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77th EAGE Conference and Exhibition - Workshops
- Conference date: June 1-4, 2015
- Location: Madrid, Spain
- Published: 01 June 2015
1 - 100 of 128 results
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4D Forward Modelling to Optimize a Closed-loop Seismic Reservoir Monitoring Work-flow
Authors D.I. Hill, D. Lowden, S. Sonika, M. Paydayesh, V. Dimova, K. Westeng and K. EggenbergerSummaryThe historic closed-loop seismic reservoir monitoring workflow is redefined to incorporate the geologic, reservoir simulation, and geomechanical models into an integrated full-field coupled Dynamic Integrated Earth Model (DIEM) to surface. From which elastic parameters for a range of reservoir simulations can be derived via the petro-elastic rock-physics model for input into the field-wide finite-difference forward-modelling with realistic calibrated noise, imaged to produce a high-fidelity prediction of the 4D signal. The Chimera geologic, reservoir, and geomechanical models were integrated into a DIEM. The Chimera turbidity type reservoir model has a maximum sand porosity of 0.25 and a maximum permeability of 200 mD with light oil and an initial gas cap supported by an aquifer from the bottom, accumulated within a structural trap segmented by normal faults. Production scenarios are simulated at the reservoir pressure corresponding to the baseline date, and four future realisations. These are transformed into elastic properties for the finite-difference forward-modelling. The modelled data with calibrated noise for each scenario is imaged, and differenced to quantify the type, magnitude and location of the 4D signal. Hence we determine if the base-line or an alternative geometry will measure the 4D signal at the required time-step and consequently if the reservoir is a candidate for seismic reservoir monitoring.
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Monitoring Changes in the Overburden – Some Observations from Using Various 2D and 3D Seismic Time Lapse Data Sets
More LessSummaryIn 1989 a major underground blow out developed when well 2/4-14 was drilled into a target reservoir at 4.5 km depth in the North Sea. 2D seismic data was acquired both prior to and during the underground blow out. In addition to these data, we use two 3D seismic data sets, one acquired 2 years after the blow out, in 1991 and another acquired in 2005. By comparing various data, we can analyse both short term and especially long term movements of hydrocarbon flows in the overburden. Since there is significant amount of shallow gas in the area, it is a challenge to discriminate between in-situ shallow gas and gas originating from the underground flow. We find that the combination of slightly dipping shallow glacial sand layers and ice berg plough marks serve as a trap for shallow gas. This can be mapped from the 3D seismic data, as well as flow of gas due to the underground blow out.
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Estimating Overburden Velocity Changes from Pre-stack Time Shifts Using Linear Tomography
Authors J.A. Edgar and T.D. BlanchardSummaryProduction from oil and gas fields induces velocity changes and strains in the subsurface, which create time shifts between vintages of time-lapse seismic data. Recovering these time shifts can be useful for reservoir management, particularly through calibration of geomechanical models. Rickett et al. (2007) and Williamson et al. (2007) have shown that these shifts can be estimated using non-linear inversion of poststack time-lapse seismic. These existing inversions of post-stack data assume that the seismic ray paths in all vintages are vertical. However, examining pre-stack time-lapse data shows that the recorded time shifts are offset dependent. To estimate the true subsurface changes we must invert pre-stack time-lapse seismic, which requires more accurate modelling of the seismic energy propagation. This abstract introduces a new tomographic inversion of pre-stack time-lapse seismic which aims at estimating the velocity changes that explain all pre-stack time shifts. We retain the assumption of the existing post-stack methods that the ray paths do not change between vintages, but discard the assumption that they are vertical. This allows a linear tomography system to be set up and solved for velocity change. We test this method on synthetic data and compare the results with an existing post-stack approach.
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3d and 4D Imaging Using a Non-destructive Ambient Noise Seismic Interferometry
Authors N.M. Shapiro, S. Singh, A. Mordret and G. TomarSummaryIn recent years there has been increasing interest in the study of so-called ambient noise seismic interferometry. This method is used to extract meaningful information from long recordings (hours to days) of ambient seismic noise. This meaningful information is extracted in the form of wavefields propagating between those receiver positions at which the noise was recorded, i.e., as if a source had been placed at one of those locations - a so-called “virtual source”. The method has found most success in global/regional seismology where low-frequency (sub-1 Hz) fundamental mode surface waves are extracted by cross-correlating months of ambient noise recorded on two or more receiver stations. Whereas the most successful applications of the method have been in recovering surface waves propagating between receiver locations, other successful applications have seen the recovery of body waves. Another very appealing aspect of the ambient noise interferometry is the possibility to use it for time-lapse or continuous un-invasive monitoring of the subsurface properties.
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A Well-to-seismic Interpretation of Interwell Connectivity Using Well Behaviour Data and 4D Seismic Surveys
Authors Z. Yin, C. MacBeth and R. ChassagneSummaryA technique is proposed to quantitatively measure inter-well connectivity by correlating multiple 4D seismic monitors to historical well production data. We make use of multiple 4D seismic surveys shot over the same reservoir to generate an array of 4D seismic differences. Then a causative relationship is defined between 4D seismic signals and changes of reservoir fluid volumes caused by injection and production activities. This allows us to correlate seismic data directly to well data to generate a “well2seis” volume. It is found that the distribution of the “well2seis” correlation attributes reveals key reservoir connectivity features, such as the seal of faults, inter-reservoir shale and fluid flow pathways between wells, and can therefore enhance our interpretation on inter-well connectivity. Application of our proposed technique proves that the well2seis attribute agrees with geological interpretations better than conventional well connectivity factors based on engineering data only. Additionally, combining with a conventional inter-well study method, this multiple 4D seismic method is found to support the conventional inter-well approaches and can provide more robust and detailed interpretation.
