method it is possible to retrieve the Green’s function of a virtual source in the subsurface from the single-sided reflection response of the medium and an estimate of the first arrival of the Green’s function. Important ingredients of the Marchenko method are the so-called focusing functions. One of the underlying ideas of the acoustic Marchenko method is that the Green’s functions and focusing functions reside in different time intervals in the time domain. We call this the causality condition. The only overlap of the Green’s functions and focusing functions occurs at one time instant, namely at the time of the direct arrival of the Green’s function.
In this paper we analyze the causality condition for the elastodynamic extension of the single-sided Marchenko method. It appears that the overlap of the elastodynamic Green’s functions and focusing functions occurs in an extended time interval. The parts of the focusing functions that overlap with the Green’s functions cannot be retrieved with the Marchenko method, and must therefore be specified separately. It appears that these overlapping parts are defined as the inverse of the forward-scattered part of the transmission response of the medium.
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Reliable Depth Imaging - A Matter of Selective Integration of Subsurface Knowledge
Authors D. Sineva, Q. Liao Yang, W. Harmony and G. GonzalezSummaryDuring the last few years significant improvements have been made in all branches of geophysical technology, from acquisition and processing all the way to depth imaging. The diversity of existing technology and variety of methodologies that can be applied for depth imaging make it difficult to choose the right technology that will result in a “perfect” image. In order to find that perfect combination of rapidly evolving technologies we need first to understand the environment: regional geology, the possible target, the known geological and geo-mechanical scenarios and all other possible challenges that will be reflected on our seismic data.
Although today we are able to acquire high resolution, high density, Multi or Wide azimuth seismic data, without proper understanding of geology and careful integration of all related geo-disciplines the final result will not be satisfactory.
The knowledge integration should happen at every stage from seismic acquisition planning to delivery of the final image. Thus what to integrate becomes a very critical question.
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Decoupled TTI P-wave Poststack RTM and Demigration Using Pseusdo-spectral Operators
Authors J. Jiao, Q. Zhang, X. Ma, P. Stoffa and G. GonzalezSummaryThe pure P-wave equation for tilted transverse isotropic reverse-time migration is free of shear-wave artifacts unlike the widely used coupled TTI equations. A new hybrid pseudo spectral and finite-difference algorithm has recently been developed to solve the TTI pure P-wave equation for prestack RTM. This algorithm applies part of the wave propagation in the wave number domain and part in the space domain and is inherently parallel improving its computational efficiency. Here we extend this hybrid algorithm to 3D post-stack de-migration. De-migration of stacked images is the reverse process of migration and generates un-migrated seismograms compatible to seismic data with zero offset. We have tested the proposed de-migration on both field and synthetic seismic data and we have also used the proposed demigration to establish a workflow to validate or build anisotropic models. After de-migration is performed with the models used in the previous migration, we select a series of anisotropic models to re-migrate the seismograms generated by the de-migration. The new models may be derived from seismic-well ties, perturbation of anisotropic parameters, perturbation of locations of interpreted horizons. By investigating a series of re-migrated images, an experienced interpreter can identify which model is the most geologically plausible.
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Cost Effective Improvements for Seismic Survey Planning Using Remote Sensing
By W.M. BaughSummarySignificant new capabilities for remotely sensed material and mineral identification were enabled with the launch of the WorldView-3 (WV-3) commercial remote sensing satellite on 13 August 2014. Most noteworthy (for material/mineral identification) are the addition of 8 high resolution super-spectral bands in the shortwave infrared (SWIR) range of the electromagnetic spectrum.
The SWIR imaging bands are collected at a native 3.7 m resolution, and cover a range of diverse and unique absorption features for materials of interest to geophysical exploration. In addition, there are 8 bands of WV-2 heritage spanning the visible through near-infrared (VNIR) range. The VNIR bands are collected at a higher 1.2 m resolution. A panchromatic band at 0.31 m resolution reveals unprecedented spatial details from space.
WV-3 is a sensor for geophysical applications. Its new capabilities look beyond the image pixels, and into materials, minerals, moisture content, and disturbance. For example, surface material information provides a natural extension to the seismic survey risk mapping proposed in the article “Satellite Sensing: Risk Mapping for Seismic Surveys” ( Oilfield Review, 2009 , v. 20, n. 4). Improved knowledge about surface characteristics benefits both seismic vehicle logistics as well as understanding of near-surface velocities.
