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- Volume 7, Issue 2, 2009
Near Surface Geophysics - Volume 7, Issue 2, 2009
Volume 7, Issue 2, 2009
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3D seismic reflection surveying at the CO2SINK project site, Ketzin, Germany: a study for extracting shallow subsurface information
Authors Sawasdee Yordkayhun, Christopher Juhlin and Ben NordenABSTRACTThree‐dimensional (3D) reflection seismic data were recorded as part of a pilot scale carbon dioxide (CO2) geological storage project (CO2SINK) with the aim of mapping the structural geometry of the site and providing a 3D baseline prior to CO2 injection. Standard processing originally focused on the storage target, a saline aquifer at 500–700 m depth and successful imaged coherent reflections from 150 ms to 900 ms in the seismic volume. However, the relatively sparse distribution of sources and receivers, the frequency content of the data and artefacts of the processing resulted in the uppermost 150 ms being poorly resolved. This depth range contains caprock, shallow faults and an aquifer system. Thus, characterizing the shallow subsurface is important in terms of site delineation of potential leakage paths and monitoring after CO2 injection. In order to study the potential of mapping the uppermost reflectors and shallow structures associated with major fault zones, a comprehensive reprocessing effort on a subset of the 3D data was performed. The challenge in imaging shallow reflections is dependent upon the separation of ground roll and refracted energy from the reflected energy, as well as compensation for time shifts due to statics. Among the processing sequences, refraction static corrections, careful muting and filtering, velocity analysis and 3D time migration were key steps for enhancing the resolution and coherency of shallow seismic reflections. This study images a previously unmapped horizon, close to the Quaternary‐Tertiary boundary, at about 95–120 ms (~65–90 m depth). Correlation of lateral variations in reflectivity along this boundary, lateral velocity variations in the tomographic image and the seismic signature in modelling studies suggest an aquifer/aquitard complex and variable lithology with associated localized silty or clayey sediments, overlying the Tertiary Rupelton clay unit. In the previous processing it was not clear if the deeper faults imaged on the 3D seismic survey extended to shallower levels than the base Tertiary. Thus, a comprehensive fault detection technique, multi‐attributes and neural networks analysis, was employed in this study to allow a more reliable fault geometry to be interpreted. Tracking of faults in the seismic image and comparisons with a tomography study indicate that some deeper faults may penetrate into the overlying Tertiary unit. These findings are important for understanding potentially risky areas and can be used as a database for future monitoring programmes at the site.
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GPR investigations to characterize Medieval and Roman foundations under existing shop premises: a case study from Chester, Cheshire, UK
Authors J.K. Pringle, J.W. Lenham and J.M. ReynoldsABSTRACTA high‐resolution, ground‐penetrating radar (GPR) investigation was performed on existing commercial shop premises within The Rows, Chester, UK to detect near‐surface features, including a suspected Medieval undercroft (cellar), the location of which was unknown. GPR 2D profiles were acquired on a 0.5 m square grid‐pattern both within the premises and on an exterior, Medieval raised walkway. Multi‐frequency (250, 500 and 800 MHz dominant frequency) repeat‐surveys were acquired in selected areas over the grid to resolve progressively deeper near‐surface structures and used to generate 500 and 800 MHz horizontal ‘time‐slices’.
Floor‐supporting wooden beams with steel supports on the exterior walkway, the Medieval undercroft, associated passageways and a hitherto unexpected access trap‐door were all successfully located. The undercroft was found to be air‐filled. Three near‐surface rectangular areas were identified and found not to be associated with the undercroft; these were interpreted to represent heterogeneous, rubble back‐fill. Separate, isolated, raised areas were also identified ~1.5 m below present floor level; these were interpreted as relict brick column supports from a long‐demolished, Roman building. GPR data acquired on the exterior raised walkway also found evidence for relict support structures from a former shopfront.
All of the targeted anomalies were intrusively investigated and provided positive confirmation of the geophysical interpretations. Archaeological investigations of trial pits confirmed the site’s varied history and depth to bedrock. GPR was therefore effective in this study to locate and characterize the modern, Medieval and Roman features without extensive intrusive investigation, whilst causing minimal disruption to commercial operations. These results demonstrate the effectiveness of carefully acquired and processed, high‐resolution, closely‐spaced GPR survey data in near‐surface characterization for geotechnical investigations.