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Effective Imaging of Reservoir Fluid Changes
Authors O. Salako and C. MacBethSummaryMonitoring of changes in brine chemistry (salinity and temperature), during water-flooding is important for injector optimisation, understanding efficiency, detecting early water breakthrough, locating bypassed hydrocarbons or detecting scaling in the heterogeneous reservoir. It is already known that water injection into the oil-leg of a hydrocarbon reservoir can be monitored by both seismic acquisition and also CSEM methods. The problem of an interfering pressure signal for seismic impacts quantitative evaluation for time-lapse analysis, whilst with EM there are the counterbalancing effects of salinity and temperature. CSEM becomes favoured when the pressure effects on seismic are dominant, or heavy oil is present with similar acoustic properties as the formation and injected waters. The quality of measurement for both methods is influenced by reservoir facies variations, acquisition repeatability and overburden heterogeneity. Time-lapse seismic is unlikely to detect brine distributions injected into the water-leg or aquifer, although it may detect associated pressure up effects. However our calculations show that it may be possible for time-lapse CSEM to distinguish the inter-mixing of different brines in the subsurface hydrocarbon reservoir. Specifically, low salinity injection or injection into a highly saline formation can clearly be detected with this technique.
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Measurement and Potential Applications of Time-lapse Attenuation
Authors T.D. Blanchard and J.A. EdgarSummaryCan we use other time-lapse seismic attributes to compliment our measurements of travel-time and reflectivity changes? In this paper we have measured attenuation changes in a seismic reservoir undergoing depletion and in some locations water injection. We will discuss these measurements in three sections, covering:
1.Potential uses of time-lapse attenuation measurements. What can we use them for? Time-lapse inverse Q? Interpretation? For this we will show an example of trying to separate water and gas saturation effects.
2.Real signal or an artefact? Could we just be looking at an apparent effect from, for example, migration using the same velocities for all vintages? Is a dedicated workflow necessary to extract what might be very subtle signals?
3.What are the underlying mechanisms controlling attenuation? What could we learn if we made these time-lapse measurements in other reservoirs?
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Application of Full Wavefield Inversion to 4D Seismic Data - Workflow and Field Data Examples
Authors G. Chen, P. Routh, X. Lu, M. Helgerud, S. Lazaratos and D. JohnstonSummaryAdvances in full wavefield inversion (FWI), 4D seismic technology, and depth migration of seismic data have led to the application of FWI to 4D seismic data in which time lapse changes of velocity model can be directly obtained from the data. It has been shown that correcting for velocity changes due to production in depth migration improves 4D imaging repeatability. In this paper, we present a detailed workflow to correct for large acoustic velocity changes due to production using the FWI 4D results. Through FWI 4D application, we achieved seismic processing cycle time reduction in a recent 4D seismic project. We have also tested the FWI 4D methodology in areas of poor seismic repeatability due to production facility obstruction to seismic data acquisition.
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Full Elastic Non-linear Inversion the Key to 4D
By A. GisolfSummaryUsually great emphasis is placed on the repeatability of seismic acquisition for time-lapse purposes. However, the non-linearity of the relationship between seismic time-lapse data differences and time-lapse property changes is usually ignored. In this paper it is demonstrated that ignoring the non-linearity can cause serious errors in time-lapse interpretation, whereas application of full elastic non-linear inversion to base-line and time-lapse surveys separately, fully accounts for the non-linearity and gives quantitatively very accurate results. In addition, it is well understood that full elastic non-linear inversion is less sensitive to noise and acquisition imprint. The method is illustrated with the help of a synthetic time-lapse example based on real logs.
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An Engineering-consistent Bayesian Scheme for 4D Seismic to Simulator Inversion
Authors S. Tian and C. MacBethSummaryA workflow is designed to integrate simulator to seismic predictions into the process of 4D seismic inversion using a coupled Bayesian scheme, and honouring the concept of reservoir engineering consistency (EC). The proposed approach is demonstrated by an application to a West Africa dataset. The results show the benefits of being EC when working across domains during a “close the loop” exercise.
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Engineering Consistent Constraints for the Inversion of Changes in Pressure and Saturation on Ekofisk
Authors M.Y. Wong, C. MacBeth and A. BertrandSummaryA high resolution, time lapse seismic inversion into pressure and saturation changes is performed. This provides insights into well performance and pressure distribution within a geo-mechanically active chalk reservoir (Ekofisk). The inversion is constrained by reservoir engineering concepts and predictions to reduce the non-uniqueness involved, and to maintain consistency with the physics of flow. At the heart of this inversion scheme is the effective union of engineering data and different seismic products such as reservoir time strain, percentage changes in elastic properties to influence the inversion. Quantitative interpretation on this field using the inversion results shows good agreement with well production data and helps to explain strong localised anomalies in both the Ekofisk and Tor formations. Analysis shows that the hardening signals around producers are due to lack of pressure support and reservoir compaction; whereas softening signals are attributed to high pressure flooding around injectors.
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4D Reservoir Analysis beyond 1D Convolutions
Authors A. Khalil, H. Hoeber, A. Jafargandomi and S. de PierrepontSummaryAnalysis of time-lapse data is performed on migrated seismic images, which represent the spatial and time-lapse variability of the medium’s reflectivity. The process of migration effectively rotates the wavelet so that it is normal to the imaged reflectors. Processes used in 4D reservoir analysis such as deconvolution, inversion and warping need to follow the structure of the data. The traditional 1D (vertical) convolutional approach does not honour this directivity. For this reason, we introduce a wave equation based approach which provides an effective platform for structurally consistent reservoir analysis. This includes applications such as wavelet extraction, warping and 4D time-strain inversion.