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Surface Microseismic Imaging - Influence of High Velocity Layers
Authors D. Price, D.A. Angus, K. Chambers and G. JonesSummaryImaging artifacts due to the influence of high velocity layers on the reduction of effective array aperture and the presence of increased multiples in the microseismic data are examined. FD full-waveform microseismic synthetics were generated that mimic a typical surface monitoring array for a range of 1D velocity models. Microseismic event locations using two different imaging techniques were compared: standard diffraction imaging (SDI) and moment tensor microseismic imaging (MTMI) algorithms. The results confirm that the presence of high velocity anhydrite layers reduce the overall aperture of a surface array, which results in poor resolution of imaged events, and a reduction in accuracy of event locations and effective monitoring area. The presence of high velocity lithological units also increases the amount of multiple and converted waves in the seismic data, resulting in an increase in coherent noise following the primary arrivals. Comparison of the two imaging procedures conclude that MTMI produces a much cleaner, less noisy image domain with more accurate and precise location estimations for similar monitoring scenarios, but both MTMI and SDI are equally affected by the presence of high velocity layers and recorded event frequency. For settings where high velocity lithological units are expected, the results of this study suggest that larger aperture arrays be deployed and the application of novel/advanced processing techniques be incorporated into the pre-processing of microseismic data to reduce multiple and converted wave noise.
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Analysis and Interpretation of Induced Micro-seismicity by Flooding of the Gardanne Coal Basin (Provence – Southern France)
Authors E. Matrullo, I. Contrucci, P. Dominique, M. Bennani, H. Aochi, J. Kinsher, P. Bernard and P. BigarréSummaryMining activity in Provence has left many underground voids that can cause land subsidence or collapses in areas of high population density. In the abandoned coal basin of Gardanne, the flooding of mine voids and fluctuations in the groundwater level induced many seismic events that have been recorded since the beginning of operational monitoring. An example is the seismic crisis of November 2012, where magnitude events > 2.5 were felt by the population.
In this context, the objective is to understand the mechanisms of the observed seismicity. This will clarify the relationships between seismicity and rising waters in old mine working, geology and local tectonics. This general aim will be pursued, thanks the availability of the high quality dataset of recording from the dense arrays managed by INERIS and BRGM.
We will focus on the spatio-temporal distribution of the seismicity, which allows to monitor the dynamics of faulting and to estimate the extension and mode of rupture of the fracture system. The present work is based on the observation and the characterization of the microseismicity in terms of the accurate hypocentral locations, local and moment magnitudes and focal mechanisms determination in order to characterize the status of the abandoned mine.
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Induced Seismicity in Underground Gas Storage - The Case of Castor, Offshore NE Spain
Authors R. del Potro and M. DiezSummaryOil and Gas operations have the potential to induce earthquakes, and this is becoming an increasing concern for operators and regulators. Here we present the case of the Castor Underground Gas Storage (UGS), offshore NE Spain, where seismicity was induced during the first phases of gas injection. The microseismicity that occurred during the injection period, and which was not felt, evolved into magnitude 4 events almost two weeks after the end of the injection. The largest earthquakes of the sequence were weakly felt by coastal populations (EMS-98 Intensity III) and, in spite of posing no risk, pressed the authorities to halt all activity in the UGS. Here we compile the results from the major studies of the Castor seismicity: earthquake relocations and source mechanisms, and use new tools to characterise the seismicity and its relation to the gas injection. Cases of induced seismicity, such as Castor, where the external forcing is known, can contribute significantly to our understanding of the response of the crust to stress perturbations caused by fluid movement and the associated rupture events.
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Production-induced Fault Reactivation and Seismicity in The Netherlands - From Quick Scan to 3D Geomechanical Modelling
Authors B.B.T. Wassing, L. Buijze, P. Fokker, B. Orlic, K. Van Thienen-Visser and J.D. van WeesSummaryRecent induced seismic events in the giant Groningen gas field have raised concern on the safety of gas production and the risk of induced seismicity in the Groningen gas field in The Netherlands. Statistical analysis of past seismic events in the Groningen Field reveals that seismicity is non-stationary and seismicity rates are increasing, which leads to the conclusion that the maximum possible magnitude of seismic events, related to gas depletion, may be significantly higher than estimated before on the basis of statistics of past seismic events alone. The non-stationarity of seismicity may have implications for the seismic hazard of the small gas fields in The Netherlands as well.Formerly the maximum magnitude for these small onshore fields was estimated via the frequency magnitude distribution of the combined seismicity for all these fields, which resulted in an estimate of ML 3.9 for the maximum magnitude. However, as seismicity is non-stationary, only a small fraction of the gas fields is seismically active and the number of seismic events associated with them is generally small, past seismicity and related statistics cannot be used very well to study the future response of those fields and implications for fields with no recorded seismicity are unclear. In this presentation we show methods to assess the seismicity potential of these fields, which are based on techniques other than statistical analysis of past seismicity alone.
