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74th EAGE Conference and Exhibition - Workshops
- Conference date: 04 Jun 2012 - 07 Jun 2012
- Location: Copenhagen, Denmark
- ISBN: 978-90-73834-28-6
- Published: 04 July 2012
1 - 50 of 156 results
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A short course in modern inversion techniques - Copenhagen 2012 v2
By N. PillarWith the change in the SEC rules in 2010 regarding the booking of Proven Undeveloped Reserves (PUD) geophysical techniques that can be proven to be reliable and repeatable will have a large potential impact on the valuation of oil companies in the future. One of the more reliable techniques is seismic inversion. In recent years with the improvement in data quality, both seismic acquisition and processing and the use of partial and angle stack data has allowed the conversion of seismic reflectivity data to rock property information, from band limited acoustic impedance to petrophysical properties such as Vshale, porosity and water saturation. The precursor to any inversion is an analysis of the rocks themselves. This is required so we can gain an insight into their possible spatial variation in the area of interest. This comes from analyzing the rock properties using well data. Comparing the trends seen with existing rock physics models, empirical or theoretical, is an important part of the process. This will highlight any differences in a particular formation that might be related to depth, some form of alteration, cementation etc. After fluid substitution plotting the different elastic properties for each fluid case against each other will allow an understanding as to whether discrimination between lithology and or fluid is possible.
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Exploration and Development of the North Sea Chalk Low Permeability System
By Joan MegsonThis talk characterises the North Sea Chalk hydrocarbon play as a low permeability system. The low vertical and lateral permeabilities are responsible for phenomena which include tilted oil-water contacts and non-structural trapping mechanisms, as seen for example in the Halfdan Field. Predicting the petroleum system requires an understanding of hydrocarbon entry points, the permeability within the Chalk at the time of hydrocarbon emplacement, and the factors controlling lateral migration, which occurs at geological time scales. It is this slow lateral migration that results in dynamic and non-structural traps. An additional consequence of low permeabilities is low recovery rates in the absence of significant natural fracturing or induced fractures.
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The Dynamic Chalk Sea: The Depositional Effects of a Collapsed Shelf-break Front, Erosional Contour Current Systems and Dry Borderlands
By Finn SurlykIn the Late Cretacous NW Europe was covered by the deep, epeiric Chalk Sea (e.g. Surlyk et al., 2003). Oceanic conditions spread across the continent as revealed by deposition of up to several kilometres of coccolithic nannofossil ooze of the Chalk Group. This was probably due to the break-down of the oceanic front located at the shelf-slope break due to very high eustatic sea level. The chalk is generally considered as deposited by a gentle rain of coccolith debris in the form of zooplankton pellets or marine snow. Local downslope redeposition by slides, slumps, debris and turbidity flows took place mainly adjacent to major fault zones which were reactivated by inversion tectonics in several episodes in the Late Cretaceous. However, recent work based on reflection seismic data shows that this paradigm is in need of major revision.
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Ecology of Lower Cretaceous Chalk
More LessChalk sediments, that is here biomicrite with a porosity of 10 – 50%, are widespread in the late Cretaceous of the Boreal Realm. Chalk, however, is uncommon in the Early Cretaceous where clays and mudstones dominate for example in the Lower Saxony Basin (LSB), northeast England (Speeton) and elsewhere. The Tuxen and Sola Formations (late Hauterivian – Aptian) of the southern North Sea (Danish Central Graben) represent this unusual Early Cretaceous chalk system.
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Chalk as a Reservoir
More LessReservoir properties of chalk depend on the primary sediment composition as well as on subsequent diagenesis and tectonic events. Chalks of the North Sea almost exclusively have mudstone or wackestone texture. Microfossils may have retained their porosity where degree of diagenesis is low, or be partly or fully cemented where diagenesis is more pronounced. It is a chalk characteristic that permea bility is controlled by the porosity and internal surface of the mud matrix, whereas the larger pores play an insignificant role. Cemented microfossils may take up a significant volume in a wackestone, and the best reservoir properties are typically found in mudstone intervals.
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Facies Change and Sequence Expression Along a Proximal to Distal Transect in the Lower Cretaceous Chalk of the Tuxen and Sola Formations (Danish Sector North Sea Basin)
More LessThe Tuxen and Sola Formations represent the earliest boreal chalk deposits in the North Sea Basin and are oil-bearing in the Danish sector of the Central Graben. These formations vary in thickness at the large scale, and show a decimetre to metre scale bedding pattern in the cored sections. The core display will show key stratigraphic surfaces and a lateral variation in the facies expression, factors that are likely to influence reservoir characteristics.
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Reservoir Facies Variation in the Upper Cretaceous and Danian Chalk of the Danish Central Graben
More LessThe Upper Cretaceous and Danian Chalks in the North Sea contain reservoir facies which are part of a world-class hydrocarbon system. The examples shown highlight some of the paradoxes of these microporous fine-grained, high-porosity, low-permeability carbonate reservoirs. Total porosity in typical chalk reservoirs is in the range of 25% to 35% with the best examples up to 45%. Permeability is typically low at 0.001mD to 10mD, but is enhanced by natural or induced fractures. Most chalk reservoirs have much better porosity than is expected from burial depths. The better than expected preservation of porosity is most likely related to an early onset of overpressure in the chalk. Apparently during burial, the overlying Tertiary mudstones provided a seal through which the pore fluids of the chalk were not able to escape. The microporous nature of the pore network is related to the size of coccoliths, the calcareous nanofossils which constitute at least ninety percent of the rock. The majority of pores are found in between the coccoliths and in coccospheres.
