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72nd EAGE Conference and Exhibition - Workshops and Fieldtrips
- Conference date: 14 Jun 2010 - 17 Jun 2010
- Location: Barcelona, Spain
- ISBN: 978-90-73781-87-0
- Published: 13 June 2010
1 - 50 of 105 results
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Integrated interactive framework for the prestack interpretation arena
Authors O. Kvamme Leirfall, S. -K. Foss, A. Osen, M. Rhodes and Ø. SkjævelandCreating an arena for interactive collaboration between geology and geophysics has for some time been identified as a field for strengthening seismic exploration. The set objective was always to release the synergies of combining these two fields of expertise. The challenges faced in providing an arena capable of true interactivity have historically limited the options for real time processing. Over the last few years there have been several publications on interactive migration, indicating that interactive processing is scalable within the current standards of production high performance computing (HPC) configurations. In this article we will present a scalable framework of visualization and interactive processing only limited to available combined memory of a HPC installation and high-speed interconnect. This in addition to interpretation software and visualization capabilities, define the hardware of an interactive prestack interpretation arena
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Optimizing interpretation knowledge with full azimuth information and data in a common canvas
More LessPre stack seismic data structures are a result of a systematic and formulated reconstruction of the recorded seismic wavefield. Seismic processing and imaging specialists spend most of their time generating, refining, analyzing, and inverting these pre stack data structures in order to attenuate noise, build velocity models, image the subsurface structure, search for direct hydrocarbon indicators and derive elastic properties. Seismic interpreters, on the other hand, spend most or all of their time performing operations on the stacked image. While modern interpretation systems are rich in data mining, visualization, and extraction procedures, these operations, and the information extracted from them, are limited to the dimensionality of the data and the loss of information suffered during the stacking process. To overcome these deficiencies, approximations in the form of limited dimensional stacks over acquisition offset or subsurface angle are frequently used as part of the interpretation process.
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Pre-Stack Pro: Scaleable Pre-Stack Computing on the Interpretation Desktop
Authors B. Shea and F. -J. Pfreundtrated by in-house and contract processors, and are forced to rely on stacking to reduce data volumes to “interpretable” dimensions. Pre-Stack Pro, a new breed of software application, harnesses several high-performance computing (HPC) technologies to bring large-scale pre-stack computing to the interpreter’s desktop. By fully exploiting parallelism throughout the system, we simplify pre-stack analysis and deliver a system whose throughput scales efficiently with total hardware investment.
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Waveform inversion from a poor starting model – using a residual ‘drip-feed’ strategy
Authors N. K. Shah, M. R. Warner, L. Guasch, I. Stekl and A. P. UmplebyWe present a new waveform inversion scheme designed to avert the need for an accurate starting
model and low frequency content in the data – a necessary key step in making the technique work on
a much wider range of exploration datasets and targets than it currently can. The scheme operates by
preceding the inversion of the field data by inversion of intermediate target datasets – synthesised out
of the curl-free (irrotational) part of the phase mismatch at the lowest useable frequency. We
demonstrate its effectiveness over the corresponding conventional approach by inverting data from the
Marmousi model with a 1-D starting model and minimum frequency of 5Hz.
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Building anisotropic models for depth imaging: from imaging parameters to Earth model
Authors A. Bakulin, O. Zdraveva, M. Woodward, K. Osypov, P. Fowler, D. Nichols and Y. LiuVelocity model is the bridge that links our data with our images of the subsurface. Therefore our images can only be as good as our velocity models. Moving to “difficult oil” in sub-salt, sub-basalt and generally deeper targets, we can no longer afford the compromises of simplistic models we did in the past. Doing that leads to poor or no image areas. To fully leverage potential of new data types (wide azimuth, long offsets), we have to put a realistic complexity into our models. To address these challenges industry moved into using anisotropic earth models as a new standard (vertical and tilted transverse isotropy or VTI and TTI). Incorporating anisotropy increases our ability to fit the data and image every single piece of it. However growing expectations requires not only focusing the image but also accurately positioning seismic images for drilling. While this is achievable with anisotropic models, it only occurs when geology and data from boreholes are intimately incorporated into velocity model building from the very start.
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Challenges of depth model building for Tilted TI media
Authors P. M. Bakker and A. StopinOver the last years, Shell has accepted anisotropic velocity model building and pre-stack depth migration as the norm for their seismic imaging. This has reduced the number of sidetracks to be drilled as a consequence of sub-optimal imaging or positioning, which is a clear demonstration of the impact of including anisotropy in the model building cycle. While VTI is a commonly accepted working model, with a workflow that is conceptually clear, this is less obvious for Tilted TI. Tilted TI might be the preferred model at the flanks of salt bodies and minibasins, where the sediments are strongly dipping. We shall demonstrate such a real data case in the Gulf of Mexico, where the application of Tilted TI simultaneously explained mis-ties of well markers and sub-optimal focusing of dipping reflectors. Mis-ties and residual moveout were in conflict for a VTI model. Modelling experiments confirm that this can be the case if the subsurface is Tilted TI, indeed. We shall also see that availability of Wide-Azimuth data should be helpful in recognizing Tilted TI in the subsurface. Usually, anellipticity causes only subtle effects in focusing, considering the trade-off between Eta and moveout-related velocity. For the Gulf of Mexico, one generally finds Eta to be small. Therefore, model building is frequently restricted to vertically elliptic models, at least in the initial phase. Reflection tomography with such a model will flatten residual moveout already to a large extent. If, subsequently, a tilt angle is introduced in such a model, this has effect on the well-ties as well as on the residual moveout in the common image gathers. A method is discussed to fit well-markers in such a situation, aiming at preservation of moveout.
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Migration velocity analysis using a transversely isotropic medium with tilt normal to the reflector dip
Authors T. Alkhalifah and P. SavaA transversely isotropic model in which the tilt is constrained to be normal to the dip (DTI model) allows for simplifications in the imaging and velocity model building efforts as compared to a general TTI model. Though this model, in some cases, can not be represented physically like in the case of conflicting dips, it handles all dips with the assumption of symmetry axis normal to the dip. It provides a process in which areas that meet this feature is handled properly. We use efficient downward continuation algorithms that utilizes the reflection features of such a model. For lateral inhomogeneity, phase shift migration can be easily extended to approximately handle lateral inhomogeneity, because unlike the general TTI case the DTI model reduces to VTI for zero dip. We also equip these continuation algorithms with tools that expose inaccuracies in the velocity. We test this model on synthetic data of general TTI nature and show its resilience even couping with complex models like the recently released anisotropic BP model.