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Quantifying Time-lapse Seismic Signal Detection for the Otway Project Using Prestack Migration
Authors M. Alajmi, A. Bona and R. PevznerSummaryDuring Stage 2 of the Otway CCS Australian project it is planned to inject a small, up to 15,000 tonnes of gas, into a saline aquifer located at depth of 1500 m. In CO2 sequestration, the ability to detect CO2 plumes is one of the main purposes of using time-lapse seismic imaging. The detectability of CO2 in seismic time-lapse surveys relies on two main factors: a sufficiently strong signal and sufficiently small noise. Therefore, to model time-lapse seismic records, we need to model not only the seismic response of the geology and the plume but also the time-lapse noise. Because plume detection is determined by the S/ N, the ability to model realistic time-lapse noise is crucial in any feasibility study. In this work, we propose a more realistic approach by adding band-limited random noise to the pre-stack data (shot gathers) to match the S/N of field data. Using these noisy gathers we then compare the detectability of CO2 plume by using pre- and post-stack Kirchhoff migrations.
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Intra-survey Pressure Variations - Implications for 4D Seismic Interpretation
Authors V. Omofoma and C. MacBethSummaryDuring the time taken for seismic data to be acquired, reservoir pressure and saturation may fluctuate as a consequence of field production and operational procedures. This has consequences for the quantitative analysis of 4D seismic data and particularly for understanding of the pressure signal that diffuses rapidly into the reservoir over a time-scale of hours or less. A modelling study using actual acquisition data (permanent seabed sensors and also towed streamers), reveals that the signature of pressure variations in the pre-stack domain is complex, and thus the resultant post-stack image is not representative of the true reservoir mechanisms that caused the pressure changes. This is of particular concern when trying to accurately resolve small pressure changes away from wells with post-stack data. It appears however that larger signals closer to the well may still be detected adequately. Our results have implications for post-stack quantitative 4D seismic analysis, as well as processing and acquisition workflows for detailed seismic time-lapse studies.
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The Implementing Geological Disposal of Radioactive Waste Technology Platform
More LessSummaryAfter decades of bilateral and multilateral cooperation, several European waste management organizations decided, under the auspices of the EC, to join their forces to tackle the remaining research, development and demonstration (RD&D) challenges associated with the implementation of their respective geological disposal programs. The main objectives of the Implementing geological disposal of radioactive waste technology platform (IGD-TP) are to initiate and carry out collaborative actions in Europe to tackle the remaining research, development and demonstration (RD&D) challenges with a view to advancing the implementation of geological disposal programmes for high-level and long-lived waste in Europe. This paper presents the organisation of the work and the main Joint activities and projects to date, initiated by the IGD-TP members and supported for some of them by the European Commission under the FP7 framework programme.
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Microseismic, Acoustic Emission and Ultrasonic Monitoring in Radioactive Waste Disposal and Feasibility Studies
Authors J.M. Reyes-Montes, W.S. Pettitt, J.R. Haycox, M. Lopez-Pedrosa and R.P. YoungSummaryConstraining the volumetric extent of excavation damage zones (EDZ) around engineered structures has benefitted from the application of remote, scaled seismic studies that passively monitor the medium using microseismic (MS) and acoustic emissions (AE) techniques combined with active ultrasonic surveys. By mapping AE/MS locations, fracturing and rock deformation can be correlated with the different operations in a working repository, e.g. gallery excavations, pressurization of containment holes and thermal stresses from the waste. These technologies provide a unique means of monitoring in real time the evolution of the EDZ and rock volumes surrounding a disposal facility and particularly the potential for creation of paths for fluid communication between the facility and its environment.
This paper present presents a series of case studies where these methodologies have been applied for the imaging of damage development and containment capability of repositories excavated both in crystalline and plastic rocks.
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Near Field Characterization of Hard Rock Spent Nuclear Fuel Repository by Seismic Reflection
Authors C. Cosma, N. Enescu, E. Heikkinen, A. Joutsen and P. KosunenSummaryPosiva and SKB conduct RD&D activities for the safety assessment and to test disposal techniques in real deep-seated conditions. Brittle deformation zones and large fractures are considered to pose a potential risk for the mechanical integrity of the spent fuel disposal canisters. These are to be avoided in positioning of the deposition holes that will host the canisters, and they need to be identified during construction of the deposition tunnels and ultimately the deposition holes. Results from two high resolution seismic surveys carried out in ONKALO and Äspö HRL in 2013 are presented here, providing continuity information for several large fractures identified through geological mapping to cut the tunnels and/or boreholes. These were recognizable in transmission and reflection images produced from the seismic data sets.
We show that reflection seismic surveys are relevant to the detailed characterisation of crystalline bedrock. Relatively small-scale features, even single fractures, can demonstrably be detected. On the other hand, the detection of some distinctive features, even large-scale, can be uncertain if the survey layout is spatially constrained. Combinations of borehole and tunnel measurements using measuring arrays with diverse orientations helped reducing the location ambiguities and should be used in the future wherever possible.
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High-resolution Seismic Characterization of the Shallow Subsurface Applied to Civil Engineering
Authors D. Martí Linares, I. Marzán, J. Sachsenhausen, J. Alvarez-Marrón, I. Cienfuegos and R. CarbonellSummaryThe construction of the future Centralised Temporary Storage facility (CTC), a surface infrastructure built to manage and storage the nuclear waste for a lengthy time, needs the characterization of the near surface to assure the suitability of the construction and operating process. In this sense, a seismic characterization to regional and local level was carried out to understand the structure and the 3D geometry of the main stratigraphic units that can be related to a safe location of the CTC. Ten
kilometers of normal incidence seismic reflection data were acquired to understand the regional context, whereas a high-resolution 3D tomographic survey were carried out around the CTC location to obtain a detailed pictures of the shallow subsurface were the main buildings have to be built. Basically the main objective was to obtain the most detailed image of the geometry of the contact between the upper shallow lutites and the gypsum units that outcrop at the Eastern part and gently dip to the West. This information is very valuable from the engineering point of view during the design and construction of the main buildings of the CTC.
