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Near Surface Geoscience 2014 - 20th European Meeting of Environmental and Engineering Geophysics
- Conference date: September 14-18, 2014
- Location: Athens, Greece
- Published: 08 September 2014
161 - 180 of 185 results
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Detailed Imaging of Seabed and Sub-seabed Geology from 3D Seismic Data Using Frequency Decomposition
Authors P. Szafian, E. Cauquil and J.L. PiazzaSummaryThe study focuses on detailed imaging of the seabed and the shallow sub-seabed sequences of a deep water area particularly affected by seabed features such as pockmarks, faults, carbonate hardgrounds and hydrate mounds. Three workflows that were applied to achieve this objective are discussed: noise cancellation, spectral enhancement and standard frequency decomposition with RGB blending. Noise cancellation was successful in attenuating much of the coherent and random noise present in the original data set. Vertical resolution, reflector continuity and event separation was improved by spectral enhancement. Frequency decomposition and RGB blending revealed a wide range of geological features on and under the seabed. With the help of these techniques one can distinguish the seabed features and identify and map different elements, such as faults, channels and pockmarks, as well as submarine landslides, mass transport complexes, outrunner blocks of varying sizes and corresponding glide tracks below the seabed. The results confirm that volumetric frequency decomposition and RGB blending lead to an improved and more reliable assessment of shallow geohazards by assisting interpreters to identify a wide range of geological features in unparalleled detail, in a reasonable amount of time.
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In Situ Resistivity of CO2 Plume at Sleipner from CSEM and Gravity Data
Authors J. Park, M. Vanneste, B. Bohloli, I. Viken and T.I. BjørnaråSummaryWe combine gravity and CSEM data acquired at the Sleipner field in order to estimate the actual thickness and in situ resistivity of the CO2 plume when the CSEM data was collected in 2008. For the sake of simplicity, we assume the CO2 plume as a homogeneous elliptical cylinder, and apply the physical properties of the CO2 plume from literature review. The result is within a range of our expectation. Through this exercise, we can see the feasibility of combining the CSEM and gravity data to improve the time-lapse interpretation for monitoring CO2 storage sites, as complementary to time-lapse seismic data.
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3D Anisotropic Modeling for Marine CSEM
More LessSummaryDue to excellent sedimentary environment in the ocean, strong layering and stratifications can be developed, resulting in distinct electrical anisotropy in the earth under the ocean. Electrical anisotropy can have serious effects on MCSEM survey data; using traditional isotropic models to interpret anisotropic MCSEM data can deliver distorted results. To understand the influence of the electrical anisotropy on MCSEM responses, we study in this paper 3D marine CSEM modeling for an arbitrarily anisotropic earth. We use the staggered finite-difference algorithm, combined with volume average or volume current average method for handling the conductivity tensor elements, accomplish successfully 3D MCSEM modeling for an arbitrarily anisotropic earth. Comparison and analysis of MCSEM responses (MVO and PVO) for different isotropic and anisotropic models reveal the characteristics related to the influence of the electrical anisotropy on Marine EM responses and identification technique from the MCSEM survey for the electrical anisotropy of the media under the ocean. The results obtained from the numerical experiments in this paper offer the technical support to the detailed interpretation of the MCSEM data.
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CSEM Data Sensitivity for Shallow Resistivity Anomaly
Authors J. Park, M. Vanneste, I.K. Waarum, P.M. Sparrevik and G. SauvinSummaryIn this study, we explore some features of marine controlled-source electro-magnetic (CSEM) data with a particular focus on shallow resistivity anomaly’s sensitivity. CSEM application to exploration purpose is related normally to rather deep (typically >1km) resistivity targets. A shallow resistivity target may screen out the sensitivity of deep resistivity targets. In the context of monitoring, particularly for offshore geohazards in overburden, the shallow resistivity targets (e.g. shallow gas, CO2 leakage) are of main interest. Through this study, we find out interesting features in the interaction between shallow and deep resistivity targets, which may help us to improve marine CSEM data interpretation. The study is based on a set of CSEM data resulting from 3D simple synthetic models.
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Detection and Characterization of Fracture Zones in Bedrock - Possibilities and Limitations
Authors G.A. Tassis, P.I. Tsourlos, J.S. Rønning and T. DahlinSummaryIn Norway, resistivity measurements have already been tested in marine environments in order to detect subsea fracture zones. However, most of these data have been processed without taking into account the special conditions the presence of seawater creates. More recent studies worldwide have also applied ERT in marine conditions, but under more favorable conditions nevertheless since they dealt with brackish water of considerably higher resistivity than pure seawater. This study summarizes our efforts to establish basic rules when considering whether or not pure sea water ERT can satisfactorily detect weak zones inside resistive bedrock, a problem engineers in Norway usually come up against in tunnel construction sites. The scope for this study is related to the construction of a sub-sea tunnels and the potential application of ERT to detect fractured zones as part of the geotechnical study. Our results indicate that ERT surveys for fracture zone detection in Norwegian marine environments can be promising under certain conditions but at the same time ambiguous since they suffer from reduced resolution and major artificial effects. Based on the modeling results, we were able to improve interpretations of ERT measurements made across the straits at Kvitsøy and plan further investigations in southern Norway.