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Extended common-image-point gathers for anisotropic wave-equation migration
Authors P. Sava and T. AlkhalifahIn regions characterized by complex subsurface structure, wave-equation depth migration is a powerful tool for accurately imaging the earth’s interior. The quality of the final image greatly depends on the quality of the model which includes anisotropy parameters (Gray et al., 2001). In particular, it is important to construct subsurface velocity models using techniques that are consistent with the methods used for imaging. Generally speaking, there are two possible strategies for velocity estimation from surface seismic data in the context of wavefield-based imaging (Sava et al., 2010). One possibility is to formulate an objective function in the data space, prior to migration, by matching the recorded data with simulated data. Techniques in this category are known by the name of waveform inversion. Another possibility is to formulate an objective function in the image space, after migration, by measuring and correcting image features that indicate model inaccuracies. Techniques in this category are known as wave-equation migration velocity analysis (MVA).
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Extended common-image-point gathers for anisotropic imaging with blended sources
More LessIn regions characterized by complex geology, the accuracy of imaging is controlled by the quality of the Earth model used to simulate wave propagation in the subsurface (Gray et al., 2001). Thus, accurate model building is a critical prerequisite for imaging of the interior of the Earth. This requirement is even more stringent in regions characterized by strong anisotropy. Furthermore, it is important to construct subsurface velocity models using techniques that are consistent with the methods used for imaging. As reported in the recent literature, in the context of wavefield-based imaging, there are two main strategies that can be used for velocity estimation from surface seismic data. Those methods can be separated into two groups: data space methods, which operate by matching the recorded data with simulated data, and image space methods, which operate by correcting image features that indicate model inaccuracies (Sava et al., 2010).
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Imaging of VTI media from wide-aperture data by frequency-domain full-waveform inversion
Authors Y. Gholami, A. Ribodetti, R. Brossier, S. Operto and J. VirieuxSeismic imaging of anisotropic media is one of the most challenging problem in exploration geophysics because of the possible coupling between the different anisotropic parameters. In this study, we develop a frequency-domain full-waveform inversion (FWI) method for imaging 2D visco-elastic VTI media from wide-aperture data. The forward problem relies on a finite-element discontinuous Galerkin method on unstructured triangular meshes that allows for accurate seismic modeling in complex media with reflectors of arbitrary shape. The inversion relies on a quasi-Newton algorithm which allows for proper scaling of misfit-function gradients associated with different parameter classes. The model parameters are either the P and S wave speeds on the symmetry axis and the Thomsen’s parameters ! and ", or the stiffness coefficients c11, c33, c13 and c44. We first present a review of the diffraction patterns of each anisotropic parameter classes to assess the best parameterization of the inverse problem. Second, we present some preliminaries examples of FWI with simple synthetic model. Results highlight the coupling between the parameters and the difficulty of imaging the ! parameter from surface data.
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Numerical analysis of parameter sensitivity in 3D TTI tomography
More LessWe investigate the resolution of anisotropic model parameters in ray-based TTI reflection tomography. Our approach is to perform numerical experiments using 3D acoustic TTI finitedifference model data. In the first part of this presentation, we introduce a 3D TTI model that is used for this study. The size of this model is x=20 km, y=17.5 km and z=10 km. Some of the important features of the model are steeply dipping layers (dips up to 70 degrees) exhibiting relatively strong anisotropy with structurally conformable orientation of the anisotropic symmetry axis. The model also contains salt bodies that allow us to investigate the effects of TTI anisotropy in overburden sediments on the imaging of subsalt events.
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Gaining insights about imaging structural uncertainty: TTI 3-D synthetic case study
Authors K. Osypov, B. Nolte, U. Albertin, M. O‘Briain, Y. Yang, D. Nichols, M. Woodward and F. QiaoOsypov et al. (SEG 2008, EAGE 2010) introduced an uncertainty analysis method for anisotropic migration velocity analysis which generates model samples from the posterior distribution by using eigendecomposition of anisotropic tomography operators and null-space projection. A realistic synthetic model is an important object for validating the uncertainty method because the ground truth is known and various “what if” scenarios could be played. This study deals with analysis of set of such scenarios for the BP TTI 3-D model to gain insights about anisotropic velocity parameter estimation ambiguity and the scale dependency of uncertainty. This analysis was done by using TTI offset ray tracing of the model for an OBC mirror geometry.
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Azimuthal anisotropy? The time and depth imaging points of view: an imaging case history
The superior imaging capabilities of wide azimuth (WAZ) are now well established. Processing such acquisitions rquires adaptation of the processing workflows and tools. For example the recorded azimuthal information must be kept and the tools should be able to deal with the increased amount of data. Concerning the velocity model building, a question remains on the need of introducing azimuthal anisotropy. In this paper we address this point with a time and depth imaging case study for a high density wide azimuth (WAZ) land surface acquisition. We show on this dataset that azimuthally varying residual move-out observed on time migrated common image gathers (CIG) definitely disappears on the depth migrated CIGs (in both cases no azimuthal anisotropy is introduced in the velocity model). This illustration highlights the limits of the assumptions of time imaging, thuis promoting the use of depth imaging when processing high-density WAZ data, even in the context of mild geological complexity.
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Modeling overburden heterogeneity in terms of Vp and TI for PSDM, Williston Basin, U.S.A.
Authors G. M. Johnson and J. DorseyWe present a Williston Basin case study solution where using a new anisotropic depth migration velocity analysis work flow and available well data, Vp and TI parameters in the overburden are modeled and validated in sufficient detail to have a direct impact on the quality of the seismic image at depth. Comparison with isotropic modeling demonstrates the need for highly detailed spatial and vertical modeling of overburden TI heterogeneity prior to azimuthal attribute analysis and other reservoir characterization flows.
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TTI model interpretation when anisotropic migration velocity analysis is impossible: A case history from the Colombian Llanos Basin
Authors R. Vestrum and I. C. FlorezThe objective of the anisotropic depth imaging was to obtain a more accurate positioning of the events for an appropriate well trajectory. The original well location was determined from the isotropic depth migration, and it was clear from early in the drilling cycle that there were lateral-position errors on the seismic image. The TTI anisotropic depth migration project ran during drilling to refine the well trajectory. We used a collaborative, geologically constrained approach that integrates all available geologic information into the interpretation of the seismic velocity model. This area has interbedded siliciclastic rocks with high dips and vertical and lateral velocity contrasts, giving a considerable lateral movement in the images in depth when we correct for seismic anisotropy and lateral-velocity heterogeneity. The position of the structure and dips on the final depth image were confirmed by two wells drilled in the area.
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High resolution surveying - Historical perspectives - how and why we got here
By A. B. ReidHigh resolution and high sensitivity potential fields surveying has a long pedigree, going back to early recognition that sediments and hydrocarbon reduction effects could have observable magnetic signatures, and that very high resolution views of Basement geology could be obtained with close-spaced survey lines. Since then, close-spaced gravity and magnetic surveying has lived up to its early promise, and delineated sedimentary and basement structure with extraordinary effectiveness.
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HiRES airborne geophysical surveys in the UK: the Anglesey magnetic perspective
Authors D. Beamish and J. WhiteThis paper provides a comparison of vintage, UK national scale aeromagnetic data and modern, high resolution airborne data using a case study across the island of Anglesey, NW Wales. Deeper responses associated with magnetic basement are masked by a near-surface Palaeogene dyke swarm. In order to extract shallow and deeper basement features both data sets are processed in a consistent manner using azimuthal and spectral filtering procedures. A joint assessment is then carried out on the resolution capabilities of both data sets using established edge and depth location methodologies.