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Upper Campanian − Maastrichtian Holostratigraphy of the Stevns-1 core, Denmark
More LessThe Upper Cretaceous chalk of the Danish Basin has been interpreted as a major contourite complex on the basis of high-resolution seismic data. The sea floor had a pronounced topography with kilometre-wide ridges and valleys up to almost 200 m deep, interpreted to have been formed by contour-parallel bottom currents. Only few ancient contourite systems have been recognised, mainly based on sedimentary facies and only rarely on architecture and morphology. Two cored boreholes, Stevns-1 and -2, 443.3 m and 345 m deep, respectively through the Danish chalk contourite complex offer a unique possibility to compare seismic and sedimentary facies. The contourite chalk is completely bioturbated except for thin intraclast conglomerates and a few thin levels, showing possible primary lamination. In terms of lithology and trace fossils the contourite chalk is similar to horizontally bedded pelagic chalk uninfluenced by bottom currents. Published contourite models cannot normally be used for the chalk due to the very fine grain size, generally complete bioturbation, and lack of any vertical trends in grain size on a millimetre- to centimetre scale. It is thus only rarely possible to document the influence of bottom currents on the basis of facies analysis alone and this can only be inferred by architectural analysis of seismic-scale outcrops.
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The Chalk of the Ekofisk Field, Norwegian North Sea – An Example From the Cores of Well 2/4-X 32
More LessWith original recoverable reserves estimated to be around 717 x 106 m3 o.e., the Ekofisk Field is the largest hydrocarbon discovery within the Upper Cretaceous–Lower Palaeocene chalk successions of the North Sea. In the Ekofisk Field, the reservoir intervals consist of overpressured and naturally fractured chalk sediments belonging to the Maastrichtian Tor and Danian Ekofisk Formation.
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Comparing Core Facies with Image Log Data, a Tool to Better Understand the Fractures Distribution in the Reservoir - An Example from the Wells 2/7-B 9A and 2/7B 7A in the Eldfisk Field, North Sea
More LessThe Eldfisk Field is third largest field in the Norwegian Continental shelf when it comes to remaining resources. The reservoir rocks of the Eldfisk Field consist of overpressured chalk sediments principally belonging to the Maastrichtian Tor Formation and the Danian Ekofisk Formation. The chalk matrix in the Eldfisk field is characterised by high porosity and low permeability, but as with other naturally fractured reservoirs, effective permeability is enhanced by fractures. Thus, mapping of fractures plays an important role in characterization ofthe dynamic behaviour of the reservoir.
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Advanced Microseismic Data Analysis and its Application to Unconventional Reservoir Characterization.
More LessThe current upswing in unconventional reservoir production, what some are calling the “shale gale”, is not a geology play. The tight rocks being turned into producers are millions of years old and have been known to be petroliferous for decades. Rather, this is a technology play. The technologies that have enabled this storm are 3: horizontal drilling, hydraulic fracturing, and microseismic monitoring. Horizontal drilling extends the pay intersection to allow for economic levels of production. Frac’ing creates the connections with the hydrocarbon molecules that give them free pathways to the wellbore and thence to the surface. Microseismic monitoring illuminates and informs as to how well the frac propagated and just what part of the reservoir has been contacted by the frac in a way that predictive models can’t.
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Novel Workflow for Shale Gas Reservoir Characterization
Authors Lars Sonneland, Geir V. Dahl, Martin Haege and Hilde BorgosShales may contain significant volumes of organic matter and constitutes the key source rock for hydrocarbon reservoirs. Today these types of organic rich shales are being economical exploited in situ and have become recognized as unconventional reservoirs. The current reservoir characterization technology has been evolved over decades focusing on sandstone - and carbonate – rocks. This technology is not directly applicable for shale reservoir characterization. One important difference is that permeability and porosity in shales are order of magnitude different from sandstone - and carbonate – rocks, another that we are describing the source - rock it. We will in this paper propose a new procedure for characterization of shale reservoirs. The workflow is highlighted in figure 1. Input data is typically cores, well-logs and surface seismic data in addition to micro-seismic data .The core, well-logs and seismic data are integrated in a hierarchical fashion that allows characterization of key features in the shales. The total organic content (TOC) in the shales is an example of such a feature. Løseth et. al has in a recent paper that acoustic impedance (AI) in organic-rich claystones decreases nonlinearly with increasing TOC percent. Further claystones mixed with low-density organic matter have significant higher intrinsic anisotropy than otherwise similar non-organic claystones.
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Geomechanics for Unconventional Resources
More LessIn this paper, we discuss the effect of geology on the resulting heterogeneous distribution of rock mechanical properties, and use this information to define key drivers of geomechanical problems of well construction and completion design. The paper explores the relationship of rock fabric on anisotropic elastic properties and on anisotropic strength, and provides examples of reasonable values across well known facies of North American organic-rich mudstone plays. The paper proceeds by defining the effect of mechanical anisotropy on nearwellbore stress concentrations, fracture width, breakdown pressure, solid production potential, and minimum horizontal stress. By discussing requirements of near-wellbore completion quality and far-wellbore completion quality and by providing means of evaluating these important sets of properties, we provide an effective workflow for hydraulic fracture design. Finally, the paper discusses important implications of time dependent behavior to organic-rich mudstone plays, including Biot’s poroelastic coefficient and creep. Principal conclusions of this work are that organic-rich mudstone plays are geologic plays, and understanding the geologic drivers of their vertical and lateral heterogeneity in mechanical and reservoir properties is of highest importance for exploration and production. Their content of stiff mineral constituents, compliant organic constituents, and compliant pores has a strong effect on the mechanical behavior of organic-rich mudstones. Thus, differentiating between the stress supporting constituents of the rock, and the non-stress supporting detrital minerals, is of highest importance to characterize their behavior. Confusion of these two constituents usually leads to misrepresentation of the shale facies. Organic-rich mudstones are also strongly anisotropic and their behavior cannot be approximated using isotropic models. For example, the near-wellbore stress concentrations in anisotropic rocks depend on the rock elastic properties. Thus, they are different for different rock types, and result in significantly different stress concentrations, breakdown pressures and fracture widths. As an additional example, changes in breakdown pressures and fracture width are strongly dependent on the ratios of elastic modulii (Eh/Ev and Gv/Eh) in the directions parallel and perpendicular to bedding. In addition, the far-field horizontal stress also depends on the anisotropic elastic rock properties, and thus changes from rock type to rock type. Finally, we found that the Biot’s poro-elastic coefficient in organic-rich mudstones is in the range from 0.2 to 0.6. These are low values and are in line of those expected for stiff rocks.