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Case Studies of Depletion Induced Seismicity Using Rate & State Modeling for Probabilistic Discrimination and Hazard
More LessSummaryEarthquakes with magnitudes M>4 have occasionally been observed close to depleted reservoirs after longer periods of production. Events of this size can be felt and pose a significant hazard for population and infrastructure. Key questions are whether such events can be associated as depletion induced, how reliable they can be discriminated against natural but rare tectonic events, and whether we can assess their seismic hazard.
We suggest a probabilistic approach based on the relative rates of natural and induced seismicity. We employ a rate and state seismicity model combined with modelled rates of Coulomb stress. The quantitative model is able to forecast the relative increase of earthquake probability during depletion as a function of space and time. We use this estimate to define a trigger potential function for the earthquake. In addition, the human induced question can be considered if a fault smoothing filter is applied to the trigger potential function.
The method is applied to different earthquakes in Europe, showing that some of the significant earthquakes were human-induced, human-triggered or caused by natural tectonic stresses. We discuss possible extensions to earthquakes occurring during the re-filling of depleted reservoirs for gas storage facilities and the assessment of time dependent hazard.
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InSAR for Induced Seismicity Monitoring
Authors A.R. Rucci, A.F. Ferretti and A.T. TamburiniSummaryWhenever fluids are injected or extracted from a reservoir, pore pressure and stress field change both in the reservoir and in the overburden. According to geomechanics, such variation in stress or pore pressure can produce a reservoir compaction, trigger pre-existing faults or even generate new ones. Al these events will translate into surface deformation which turns to be a valuable information to better understand the subsurface phenomena.
In the last few years, the analysis of multi-temporal SAR data sets represents an important layer of information, for reservoir monitoring and management.
The possibility to provide an high spatial density of accurate measurement points make InSAR an additional information to be integrated with subsurface data to calibrate geomechanical models, to identify fault/fracture reactivation providing also information about the fault geometry and mechanism.
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Geomechanical Assessment of Seismic Potential from Hydraulic Fracturing
Authors S.C. Maxwell, F. Zhang and B. DamjanacSummaryThe impact of hydraulic fracturing on a Horn River Basin fault was examined to quantify seismic potential or hazard. A geomechancial model was designed to investigate the mechanism of fault activation and the impact of fracturing at different locations around the fault. The model is used to simulate hydraulic fracture growth through a discrete fracture network, examining the pore pressure diffusion and associated fracture dilation and shearing. Based on the geomechanical deformation, the seismic activity can be predicted and used to compare with the actual seismicity monitored during the fracture treatment. The microseismic geomechanics study indicated that the stimulated fracture network had to grow directly into the fault in order for the injection pressure front to trigger fault slip. Geomechanical assessment of absolute seismic potential can be used to modify the engineering design prior to operations to minimize the seismic hazard including the placement of the well, and modifiy staging along the well to avoid fracturing in the regions likely to lead to fault activation. In scenarios where induced seismicity occurs during the treatment, the method can also be used to examine operational changes to lessen the relative seismic hazard.
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Real Time Forecasting of Maximum Expected Induced Seismicity
Authors J.P. Verdon, M.J. Werner, A.L. Stork and J.M. KendallSummaryFluid injection into the subsurface is performed for a variety of reasons, such as hydraulic fracturing, and waste storage. It is well established that fluid injection can trigger seismic activity of sufficient magnitude to be felt by local populations. The industry wishes to avoid such events. We develop a statistical model to forecast the largest event that might be induced by a given injection operation. The model is continuously updated as microseismic data is processed in real time. We use this model in a prospective sense, updating our forecasts through the injection period. We apply our model to two case studies: a hydraulic fracture stimulation in a tight gas reservoir; and at the In Salah CCS project, Algeria. In both cases, our model is capable of forecasting the largest event prior to its occurrence, providing an early-warning for the operator.
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Fracture Corridors in Carbonates
Authors S. Chatelée, J. Lamarche and B.D.M. GauthierSummaryAmong fractures, Fracture Corridors (FC) are anomalous structures made of highly persistent fracture clusters having a strong effect on multi-phase fluid flow in subsurface. The aim of this study is to reveal the genetics factor for FC development, their global geometry and internal morphologic variations, but also to clear the impact of fracture corridors on diffuse fracture. For that, we studied fractures in a 400×300 m wide quarry (Calvisson, SE France) dug in homogeneous marly limestones of Hauterivian age. The quarry exhibits diffuse fractures and FC. we measured >2500 fractures (strike, dip, spacing, filling, aperture, etc.) and studied microstructures in 80 thin sections. We calculated fracture density and acquired LiDAR data with >90 million points with a resolution of 4 to 15mm.