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An equivalent source approach to the removal of cultural noise from HRAM survey data: examples from Midland region of central Ireland and Haynesville, Louisiana, United States
Authors A. Salem, K. Lei, C. Green, D. Fairhead and G. StanleyHigh-resolution aeromagnetic surveys are commonly flown close to the ground surface (~80m) and sample the magnetic field at about 10m spacing. Such surveys are thus susceptible to magnetic anomalies resulting from populated and/or industrial areas (rigs, pipelines etc). These anomalies are generally called “cultural noise” and need to be removed from the survey data if one wishes to carry out accurate microlevelling to identify and interpret subtle anomalies resulting from subsurface geological structures. Conventional algorithms of cultural noise removal tend to be based on Fast Fourier Transform (FFT) operations either on their own or together with identification and removal of cultural noise signals, using either manually or nonlinear filters methods. These algorithms can have difficulty interpolating across the profile edited sections (i.e., data gaps where cultural noise has been removed) and can introduce artificial anomalies. For these reasons, we have developed a semiautomated method that both identifies sections of profile data containing cultural noise and use the equivalent source approach to recover the magnetic responses of subtle geological anomalies and interpolate their field across the cultural noise gaps in the profile. Theoretical examples of combined subtle magnetic anomalies and cultural noise are used to test the effectiveness of the proposed method, which is shown to provide results that are closer to the original magnetic data without the cultural noise. We demonstrate the practical utility of the approach using highresolution aeromagnetic data from Ireland and United States.
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Aeromagnetic mapping of Norway and adjacent ocean areas - from the Quaternary to the Precambrian
Authors O. Olesen, M. Brönner, J. Ebbing, L. Gernigon, J. Koziel, T. Lauritsen, R. Myklebust and S. UsovAeromagnetic surveys have in the past mainly been used for mapping depths to magnetic basement and igneous units in sedimentary basins. NGU has since 1994 acquired high-resolution aeromagnetic surveys and has also revealed the existence of significant magnetic anomalies arising from sedimentary layers. We have recognized that susceptibility measurements on core samples, hand specimens and in situ on bedrock exposures are essential for the interpretation of these anomalies. Petrophysical data (magnetic susceptibility and remanence) of 40.000 rock samples from the Norwegian mainland and offshore wells and drill holes have been acquired in order to constrain the interpretation of the aeromagnetic data. Sub-cropping Late Paleozoic to Tertiary sedimentary units along the Trøndelag-Nordland coast produce a very distinct anomaly pattern. The asymmetry of the anomalies, with a steep gradient and a negative anomaly to the east and a more gentle gradient to the west, relate the anomalies to a
strata gently dipping westward. The susceptibility measurements on Sintef’s cores indicate that these coast-parallel anomalies are caused by 1) alternating beds of sandstone and claystone/siltstone/mudstone [mean suscept. 0.00013 and 0.00025 SI], 2) siderite-cemented sedimentary rocks [mean suscept. 0.00135 SI], and 3) sedimentary units containing detrital Fe-Tioxides [suscept. 0.00100-0.01000 SI]. Negative anomalies are caused by low-magnetic gypsum, anhydrite, salt or coal [mean suscept. 0.00007 SI]. Recent aeromagnetic surveys in the Barents Sea have also revealed distinct negative magnetic anomalies clearly associated with salt diapirs.
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Full tensor magnetic gradiometry processing and interpretation developments
Authors D. Fitzgerald, H. Holstein and S. LettsIn recent years, Anglo/De Beers have championed the development of a Full Tensor Magnetic gradient (FTGM) signal instrument from IPHT. Multiple surveys of this quantity have been made in Southern Africa. With the advent of this new potential field full tensor
gradient instrumentation, new methods have been developed to de-noise and process these curvature gradients. Traditional Fourier domain and minimum least squares residual of the linear differential tensor relationships have been adapted. This leads to levelling, gridding and grid filtering innovations. The result is a full tensor grid representation of the curvature gradients that is coherent and compliant with the physics at all points in the grid. All of the observed data is thus honoured in the Tensor grid. Isolating the signal and then refining it to be sure there are no distortions have dominated efforts to date. Superior anomaly interpretation regarding the full magnetic history and inferences can then be made. A survey from the Groblersdal Platinum mine is shown in the context of the structural geology interpretation. In particular, the dolerite dykes and faults are seen. The Hornfels footwall contact is very strong. The phase map traces the Platreef contact. The Upper Zone magnetites are more pervasive and fine layered structure is revealed there. None of the granites can be seen. A 3D geology model is in preparation. The observed FTG signal will be compared to the predicted thin-body responses from the model. There is more directly inferable structural geology in this tensor signal than can be found in a conventional TMI signal.
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Sensitivity of gravity and magnetic data to basalt and subbasalt structures – A case study from the Møre margin, mid-Norway
Authors J. Ebbing, M. Aarset, R. F. Reynisson and T. VattekarThis study investigates the feasibility to use gravity and magnetic inversion to image basalts and sub-basalt structures in sedimentary basins affected by volcanism. Multiple gravity and magnetic data sets are available fro the Møre margin, mid-Norway, which are flown at different altitudes and with different line spacing. Previously, a 3D model was constructed for study area based on a wealth of seismic and petrophysical information, but the resolution of the regional 3D model prevented detailed imaging of the basalts and sub-basaltic structures. While it is difficult to identify the lateral extend of the volcanic features (at depth of 6 km) in the gravity and magnetic data, as well as in Full Tensor Gravity (FTG) data, the sub-basaltic basement architecture can be identified. The gravity gradients provide valuable information on the vertical and lateral extent below the basalts, despite the small density contrast to the surrounding, if the basalts are located in depth of less than 4 km. In such cases, inversion of the gravity and magnetic residuals, gives a better insight into the extent and thickness of the basaltic and sub-basaltic layer.
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When resolution is the key – a review of modern airborne magnetic, radiometric and gravity survey
More LessWhile standard airborne magnetic, radiometric and gravity technologies have not undergone significant changes over the last decade, the techniques have each advanced incrementally. Most of the improvements are related to better acquisition systems which allow for higher speed sampling, improved processing techniques, and, more than often, the choice of aircraft platform which can benefit lower and slower survey flying. Using a series of example datasets, we review the current “state-of-the-art” in each technology demonstrating what can be achieved from a resolution perspective and ultimately setting a bench-mark for the sort of data quality industry should demand when resolution is key to project success.