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Reservoir Simulation for Unconventional Reservoirs - Advances and Future Challenges
More LessDevelopment of unconventional resources is accelerating, and the technology for the recovery of unconventional gas and oil is rapidly advancing, being in part responsible for continuing low price of the hydrocarbon-based energy. The term “unconventional” can include a variety of resources, such as coalbed methane gas, tight and shale gas, CO2 injection based enhanced recovery, thermal recovery (Cyclic steam stimulation (CSS) and Steam/Solvent Gravity Assisted Drainage (SAGD)), gas from hydrates, in-situ coal gasification, etc. Of these, the main commercial impact for oil production comes from the CSS and SAGD operation, and for gas (and condensate) production from tight, and recently shale, plays. Since shale gas reserves are geographically more pervasive compared to heavy oil and bitumen deposits, this presentation will deal primarily with the unconventional gas resources and the methods for their simulation analysis and recovery optimization. While commercial applications of tight gas date back to 1970’s, continued improvements in exploration/imaging tools, horizontal drilling, and advances in reservoir engineering interpretation techniques in the following decades have made tight gas a significant part of the current North American gas production. Kuuskraa (2006) estimated that in 1994, unconventional gas (including tight gas, CBM and shale gas) already accounted for 40% of US natural gas production. Recently the multi-stage fracturing technology for horizontal wells spurred unprecedented increase of production from shale gas and oil deposits in North America and today the trend is expanding worldwide.
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Developments in Strategic Decision-Making for Water Management in Unconventional Gas Plays
More LessEnvironmental issues in unconventional gas plays are dominated by the water supply chain, including supply, competition with other water uses, transportation, storage, reuse, recycling, treatment, and disposal. The limiting factor within the supply chain varies from play to play, so no single solution is the right one for every geographic location. Application of well-reasoned decision-making tools that assess and quantify the risk-benefit comparison between water management options throughout the water supply chain, results in more cost effective solutions for operators, that are at the same time compliant with regulations and minimize impact on the local community and environment. Simulated case studies will be used to demonstrate decision-making tools ranging from regulatory register risk evaluation to triple bottom line modelling to incorporate social and sustainability aspects of technology selection.
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DS3 Recording for Faster Seismic Acquisition Rate Without Data Compromise
More LessThe main challenge for seismic data acquisition today is to design a survey which accurately images the subsurface in line and cross line, despite the presence of a variety of noise contaminants. This challenge must encapsulate the raising of data quality and the provision of adequate seismic areal coverage, whilst keeping unit acquisition costs low and ensuring no harm to people or the environment. Recent developments in acquisition technologies, like simultaneous recording (Bouska, 2008), and dramatically increased channel counts, help to facilitate high productivity and thus allow the acquisition of properly sampled data in both source and receiver domains, over large areas with fast turn-around.
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Broadband Full-Azimuth 3D Land Seismic Acquisition - Managing 100,000 Channels, Over 20,000 Vibrator Source Points, and 24 Terabytes of Uncorrelated Data per Day
Authors Peter I. Pecholcs and John QuigleyOn land, broadband simultaneous vibroseis acquisition methods and high-channel count recording systems provide key technologies for the acquisition of full-azimuth 3D seismic surveys. This paper focuses on the experience gained in the acquisition and processing of two high density source (1,280 and 3,200 VPs/km2) and receiver (640 stations/km2) surveys, and reviews a new high-productivity field test for optimal de-noising.
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Acquisition and Processing of Dithered Vibroseis Data: A Case History
Authors Claudio Bagaini, Mark Daly and Ian MooreWe present a 2D and a 3D case history of acquisition and processing based on a simultaneous vibroseis acquisition technique we call dithered vibroseis acquisition. This technique is designed to achieve high productivity whilst maintaining the prestack amplitude fidelity required for beyondimaging applications of the seismic exploration method. A land seismic acquisition system with a very large channel count was used to acquire the data for the 3D example. We compare the processing results obtained with conventionally acquired datasets and those obtained with the dithered datasets using three different processing routes. We observe that the poststack results are not substantially deteriorated using this high productivity acquisition technique, particularly in the case of the 3D example, even if the standard processing route is adopted. However, the prestack separated data are significantly better if the processing route that includes algorithms specifically designed for a dithered dataset is adopted.
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Simultaneous Shooting, Today and Tomorrow
Authors Jim Keggin and Ray AbmaSince 2006 in North Africa and the Middle East, BP has faced the problem of having to evaluate and meet drilling commitments in several very large onshore exploration licences within the relatively short timeframes set by the terms of exploration license agreements. Historically, time and cost considerations would have precluded the use of 3D and large exploration areas would have been evaluated using 2D data. Our goal was to acquire simultaneous seismic source data as good as or better than conventional data while reducing the cost and time of the operation. Moving from conventional to simultaneous source acquisition has required significant changes to the way we think about seismic acquisition. For example, our usual approach of discontinuous recording of shots separated in time needs to be replaced with continuous recording with careful scheduling and recording of the location and times of all shots (Howe, et al. 2008). Reduced field effort is offset by increased processing effort and computational costs, and new processing approaches and mathematical techniques, especially those of sparse inversion and compressive sensing, will become more important in dealing with simultaneous source data. Between 2008 and 2012, BP acquired around 25,000 square kilometers of high fold wide azimuth 3D in the desert regions of North Africa and the Middle East. In every case, simultaneous source technology resulted in better data at less cost. Furthermore, less time in the field has meant a significant reduction in HSE exposure. Whilst the bulk of these wide azimuth onshore data had fold in the 500-1500 range, high channel-count crews and simultaneous source techniques now make it possible to acquire super-dense 3D data with regular ‘carpets’ of sources and receivers on a 50m*50m grid (or less). This approach resulting in 20,000 fold data, has been tested over a small focussed area for field development purposes (Howe, et al. 2009). Onshore success naturally suggests the potential for application offshore. Certain challenges need to be overcome before simultaneous source acquisition becomes more than a simple compromise between cost and quality. These new methods of acquisition have and will continue to require modifications to traditional methods of acquiring and processing seismic data.