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Application of Diffrction Imaging and Steered Migration to 3D Seismic Data from the South West Hub CCS Project
Authors K.V. Tertyshnikov, S. Ziramov, A. Bóna and R. PevznerSummaryThe assessment of subsurface architecture and location of faults is key information in CO2 geosequestration projects. The 3D seismic survey has been acquired as a part of the SW Hub CCS project in vicinity of potential future CO2 injection sites. The acquisition was carried out within the Harvey and Waroona Shires about 150 km south-east from Perth, Western Australia. In order to support the fault detection and decrease uncertainties on the structural framework two additional imaging techniques have been applied to the 3D seismic volume. One is a diffraction imaging algorithm, the other is an alteration to the post-stack Kirchhoff migration – the steered migration. The application of these methods to the seismic volume enhanced the signal to noise ratio of the final migrated images and supported the understanding of fault distribution in the study area.
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Application Seismic Multiattributes Fuse Method in Shale Play
By W. LiuSummaryShale gas "sweet spot" usually refers to an area that contains high total organic carbon content (TOC) and has high brittleness, relatively developed fractures and thick favorable shale, high core pressure. High TOC ensures a high gas content of shale gas, and high brittleness and fracture development guarantees that an effective stimulated reservoir volume (ESRV) can form after fracturing, thus achieving fracture network communication. In this paper,we using seismict attribute to identify shale sweet spot.By elastic parameters crossplots analysis,we established the relationship between the TOC, brittleness and elastic parameters, and extrapolated to a three-dimensional block, we can getting the brittleness and TOC distribution.we can defined different sweet spots by how much the content of TOC&brittleness& pore pressuer.finally,we jointed all parameters by seismic attributes fuse method to identify shale sweet spot.
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Construction of Synthetic Shale Based on Physical Simulation Method
More LessSummaryIn this paper, a construction method of synthetic shale sample is proposed. Fine particles (3000–5000 mesh), brittle mineral, clay, carbonate, organic carbon and cement powder are mixed together in different amounts and then cold-pressed the mixture to obtain solid synthetic samples. And basic measurements of the finished samples are conducted in the laboratory to verify whether they are able to meet the requirements of relevant studies by simulating natural shale. The final sample similar to real shale cores in aspects of mineral composition, pore structure, seismic characteristics, etc. is constructed. The biggest characteristic of synthetic shale samples can be simply summarized as controllability, i.e., their mineral composition, diagenetic pressure, cementation degree, porosity, permeability, etc. can all be set according to experiment requirements, and an arbitrary combination of different factors is possible to meet multiple research needs.
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Decline Curve Analysis for Low Permeability Gas Condensate Reservoirs - Effect of Fluid Richness, Inertia and Coupling
Authors C. Johnson and M. JamiolahmadySummaryDecline curve analysis has been proposed as an efficient approach for estimating reservoir and well parameters especially in unconventional gas reservoirs where the use of conventional pressure-transient analysis is often technically and economically challenging. This study investigates the applicability of the widely accepted Fetkovich type curves to low permeability gas-condensate reservoirs under various operating conditions. A synthetic reservoir model based on one of the Fetkovich et al. (1987) case histories was first constructed and validated. Then the impacts of condensate gas richness, different rock types, relative permeability (kr) including coupling or capillary number (increase in kr as flow velocity increases and/or interfacial tension decreases) and inertia (decrease in kr as flow velocity increases) on estimated parameters were examined. The results show that the quality of match (and parameter estimates) between the generated decline curve and type curves is dependent on the condensate saturation level around the wellbore and the nature of the kr curves. In those cases where non-Darcy effects are strong enough to affect the results, use of Fetkovich type curves in the presence of inertia (coupling) results in pessimistic (optimistic) estimates of permeability, and higher (lower) skin estimates than the non-rate-dependent values input into the simulations.
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Towards a Joint Programming on Geological Disposal – The Joprad Project
More LessSummaryThe goal of The JOPRAD project is to prepare the setting up of a “Joint Programming on Radioactive Waste Disposal” that would be established to coordinate at the European level, national research programmes and the associated research and development (R&D) activities on geological disposal for high activity long lived radioactive waste. This action includes reviewing of all strategic aspects linked to a stepwise move towards a Joint Programming in this field. This project will involve organisations that are active in the safety, management and disposal of radioactive waste and research entities. The first step of this project will be to engage in discussion with Member States representatives in order to clarify the organisation of their national R&D consistent with the implementation of the Council Directive. The second step will be to identify existing research programmes that could contribute to the identification of common scientific objectives and activities as well as specific aspects that the organisations would like to develop in the Joint Programme. The third step will be to draft the joint “Programme Document” that should be the technical background of the Joint Programming. This action will lead to the further integration of the interested research community and hence help to maintain and develop the EU leadership in knowledge and expertise for innovative radioactive waste management solutions that effectively matches public expectations. Moreover, it will further reinforce and make the interaction at EU level between WMOs, TSOs, industry, policy makers and the research community more effective.
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How Rift-related/Plate-forming Processes Control Structure and Shortening in Fold-thrust Belts
More LessSummaryFold-thrust belts (FTBs) are archetypical tectonic features adjacent to collisional orogens, where structural traps account for 14% of discovered oil reserves. After decades of field and seismic surveys, detailed geological interpretations have emphasized their high degree of structural complexities. This has dampened academic research interest and impetus towards the understanding of larger scale geodynamic processes related to their development.
FTBs express processes by which plate convergence in collisional orogenic belts is accommodated by accretion within the continental lithosphere. Two dominant tectonic modes are observed in FTBs, thin-skinned and thick-skinned. Thin-skinned style characterized regions dominated by underthrusting, suggesting simple-shear subduction of the underlying crust. A recently discovered relationship between structure of FTBs and plate forming processes ( Mouthereau et al., 2013 ) shows that styles of tectonic deformation at the front of orogenic belts are controlled by age-dependent, deep-seated, properties of the crust and sub-continental mantle. More generally, the role of inherited thermo-tectonic properties of the lithosphere and the architecture of former continental margins is increasingly being shown to be a major driver in the evolution of mountain belts. This research should stimulate novel R&D integrated efforts on both rifted margin and thrust belt.