The LiDAR acquisition allows to visualize the 3D lateral continuity of corridors with a minimal extension of 30m. We also distinguished several internal morphology types of FC.Fracture density study shows that diffuse fracture increase around FC.
FC growth and variability was not dependent on facies variations, as they are inexistent in the quarry. The result of this study allows to interpret geomechanical behaviors and geological history of fractures and fracture corridors in carbonates.
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Fracture Modeling in a Complex Carbonate Reservoir - The Lubina-Montanazo Field, Offshore Spain
Authors O. Fernandez, M. Masini, R.M. Aguilar, M. Victoria, A. Briceño and P. CalderónSummaryThe Lubina-Montanazo field is a carbonate reservoir producing from Tertiary clastics and Cretaceous limestones. An outcrop analogue study has been integrated with subsurface data (image logs, well test interpretations, seismic attributes) to characterize the static and dynamic behavior of the fracture network and its interplay with diagenetic facies.
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The Kurdamir-Topkhana Field of Kurdistan - Modelling Clinoforms, Dolomitisation, Porosity Distribution and Fracturing
Authors R.B. Owens, J. Hsieh, D. Nolan and B. VeilleuxSummaryThe Kurdamir-Topkhana reservoir consists of carbonates deposited in a passive margin ramp setting during the Oligocene. Six principal facies types have been identified that differentiate between inner, middle and outer ramp facies, as well as differing degrees of dolomitisation. Subtle evidence of most of these features can be identified on seismic inversion (AI) data, which also provide good indications of porosity distribution throughout the reservoir. In addition, multiple fracture sets have been identified in image log data and a discrete fracture network constructed for the field. It presents a considerable challenge to construct a geocellular model that successfully intergrates all of these differing reservoir characteristics and which can be used for history matching and overall reservoir management purposes.
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Seismic Full Waveform Inversion for Characterizing Near-surface Structures - Potential Problems and Solutions
Authors H.R. Maurer, A. Nuber, E. Manukyan and S.A. GreenhalghSummaryFull waveform inversion (FWI) of seismic data has great potential to image the shallow subsurface, but the specific nature of the associated data sets requires that several problems be first addressed. One problem is the predominance of high-amplitude surface waves, which enforces changes primarily in the shallowest part of the subsurface model when matching observed and predicted data. A more uniform model update over the full depth range of interest can be obtained using Jacobian matrix scaling techniques. A further issue with FWI concerns the occurrence of local minima in the model space. We demonstrate the use of joint inversion of travel times and waveforms to alleviate this problem. Finally, we address the problem of variable source and receiver coupling and propose an estimation procedure that allows such variations to be accommodated and subsequently determined during the inversion.
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Towards Robust Acoustic Full Waveform Inversion
Authors E. Manukyan, H. Maurer and A. NuberSummarySeismic full waveform inversion (FWI) is a powerful method for obtaining high resolution subsurface images, but the objective function of FWI algorithms typically contains many local minima, which may lead to erroneous solutions. This can be avoided by choosing an initial model that is close to the true model, and by incorporating only low frequencies at an early stage of an inversion run. Here, we demonstrate that the local minima problem can be mitigated by removing, at each iteration, all data that have more than half a cycle mismatch in the traveltimes compared with those predicted for the current model. Using a simple two parameter example, we show that filtering waveform data by this traveltime criterion can successfully circumvent local minimum trapping. Inversions of synthetic 2D acoustic data show that our novel traveltime-directed FWI approach is more robust and far less dependent on having an accurate starting model.
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Alternative Multicomponent Observables for Robust Full-waveform Inversion - Application to a Near-surface Example
Authors R. Valensi, R. Brossier, D. Leparoux and P. CôteSummaryIn this study, using the framework of the Full Waveform Inversion (FWI) method we compare three different multicomponent cost-functions : the conventional multicomponent cost-function, a cost-function based on the normalized particle motion and a cost function only sensitive to the particle motion polarization.
With a synthetic test, it is showed that even if the attenuation model is poorly estimated the normalized particle motion misfit function and the polarization based cost functions are able to accurately recover the shear wave velocity parameters whereas the conventional multicomponent misfit function fails. Furthermore in context of near surface imaging, the proposed polarization based cost-function has the advantage to have a great sensitivity to the near-surface and to be independent of the knowledge of the source wavelet.