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Qualitative and quantitative interpretation of airborne gravity and HRAM targets structures for prospect-scale 2D seismic: Southern Maranon Basin, Peru
Authors H. D. Geiger, E. Velasquez, J. F. Ceron and D. M. DolbergIn conjunction with our partners Ecopetrol, Talisman flew a 12,220 km2 aerogravity/aeromag survey (Sander) in Blocks 134/158, Southern Maranon Basin, Peru in mid-2008. The primary objective of the survey was to reduce exploration cycle time and costs
by eliminating a round of regional reconnaissance 2D seismic, moving directly to prospect scale 2D seismic over the most promising anomalies. The survey coverage included areas to the north and west of the area of interest with existing structures
defined on 2D seismic, some with well control. 2D and simple 3D modelling studies were used to evaluate airborne gravity vs airborne gravity gradiometry. A careful analysis of expected signal to noise performance over the wavelengths of interest suggested that both methods would perform well. Airborne gravity was selected as the most cost-effective, and for reduced contamination of signal from near-surface geologic noise. Qualitative interpretation was based primarily on bandpass filtered Bouguer anomaly maps. In areas with 2D seismic, there was a strong correlation between structural closures mapped from seismic and anomalies identified on specific filter products. The spatial scale-length of structures appears to be consistent and extends into the area of interest where there is no previous subsurface mapping. The revised structural interpretation agrees with preferred tectonic models. Existing structures on key seismic lines were modelled to determine the 2D anomalous Bouguer gravity response, and compared with profiles extracted from filtered products. Anomaly amplitudes and corresponding contour intervals were chosen as proxies for prospect areas. The prospect areas associated with mapped anomaly contours were then used to design a prospect-scale 2D seismic program. Constrained 3D gravity inversion has since confirmed the prospect locations and scales. The dataset was decimated and reprocessed to estimate signal to noise ratios, which confirming the signal to noise estimates.
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3D and 4D high resolution microgravity – case stories from mining and geoengineering
By J. MrlinaGravimetry has a long tradition in various geological investigations. For regional exploration projects the accuracy of 0.2 – 0.5 mGal was sufficient for decades. Detailed surveys, on the other hand, and especially microgravity require not only high resolution of gravimeter’s readings, but also high accuracy/repeatibility in the range of 0.00X mGal (X μGal). Therefore, we also focused on the reduction of disturbing effects affecting microgravity measurements (wind, sun, rain, vibrations, etc.), but mainly on data processing. We developed special software for accurate determination of gravimeter system drift from field observations. This technique allows significant reduction of repeatibility error. As the presented case stories represent about three decades, the resolution of gravimeters in contrast to expected and observed signals will be discussed. Case histories will be presented on the detection of voids that often exist in mines as result of natural processes, or unknown historical mining. This will include the monitoring of surface collapse risk. Another example of microgravity monitoring of stress evolution in a deep coal mine will be shown. As well the monitoring of groundwater variations in an open pit mine waste dump body will demonstrate the efficiency of repeated microgravity in hazard control. Microgravity indicated critical increase of groundwater before a damaging slide of the waste dump body mass. Special case is represented by 4D gravity in oil EOR. The results suggest that applied high resolution microgravity may contribute, beside others, to mine monitoring systems aimed at risk mitigation.
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High resolution AIRGrav surveys: Advances in hydrocarbon exploration, mineral exploration and geodetic applications
More LessSander Geophysics has now operated its AIRGrav airborne gravity system for over ten years. More than 2,500,000 lkm of AIRGrav surveys have been flown, predominantly for hydrocarbon exploration. Recent advances in SGL's gravity data processing, involving advanced analysis of system dynamics and improved filtering, help to further reduce system noise and allow for the generation of high quality, low noise gravity data through a wider range of survey conditions than was previously possible. In the past year, a number of AIRGrav projects with innovative survey design parameters have been successfully completed. Mineral exploration projects have been flown using a helicopter at extremely slow acquisition speed (30 knots) combined with tight (50 m) line spacing to produce data sets with higher resolution and higher accuracy. On the other end of the spectrum, the AIRGrav system produced excellent results when installed in a NASA DC 8 flown from 500 to 11000 m altitude at 300 knots, covering approximately 9,000 km in 12 hour flights, with differential GPS baselines as long as 3,000 km. New data processing techniques have allowed the extraction of the horizontal gravity components of the airborne gravity data in addition to the traditionally used scalar gravity measurement.
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The Value of Integration of Gravity Gradiometry with Seismic and Well Data – an example from a frontal thrust zone of the UAE-Oman Fold Belt
Authors J. A. Protacio, J. Watson, F. van Kleef and D. JacksonThe east of the Emirate of Dubai is dominated by the geologically complex western thrust front of the northern Omani Mountains. This deformation front is the boundary between the western foredeep basin and the eastern Omani fold-and-thrust belt. Complex geology makes conventional exploration challenging. The reservoir (Thamama Group) structures are thrusted anticlines with the overlying Tertiary units showing large-scale thrusting as well. The Lower Cretaceous Thamama Group limestone is one of the main hydrocarbon reservoirs in the Middle East. It forms a major hydrocarbon-producing reservoir in the U.A.E., Iraq, Bahrain and Oman and has a high hydrocarbon potential in southeast Iraq, offshore Oman and offshore northeast Saudi Arabia. Due to the significant density contrast between the reservoir and the overlying sediments, Margham Dubai Establishment commissioned an airborne gravity gradiometry (GG) survey to improve the confidence in top reservoir location and to aid in ongoing exploration activity. GG, magnetic and LiDAR data were acquired and used in an integrated interpretation with existing seismic and well data. The integration of these data allowed a better understanding of the thrust linkages at different levels, and a better insight into the interaction of thrusts, backthrusts, detachment levels, imbricated zones, and lateral ramps. The survey is designed around the airborne GG technology known as the Full Tensor Gravity Gradiometer (FTG). GG measures the rate of change of the Earth’s gravitational field
while conventional gravity (CG) measures the vertical acceleration. Acquired from an aircraft, GG has a strategic advantage over CG due to the resolution limitation imposed by the Differential Global Positioning System (DGPS). The DGPS limits airborne gravity
resolution to > ~4000m wavelengths while GG can resolve wavelengths of > ~300m. The shorter wavelengths are crucial to accurately model the geology above the reservoir. In complex geology, multiple lithological units contribute to the GG signal. To map the
reservoir, it is vital to isolate its response from the overlying geology. The high resolution GG data facilitated an accurate investigation of the 3D Shallow Earth Model (SEM). Modelling of seismic sections constrained by GG and magnetic data exploits their complementary nature. Seismic data respond well to horizontal discontinuities while potential field data respond to vertical discontinuities. This produces a geologically realistic SEM. Forward calculation of the GG signal from the 3D SEM was performed and then subtracted from the observed signal. The SEM corrected data were then used to interpret the Thamama reservoir structure.
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Exploration Play Models and FTG Gravity data
Authors C. A. Murphy and J. DickinsonExploration play models are employed by mineral and hydrocarbon exploration companies to establish and help explain the geological setting of their targeted resource. Such models are routinely enhanced through the deployment of geophysical technologies to better improve the understanding of the geological setting and size, shape and depths of the target. FTG Gravity data offers an additional layer of information to consider the fortunes of many exploration play models from salt, fault block closure and igneous intrusion models to more mainstream fault mapping. Many have been confirmed or tweaked through direct involvement of the FTG data into their exploration workflow and are a testament to the acceptance of this innovative technology by leading exploration
companies. This paper will present data examples demonstrating the usefulness of FTG Gravity when investigating the prospectivity of exploration play models. Examples from survey work targeting fault block structural closures and igneous intrusives will be
shown.
