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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
21 - 40 of 156 results
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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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