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Near-surface Characterization by Full Elastic Wave-field Inversion Based on the Scattering Integral Equation
Authors G. Rizzuti and A. GisolfSummaryThe near-surface can leave a strong imprint on seismic data. In order to be able to image deeper targets, we propose a full wave-field inversion approach to be followed by redatuming to the specific target level. The inversion scheme presented in this paper estimates iteratively the elastic properties of the model by trying to predict, at each iteration, one order of scattering of the data. We demonstrate the effectiveness of this idea on near-surface synthetic examples.
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Semi-discrete Matrix Free Formulation of 3D Full Waveform Elastic Modeling and Inversion
SummaryIn this work we derive the adjoint equations for the SEM model, which enables the incorporation of a generic data misfit functional. The key contributions of the study are a scalable computational strategy and a novel approach for matrix-free evaluation of the adjoint wavefield. The formulation enables computation of the adjoint wavefield, and consequently the gradient (or proximal function of which) of the objective function, while avoiding storage of the forward displacement field in memory. This algorithmic setup grants scalability in both weak and strong scaling tests.
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Simultaneous GPR Reconstruction of Electrical Conductivity and Permittivity
Authors H. Pinard, M. Dietrich, S. Garambois, F. Lavoué, L. Métivier and J.M. VirieuxSummaryGround-penetrating radar (GPR) is a non-invasive prospecting technique based on the electromagnetic waves sampling of the near surface. Building quantitative images through these waves requires the reconstruction of both
electrical permittivity and conductivity. This multi-parameter reconstruction is performed through the minimization of a misfit function measuring the discrepancy between observed and synthetic data. The minimization is achieved with a local descent method based on the Newton equation. Both the gradient and the product of the
Hessian matrix with a model vector are necessary for avoiding any trade-off between parameter classes, especially when high contrasts are encountered by electromagnetic waves. This presentation is devoted to the design
of these two key ingredients needed when updating the model, based on efficient first- and second-order adjoint methods. We formulate the problem in the frequency domain and we show that we need two forward modeling for the gradient and two additional forward modeling for the product of the Hessian matrix and a model vector. Our
formulation is such that these quantities are obtained through solution fields, regardless of the numerical scheme used to obtain them.
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Application of Crosshole GPR Full-Waveform Inversion to Experimental Data - Overview of Recent Developments and Possible Pitfalls
Authors A. Klotzsche, J. van der Kruk and H. VereeckenSummaryMany theoretical papers show the potential of the crosshole GPR full-waveform inversion, however to apply the full-waveform inversion to experimental data is still challenging. We have applied the crosshole GPR full-waveform inversion to many different datasets in Germany, Switzerland and USA. Here, we will give an overview of all the important steps and developments of the full-waveform inversion during the last years that are necessary for a successful application to experimental GPR data. Thereby, we will discuss important pre-processing steps including time zero correction, source wavelet estimation and different inversion strategies.
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Near-surface 2D SH-waveform Inversion - A Case Study from Slovakia
More LessSummaryWe apply elastic time-domain 2D full waveform inversion (FWI) to a near surface SH-wave data set recorded in the village Cachtice in northwestern Slovakia. Aim of the seismic survey was the detection of possible basement structures. The recorded data, consisting of 42 shots and receivers distributed along a 21 m long profile, covers a wide frequency range from 5 Hz to 100 Hz with high signal-to-noise ratio well suited for FWI. In addition to the direct SH/Love-wave a distinct refracted wave is visible. The trend of the first arrivals suggests an approximately 1D velocity gradient in the subsurface. Therefore a 1D initial model for the FWI is estimated by the Wiechert-Herglotz method. For the initial density model a similar gradient medium is assumed. The 2D FWI is based on the global correlation norm as objective function in combination with sequential frequency inversion. The final FWI S-wave velocity and density models reveal a heterogeneous underground with a prominent low-velocity weathering layer and isolated highvelocity anomalies. The structures of the density model correlate well with the velocity model.
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Applications of 2-D Elastic Full Waveform Inversion to Shallow Seismic Rayleigh Waves
Authors T. Bohlen, T. Forbriger, L. Groos, M. Schäfer, C. Wolf and M. BinnigSummaryShallow-seismic Rayleigh waves are attractive for geotechnical site investigations. They exhibit a high signal to noise ratio in field data recordings and have a high sensitivity to the S-wave velocity, an important lithological and geotechnical parameter to characterize the very shallow subsurface. Conventional inversion methods assume local 1D models and invert the dispersion curves of Rayleigh waves. In this work we present a workflow for 2-D elastic FWI that allows for the reconstruction of small-scale lateral velocity models. We apply a geometrical correction and an a priori estimation of the quality factors which used as passive modelling parameters during the multi-parameter FWI. We invert for the seismic velocities and densities, but only the S-wave velocitiy seems to be reliable due to its high sensitivity. Different field data examples demonstrate that realistic small-scale lateral S-wave velocities can be inferred by this approach.