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Integration of High Resolution Falcon TM Gravity Gradiometry and Seismic data: an example from Northern Argentina
Authors L. Braga, M. L. Fernandez, J. C. S. de Oliveira Lyrio, S. V. Yalamanchili and A. MorganNewly acquired airborne gravity gradient and magnetic data were used to model the tectonic framework and basement configuration beneath the Chirete concession area, onshore, northern Argentina, using the existing seismic data as constraints. The study area is comprised of approximately 3675 square km and covers approximately 35 km in an east-west direction and 105 km in north-south direction. The main project goal was to modeling the basement and associated major structural elements for selection of oil & gas prospects. The basement related faults/lineament maps were generated using several enhancements of gravity gradient and magnetic data. 2D seismic depth sections were used as initial constraints for 2.5D and 3D gravity and magnetic modeling. Well log densities and velocities are also constrained in this modeling process. The basement depth estimates are computed from the total magnetic intensity profile data. The Werner, Euler, and Peters half slope techniques are used in basement depth computations as well as the depths to the magnetic sources within the sedimentary section. This interpretation was then refined by utilizing the gravity gradient data and seismic data using 2.5 D and 3D modeling. Several of these depths are related to volcanic
sources and were identified as igneous provinces and probably related to the rift and post rift periods. The basement depths show significant variation from south to north and ranging approximately 4-13.5 km below sea level. The enhanced potential field data yielded basement faults and structural framework of the area. The general basement faults trends are striking mostly in ENE-WSW direction. Two major East-West faults were identified as being the major bounding faults of the Lomas de Olmedo rift. There exists a remarkable correlation between these faults derived independently from seismic reflection data and enhanced Airborne Gravity Gradiometry data. A number of positive structural features were identified with the associated faulting and may provide new prospects for well drilling.
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Interpretation of Gravity Gradiometry data and integration with PSDM workflow – imaging sub-salt structures in Gabon
Authors J. Watson, J. Barraud, F. Assouline and N. DyerFTG data were acquired over an onshore Gabon Block. The objectives of this survey were to delineate accurately the salt structures; to derive a base salt structure map and to map basement structures associated with rifting. The survey area is situated along a clear trend of oil & gas subsalt fields that runs roughly north-south from offshore fields in south Gabon to the Lambaréné horst in the north. As a proven reservoir rock, the primary objective is the Gamba Sandstone. It is overlain by the Gamba Formation Vembo Shale which in turn is overlain by the Ezanga Salt formation. The trap is formed by reactivation of Cocobeach Formation faults due to the sediment load of the overlying Gamba and Ezanga Formations and the Madiela Group limestone and dolomite. The Ezanga salt also provides the top seal. Prior to the FTG survey three wells were drilled to target the Cretaceous Gamba sandstones. Although one of the wells appeared to be gas-bearing, the other two wells were dry. In addition, the significant discrepancies between predicted and actual depths demonstrated the need for new and independent geophysical data that would shed a new light on the “salt problem”. Acquisition of seismic data in this area is difficult given the environment, described as coastal, marshy, tree covered areas with small rolling hills. Processing and depth migrating the 2D seismic data is also difficult for several reasons, including a thick and variable weathering layer, 3D nature of salt structure and uncertainties in velocities. These problems resulted in misinterpretations of the seismic data and significant errors on the predicted depths of formations tops. The excellent quality of the FTG data and the large density contrast between salt and the surrounding sediments (clastics and carbonates) ensured that the workflows employed returned a successful result. Compared to traditional land gravity techniques, airborne FTG data has the advantage of offering fast and complete horizontal coverage, and providing the high resolution and bandwidth that is necessary to image the shallow salt structures with confidence. The workflow involved a back-stripping approach in which the model is constructed from top to bottom. Independent data (seismic and magnetic data) was also used to constrain the results. Following on from this an area of interest was defined to focus the interpretation effort on the most promising targets. In this core area, eleven seismic lines were selected for reprocessing and depth migration. The new PSDM seismic data were then used as constraints, together with well data, to build a model in 3D. Manual 3D forward modelling was used to build detailed surfaces with a maximum control on the geometry.
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Nigeria’s Nationwide High Resolution Airborne Geophysical Surveys
By S. RefordNigeria has gained near nationwide airborne geophysical coverage, with high resolution horizontal gradiometer magnetic and radiometric surveys, flown at 500 m line spacing and 80 m mean terrain clearance, totalling almost 2 million line-km. The surveys were flown, per the index map, as follows: 2003 – Pilot Project – Ogun State, 2005-07 – Phase 1 – Blocks A+C and B, 2007-09 – Phase 2 – Blocks D1, D2, D3 and D4. All surveys were carried out by Fugro Airborne Surveys on behalf of the Nigerian Geological Survey Agency. Phase 2 forms part of the World Bank-supported Sustainable Management for Mineral Resources Project. As part of Phase 1, time-domain electromagnetic surveys were flown at 200 m line spacing in 2008-09 with the Tempest system over three blocks, totalling 24,000 line-km. Additional TDEM surveys are planned for Phase 2. To complete the airborne coverage, the Niger Delta block will be flown in 2010 with magnetics at 1 km line spacing. In addition, a quarter of the block will incorporate airborne gravity. The survey data are currently being interpreted by Fugro Airborne Surveys (Phase 1) and by Paterson, Grant & Watson Limited (Phase 2). PGW has prepared nationwide merged grids, and will integrate the two interpretations. The data have proven extremely valuable for: -Depth-to-source mapping of the inland sedimentary basins, delineating areas of interest for oil & gas exploration, as well as mapping shallow basement with extensions of known mineral belts -Determining signatures of known occurrences such as gold deposits, lead-zinc deposits and kimberlite pipes -Mineral potential mapping -Characterization of the “Older” and “Younger” granites -Mapping intrasedimentary igneous sources, sedimentary horizons and structure -The complementary mapping capabilities of radiometric and electromagnetic data in both hard rock terrains and exposed sediments (e.g. Benue Trough). An oral presentation will provide an overview and key highlights. It will also discuss the challenges of compiling and integrating data from a multi-year campaign, utilizing as many as seven aircraft in a survey block. A poster presentation will provide a more in-depth analysis of specific areas of geophysical and geological significance, as well as the contrast of the new surveys with the low resolution magnetic data from the 1970s.
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Waveform inversion for shallow velocity recovery
Authors U. Albertin, J. Mika, M. Zhou, A. Ford, N. Robinson and G. SchurterWe demonstrate the effectiveness of frequency-domain waveform inversion for shallow velocity recovery in a number of geologic settings around the world. Waveform inversion uses the entire wavefield, including direct-arrival information, in order to achieve high resolution, and we find it remains stable in areas historically known to have difficulty with coherent noise such as multiples. We demonstrate the effectiveness of the technique in offshore Trinidad, offshore Pacific Asia, and the Caspian Sea.
