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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
21 - 40 of 105 results
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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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