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Full Waveform Inversion for Shallow Hazard Identification on a Narrow Azimuth Dataset
Authors D. Bright, C.E. Jones, Y. Belhassen, R. Brasil, H. Macintyre and C. FrançaSummarySite survey seismic is usually acquired as 2D seismic lines with specific acquisition configurations for imaging close to the seabed at high frequencies. The resulting images have excellent detail and they are then interpreted and potential drilling hazards identified largely from the stack amplitudes. However we lack other methods by which to constrain the interpretation of high amplitudes, such as being able to map their extent in 3D or use other seismic attributes derived from pre-stack data.
In many prospects around the world, shallow gas is a known issue for drilling and as an experiment we ran a full waveform inversion study to test if we could detect velocity anomalies caused by these accumulations.
To isolate anomalous velocities, the FWI velocity model was smoothed in a horizon-consistent manner and the smoothed model subtracted to give an “anomaly volume”. This was then compared with conventional site survey seismic data. We also verified the FWI velocity model by remigrating the 3D seismic data using the original tomographic model, the smoothed FWI model and the full FWI model.
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Application of 2D (visco)-elastic Rayleigh Waveform Inversion to Ultrasonic Data from the Porta Nigra in Trier (Germany)
More LessSummaryBeside geophysical applications from the near-surface to global scale, seismic full waveform inversion (FWI) can be applied to ultrasonic data on the centimeter and decimeter scale for non-destructive testing (NDT) of pavements, facades, plaster, sculptures and load-bearing structures like pillars. Classical NDT approaches are based on the inversion of body-wave travel-times to deduce P-wave velocity models. In contrast, surface waves (Rayleigh or Love waves) are well suited to quantify superficial alterations of material properties, e.g. due to weathering. In this paper we demonstrate the potential of 2D Rayleigh waveform inversion on the ultrasonic scale using a very low coverage acquisition geometry consisting of 1 shot and a few dozen receiver positions. For a 2D elastic FWI with a passive visco-elastic modelling approach the resolution is illustrated using a ultrasonic field data example from the weathered facade of the Porta Nigra, a large Roman city gate from the 2nd century AD, in Trier (Germany).
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The Sensitivity of a Seismic Sweep to the Near Surface
By W. KimmanSummarySurface consistent deconvolution compensates for phase perturbations in the near surface. Some of these perturbations are the result of wave propagation near the source or receiver. Using a Born based approach in a highly idealized representation of the near surface I derive analytical sensitivity kernels based on the instantaneous phase of a seismic sweep. This allows predicting changes in instantaneous phase due to velocity perturbations in the very near surface. One interesting observation is the relative large effect that small perturbations near the source/receiver can have on the instantaneous phase. This calls for further study on the relation between surface consistent source/receiver terms and more realistic structures with large velocity contrasts.
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Early-arrival FWI - Potential for the Near-surface
More LessSummaryI will discuss an exemplary application of early-arrival FWI to near-surface data from a highly complex environment and the benefit of using the high-resolved FWI model for reflection imaging. To further assess the potential of the method I will summarize some of the experience from early-arrival FWI and derive scaling relationships that allow for constraining the required frequencies. I will discuss the limitations of acoustic FWI and some of the problems and solutions towards fully elastic FWI.
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Noise-based Seismic Tomography at the Valhall Oil Field with Using Scholte and Love Waves
Authors G. Tomar, N.M. Shapiro, A. Mordret, S. Singh and J.P. MontagnerSummaryWe present here Scholte and Love wave phase velocity tomography at the Valhall Oil Field using ambient noise recorded by a network of 3D multi-component ocean bottom cable. We have cross-correlated 6.5 hours of continuous recording of noise between vertical-vertical (ZZ), radial-radial (RR), and transverse-transverse (TT) components. After applying an F-K filter, we were able to extract the first overtone of Scholte waves at Valhall from the RR cross-correlations. We then used the filtered overtone waveforms to measure inter-station frequency-dependent phase time delays and constructed 2D phase-velocity maps with the Eikonal tomography method.
Furthermore, we compute average dispersion curves for Scholte and Love waves by combining information from more than 10 millions of individual cross-correlations. We use the Neighbourhood algorithm to invert jointly these dispersion curves and to obtain an average 1D anisotropic model of the Valhall overburden down to depths of ~1 km. We find a significant radial anisotropy at depths below 600 m. This average 1D model is used as a reference model for the 3D inversion.