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Waveform Tomography of Land Data from a Complex Thrust-Fold Belt in Western Canada: What Works, What Doesn’t, and What Needs to Improve
Authors A. J. Brenders, R. G. Pratt and S. CharlesIn the Canadian Foothills, long-offset seismic data are occasionally acquired in an effort to "undershoot" areas of steeply dipping faults and a severely weathered near-surface. Since long-offsets are one method of acquiring the low-wavenumber information necessary for Waveform Tomography (diving wave tomography followed by full-waveform inversion), these data provide an excellent opportunity for demonstrating the efficacy of the method in building velocity models in with land seismic data, in areas possessing large, lateral velocity variations. However, the acquisition of lowfrequency field data remains a challenge: the majority of land-seismic field crews use 10Hz geophones. The use of MEMS accelerometers (with a broadband response in the acceleration domain) was proposed as a solution to this problem, but the results of a recent high-effort, long-offset acquisition demonstrate that the records possess sub-optimal low-frequency data, dominated by instrument noise. In addition, full-waveform inversion of land seismic data must account for the effects of elastic modes such as ground roll. Without an efficient, multi-parameter, elastic inversion scheme, we must use one based upon the acoustic wave-equation, and care must be taken to mitigate the effects of elastic modes
during pre-processing of the seismic data. If seismic data are recorded with an appropriately designed acquisition (i.e., long-offsets and recording instruments capable of recording the low-frequencies), and sufficient, appropriate preprocessing is applied to the input data, acoustic full-waveform inversion can produce complex velocity models of the sub-surface from field seismic data acquired on land in complex geological settings.
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Elastic wavefield inversion in three dimensions
Authors L. Guasch, I. Stekl, A. Umpleby and M. WarnerAlthough acoustic wavefield inversion is widely used, a complete solution of the seismic inversion problem requires that we account properly for the physics of wave propagation, and so must include elastic effects. We have developed a 3D tomographic wavefield inversion code that incorporates the full elastic wave equation. The code uses explicit time-stepping by finite differences that are 4th order in space and 2nd order in time.
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Acoustic waveform tomography of elastic wavefields: Application to marine OBS data
More LessAmong varieties of waveform inversion techniques, acoustic waveform inversion has been a popular choice because of its simple formulation and modest computational costs. However, the earth consists of elastic materials, and thus there remain concerns about reliability, since behaviors of elastic wavefields, such as P-S convergence, are not properly accounted for. We demonstrate the practical validity of acoustic waveform tomography in marine settings using real data and synthetic studies. Ocean Bottom Seismograph (OBS) data in the seismogenic Nankai subduction zone were inverted with the acoustic implementation. We clearly delineated major geological features including the mega splay fault, and thrusts in the accretionary prisms. The mega splay fault accompanies a low velocity layer, which indicates fluid migration or a lithology change. The fault structure underneath the ridge could be debatable, due to the similarity to the topography. Synthetic waveforms kinematically well coincides with observed waveforms, but there remain discrepancies in amplitudes. The results validate the applicability of waveform tomography to elastic wavefields, but the elastic and attenuation effects need to be investigated further. In order to validate the real data results, preliminary 1D evaluation of the inverted results was conducted with synthetic elastic wavefields. The recovery of major structures was verified, but degradation was admitted in vertical velocity contrasts. 2D synthetic results will be computed to further investigate the ability of the acoustic implementation to retrieve spatial velocity contrasts, and the contamination by topography effects.
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Examples of full-waveform inversion that utilize the low-frequency content available from dual-sensor, single-streamer acquisition
Authors S. Kelly, J. Ramos-Martínez and S. CrawleyIn this abstract, we present both 2-D and 3-D examples of acoustic, full-waveform inversion using synthetic recordings for up-going pressure. These data are representative of those obtained through de-ghosting, by utilizing dual-sensor, single-streamer recording. We compare the results of “conventional” full-waveform inversion with those obtained from a method based on impedance, which we have found to be useful for improving recovery of the lowest wavenumbers in the inverted model. We also present results for the 2-D inversion of a line of dual-sensor, field data recorded from offshore Cyprus. Both this inversion and the synthetic inversion study indicate that features ~ 0.5 km can be accurately recovered at depths of 3.5 km using maximum offsets of only 8 km.
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Fast Three-Dimensional Full Wave Seismic Inversion using Source Encoding
Authors J. R. Krebs, J. E. Anderson, D. Hinkley, S. Lee, A. Baumstein, Y. Ho Cha R. Neelamani and M. -D. LacasseIn this paper we will demonstrate that the computational effort of FWI can be reduced significantly by applying it to data formed by encoding and summing source gathers, if the encoding of the sources is changed between iterations. Changing the encoding between iterations changes the crosstalk noise caused by the summation of the sources. Thus, the source crosstalk-noise stacks out of the inverted earth model, allowing summation of a large number of encoded sources. We call this method encoded simultaneous-source FWI (ESSFWI). We demonstrate this technique on 2D and 3D synthetic data.
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Deep Ocean 3D tomography on field data
Authors J. Morgan, G. Christeson and M. WarnerWe have run a suite of 2D and 3D wavefield inversions to recover fine-scale velocity structure in upper oceanic crust. The data were acquired on a plateau, close to a transform fault in deep water in the Pacific ocean. At the fault, a vertical section of oceanic crust is exposed and has been mapped using submersibles, hence our inverted velocity models can be directly compared with adjacent outcrop data. Synthetic tests using the 2D inversion code suggest that the inverted velocity structure may contain artefacts caused by offline arrivals and feathering of the streamer. Synthetic tests using the 3D inversion code show that true velocity structure can be recovered, and 3D inversions of the real data suggest that there is a velocity inversion in the upper oceanic crust in the area modelled.
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Lithospheric imaging from teleseismic data by frequency-domain elastic full-waveform tomography
Authors D. Pageot, S. Operto, M. Vallée, J. Virieux and R. BrossierWe shall present a 2D elastic frequency-domain full-waveform tomography method suitable for lithospheric imaging from teleseismic data. In the teleseismic configuration, the source is a plane wave impinging the base of the lithospheric target located below the receiver netwok. The plane-wave source is implemented in the frequency-domain forward problem using a scattered-field formulation. The wave modeling is performed with a finite-element discontinuous Galerkin method on unstructured triangular meshes. The inverse problem is solved in the frequency-domain using a quasi-Newton LBFGS optimization and the adjoint-state method. Preliminary applications in the framework of the acoustic approximation were presented to highlight the resolution improvements provided by the inversion of topside reflections after the first reflection at the free surface. Theses shorter-aperture converted phases increase dramatically the high-wavenumber coverage in the model space which would have been rather poor otherwise. We assess in a realistic teleseismic setting for a 0.2-2 Hz source bandwidth the frequency sampling required for avoiding wraparound of lithospheric reflectors, which result from the narrow aperture illumination provided by plane wave sources when temporal frequencies are not sufficiently finely sampled. Before considering application to real data, obliquity of plane wave sources with respect to the imaged section must be adressed either by implementation of the 2.5D wave equation or by applying empirical corrections to velocities.