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Integrated Workflow for Risk Assessment in Fold-and-thrust Belts
By F. RoureSummaryOur understanding of sedimentary basins and thrust belts has greatly benefited from recent improvement of imagery techniques, accounting for a renewed interest for hydrocarbon exploration in fold-and-thrust belts.
This paper describes, using various case studies worldwide, the integrated workflow developed at IFP-EN to reconstruct the kinematic and thermal evolution of fold-and-thrust belts (foothills) and adjacent forelands, and the way numerical modelling and analytical work can improve our predictions in terms of energy resources, hydrocarbon potential and reservoir risk assessment.
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Coupling Tectonics and Surface Processes in Fold and Thrust Belts - Insights from 30 Years of Analog Modeling
More LessSummaryThrough a rapid overview of 30 years of analog modeling studies, we outline the role of the primary mechanisms and processes that exert a strong control on fold and thrust belts evolution. Following this general approach, we address major open questions regarding the global and local responses (i.e., at orogenic scale and at the scale of faults or ridges) of a fold and thrust belt under the impact of tectonic or climatic forcing at different time scales. Insights from analog models are used to: a) characterize the behavior of wedges subject to different geometric, kinematic and rheologic boundary conditions and b) show how the interactions between surface and tectonic processes influence the structures, kinematics of deformation, exhumation mechanisms, and long-term evolution. Impact of first order parameters such as the initial tectonic setting, material transfer in the wedge, structural inheritance (OCT and inherited extensional structures), and their role on the tectonic evolution of fold and thrust belts will be successively reviewed. Several case studies of active or fossil orogens (Taiwan, the western Alps and the Variscan belt) representative of first order tectonic processes are presented in the light of field observation and analog experiments comparison.
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What are the Limits of Balanced Sections in Fold and Thrust Belts
By J. VergesSummaryIn this paper the limits of the geological cross-sections are explored as well as their validity depending on the used datasets and multidisciplinary approaches. Selected cross-sections from different orogenic belts are discussed and framed within plate tectonic reconstructions. The Pyrenees formed during convergence between Iberia and Europe producing the inversion of the relatively narrow basin system between the two plates. Contrarily, the Zagros fold belt is a more complicated orogenic chain produced by an initial obduction phase and a later collision between Eurasia and Arabia. The uncertainties and variability of published balanced cross-sections through these orogenic systems need further constrain to solve problems that cannot be interpreted only with existing structural information.The integration of multidisciplinary studies at different scales of work might help constraining the evolution of the orogenic events and might provide solutions for regions that cannot be addressed solely by the construction of the balanced cross-section using structural information.
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Apatite (U-Th)/He Thermochronology Data Interpretation in Fold and Thrust Belt - New Insights from Physical Point of View
Authors C.G. Gautheron, D. Mbongo-Djimbi, C. Gerin, J. Roques, C. Bachelet, E. Oliviero and L. Tassan-GotSummaryThe apatite (U–Th)/He (AHe) system has rapidly become a very popular thermochronometer to constrain burial and exhumation phases in a variety of geological contexts, including fold and thrust belt. In this particular case, detrital apatite crystals come from different sources and are characterized by different pre-deposit ages and grain chemistry. However the actual He diffusion models are not well constrained and do not fully explain the mechanism of He retention. This is particularly true for detrital apatite crystals and AHe age interpretation can be tricky. In order to have a deeper insight on this issue, multidisciplinary studies on apatite combining physical studies were performed. We propose new He diffusion coefficient, so closure temperature and a physical He diffusion model. We will propose a new way of interpreting AHe datasets on fold and thrust belt samples more particularly, because of their own apatite characteristics. Two examples from Peruvian Subandean zone and south Pyrenean foreland will be given. We will show how a better interpretation of He diffusivity will improve significantly AHe data and thermal histories obtained by data inversion.
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Balanced Cross-sections, Thermochronology and Unexplored Plays - The Peru Case Study
Authors P. Baby, Y. Calderon, C. Hurtado, N. Espurt, M. Louterbach, A. Eude, S. Brusset, M. Roddaz, S. Brichau and R. BolanosSummaryIn the Peruvian Subandean fold and thrust belt, the construction of 4 serial balanced cross-sections from a good set of structural data and an extensive knowledge of the stratigraphy and geodynamic evolution allow a more refined definition of the unexplored plays, as sub-thrusts, duplexes or pre-Cretaceous structures. Sequential restorations are proposed by coupling thermochronologic analyses with growth strata studies. The results show significant north-south variations in the geometry, the timing and the rate of the deformation. This latitudinal variation is related to the pre-Andean basins inheritance, but also to the Andean slab geometry. Thermochronologic ages correspond to the most recent tectonic uplifts and cannot be disassociated from the study of growth strata that might have recorded oldest events. North of the Peruvian Subandean zone, the thrusts propagation seems continuous since the Late Cretaceous. In the south, two stages of Cenozoic deformation are clearly indentified.
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From Andes to Zagros - Variations in Structural Styles and Seismic Data from Two Processors’ Perspectives
Authors R.W. Vestrum and J.M. GittinsSummaryAn overview of data examples from a variety of settings illustrates the similarities and differences in structural style, velocity structure, and acquisition conditions. We have observed that younger rocks typically show lower velocities and the lower compressibility of these strata results in velocities more sensitive to depth of burial. Velocity gradients need more testing in younger basins with lower velocity rocks. Another observation is that more topsoil with wetter surface conditions appear to offer better coupling for seismic sources and receivers, which could explain why we see better signal-to-noise ratios on seismic data from rainforest versus desert environments. Desert outcrops may read like a geology textbook, but visual clarity at the surface comes at a cost of clarity in the subsurface.