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A Cascaded Approach to Surface Wave Noise Attenuation
By P.J. BilsbySummaryThe removal of source-generated coherent energy propagating in the near-surface continues to be a fundamental step in land surface seismic data processing and one of key importance for subsequent reservoir characterisation. Such noise overlays and obscures a significant portion, in time and space, of the reflection signal and requires effective removal to maximise the value delivered to the reservoir geologist, as well as the perceived value of onshore seismic itself. Common strategies for the attenuation of this complex coherent noise have historically included both acquisition and data processing techniques, with the former using large areal source and receiver arrays and the latter using aggressive velocity filtering, both providing limited effectiveness. We describe a new systematic, cascaded, approach for the progressive attenuation of both the complex direct-arrival and scattered surface wave energy prevalent in land seismic data.
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Understanding the Impact of Karst on Seismic Wave Propagation - A Multi-method Geophysical Study
SummaryKarstified areas are known to be difficult ground for seismic exploration. We conducted a combined numerical-modeling and field-experiment study with the objectives to study the impact of karst on seismic wave propagation and to advance geophysical characterization of karst with seismic as well as non-seismic methods (electric and electromagnetic techniques). Finite-difference simulations using models with realistic topography illustrate the pronounced impact of topographic variations in high-velocity carbonate-bedrock environments on the scattered surface/guided waves. Wavefield complexities such as strong lateral changes in the strength of surface/guided waves, which were observed in a Vibroseis gather from a karst terrain in the Middle East, were also evident in our data recorded in Switzerland. In the latter case, amplifications of surface/guided waves could be correlated with low-velocity zones, which are probably due to more intensively karstified zones. Our study demonstrates that because of the strong heterogeneity of karst terrains, dense sampling is required to properly comprehend and disentangle the observed wavefield. Furthermore, we observed in our field study that the electrical-resistivity models correlate more closely with the mapped lithology, whereas karstification seems to more strongly affect the P-wave velocity models.
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Quantitative Integration of Measurements in Near Surface Characterization
By M. MantovaniSummaryA valid support to seismic in shallow anomaly detection is offered by additional geophysical measurements such as potential fields or electromagnetic induction soundings. In fact, gravity and electromagnetics (EM) measure complementary properties and, even though they do not sense any quantity directly related to P-velocity, they can be successfully used to improve the statics solution. Since the different geophysical domains are sensitive – to a large extent – to the same geometry structure of subsurface rock formations, there exists a structural link between the properties measured by the various methods, which are supposed to sense consistent anomaly shapes.
A high confidence solution for the near surface is therefore derived through a Simultaneous Joint Inversion (SJI) process ( De Stefano and Colombo, 2007 ). Within this process, seismic and non-seismic data are inverted simultaneously together with the structural (or petrophysical) link established between the corresponding physical properties (velocity, density, resistivity, etc.). The solution explains seismic as well as plenty of logging information other than the simple sonic curve. Moreover, EM and potential field data are also explained by the output model simultaneously with seismic and borehole data. Thus, each of the measurements complements the others, as each is affected by different and uncorrelated noise.
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Mitigating Uncertainties in Towed Streamer Acquisition and Imaging by Survey Planning
More LessSummaryUncertainties in seismic images or reservoir characterisation can very often been associated with lack of resolution, illumination problems, or the inability to invert for accurate velocity models. Uncertainties may also be caused by general data quality issues like noise content as well as acquisition and processing footprints. Provided the geophysical challenges are well understood, such uncertainties can be mitigated already in the planning phase of a seismic acquisition project. Geophysical survey planning typically considers requirements for, e.g., temporal and spatial resolution, illumination at target level and also looks into the suppression of incoherent and coherent noise (e.g., multiple energy). Typically, the geophysical objectives and quality requirements have to be traded against survey cost and may be constrained by operational aspects.
Combining state of the art towed streamer acquisition technology with advanced processing and imaging methods enables us to mitigate some of these uncertainties. This paper discusses issues and solutions related to illumination, footprints, and resolution. Also the benefit from utilising seismic wave types in addition to primary reflections and other geophysical methods is addressed.