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Offshore Geohazards - Industry Implications and Geoscientist Role
Authors M. Calarco, F. Zolezzi and W.J. JohnsonSummaryThe research on offshore geohazards has increased greatly also driven by the needs of the offshore petroleum industry. In fact, industry is interested in understanding if a given submarine feature represents a risk to a proposed structure and if so how to minimize the associated risk.
Assess the geohazards for a deepwater project is particularly challenging, however the development of new technologies to investigate and monitor the seafloor is providing new insights into these unexplored regions.
Our contribution presents a brief summary of what industry requires from the geoscientists and illustrates the approach that shall be used while approaching a geohazard study.
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Simultaneous Acquisition of Airgun Seismic and High Resolution Sparker Data – Combining the Two Types of Data
Authors E. Nørmark, C. Bendixen, J.B. Jensen and O.R. ClausenSummarySeismic data with different resolution are normally acquired in separate surveys. In the present survey both conventional reflection seismic data with airgun source and high resolution sparker data have been acquired in the Kattegat, - the costal waters of Denmark. The data are recorded with a small time delay between activating the sparker and the airgun. Data have been submitted to a conventional seismic processing. Since the two datasets are acquired almost exactly at the same positions it facilitates a combination of the two seismic sections. Wavelet conversions have been made and a weighted combined stack of the two datasets has been established. This has produced a stack with all essential information, with a wide spectral content for the near surface intervals and a low frequent content for the deeper parts, where otherwise the sparker data shows no significant energy.
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Shallow Marine EM Receivers for Coastal Shelf and Transition Zones
By I. IngerovSummarySubstantial costs associated with offshore drilling highlight the importance of additional geophysical information for the decision makers. This information can be provided by seabed MT (natural source EM field) or seabed logging (MCSEM). These techniques are fairly well developed for relatively deep water but depths of less than 200m, and particularly less than 20m, cause certain difficulties. Both AUSS systems (10–200m) are designed to reduce the influence of induced noise when deployed in shallow marine environments. The main advantage of these systems is their low weight, which allows them to be deployed from relatively small vessels. A unique feature of the 5-component (2E+3H) system is the automatic leveling of the platform and, most importantly, of all three magnetic sensors at the seabed, which allows accurate measurement of a vertical magnetic component. The SMMT system (0–50m) has a very low profile, aerodynamic shape not affected by currents and waves, and a unique recovery system that does not leave an environmental footprint on the seabed unlike common industry practice. In addition to oil and gas exploration, the equipment can be successfully applied to tectonics, geodynamics, pipelines, underwater cable routes and geological engineering investigations.
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Seismic Character, Lithology and Age Correlation of the Aberdeen Ground Fm. in the Central North Sea
By F.A. BuckleySummaryTop-hole drilling conditions in the Central North Sea (CNS) are comparatively little understood, but occasional wells, sampled and dated from the shallow sub-seabed, offer opportunities to rectify this. Industry acquired seismic and well data have been used to reconstruct environments of deposition for an area of Quad 30 in the CNS. The Early to Middle Pleistocene Aberdeen Ground Fm. dominates the top-hole section in the CNS, extending down to a dated, near-base Quaternary surface, characterised by iceberg ploughmarks, at approximately 890m MSL. The lower part of the unit comprises predominantly clay lithologies and northwesterly dipping clinoforms, interpreted to represent a delta-front environment. Above 580m MSL the clay package is replaced by heavily ploughmarked sands, clays and occasional coals, interpreted as a shallow water delta-top environment with the coals resulting from deposition of peat in periodic emergent conditions. The Aberdeen Ground Fm. records the filling of the Quad 30 depocentre by the Eridanos delta in the lower Early Pleistocene leading to much shallower waters and occasional sub-aerial conditions in the later Early Pleistocene prior to establishment of more glacially dominated conditions in the Middle Pleistocene. These findings have implications for top-hole drilling strategies.
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Integrated Acoustic and Sampling Data for Seabed Mapping in Kandalaksha Bay the White Sea
Authors Y.E. Terekhina and M. TokarevSummaryThe main task of the research was seabed mapping. Very high resolution seismic data, sampling and underwater observations procure in Kandalaksha bay the White sea were used.