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Broadband Seismic Imaging of Thrust Belt along the Outer Banda Arc in Indonesia
Authors M. Fujimoto, Y. Sasaki, Y. Guo and M. OharaSummary3D broadband seismic technique and appropriate PrSDM imaging technique provided us significant improvements in seismic imaging for both thrust complex and sub-thrust structures. Broadband 3D seismic imaging contributed better recovery of seismic energy from deep sections where conventional seismic imaging could not reveal any reflections. Pre-migration deghosting technology provided us the stable phase of wavelets and desired broadband frequency spectrum in the seismic image. The velocity model constructed from the combination of Kirchhoff PrSDM and Controlled Beam Migration provided better resolution and continuity of seismic images under thrust complexes and large fault surfaces by overcoming the poor illumination issue. These technologies contributed to revealing subtle fault displacements, reducing depth structure uncertainty below thrust complex and increasing the understanding of petroleum systems and risk elements of exploration target in a frontier region.
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Fast-track 2D Seismic Processing While Drilling to Ameliorate Foothills Exploration and Optimize Side-track Trajectory
Authors F. Sapin, H. Allouche, G. Sterbecq, B. Chevallier, G. Guerin, B. Eriksen and P. RochatSummaryThe petroleum exploration in foothills domains remains challenging in terms of positioning a well in an optimal manner to target different objectives in places where there are several mechanical discontinuities in the stratigraphy separating them and inducing often shifted structural tops. In consequence, seismic imaging, as well as a good geological knowledge, is mandatory to predict a well trajectory suited for all the targeted objectives. In Kurdistan Region of Iraq, the mechanical stratigraphy is rather simple but the tightening of the structures often leads to the activation of internal mechanical discontinuities and a shift of the main objectives tops. In consequence, and as in any foothills environment, side-tracked wells have to be carefully planned.
After drilling the first leg of a well, and finding out that we were clearly out of our predictions in terms of structural positioning and in the evidence that the deepest objective cannot be drilled with the first leg, the decision to drill a side-track came quickly. Meanwhile, we performed a fast-track processing on the closest 2D line in order to image the high dips of the drilled formations. The output allowed us to help optimizing the side track trajectory and to ahead with better constraints.
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Inversion, Detachment Folds, and Out-of-sequence Thrusts in the Papua New Guinea Fold Belt
Authors K.C. Hill and R. WightmanSummaryAnalysis of structural profiles through oil- and gas-fields of the Papuan Fold Belt indicates that the preexisting configuration of basement played a significant role in the compressional deformation. The Moran-Paua structures overlie upthrust basement and comprise small asymmetric detachment folds subsequently decapitated by out-of-sequence thrusts, causing shearing of the forelimb. Interpretation of reflection seismic data in the Agogo area suggests a 10° regional dip in basement and a thick syn-rift sequence in which a large detachment fold was formed, probably associated with early basement thrusting at Moran. The forelimb of the fold was then cut through by low-angle thrust faults creating oilfields in both the gentle hangingwall and steep footwall-forelimb. Along strike at Usano, in contrast, seismic interpretation suggests that the basement is shallower dipping and rift sediments appear to be thinner. There, the detachment folds were not developed and early basement thrusting/inversion resulted in a shear zone. Subsequent thrusting along Jurassic and Upper Cretaceous décollements created duplexes and cut through the shear zone such that it was oversteepened creating the Usano oilfield in the hangingwall. Understanding these relationships is important in defining new hydrocarbon plays.
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Plio-pleistocene Thrusting in the Eastern Amazonian Orogenic Wedge (South Peru)
Authors M. Louterbach, M. Roddaz, P. Baby, S. Brichau, J. Bailleul and Y. CalderónSummaryQuantifying interactions between uplift, climate, deformation and exhumation processes remains difficult, mainly due to a paucity of data relevant to all processes. Central Andes are a prime example to study the respective role of climate forcing and tectonic activity on mountain building. In this study we propose new data (thermochronology, biostratigraphy, field observations and seismic interpretations) focused on the Eastern Cordillera (EC) and the Sub-Andean Zone (SAZ) of southern Peru between 12ºS and 14ºS, in order to i) constrain the timing of deformation, and to ii) demonstrate that contrary to recent studies, our new data outline that the Plio-Peistocene uplift and erosion of the Andes are more likely related to thrust induced exhumation than to a climate forcing. Low temperature thermochronological results obtained for the EC and the SAZ, as well as the growth strata documented in seismic and on the field allow to determine two main deformation periods: i) from the Oligocene to Middle Miocene (~25-14 Ma), and ii) from the Late Miocene to Pleistocene (~10-2.8 Ma). The Plio-Pleiostocene cooling induced by the erosion of the EC in southern Peru is not controlled by climatic change, but better by the development of duplex in the Andean front.
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New Exploration Concepts in Zagros Fold Thrust Belt and Persian Gulf, Iran
Authors S. Jahani, I. Abdollahie Fard and S. SherkatiSummaryAfter more than 100 years hydrocarbon exploration, The Zagros Fold-Thrust Belt (ZFAB) is still remained as one of the most talented regions for future hydrocarbon discovery. Three main hydrocarbon structures originated from the Neogene folding, inherited deep-seated faults and buried salt diapirs. Remained many complex and small structures with potential prospect in the ZFTB which has not completely studied yet. They could be categorized in: (1) Fault related structures as subtle traps, which formed in response to intermediate detachments during Zagros folding or formed below the thrust faults, (2) Long time halokinesis prepared suitable small traps around the diapirs or under the salt sheets, (3) Inherited deep-seated fault movement and see-level changing formed stratigraphic-structural traps towards the palaeohighs, (4) Results of few drilled wells in pre-Permian sediments (Faraghan Formation) proved new gas reserves (5) and oil rings developed in some of onshore folded anticlinal. Consequently for future oil and gas exploration has to take two strategies. For oil exploration focuses more on the complex and small structures including subtle traps, stratigraphic-structural traps around salt diapirs and Palaeohighs. Whereas, for gas exploration remained many huge anticlines to the eastern part of ZFTB which has not been investigated in details.