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Some 3D applications of full waveform inversion
Authors R. -E. Plessix, S. Michelet, H. Rynja, H. Kuehl, C. Perkins, J. W. de Maag and P. HatchellSince 25 years, several synthetic and real examples of Full Waveform Inversion (FWI) have been published. The main advantages and limitations of this approach have been explained in the 80’s. It was soon realized that FWI requires long offsets and low frequencies to update the velocity background. Thanks to the improvements in acquisition over the last 20 years, long offset data became available and several research groups could demonstrate the relevance of FWI with 2D data sets. Over the last 5 years,
the increase in computer power made 3D FWI affordable at least with low frequency data. In this presentation, we will discuss a few applications of FWI with marine data sets in the context of velocity model building and time laspe when we invert only the low frequencies. We will also illustrate the relevance of an anisotropic FWI to correctly handle short and long offsets.
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Waveform tomography – Marine vs Land: Targets, Challenges and Opportunities
Authors A. J. Brenders, R. G. Pratt, R. Kamei and S. CharlesWaveform tomography yields sub-wavelength scale velocity resolution through formal inversion of the recorded seismic wavefield. Velocity estimation through waveform inversion works best where wide angle (large offset) refractions and reflections are available in the input data, where low frequencies with good signal-to-noise rations are available, and where sources and receivers are adequately coupled and are reliably consistent. Waveform tomography results have been obtained by our group for both Marine and Land data settings. Marine seismic data, in most cases, lend themselves well to waveform tomography. Waveform tomography, especially where OBS recording are available, is ideally suited to provide geologically significant velocity images of deeper structures. Land seismic data, in contrast, pose significant challenges for waveform tomography particularly when topographic relief, weathering, and near-surface conditions are severe. The challenges afforded by land data mean that waveform tomography can require extensive manual intervention and repeated parameter testing, driving the costs up dramatically.
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Building starting model for full waveform inversion from wide-aperture data by stereotomography
Authors V. Prieux, G. Lambaré, S. Operto and J. VirieuxBuilding a reliable starting model remains one of the most topical issues for successful application of full waveform inversion (FWI). In this study, we assess stereotomography as a tool to build a reliable starting model for frequency-domain FWI from long offset (i.e., wide-aperture) data. Stereotomography is a slope tomography method based on the use of traveltimes and slopes of locally-coherent events in the data cube. A key feature of stereotomography is that it can be coupled efficiently with semi-automatic picking, which partially frees one from the tedious and difficult interpretive traveltime picking. We assessed a tomographic workflow based on stereotomography and frequency-domain FWI with the 2D acoustic synthetic Valhall case study. The Valhall model is
mainly characterized by a large-scale low velocity zone associated with gas layers above the reservoir level. We first computed an acoustic full-wavefield dataset using a finite-difference time-domain modeling engine for a wide-aperture survey with a maximum offset of 16 km. The source bandwidth is between 10 and 45 Hz. Compared to the conventional application of stereotomography, we assess in this study the benefits provided by the joint inversion of refraction and reflection traveltimes from long-offset data. Use of refraction traveltimes is expected to stabilize and improve the reconstruction of the shallow part of the model. In a similar manner for frequency-domain FWI, we design a multiscale approach which proceeds hierarchically from the wide-aperture to the short-aperture angles to mitigate the non-linearity of the inversion. Starting models for FWI were built by stereotomography using two sets of picked events. For the first data set, the picking is limited to reflection traveltimes with a maximum offset of 4 km, while both refracted and reflected events were picked in the second case using the full range of offsets (± 16 km). We highlight the improvements of the FWI results obtained from the starting stereotomographic model built from the long-offset data set. The improvements are observed at the reservoir level below the gas layers but also in the upper part of the model where the joint use of refraction and reflection traveltimes is helpful to improve the ray illumination.
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Full Waveform Teleseismic Tomography: Theory and Applications
Authors S. Roecker, B. Baker and J. McLaughlinWe have adapted a 2D spectral domain finite difference waveform tomography algorithm previously used in active source seismological imaging to the case of a plane wave propagating through a 2.5D viscoelastic medium in order to recover P and S wavespeed variations from body waves recorded at teleseismic distances. A transferable efficacy that permits recovery of arbitrarily heterogeneous models on moderately sized computers provides the primary motivation for choosing this algorithm. Synthetic waveforms can be generated either by specifying an analytic solution for a background plane wave in a 1D model and solving for the source distribution that would produce it, or by solving for a scattered field excited by a plane wave source and then adding the background wavefield to it. Because the former approach typically involves a concentration of sources at the free surface, the latter tends to be more stable numerically. We adapt a gradient approach to solve the inverse problem to maintain tractability; calculating the gradient does not require much more computational effort than does the forward problem. The waveform tomography algorithm can be applied in a straightforward way to perform receiver function migration and travel time inversion. We will discuss an application of this technique to imaging the crust and upper mantle beneath the Tien Shan range in central Asia.
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Full waveform inversion in the Laplace and Laplace-Fourier domains
Authors C. Shin, W. Ha, W. Chung and H. Seuk BaeWe present a review of Laplace and Laplace-Fourier domain waveform inversion. The wave equation in the Laplace and Laplace-Fourier domains can be solved by changing the real frequencies from the Fourier transform into imaginary frequencies. The initial model of Laplace-domain inversion can be a scratch such as a homogeneous velocity model. The inversion provides a long-wavelength velocity model that can be used as a starting velocity model for conventional waveform inversion, which uses the zero-frequency components of the damped wavefield. Laplace-Fourier domain inversion can recover long-, medium- and short-wavelength velocity models by adjusting the complex frequencies. Careful muting of noise should be applied before the first arrival because the damped wavefield is sensitive to random noise. Numerical experiments and real data examples show that full waveform inversion in the Laplace and Laplace-Fourier domains can provide an alternative for seismic velocity estimation.