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Lessons Learned from Commercialization of Marine Simultaneous Sources
More LessSimultaneous source acquisition in marine environments has been much slower to develop than it has for land. Land has several key enablers not found in the marine environment such as a relative ease of adding extra sources, use of complex source signatures as well as a history of related highproductivity acquisition schemes. Marine acquisition also has a long history now of simultaneoussource studies but has only recently seen the first fully commercial marine simultaneous source survey acquired. This paper will examine the key issues, barriers and solutions that have led to this significant step and speculate on future simultaneous source marine activity.
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A Synthetic Simultaneous Source Wide-azimuth Acquisitions Study over a Complex Marine Environment
Marine blended source acquisition is becoming increasingly important in the seismic industry due to the possibility of reducing costs through higher productivity and improving seismic data quality through denser source sampling. One major drawback of marine blended acquisition is the crosstalk noise generated by the nearly simultaneous firing of the source arrays. It is essential to understand the characteristics of this type of noise interference and identify proper acquisition techniques and processing workflows to reduce its effects on image quality. We start by 3D finite difference modelling of a complex subsurface. We then combine the synthetic shots to simulate a four boat, wide azimuth (WAZ) marine seismic survey comprised of two streamer vessels with sources, and two additional source vessels located between the streamer spreads and off the tails of the streamers. All four sources fire nearly simultaneously with a randomized time lag of up to 500 milliseconds between sources. Overall, the gain from near-simultaneous source firing versus a conventional four-vessel WAZ marine design is an increase of approximately 2.67 times in terms of both source density and fold. This type of blended acquisition survey can also be acquired in about the same amount of acquisition time as a conventional four vessel WAZ survey. In general, most of the processing techniques for the simultaneous source blended data rely on the fact that crosstalk noise exhibits coherency in the shot domain, but appears random when viewed in a different data domain such as common channel, offset and midpoint domains. A number of processing techniques are applied in order to optimally deblend the data. These techniques are tested in several different domains and also in a cascaded manner. The results and effectiveness of each technique is evaluated and compared against the original non-blended synthetic data. Direct comparisons between the processed blended data and the single source non-blended data reveal comparable seismic images in both the prestack and post-stack domains.
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Compressive Sensing in Marine Acquisition and Beyond
Authors Felix J. Herrmann and Haneet WasonSimultaneous-source marine acquisition is an example of compressive sensing where acquisition with a single vessel is replaced by simultaneous acquisition by multiple vessels with sources that fire at randomly dithered times. By identifying simultaneous acquisition as compressive sensing, we are able to design acquisitions that favour recovery by sparsity promotion. Compared to conventional processing that yields estimates for sequential data, sparse recovery leads to significantly improved results for simultaneous data volumes that are collected in shorter times. These improvements are the result of proper design of the acquisition, selection of the appropriate transform domain, and solution of the recovery problem by sparsity promotion. During this talk, we will show how these design principles can be applied to marine acquisition and to other problems in exploration seismology that can benefit from compressive sensing.
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Seismic Full-waveform Inversion Using a Source-receiver Compression Scheme
Authors Aria Abubakar, Tarek M. Habashy and Guangdong PanIn seismic full-waveform inversion (FWI), one usually deals with a large size data set. This is one of the major bottlenecks for inversion, particularly if a significant part of this data set is lacking sensitivity (redundant) to the unknown model parameters. Such redundancy is usually the result of collecting data employing a non-optimally designed survey. A large number of sources in the survey contributes to a large computational cost in running the forward simulator a number of times corresponding to the number of these sources. On the other hand, a large number of receivers contributes to the computational cost of constructing the Jacobian matrix (the derivative of the simulated data with respect to the unknown parameters), as well as in inverting the Hessian matrix in a Newton-type inversion approach. To deal with these issues, the simultaneous-source encoded FWI approach was proposed to reduce the number of sources used in the inversion, see Krebs et al. (2009). In this approach, a large number of physical sources are converted into one simultaneous source or several simultaneous sources by summing the individual physical sources using a phase-encoding technique. Another approach to reduce the computational time and memory usage of an inversion algorithm is the so-called source-receiver compression scheme, see Habashy et al. (2010). This approach constructs compressed simultaneous source and receiver arrays that have minimum redundancy and maximum sensitivity to the unknown model parameters. The synthesized simultaneous source array has a reduced number of sources that decreases the number of forward model simulations required to carry out the inversion. In addition, the synthesized simultaneous receiver array has a reduced number of receivers that further decreases the size of the Jacobian matrix. Hence, this compression approach significantly reduces the computational time and memory usage of any inversion method. Moreover, because this approach removes the small eigenvalues in the data, the effects of noise are also suppressed. In this paper, we apply the source-receiver compression approach for solving three-dimensional (3D) acoustic FWI for obtaining the compressional (P-wave) velocity. The forward problem is formulated using a frequency-domain finite-difference (FDFD) approach with fourth-order accuracy. For the inversion method, we employ the Gauss-Newton framework described in Habashy and Abubakar (2004) combined with the multiplicative-regularization technique described in van den Berg and Abubakar (2001). As a demonstration, we show inversion results of 3D SEG/EAGE salt model.