Analysis of amplitudes and shape of bottom reflection signal gives the information of seabed sediment properties. The obtained data provide the geological structure and the paleorelief influence to modern seabed processes. Understanding these processes is a key for geomorphological analysis. In conjunction with sampling and underwater observations these data provide the basis for lithological map.
Resulting maps are not sufficient for habitats mapping, otherwise living organisms as mollusks Arctica islandica contribute to strength and other side scan sonar data characteristic. Combination of the methods could be used for the mollusk distribution mapping.
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The Successful Design, Development and Acquisition of a UHRS 3D Seismic Dataset
Authors K.P. Games and N.D. WakefieldSummaryExploration 3D has become commercially beneficial because vessels can tow multiple streamers, often with multiple sources. The data can also be acquired in relatively rough seas, with large areas covered in a short time. There are two main downsides to this 3D data acquisition. Firstly, the frequency content of the data is low - typically ~ 60Hz. This limits the vertical resolution of the data. Secondly, because of the large offsets between the source and streamers, any ‘shallow’ data will be very poorly imaged. This lack of high frequency content cannot be completely solved due to the nature of seismic waves. The frequency can be enhanced by processing techniques but these have limitations and can only achieve a limited improvement in resolution. So if higher resolution is required, then the only solution is to use HR or UHR techniques. This may be needed for shallow reservoir delineation, geohazards including shallow gas, carbon capture in salt caverns, and foundation studies. However, while the benefits of HRS or UHRS data are well known and accepted, there has been a reluctance to move from 2D to 3D acquisition. This paper describes the successful planning, development and application of just such a 3D spread.
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Deghosting of High Resolution Marine Seismic Data by Adaptive Filtering Algorithm
Authors S.A. Vakulenko, S.V. Buryak, P.A. Gofman and D.B. FinikovSummaryEffective deghosting algorithm of marine data can significanlty improve obtained information by removing notch frequencies from the recorded data.
We propose an efficient pre-stack deghosting algorithm, based on adaptive filtering. We demonstrate on real data, acquired with different sources, that developed algorithm can successfully remove ghost reflection from data and fill in notch frequencies in the recorded spectrum. Our algorithm can be applied both on single channel and multichannel high resolution marine seismic data.
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Multi-modal Surface Waves for Site Characterization - Results from a Unique Marine Shear Wave Experiment
Authors J. Park, M. Vanneste, L.V. Socco, C. Madshus, G. Sauvin and E. SkomedalSummaryWe revisit a unique marine seismic data set collected with a densely populated ocean bottom cable (2.5 m effective channel spacing) and a prototype shear wave vibrator at the Gjøa field, offshore Norway. Whereas the survey was primarly designed for reservoir illumination, multi-modal surface waves stand out on the seismic data (both time-offset and frequency-wavenumber domains), in the frequency band between 3 and 35 Hz. As the source was operated in both in-line and cross-line direction, we identify multiple Scholte and Love waves that were subsequently used for a constrained inversion for obtaining a detailed shear wave velocity with depth, down to 50 m. In addition, a comparison of the velocities yields an estimate on shear wave anisotropy in the shallow sub-surface, with up to 15%. The data are further used to determine attenuation, a second critical parameter affecting wave propagation and dynamic response of seabed-founded structures, which can also be important for soil characterization. Tuned forward modelling of the fundamental surface wave mode yields shear wave damping of less than 1% for the deepwater soft soils. The attenuation coefficient increases linearly with frequency, and indicates that the damping is to a large degree viscous rather than due to hysteresis.
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Shear Wave Seismic – The Missing Link?
By E. SkomedalSummaryChallenged by a statement from Terzaghi on the shortcoming of geophysical surveys to be helpful in geotechnical characterization, a systematic development to qualify shear waves as the misssing link is described. The potential of the shear waves for this purpose is the unique relation between effective stress, the grain skeleton, the shear stiffness and the shear velocity. The potential is demonstrated in practice in wellbore log scale and in seismic scale. Examples of the strategic implication for the oil industry are given.
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Seismic Characteristics of Gas Accumulations in Sediments of Lake Balaton, Hungary
Authors F. Visnovitz, T. Bodnár and F. HorváthSummaryHigh-resolution water seismic investigations are often limited by the presence of shallow gas in marine and lacustrine environments. Lake Balaton, Hungary is no exception. Gas is a major obstacle for deeper stratigraphic mapping but also an important indicator of ecological and geological processes, hence it deserves a detailed study.
This work, applying single- and multichannel profiles from the last 3 decades from the area of Lake Balaton, presents data about the general shallow gas characteristics of this lacustrine environment. Various anomaly types have been observed that were grouped into 3 distinct (upper, middle and lower mud) gas fronts that are sometimes superimposed on each other. The specific gas fronts are showing different origin and temporal behavior. We have concluded that the most important sources of the gas are the biodegradation of recently deposited organic matter including anthropogenic pollution, gas upwelling helped by tectonically amplified groundwater discharge and the decomposition of Pleistocene peat.