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Elastic (Visco) Full Waveform Inversion of multi-component marine seismic data in the time domain: A tribute to Albert Tarantola
Authors S. C. Singh, G. Royle, T. Sears, M. Roberts and P. Barton(AVO) analyses can be used to estimate P and S-wave impedances. Since the method is local, i.e. assumes 1D media, linear approximation to the reflection coefficient, and ignores interference effects, the results are very approximative. In 1980s Tarantola’s group in Paris started developing elastic full waveform of near offset, while other groups were focusing on different types of migration algorithm using more sophisticated mathematical techniques. Tarantola (1986) first set-up the mathematical foundation of full waveform inversion in acoustic media and then extended it to full elastic media (Tarantola, 1988). In early 1990s our group started working on 1D elastic full waveform inversion (Singh et al, 1993) but used long offset data to get medium to large-scale velocity of the sub-surface. We showed that wide-angle reflection data (Neves and Singh, 1996) has sensitivity to intermediate wavelength information. Joint inversion of near- and post-critical angle reflections data allowed convergence towards the global minimum (Shipp and Singh, 2002). Since then we have extended the algorithm to multi-component OBC data to invert P and S-wave velocity (Sears et al., 2008; Roberts et al., 2008) and recently for attenuation (Royle and Singh, 2010). We start inverting wide-angle data first, followed by critical angle and then near offset data. For a stable inversion, we invert P-wave velocity first from vertical component data, then medium scale S-wave velocity vertical component and finally short wavelength S-wave velocity from horizontal component data. Although, our group has made significant progress, computation remains a main issue in applying elastic full waveform inversion on a routine basis. In this talk, I will give a historical prospective of elastic full waveform inversion, particularly those related to work of Albert Tarantola, and then present state of the art techniques of full elastic waveform and then propose a strategy for future waveform inversion. I will particularly highlight the importance of elastic inversion for reservoir characterization, and show how the full elastic waveform inversion could be extended to 3D media in a time-lapse mode (Royle and Singh, 2010; Queisser and Singh, 2010). We are presently taking full waveform a step further by jointly inverting both seismic and controlled source electromagnetic data (Brown et al, 2010).
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Improved Near-surface Velocity Models from Waveform Tomography Applied to Vibroseis MCS Reflection Data
Authors B. Smithyman and R. ClowesMultichannel vibroseis reflection surveys are prevalent in the land exploration seismic industry because of benefits in speed and cost, along with reduced environmental impact when compared to explosive sources. Since the downgoing energy must travel through the shallow subsurface, an improved model of near-surface velocity can in theory substantially improve the resolution of deeper reflections. We describe techniques aimed at allowing the use of vibroseis data for long-offset refraction processing of first-arrival traveltimes and waveforms. Waveform tomography combines inversion of first-arrival traveltime data with full waveform inversion of densely-sampled refracted arrivals. A number of challenges are presented by the characteristics of vibroseis acquisition; we discuss some of these challenges and techniques to mitigate them. Through the use of waveform tomography, we plan to build useful, detailed near-surface velocity models for both the reflection work flow and direct interpretation.
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Seismic anisotropy effects in 3D wavefield tomography
Authors I. Stekl, A. Umpleby and M. WarnerWe are presenting results how seismic anisotropy may affect waveform inversion images. Result from our Marmousi model extended to 3D as 2.5D morel show that not including appropriate anisotropy in the modelling algorithm can lead to mispositioning of anomalies in the images.
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Time vs frequency for 3D wavefield tomography
Authors A. Umpleby, M. Warner and I. SteklUnlike the situation in two-dimensions, where direct factorisation of the matrix equations makes frequency-domain methods much faster than explicit solution in the time-domain, the computational resources required for practical wavefield tomography in 3D can be rather similar in the two domains. We have developed and optimised schemes that undertake wavefield tomography using explicit time stepping in the time domain and that iteratively solve the matrix equations of the implicit problem in the frequency domain.
We have applied these two methods systematically to the same suite of problems. In the frequency domain, the principal advantages are that the initial tomographic updates for lowest frequencies are often seen more quickly, and spatial resolution can be better at the highest frequencies. In the time domain, one of the principal advantages is that it is possible to mute and/or weight the field data in time, and consequently the method can be made to work more effectively with difficult datasets. In practice, both approaches are useful, and both should be available within a comprehensive suite of inversion tools.
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3D GOM WAZ survey experiment using Full Waveform Inversion
More LessExploration in geologically more complex areas requires new tools/methodologies in order to address these challenges. The recently introduced wide-azimuth data acquisition method offers better illumination, noise attenuation and lower frequencies to more accurately determine a velocity field for imaging. The methodology in this paper follows the layer striping approach where we developed the supra salt sediment followed by the top of salt, salt flanks, base of salt and finished with a limited subsalt update. The inversion stages were carefully QC-ed through gather displays to ensure the kinematics were honoured. In order to approximate the observed data, the acoustic inversion had attenuation, anisotropy, acquisition source and receiver depth incorporated in the propagator. The final results were validated by reverse time migration using the inverted velocity field versus the final tomography velocity regime.
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An overview of the SEISCOPE project on frequency-domain Full Waveform Inversion Multiparameter inversion and efficient 3D full-waveform inversion
Authors J. Virieux, S. Operto, H. Ben Hadj Ali, R. Brossier, V. Etienne, Y. Gholami, G. Hu, Y. Jia, D. Pageot, V. Prieux and A. RibodettiWe present an overview of the SEISCOPE project on frequency-domain full waveform inversion (FWI). The two main objectives are the reconstruction of multiple classes of parameters and the 3D acoustic and elastic FWI. The optimization relies on a reconditioned L-BFGS algorithm which provided scaled gradients of the misfit function for each classes of parameter. For onshore applications where body waves and surface waves are jointly inverted, P- and S-wave velocities (VP and VS) must be reconstructed simultaneously using a hierarchical inversion algorithm with two nested levels of data preconditioning with respect to frequency and arrival time. Simultaneous inversion of multiple frequencies rather than successive inversions of single frequencies significantly increases the S/N ratio of the models. For offshore applications where VS can have a minor footprint in the data, a hierarchical approach which first reconstructs VP in the acoustic approximation from the hydrophone component followed by the joint
reconstruction of VP and VS from the geophone components can be the approach of choice. Among all the possible minimization criteria, we found that the L1 norm provides the most robust and easy-to-tune criterion as expected for this norm. In particular, it allowed us to successfully reconstruct VP and VS on a realistic synthetic offshore case study, when white noise with outliers has been added to the data. The feasibility of 3D FWI is highly dependent on the efficiency of the seismic modelling. Frequency-domain modelling based on direct solver allows one to tackle small-scale problems involving few millions of unknowns at low frequencies. If the seismic modelling engine embeds expensive source-dependent tasks, source encoding can be used to mitigate the computational burden of multiple-source modelling. However, we have shown the sensitivity of the source encoding to noise in the framework of efficient frequency-domain FWI where a limited number of frequencies is inverted sequentially. Simultaneous
inversion of multiple frequencies is required to achieve an acceptable S/N ratio with a reasonable number of FWI iterations. Therefore, time-domain modelling for the estimation of harmonic components of the solution can be the approach of choice for 3D frequency-domain FWI because it allows one to extract an arbitrary number of frequencies at a minimum extra cost.
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3D full-wavefield tomography: imaging beneath heterogeneous overburden
Authors M. Warner, A. Umpleby and I. SteklWe have developed computer codes and work-flows for 3D acoustic waveform inversion in both the frequency and time domains. We have applied these methods to several 3D field datasets with a variety of acquisition geometries and target depths. In each case, wavefield tomography was able to obtain a high-resolution high-fidelity velocity model of the heterogeneous overburden, and consequently to improve subsequent depth imaging of an underlying target.
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