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Understanding and Analysing Seismic Images - Insight through Appropriate Modelling
Authors I. Lecomte, P. Lubrano-Lavadera and D.W. SchmidSummarySeismic images are often provided to interpreters as the only element to work with, besides well logs. However, the possible geological complexity of both overburden and reservoir, as well as survey, wavelet, and processing are all parameters affecting seismic images. An ideal seismic modelling would be using full-waveform methods to generate complete synthetic seismic records, then process the latter in the same manner as the observed data. But it is a complex and time-consuming task, impossible at production and routine levels. Interpreters often use the simplest method of all, i.e., “trace modelling” or “1D convolution”, but this corresponds to strong, often forgotten, model assumptions: homogeneous horizontal layers. Using a 3D-spatial ray-based prestack convolutional method allows instead accounting for many of the above-mentioned effects, at a similar cost than 1D convolution, but in a flexible and more complete manner, permitting sensitivity tests on various parameters. This would help interpreters better understanding what seismic can/cannot see and assessing their results in each specific case.
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5D Local Angle Domain Gathers as an Ideal Representation for Directivity Driven Imaging
More LessSummaryIn seismic imaging, 5D recorded seismic data is migrated to the depth domain, forming a much lower dimensionality, the extra dimensions being reduced by integration. As a consequence much information is irretrievably lost, information that could be of use in characterizing with minimal uncertainty the subsurface geological features. In order to retain the full richness of the collected data in a form which can be of use in achieving an unambiguous interpretation of the geology of the subsurface, an approach is presented here that, using migration, maps data from the surface acquisition domain to that of the physically derived subsurface 7D domain. The gathers thus derived can be considered to be complete representations of the scattering event and as such can be processed subsequent to migration, to accentuate any aspect required. We have demonstrated the directionality derived images among many other possible applications.
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Seismic Imaging in and around Salt Bodies - Problems and Pitfalls Update
Authors I.F. Jones and I. DavisonSummarySalt movement often results in steeply-dipping complex structures, which pose significant challenges for model building and migration. In recent years, advances in seismic imaging algorithms have permitted imaging of steep structures by exploiting the two-way wave equation via the introduction of reverse time migration (RTM). With such imaging algorithms, double bounces and turning wave reflections can be imaged, thereby enabling the imaging of vertical and overturned salt flanks. However, despite advances in the migration algorithms, the derivation of a suitable earth model incorporating the anisotropic behaviour of the velocity field remains a significant challenge, requiring tight integration of geological interpretation, and geophysical skills.
A major contributing factor to the successful execution of a complex salt imaging project, is the understanding of the many and varied pitfalls involved at every stage of the process. Here we describe and discuss some of these issues, building on the observations made at the predecessor to this workshop which took place at the EAGE in London in 2003.
The various aspects described in this review paper all contribute to the overall uncertainty associated with creating and understanding images of complex structures.
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Imaged Depth Versus True Depth
More LessSummaryThe use of the term ‘velocity’ to describe both the true propagation velocity and the modelling “velocity” used in processing (what is here termed ‘pro-velocity’) has given rise to a damaging level of confusion whereby a great majority of geoscientists take the depth image to be a good representation of depth in the subsurface. The relationship between the pro velocity and true velocity is unsystematic. It essentially amounts to a dichotomy between the two quantities. This form of relationship arises as a result of a number of fundamental factors. The depth image could therefore contain considerable structural distortion relative to the actual subsurface. The misconception that anisotropic depth imaging produces true depthing has been behind the significant decline in the use of the solidly sound approach for obtaining an optimal representation of the subsurface, namely: to convert the depth image vertically to time and to carry out a proper vertical depth conversion using true velocities. The application of this approach to practically every PreSDM process is strongly recommended.
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Master the Uncertainty with Bayesian Approach -Case Study, Multi-Azimuth Depth Imaging Deep Water, Offshore Brazil
More LessSummaryInterpretation in the depth domain is controversial. Sometimes, the problem comes with constantly changing models. Geologists have found that structures which have been focused come and go along with the model update.
One of these tough imaging challenges occurs when a carbonate feature lies above a salt feature.
Overcoming this carbonate feature is a good challenge to exploration today due to the high variation in velocity and the homogeneity of anisotropic character.
Most of the current depth imaging processes faces uncertainty with an arbitrary and variables. This judgement of adequate volume and pattern of distribution remain controversial. With limited well information in the area, the depth model from the same seismic data could be non-unique. In consequence this uncertainty of the structure causes problems in deep water exploration.
Repsol intelligently designed its proprietary multi-Azimuth acquisition in Campos basin, Offset Brazil. By using an advanced imaging study combining technologies of advanced grid based tomography and inversion tools with structure control and multi-discipline joint inversion processes, we achieved the goal of the imaging the structure with outstanding resolutions.
The imaging results are confirmed by the latest well marker and the accuracy of production structure both approach the limit of resolution by inversion theory. This project sets an outstanding benchmark for computing resource utilization and demonstrates the benefit of advanced migration technologies and well-planned acquisition patterns.
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