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Analogue Modelling of the Papua New Guinea Fold and Thrust Belt
Authors R. Darnault, K.C Hill, J.M Mengus, J.M Daniel, J.P Callot and J.C RingenbachSummaryIn the PNG Fold Belt the jungle-covered mountains limit data acquisition so that the internal geometry and evolution of the large anticlines are poorly understood. It is postulated that the anticlines formed above old normal faults in basement, which has been tested by analogue modelling and 3D computerized X-ray tomographic scanning. Layers of corundum, sand, silicone and sand/silicone mixes were used to represent the PNG stratigraphy of 2 km of molasse, 1 km of carbonate, 1 km of mudstone, 500m of sandstone and shale reservoirs and 0.5-3 km of syn-rift clastics. The thicknesses, strengths and velocities of deformation were all scaled appropriately and erosion/deposition was modelled by adding or removing ‘molasse’. It was found that the carbonate deformation was often detached from that of the underlying reservoirs and that the structural style was critically dependent upon the strength of the intervening mudstone and of the basal detachment. Structural style was also strongly correlated to deformation rate, in that slower convergence rates yielded a single large fold, whereas optimum rates yielded more realistic detachment folds. It was observed that the development of an overturned detachment fold was greatly enhanced when the basement fault was first partially inverted.
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Development of Detachment Folds on Syn-orogenic Sequences Including Salt Layers
Authors O. Pla, J.A Muñoz, O. Ferrer, E. Roca and O. GratacósSummaryThe southern Pyrenean foreland basin is formed by a thick syn-orogenic sequence that includes several evaporitic layers with salt that are involved in the fold-and-thrust belt. The Oliola anticline is a detachment fold probably developed at the southern pinch-out of the syn-orogenic Cardona salt unit that also involves younger syn-contractional evaporitc unit (Barbastro Fm.).
Based on this natural analogue and using an experimental approach (sandbox models) we propose a new kinematic model for the development of detachment folds with multilayered evaporites in the Ebro basin. Analogue modelling of detachment folds suggests a new scenario for diapirs formation in compressional tectonic settings.
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Genesis and Tectono-thermal Evolution of the Altiplano (Central Andes), from the Basin to the Orogen
Authors A.M.M. Robert, P. Baby and M. RoddazSummaryWe would like to present our project that aims to investigate the tectono-thermal evolution of the Altiplano in the Central Andes from the basin to the orogenic scale and to study the interplay between Earth's deep and surface processes in convergent context.
We aim to unravel the sedimentologic, structural and thermal evolution of the hinterland basins of the Altiplano in order to provide critical evidences for the history and style of the deformation in that region. In fact, these basins remain attractive frontier basins for petroleum exploration.
At the orogenic scale, this study projects to compare regional cross-sections at different latitudes. According to the available seismic and seismologic data, we propose to produce 2 regional lithospheric-scale cross-sections mainly based on detailed balanced cross-sections and geophysical modeling.
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Algorithmic Aspects of Extended Waveform Inversion
By W. SymesSummaryExtended waveform inversion uses inference of an extended model, depending on non-physical parameters, to extract velocity information directly from waveform data. It has been observed that fitting data at all stages of velocity updates, a possibility offered by model extension, appears to be important to assure reliability of algorithms of this type. An analysis of the modeling operator derivative provides an explanation for this observation, and reveals the link between extended waveform inversion and traveltime tomography.
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Inversion Velocity Analysis - The Importance of Regularisation
Authors H. Chauris, C.A. Lameloise and E. CocherSummaryInversion Velocity Analysis has been recently proposed as an alternative to Migration Velocity Analysis. Under the Born approximation, it consists of first determining the optimal reflectivity model such that the synthetic data set nicely fits with the observed data set. Then, a standard velocity analysis is applied to the inverted reflectivity. The main differences with respect to the classical approach is the use of iterative migration versus standard migration. We propose here an alternative way to compute the gradient of the objective function and demonstrate the importance of the regularisation term introduced to determine the optimal reflectivity model.
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Optimizing the Input Model for Waveform Inversion Using Image-domain Wavefield Tomography with Illumination Compensation
Authors E.F. Diaz Pantin and P.C. SavaSummaryImage domain wavefield tomography exploits focusing characteristics of extended images for updating the velocity field. In order to make good use of this information, we must understand how such images behave if the migration velocity is accurate. This is not trivial since focusing depends on the acquisition setup, as well as on illumination variation caused by the geology separating the acquisition array from the imaged structure, the data bandwidth, etc. We address this problem using a combination of migration/demigration to construct penalty functions that characterize focusing by incorporating acquisition parameters and data bandwidth. Moreover, instead of sampling the extended images at preset distance along the surface, we sample the image by constructing common image-point gathers, which are also much more economical from a computation point of view. Coupled with image residuals exploiting illumination-based penalty functions, we obtain robust wavefield tomography in areas of poor or uneven illumination. Models obtained with this type of methodology are good starting points to more sensitive, but less robust waveform inversion methods.
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Migration Velocity Analysis Combining Reflected and Direct Waves in a Crosswell Configuration
Authors C.A. Lameloise and H. ChaurisSummaryMigration Velocity Analysis is classically restricted to reflected wave fields. In Crosswell Seismic Profiling (CSP) configuration, transmitted waves are usually considered. Here, in a CSP configuration, we combine direct and reflected waves in order to perform a Migration Velocity Analysis. The idea is to take advantage of the different illuminations of the subsurface provided by these different wave fields. The velocity update shows that the part related to reflected waves mainly emphasizes zones around reflectors, whereas transmitted waves illuminate the subsurface in a more homogeneous way. We discuss in a heterogeneous faulted model an automatic velocity method, based on different waves, separately or combined.
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Is Image-warping a Robust Tool for Image Domain Tomography?