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Multisource Least-squares Migration of Marine Streamer with Frequency-division Encoding
Authors Yunsong Huang and Gerard T. SchusterThe computational cost of conventional migration can be reduced by multisource migration of supergathers, and the accompanying crosstalk noise can be reduced through least-squares inversion. This approach is not directly applicable to marine streamer data, however, due to the mismatch between the limited number of live traces/shot recorded in the field and the pervasive number of traces generated by computer simulation. This leads to a strong mismatch in the misfit function, and results in strong artifacts (crosstalk) in the migration image. To eliminate this noise, we present a frequency division multiplexing (FDM) strategy with iterative least-squares migration (ILSM) of supergathers. The key idea is, at each ILSM iteration, to assign a unique frequency band to each shot gather. This assignment ensures that there is no spectral overlap among shots, and therefore their crosstalk is zero. Our results in applying this method to 2D marine data for the SEG/EAGE salt model show better resolved images than standard migration computed at about 1/10 the cost. We conclude that multisource migration for a marine geometry can be successfully achieved by a frequencydivision encoding strategy, as long as crosstalk-prone sources are segregated in their spectral content. This is both the strength and the potential limitation of this method.
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Analysis of a Coherency-constrained Inversion for the Separation of Blended Data: Discovering the Leakage Subspace
Authors Kenneth Bube, Panagiotis Doulgeris, Gerrit Blacquière and Gary HampsonSo-called blended or simultaneous source techniques are interesting because they offer the potential for better acquisition sampling, improved illumination and greatly enhanced computational efficiency. The procedure of retrieving data as if they were acquired in the conventional way is called deblending. Several inversion techniques have been proposed for solving this ill-posed problem with the iterative estimation and subtraction algorithm being one of them. This particular method uses a dedicated iteration that integrates a coherency-pass filter. However, fundamental questions regarding the convergence and limitations of this method have not previously been addressed. The notion of the leakage subspace is hereby introduced to aid in this quest.
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Separation of Multiple Interfering Sources in Wireless Communications from a Geophysical Perspective
More LessRedundancy in array processing enables the separation of multiple correlated signals. In wireless communications, the adoption of multiple antennas has increased the spectrum efficiency by enabling the transmission of high data-rate over limited spectrum allocations. Over the last decade, one of the most disruptive innovation in wireless communications has been the revisiting of multiple antennas communications as Multiple Input Multiple Output (MIMO) system (Paulraj et al, 2004). There is such a strong parallelism between the array processing adopted in wireless communication systems and seismic acquisition and processing that in depth investigations of the communalities can only be beneficial. Seismic acquisition for simultaneous sources can be modelled as MIMO system when accounting signals from multiple receivers. In this paper we introduce the MIMO modelling and processing algorithms to show that MIMO processing tools can be transferred and tailored for acquisitions of simultaneous sources with minimal efforts in the model abstractions.
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Submarine Slope Instabilities – Key Questions and Natural Field Laboratories
More Less, impact, risk assessment). Following a brief introduction on the – typical – three-phase landslide development, we address a number of issues at stake (e.g., gas, hydrate, excess pore pressure) that should be addressed in more detail in future research activities within this field. These include in situ measurements, but also the advanced use of geophysical methods to derive soil properties in the shallow sub-surface. Finally, we highlight activities conducted to develop the Finneidfjord area (northern Norway) as a natural field laboratory for submarine landslide investigations. When it comes to smaller-scale landslides, high lateral and vertical resolution is paramount to understand such landslides, which still can have devastating consequences.
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Seismic Characterization of Landslides and the Emergence of Seismic Monitoring
Authors Stphane Garambois, Denis Jongmans and Agns HelmstetterAfter a short introduction on the main problems that need to be better understood in order to better model landslide dynamics, this contribution focuses on how geophysical methods and in particular seismic methods can help in better characterizing the geometry and heterogeneities of landslides. Indeed, these parameters constitute fundamental inputs that need to be precisely addressed before any numerical modelling attempt. We will notably show the sensitivity of shear wave velocity to the variation of shear strength in soft landslide. In a second part, a focus is done towards the emergence of seismic monitoring methods which are now used for different purposes. They include seismic activity assessment and characteristics (location, magnitude, classification), which permit to study the correlation of this activity with other observables (meteorological, displacements) in order to better understand hydro-mechanical coupling at a large scale on landslides. Seismic observatories also enable characterizing the dynamics properties of seismic site effects due to the presence of disturbed materials. Again, this property must be taken into account in any numerical attempt that aims at evaluating the potential sensitivity of the landslide stability to different earthquake scenario. Finally, the use of correlation of continuous ambient seismic noise recorded by at least two different sensors can permit to detect very weak shear wave velocity changes between these sensors. Strong variations of shear wave velocity might be a precursor to rupture.
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Geotechnical Characterisation of Submarine Slope Movements, Example from the Area off the Nice Airport (SE France)
Authors Sbastien Garziglia and Nabil SultanIn order to present the geotechnical approaches and tools developed and deployed in IFREMER, light is shed on the shelf and upper slope off the Nice airport. The complex problem of submarine mass movements is split into two distinct aspects that are the mechanical behaviour of sediments and the movement stages. It is hence shown that the area off the Nice airport remains predisposed to failure due to fresh water infiltration and gas occurrence in the sediments. The steep continental slope is recognised as a factor predisposing both failure and subsequently large and rapid movements of sediments. Given the high probability for a 30-m thick slide to occur due to the occurrence of shear zones, conclusions point toward the need to deploy an array of sensors for the monitoring of changes in pore pressures and potential pre-failure movements.