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Integrated analysis and interpretation of cross‐hole P‐ and S‐wave tomograms: a case study
Authors Peter Dietrich and Jens TronickeABSTRACTWe present cross‐hole P‐ and S‐wave seismic experiments that have been performed along a ~100 m long transect for the detailed characterization of a contaminated sedimentary site (Bitterfeld research test site, Germany). We invert the corresponding first break arrival times for the P‐ and S‐wave velocity structure and compare two different strategies to interpret these models in terms of pertinent lithological and geotechnical parameter variations. The first (common) approach is based on directly translating the tomographic velocity models into the parameters of interest (e.g., elastic moduli). The second (zonal) approach first reduces the tomographic parameter information to a limited number of characteristic velocity combinations via ‐means cluster analysis. Then, for each zone (cluster) further parameters including uncertainties can be estimated. In the presented case study, our results indicate that the zonal approach provides an effective means for the integrated interpretation of different co‐located data.
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Markov random field image processing applications on ruins of the Hittite Empire
Authors Osman N. Uçan and A. Muhittin AlboraABSTRACTIn this paper, we applied Markov random field processing to geophysical data as an alternative to classical deterministic approaches. Markov random field processing is an unsupervised statistical model‐based algorithm, which does not require a priori information. We present a dynamic programming based on evaluation of noisy and superpositioned effects of the various geological structures considering a statistical maximum a posteriori criterion. The objective of the proposed modelling is to capture the intrinsic character of the input potential anomaly map in a few parameters, so as to understand the nature of the phenomenon generating the anomaly. In order to decrease processing time and to enhance image performance of the Markov random field, we introduce a preprocessing step. The preprocessing step is crucial and it helps us to solve difficult multi‐disciplinary problems such as separation, enhancement of magnetic anomalies and border detection. We also decrease the noisy peak values of pixels by this smoothing process and emphasize the discontinuity properties of the noisy data by an absolute differentiation procedure. Here, the magnetic field of the input data is considered as a two‐dimensional image with a matrix composed of pixels. We evaluate each pixel of the matrix using the Markov random field approach, regarding the neighbouring pixels and their locality in real time with no a priori training procedure. As synthetic examples, various prism models are considered and the separation and edge detection performance of the Markov random field is tested. As real data, we have evaluated the magnetic anomaly map of the Hittite civilization in the Sivas‐Alt1nyayla region of Turkey. We have obtained satisfactory results in both synthetic and real data and concluded that the Markov random field is a compromising approach for the separation problem of regional‐residual anomalies and edge detection of various geological bodies.
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DC resistivity sensitivity patterns for tilted transversely isotropic media
Authors Tim Wiese, Stewart Greenhalgh and Laurent MarescotABSTRACTIn this paper we present and analyse DC resistivity sensitivity patterns for uniform anisotropic media and for various surface electrode arrays. The sensitivity functions (or Fréchet derivatives) give the responsive change in measured electric potential for a perturbation in a model parameter at a particular point in the subsurface for a specific electrode configuration. The anisotropic model investigated is the common tilted transversely isotropic medium, which is defined by four model parameters. We examine the changes in the Fréchet derivatives of the Green’s functions with respect to both the longitudinal and transverse conductivity and the dip and azimuth angle of the symmetry axis for varying model parameters. Anisotropic sensitivities are vastly different in strength and shape compared to the isotropic sensitivity pattern. The various arrays (pole‐pole, dipole‐dipole, Wenner and square array) produce distinctive patterns that are important in assessing resolution. Valuable insights were gained: the anisotropic Fréchet derivatives can vary greatly from the isotropic pattern in both strength and shape depending on the nature of the anisotropy. Also it was found that the relationship between components of current density (parallel and perpendicular to the symmetry axis) and the sensitivity patterns is somewhat akin to the dot product of the true source and adjoint source current density vectors in isotropic media. Understanding of the anisotropic sensitivity pattern variations will enable, through further analysis, computation of model resolution matrices and improvement of experimental design. Appreciation of the significant differences between isotropic and anisotropic sensitivities should help avoid errors in using inappropriate isotropic inversion schemes to interpret measurements made over anisotropic ground.
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Volumes & issues
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Volume 22 (2024)
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Volume 21 (2023)
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Volume 20 (2022)
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Volume 19 (2021)
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Volume 18 (2020)
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Volume 17 (2019)
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Volume 16 (2018)
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Volume 15 (2017)
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Volume 14 (2015 - 2016)
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Volume 13 (2015)
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Volume 12 (2013 - 2014)
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Volume 11 (2013)
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Volume 10 (2012)
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Volume 9 (2011)
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Volume 8 (2010)
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Volume 7 (2009)
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Volume 6 (2008)
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Volume 5 (2007)
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Volume 4 (2006)
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Volume 3 (2005)
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Volume 2 (2004)
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Volume 1 (2003)