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Morphological Controls on the Occurrence of Submarine Slab Failures
Authors O.J.N. Dabson, J. Barlow, R. Moore and A. MillsSummaryThis paper describes a statistical analysis of an inventory of 267 shallow slab slides in the West Nile Delta to identify significant combinations of morphometric parameters deemed conducive to failure. It was found that slopes with a planar plan curvature and a slope angle of less than 6 degrees account for approximately 95% of the observed landslides, and that increasing plan concavity acts to stabilise submarine slopes beyond this 6 degree threshold. It is hoped that these findings will provide a useful starting point in ultimately reconciling geomorphological observation with geotechnical modelling of the sea floor, and thus provide an increased understanding of the controls on submarine slope instability.
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Determination of Slope Failure Mechanics from an Integration of High Resolution Seismic and Sedimentological Core Data
Authors M.A. Clare, A. Mason, C. Mason and J. NichollsSummaryDiagnosis of slope failure deposits is often based on an acoustically transparent to chaotic character, contrasting with the surrounding well bedded seismostratigraphy. It may seem appropriate to directly relate sub-bottom profiler observations to an interpretation of internal deformation or flow rheology; however, we outline some cautionary notes on this approach and stress the need for ground-truthing by acquisition of sediment cores. We present examples of AUV-Chirp data where slope failure deposits were interpreted.
Where failure primarily involved translation on a planar surface, the observed deformation at core-scale may be negligible – to the point that sediments may be described as undisturbed. The acoustic response of only relatively limited lateral displacements can however be dramatic, resulting in a complete loss of acoustic structure. In instances where failures become frontally emergent and develop into debris flows, there can be a vast range of sedimentary structures and deformation. Acoustic structure may, however, be quite similar to that of translational failures. Acoustic character must be considered alongside failure geometry, its interaction with the slope, and sedimentological core observations for a full integration for submarine landslide studies.
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Morpho-bathymétric 4D Study of Cap Lopez, Gabon
Authors D. Robert, C. Labaune and A. OyoubiSummaryThe oil terminal of the Cap Lopez in Gabon is situated on the extremity of a sandy arrow which is subject to strong sedimentary mobility phenomena’s.
To understand the hydrodynamic system of the site, a 4D morpho-bathymetric study is conducted since 1958. The difficulty of this study is mainly due to the lack of homogeneity in terms of precision and wedging of the data.
The old data (1958 to 2003) consists of sounding maps acquired with mono-beam echo-sounders. From 2004, digital data were acquired with mono-beam echo-sounders (2004 to 2008) then with multi-beam echo-sounders. Besides the diversity of the operated tools, the quality of the surveys and their precision are variable depending on the weather conditions, the calibration processes and the validation of the data during the acquisition.
After data homogenization and selection, a 4D analysis was conducted using GIS tool in order to compare all the bathymetric surveys but also the shoreline evolution. A quantitative and qualitative analysis of the evolutions of the site is produced. The objectives of this regular follow-up are to better understand the sedimentary processes and to delineate the potential geohazard zones to anticipate slope failures.
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Characterisation of a Submarine Glacial Channel and Related Linear Features
Authors A.K. Furre, P. Ringrose, A. Cavanagh, A.D. Janbu and S. HagenSummaryWe study a linear feature observed at the seabed using side-scan sonar. We investigate potential causal mechanisms for the feature and test a range of possible hypotheses. The origin of this feature is important for determining the nature of possible fluid flow regimes in the shallow sub-surface, past and present.
Our main finding is that the linear feature of interest is on the margin of a large sub-glacial channel or tunnel valley. This channel is mainly sand filled and overlain by clay-rich strata. The channel appears to be partially gas charged (based on evidence from seismic amplitudes and seabed observations), and small pressure changes due to accumulated gas within the channel have most likely led to some surface displacements and gas escape at the seabed.
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Characterization of Shallow Seal Complexes for CO2 Storage Sites - Example from the Greater Sleipner Area
Authors K. Landschulze, J. Tveranger and R.B. PedersenSummaryBuilding on the successful Sleipner CO2 storage project in the Norwegian North Sea, the Utsira Formation has been proposed as large-scale CO2 storage reservoir. Given its shallow depth with regard to phase transition of the injected CO2, the integrity of the approx. 800m thick seal is of great importance. We show a case study of the Greater Sleipner Area where acquisition of sonar data revealed a fracture on the seafloor. This has led us to investigate the shallow subsurface in this area with regard to seal integrity using 3D seismic data. This paper presents results from seismic investigation of the seafloor fracture and its surrounding.
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