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Electrical Imaging for Landslide Monitoring: State-of-the-art and Future Challenges
Authors Vincenzo Lapenna, Angela Perrone and Sabatino PiscitelliIn this paper we briefly resume the current start-of-the-art of the electrical imaging techniques for monitoring hydro-geological instability phenomena. An overview of the more interesting results obtained in the Southern-Apennine chain (Italy), that is one of the Mediterranean areas affected by large and diffuse landslide events, is presented and discussed. Furthermore, we focus the attention on novel and challenging applications of the electrical imaging. To-date, combining robust techniques for tomographic data inversion and new distributed sensor network for field data acquisition is possible to obtain 3D high-resolution electrical images and time-lapse 4D tomographies able to follow the spatial and temporal dynamics of electrical parameters (i.e., resistivity, self-potential) in landslide bodies.
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Marine Drilling/Construction Hazard Site Investigations - Current Practices and Techniques in the Oil E&P Industry
More LessThis presentation will provide an overview of the current practices and techniques used in the oil exploration and production (E&P) industry for mapping and assessment of constraints and hazards to marine drilling and construction operations. This presentation will also provide an overview of the various types of hazards and the equipment and techniques that are used to identify and map them and include a brief overview of industry standards and guidelines for drilling and construction site investigations.
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The Role of Sediment Composition in the Flow Behaviour of Submarine Landslides
More LessLaboratory experiments on subaqueous mudflows at St. Anthony Falls Laboratory, the University of Oslo and NGI/ICG showed that the flow regime undergoes a remarkable transition as the slurry’s cohesion is varied. Flows of highly cohesive slurries exhibit plug flow in the head, hydroplaning, and rapid stretching, but create only a feeble turbidity current. Flows with low clay content generate a turbulent head and a pronounced turbidity current. Their body is fluidized, with the sand fraction settling out progressively. A simple model indicates that low-to-intermediate cohesion mudflows are a candidate for explaining extended and massive deposits of clean sand that have been found in abyssal plains far from their possible origin. The viability of this proposal depends crucially on the scaling properties of the system, which are discussed in some detail. Adapting the definition of the Reynolds number to flows of cohesive materials, the impact forces of debris flows on structures like pipelines can thus be described in a simpler and more universal way than hitherto, elegantly combining aspects of solid and fluid behaviour. Time permitting, the implications for numerical modelling of such flows and the reasons for the striking difference with sub-aerial debris flows will be briefly touched upon.
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Assessment of Unstable Slopes in Mountain Regions Using Geophysical Methods
Authors Hansruedi Maurer, Christian Hauck and Thomas SpillmannAs transport routes, power networks, communication lines and human habitats expand in mountain valleys, the risks associated with landslides are growing on a yearly basis. To minimize these risks, there is a need to identify and characterise mountain slopes that are susceptible to failure. We assess the potential and limitations of geophysical techniques for investigating potentially unstable mountain slopes. Our assessments are based on two case studies involving mountain permafrost in form of a rock glacier and a steep rock slope well below the occurrence of permafrost. The internal structure of the rock glacier is delineated with a combination of seismic refraction tomography, geoelectrical tomography and ground penetrating radar measurements, whereas the steep rock slope is characterized with 3D seismic and ground penetrating radar measurements in combination with microseismic investigations.
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Seismic Full-waveform Inversion and Data Fusion on Clayey Landslides
Authors Gilles Grandjean, Anouard Romdhane and Adnand BitriFrom numerous studies, geophysical methods based on seismic surveying appear to be well-adapted to investigate the morpho-structure of landslides and to progress in understanding the related mechanisms. Indeed, these methods allow direct and non-intrusive measurements of acoustic (Vp) or shear (Vs) wave velocity, two important physical parameters for estimating mechanical properties of reworked moving materials. Different processing techniques and inversion strategies were applied on the La Valette and Super-Sauze mudslides (French South Alps) as well as on the Ballandaz landslide (Savoie, France) to retrieve these parameters. On each of these sites, measurements were recorded along 2D profiles of several hundred meters length, with sensor spacing from 2 to 5m (of about few meters). A first approach, based on first breaks acoustic inversion for estimating Vp distribution on the Super-Sauze and La Valette sites was carried out; then, SASW (spectral analysis of surface waves) was performed to image Vs distribution on the same site. In order to produce a more geotechnical diagnosis of these sections, a fuzzy logic fusion was used to assimilate both of these parameters into a highest level of interpretation. This approach has (also) the advantage to take into account the resolution and accuracy of each individual method. Finally, a 2D elastic full-waveform inversion test was carried out on a synthetic seismic data set computed from a conceptual Super-Sauze velocity model. This test highlights the difficulty to manage highly contrasted media in terms of velocity but also of topography. Nevertheless, the integration in the inversion process of the whole seismic signal produce a more coherent model in terms of Vp and Vs distribution compared to abovecited conventional techniques.
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Characterization of Clayey Landslides Using Multidisciplinary and Multiscale Dataset
Authors Grgory Bivre and Denis JongmansSlow-moving landslides frequently affect gentle slopes made of clayey formations, with volumes which can range from a few m3 to several tens of millions of m3. These landslides frequently exhibit sudden acceleration phases and flows, which can be triggered by changes in the stress field or modifications in the soil characteristics. We present here an integration of multidisciplinary and multiscale data. The combined interpretation of continuous geophysical data acquired over the whole Avignonet landslide (OMIV observatory on landslides in the French western Alps) with punctual measurements (boreholes and geotechnical tests) allowed building a robust 3D morpho-structural model of the slide down to a depth of around 300 m. It has been possible to characterize the main lithological features along with the location of slip surfaces. We propose here that, in complement with shear-wave velocity which gives information on the mechanical effect of the slide, resistivity variation observed at sub-surface can be linked to grain size variation which seems to partially control the landslide kinematics via the regulation of overpressure. Combining geophysical data allows to take into account both lithological variations and landslide activity to account for surface displacements.