Authors F. Perrone and P. SavaSummaryMigration Velocity Analysis in the subsurface-domain measures velocity errors via (extended) imagedomain residuals with respect to an ideal reference image and then updates the velocity model in order to minimise those residuals. Because of the similarity between images with similar extension parameter (shot number, offset, incidence angle, etc.), image-warping presents a robust approach to compute image residuals in different subsurface domains. However, since similarity measures cannot in general distinguish between signal and coherent noise, kinematic artefacts that contaminate the migrated images in certain extended domains can lead to strong and spurious events in the image perturbations and hinder the robustness of the tomographic inversion.
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Utilization of Multiples in Simultaneous Image Building and Velocity Estimation
Authors A.J. Berkhout, D.J. Verschuur, X.R. Staal and M. DavydenkoSummaryIn the past the seismic industry has made large investments in the removal of multiples from the seismic response ('data linearization'), with the purpose to make the data suitable for our linear velocity estimation and linear migration algorithms. Now we are beginning to understand that multiples contain most valuable information that should not be removed.
Instead they should be utilized, making velocity estimation and migration fully consistent with the underlying physics. In this presentation it is shown that velocity estimation can function as an integral part of the nonlinear migration process: Joint Migration Inversion (JMI).
It is also shown that JMI is insensitive to the initial velocity distribution.
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Affordable Full Subsurface Image Volume - An Application to WEMVA
Authors T. van Leeuwen, R. Kumar and F. J. HerrmannSummaryCommon image gathers are used in building velocity models, inverting for anisotropy parameters, and analyzing reservoir attributes. In this paper, we offer a new perspective on image gathers, where we glean information from the image volume via efficient matrix-vector products. The proposed formulation make the computation of full subsurface image volume feasible.
We illustrate how this matrix-vector product can be used to construct objective functions for automatic MVA.
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3D RTM-based Wave Path Tomography Tested at a Realistic Scale
Authors P.M. Bakker, S. Gerritsen and Q. CaoSummaryA new method of 3D RTM-based Wave Path Tomography is presented. Input of the method are RMO picks from wide-azimuth RTM angle gathers. The depth sensitivity of picked events to a velocity perturbation is derived by Born approximation of the associated wave fields and the images. For reasons of efficiency, this is done only for the stationary shots, pertaining to selected picks at certain angle slots in the gather. In the sensitivity kernel calculations, the corresponding recorded data are restricted to those in the vicinity of the estimated stationary receivers. Each branch of a sensitivity kernel, linking stationary shot or receiver to the picked event, is computed by RTM imaging. The method is successfully tested at a realistic scale for a wide-azimuth synthetic example, and for a case with real data from an OBS survey.
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Application of Plane-wave Extended Waveform Inversion to an Onshore Seismic Dataset
Authors Y. Liu, W.W. Symes, C.H. Zhu, Y.Q. Chen and M.Q. LuoSummaryExtended waveform inversion (EWI) fills the gap between migration velocity analysis (MVA) and waveform inversion (WI). It’s less prone to local minimum when the starting velocity is inadequate for conventional WI. To improve the efficiency of EWI, we extend the velocity model in the plane-wave source domain. To weaken the absolute amplitude information and make plane-wave encoding applicable in the real data processing, we use normalized wavefields in the data-misfit term of objective function. Numerical tests show that EWI can successfully and efficiently build a high-resolution velocity model for the onshore seismic dataset.
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Strength and Limitation of FWI - What Can Tomographic Approaches Bring?
Authors G. Lambare and T. AllemandSummaryFull waveform inversion is now well established for velocity model building in areas investigated by recorded diving waves. At larger depths, significant limitations appear associated with a lack of resolution in the long vertical wavelengths of the velocity. To remedy this, new wave equation based approaches have been proposed. They reveal strong connections with migration velocity analysis and tomographic techniques. In the new context of an overlap between the resolution of velocity analysis and of imaging (created by the progress of migration velocity analysis and of low frequency acquisitions) the challenge is to combine these approaches with full waveform inversion in order to get high-resolution structurally conformable velocity models. We review and analyze the proposed methods.
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Joint Full Waveform Inversion of Diving Waves and Reflected Waves for Velocity Model Building
Authors W. Zhou, R. Brossier, S. Operto and J.M. VirieuxSummaryAt depths where there is no sampling by diving waves, FWI behaves as a least-squares migration of the short-spread reflections, hence providing a reconstruction of the short-scale reflectivity at the expense of the long wavelengths of the velocity. Recently, it has been proposed to modify the FWI formalism such that the long wavelengths of the velocity can be updated from reflected waves using some prior knowledge on the reflectivity and an explicit scale separation between the velocity macro-model and the reflectivity. This scale separation allows one to emphasize the forward-scattering regime in the sensitivity kernel of the FWI, referred to as reflection FWI (RFWI). The drawback of the RFWI is to discard the valuable information on the shallow subsurface carried out by diving waves. A new FWI formalism, referred to as joint FWI (JFWI) is proposed and takes advantage of the long-wavelength information carried out by both diving waves and reflected waves to build a smooth velocity model. This formalism leads to a workflow which iteratively cycles the update of the smooth velocity model by JFWI and the update of the short-scale impedance model by classical FWI of short-spread reflections. Application to a synthetic Valhall model illustrates the performance of JFWI.
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An Auxiliary Bump Functional to Overcome Cycle Skipping in Waveform Inversion
Authors P. Bharadwaj, W.A. Mulder and G.G. DrijkoningenSummaryTo overcome the local minima problem in FWI, we propose to use an auxiliary data-domain objective function during inversion. It reduces the data to a simpler form by squaring, followed by blurring to ensure that events that are too far apart can still interact during the inversion. As it effectively replaces seismic arrivals by bumps, we call it the bump functional. This objective function is less sensitive to cycle skipping. Its rôle is to guide the inversion towards the global minimum by pulling the trapped solution out of the local minima associated with the least-squares functional whenever necessary.
Waveform inversion cannot be performed with only the auxiliary objective function because it is insensitive to the polarity of the arrivals and the source signature. Therefore, we alternate between minimization with this and the classic least-squares functional. We confirm the validity of the approach using a simple numerical example with reflection data.
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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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