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Quantifying the Effect of Fluids and Mechanical Weakening of Fractures and the Implications for the Rupture of Large Landslides
Authors Benoit Derode, Frdric Cappa and Yves GuglielmiLandslides are sensitive to fluid pressures generated by rainfall and snowmelt. Correlations between landslides accelerations and seasonal infiltrations have been widely observed, and models have shown that fluid pressurization in fractures is a dominant driving factor. Nevertheless the precise mechanisms relating fluid pressures and rupture are poorly constrained. Here, we quantified in-situ the effect of fluids and strength loss related to pressure increase and deformation in fractures. Then, using hydromechanical modeling, we analyzed these data and explained the evolution of permeability with deformation. Finally, based on these novel observations, we used improved hydromechanical simulations to model the behaviour of the Schilienne landslide in France.
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Geophysical and Geotechnical Investigation of a Complex Offshore Foundation Ground Affected by Glacial/Postglacial Sedimentation Processes (German North Sea)
Authors Wenfang Fan, Hanno Keil, Stefan Kreiter, Volkhard Spiess and Daniel Hepp and Tobias MrzThe complexity of the North Sea sediments resulting from the Quaternary glacial history requires a thorough assessment of the local depositional environment to determine appropriate foundation dimensions or to even exclude specific areas from construction. In this study geophysical and geotechnical methods were combined to assess a potential offshore foundation ground in the southern German North Sea. Due to a dense net of multichannel seismic lines the very heterogeneous subsurface structures were mapped and prominent glacial buried valleys were identified in the study area. Geotechnical methods applied at selected locations were used to investigate the bearing capabilities of the soil. The results show, that the infill types of glacial buried valleys have to be taken into account regarding the suitability of foundation ground.
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Monitoring the knes Rock Instability, Norway
Authors Lene Kristensen and Lars Harald BlikraThe knes rock instability at Storfjorden, western Norway, has the potential to form a major rock avalanche and create a tsunami with devastating impact to the settlements around the fjord. The most feasible mitigation measure to this hazard is monitoring as large rock avalanches are usually forecasted by an acceleration of displacement. The knes/Tafjord Early Warning Centre is responsible for monitoring of knes rock instability. Here we present the monitoring concepts, measurements of displacement and the early warning system.
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A Monitoring System for Time-lapse Electrical Resistivity Tomography (ERT) and Time Domain Reflectometry (TDR) Measurements
More LessA permanent monitoring system for time–lapse acquisition of 2D Electrical Resistivity Tomography (ERT) and Time Domain Reflectometry (TDR) measurements was installed in a test site affected by a landslide in Basilicata region (southern Italy). Time-lapse acquisition allows us to monitor in real time the variation of resistivity values in the first layers of the subsoil, closely connected with variation of the water content and potentially associated to the reactivation of the landslide. A weather station installed in the test site gives information about the rainfall intensity and frequency and suggests the acquisition time interval. Resistivity trend analysis was carried out in different ways: at first acquiring many sets of data that were inverted by the software RES2DINV, in order to obtain a series of ERT representative of the resistivity distribution in the subsoil during different periods of the year; then, considering resistivity data coupled with TDR ones and rainfall, realizing a statistical study in the time and space. Statistical study has been made by using a MATLAB script specifically created for this work. The installed system and the results are presented.
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Geophysical Characterization of the Furggwanghorn Rock Glacier, Switzerland
Authors Kaspar Merz, Lasse Rabenstein, Thomas Buchli and Hansruedi MaurerDegradation of alpine permafrost due to changing mean annual air temperatures can act as trigger for landslides and other ground instabilities. For a better understanding of the underlying thermo-hydromechanical processes an interdisciplinary research project has been set up. An extensive geophysical and monitoring campaign was carried out on rock glacier in the Turtmann valley, Canton Valais, Switzerland over the last two years to investigate its internal structure. We employed seismic refraction tomography, electrical resistivity tomography and ground-penetrating radar. Additionally seven boreholes were drilled to a depth of 25m and equipped with temperature sensors and inclinometers. Results from the seismic tomography show a lateral very heterogeneous zone below an active layer of 3-4m thickness. The bedrock depth could not be detected over large parts of the profiles. On the electrical tomograms we can clearly distinguish between ice-free zones at the front and the flanks of the rock glacier and an ice-rich zone in the central part. Several internal shear horizons could be identified on the radar profiles. Most of them could be tracked over several profiles. Deformation measurements in a nearby borehole show that the horizon at about 15m depth is currently active.
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Quick-clay Landslide-prone Grounds in Norway and Sweden:A Complex Problem Requiring a Combined Geophysical and Geotechnical Approach
Quick-clay sliding occurs in formerly glaciated coastal areas in, e.g., Norway, Sweden and Canada. The soil was originally deposited in shallow marine environments which emerged following isostatic rebound and fall of the relative sea level since the last glacial maximum. Long-term leaching of salt, due to groundwater flow and percolating surface water, affects clay-particles bonding and makes the soil highly susceptible to failure when disturbed. We review the properties of quick-clays in order to define a suitable, integrated and multi-disciplinary approach to improve identification and mapping of quick-clay areas. Though electrical resistivity tomography is actually the geophysical method of choice, it is paramount to combine a range of geophysical and geotechnical approaches for a better assessment of a given quick-clay site. The discussed integrated approach is here presented for 2 Norwegian and 1 Swedish quick-clay sites. The collected data and preliminary site characterization will illustrate the high diversity of quick-clay grounds as well as the complexity related to an integrated approach.
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A Multidisciplinary Study of Near-shore Landslides in the Trondheim Harbour, Mid Norway
The geological and historical record in the bay of Trondheim, mid Norway, illustrates that landslides are recurrent phenomena. Recent and ongoing development of the area, including land reclamation and extension of harbour facilities, have increased concerns about the stability of the shoreline slopes and highlighted the need for better understanding of these mass-movement processes. Herein, we summarize previous and ongoing work in the area that shed new light on the origin and development of mass wasting processes in the bay of Trondheim. Intergration of geotechnical and geophysical data (including shear wave reflection profiling) from both on- and off-shore shows that the presence of softer and more sensitive laminated clay-rich beds facilitates translational slope failure, by acting as slip planes. Additional pre-conditioning factors promoting instability include the loading of weaker clay-rich beds by delta progradation, over-steepening from erosion and/or sediment accumulation and artesian groundwater pressure at different stratigraphic levels. For the more recent landslides, anthropogenic factors like embankment fillings and vibrations from construction work are considered the most important triggering mechanisms. Finally, the results presented illustrate the importance of detailed morphological analyses, combined with a geological model including the physical/geotechnical characteristics of sediments on- and off-shore, in order to perform proper shoreline slope stability assessment.
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Ray Based Approaches for Velocity Model Building: Past Present and Future
More LessBy mid 2000’s emergence of full waveform inversion (Virieux and Operto, 2009) has somehow occulted the progresses and perspectives of ray based velocity model building. Whatever by now velocity model building in industry remains still largely done using ray based approaches. Moreover, if breakthroughs have not been strongly put forwards for these techniques, they provide now solutions with unexpected resolution and accuracy. My aim here is to review this approaches and to investigate their future. The success and power of ray based approaches is strongly connected to a theoretical and numerical frame, which has been built around ray theory, and which offers a so powerful frame for the physical understanding of velocity model building. I first recall them before reviewing the various ray based velocity model building tools. Finally I discuss their connection with full waveform inversion approaches and discuss the perspectives.
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Objective Functions for Full Waveform Inversion
More LessSeismic full waveform inversion as conventionally formulated imposes very strict constraints on data acquisition: high signal-to-noise at very low frequencies and/or very long offsets. In this workshop presentation, I will explain how some of these constraints arise in the mathematics of wave propagation, and describe some alternative optimization formulations of seismic inversion, which may allow fitting of less constrained data. The extreme heterogeneity of the earth’s sedimentary crust still poses many unresolved challenges for inversion.
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Challenges and Value of Applying FWI to Depth Imaging Projects
Authors Laurent Sirgue, Bertrand Denel and Fuchun Gaol Waveform Inversion (FWI) has now been established a few decades ago (Lailly 1983; Tarantola 1984; Pratt et al. 1996). Until recently however, the application of the technique on full scale 3D seismic was made impossible due to the computational requirements of modeling thousands of shot. Recent advances in high performance computers along with the development of efficient 3D modeling algorithms has lead to the first successfull full scale application on field data (Plessix et al. 2009, Sirgue et al. 2009, Vigh and Star 2009). However, the added value of performing FWI to Prestack Depth Imaging Projects is yet to be clearly defined by the industry. This task is particularly challenging since the type of seismic acquisition largely varies from one asset to the other; along with the geophysical environment (presence of salt, water depth etc…). In this workshop, we will present a wide range of application of FWI to streamer data in both deep offshore and shallow water environments. We will show that FWI may be applied to a wide range of seismic data: from an isotropic application in the Caspian Sea to more complex application such as deep offshore of Gulf of Guinea.
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Full-waveform Inversion Aided Depth Imaging
By Denes VighFull-waveform inversion based on the finite difference approach was originally introduced in the time–space domain (e.g.Tarantola, 1984, Pica et al., 1990, Sun & McMechan, 1992). Inversion can also be implemented in the frequency-domain (Pratt et al., 1998, 1999, Ben-Hadj-ali et al., 2008). The advancement in hardware over the past few years has allowed us to execute 3D FWI on real datasets in marine (Plessix,2009, Sirgue at al.,2009,Vigh et al.,2009,2010) and in land (Plessix,2010) environments. They demonstrate that FWI can be used for velocity updates if the acquired data has adequate low frequency signal and long offsets. The shallow section of the model could be significantly enhanced by the use of FWI, which can result in a much improved depth image over all. One of the challenges with FWI is converging to the local minima, which makes the technique very sensitive to the starting velocity model especially when 3D is considered. To lessen the sensitivity of the initial velocity field, the implementation of FWI at low frequencies using long offsets is required (Bunks et al., 1995, Pratt, 1999).
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A Priori Model Estimation for FWI from Constrained Kinematic Inverse Problem
Authors Christophe Barnes and Marwan ChararaExploration inverse methods such as Monte Carlo could provide the general solution for the fullwaveform inversion (FWI) inverse problem allowing incorporating complex a priori information and data with arbitrary noise distributions. Such an approach applied directly to the FWI problem is computationally not tractable. The alternative is to solve the highly non-linear part of the problem with a fast forward problem such as traveltime with a constrained random exploration. The obtained a priori models and uncertainties help greatly in solving the FWI. This approach was successfully applied to an offset vertical seismic profile (OVSP) case.
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Investigating the Differential Waveform Inversion
Authors Herve Chauris and Ren-douard PlessixIn the context of velocity estimation, we propose the Differential Waveform Inversion approach defined in the data domain. For reflected data, it first consists of migrating a single shot. From the updated velocity model, we then compute the data at the next shot position and compare it with the observed data. The minimization of the misfit appears to be equivalent to the Differential Semblance Optimization approach formulated in the depth migrated domain, at least for reflected data.
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Traveltime Inversion, an Integration Tool in Complex Structures: Lessons from Case Studies in Thrust Belt Setting
Authors Anne Jardin and Karine BrotoWe have developed a traveltime tomography software whose flexibility enables to tackle complex subsurfaces. Applications of this software in thrust belt setting are presented. The role of geology and seismic data integration to validate the final model is analyzed.
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Sensitivity Analysis of Joint Diving Wave+Reflection Tomography in Anisotropic Media
Authors Jacopo Panizzardi and Nicola BienatiIt is more and more evident that the transmitted component of the wavefield recorded by reflection acquisitions does contain precious information about velocity. Indeed, the results obtained from firstarrival traveltime tomography, and in general those obtained by the various works recently presented on Full Waveform Inversion are motivating an increasing interest for transmitted waves, i.e. refractions and diving waves.
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