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- Volume 18, Issue 6, 2020
Near Surface Geophysics - Volume 18, Issue 6, 2020
Volume 18, Issue 6, 2020
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Downhole nuclear magnetic resonance logging in glaciomarine sediments near Ottawa, Ontario, Canada
Authors Heather L. Crow, Randolph J. Enkin, Jeanne B. Percival and Hazen A.J. RussellABSTRACTDownhole nuclear magnetic resonance technology was applied in four boreholes intersecting glaciomarine sediments of the Ottawa Valley, Ontario. The study evaluated the ability of slim‐hole nuclear magnetic resonance tools to measure in situ volumetric water contents (porosities in saturated sediments) for geohazard and hydrogeological applications. The sediments are composed of clay‐ and silt‐sized grains of glacially eroded rock flour derived from the Precambrian Shield containing trace amounts of magnetic minerals, and porosities ranging from 40 to 74 porosity units (PU, 1% porosity = 1 PU). Two nuclear magnetic resonance instruments were deployed with echo times of 0.5 and 1.0 ms, and diameters of investigation ranging from 14.0 to 30.5 cm. Quantitative nuclear magnetic resonance porosities in the sediments were typically within ±5 PU (95% within ±10 PU) of core calibration data sets in the silty clays where threshold bulk magnetic susceptibility values were <30 × 10−4 SI. This was found to deviate, however, where the concentration and mineralogy of magnetic particles changed, interpreted to be shortening relaxation times which led to underestimation of true water contents. This effect is correlated with a change in depth from magnetite to superparamagnetic nanoparticles of greigite (low‐temperature iron monosulphide) magnetism, interpreted to occur at the sulphate‐methane interface. Clay mineralogy and pore water chemistry were also examined as contributing factors, but were not found to significantly shorten nuclear magnetic resonance relaxation responses. Very short T2 times (<2 ms) are typical in these particular silty clays, requiring a tool with an echo spacing of <1.0 ms. Due to increased potential for sediment disturbance around a well caused by the geotechnical sensitivity of the sediments, a nuclear magnetic resonance instrument with multiple frequencies provided signal penetration out to various diameters around the tool, giving needed information about the size of the disturbed zone surrounding the casing.
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A full‐range gradient survey for 2D electrical resistivity tomography
Authors Bing Zhou, Youcef Bouzidi, Saif Ullah and Muhammad AsimABSTRACTIn this paper, we propose an enhanced version of the multi‐gradient measurement technique, called full‐range gradient survey, for 2D electrical resistivity tomography. To demonstrate its effectiveness, we conducted numerical simulations and field experiments to highlight the advantages of the new data‐acquisition technique on the traditional electrode arrays and the original multi‐gradient measurement technique. A comparison of the imaging capabilities of the full‐range gradient technique with the dipole–dipole, Schlumberger and multi‐gradient shows that the former inherits all the advantages of the multi‐gradient technique as well as the sensitivity of the traditional three‐ and four‐electrode arrays. The new survey technique has a better pseudo‐section coverage, a much smaller geometrical factor, and less noise contamination than the dipole–dipole survey. It significantly improves the subsurface images of the Schlumberger and original multi‐gradient data. The numerical and field experiments both demonstrate that the full‐range gradient survey can be an alternative to all the traditional electrode arrays and original multi‐gradient surveys of 2D electrical resistivity tomography to obtain better quality and higher resolution images of near‐surface targets.
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An unconventional method for calculating porosity of marine clay deposits using the 2D resistivity method
Authors Najmiah Rosli, Rosli Saad, Nazrin Rahman and Nur Azwin IsmailABSTRACTVarious methods were earlier designed to calculate porosity of a formation, but many of them are reliant on physical soil sampling and/or laboratory measurements. The present work examines a recently proposed method, which we call here as Sample‐free Porosity Calculation from Resistivity Imaging Data (SPyCRID), to calculate porosity of unconsolidated soils. By conjoining Archie's and Waxman–Smits’ equations, SPyCRID can achieve high accuracy in porosity calculation, while requiring no physical soil‐sampling data. Two‐dimensional resistivity data acquired at Segantang Garam were modelled to calibrate SPyCRID's workflow and to devise data constraints for marine clay and brackish pore‐fluid conditions. Measured porosities from soil samples were used only for calibration and validation purposes. With data transformation added into the workflow, the performance of SPyCRID was improved as it was possible to achieve more precise value for the calculated porosity (error ≤3.1%). The final step to establish SPyCRID's competency was implemented through testing SPyCRID at the Nibong Tebal test site that has similar geological conditions to the model. Results from this test site showed nominal errors (≤3.9%) in the calculated porosity. SPyCRID could successfully calculate the porosity of the unconsolidated, marine clayey soils with a confidence limit exceeding 96%, while requiring no physical sampling data.
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Misinterpretation of velocity pull‐ups caused by high‐velocity infill of tunnel valleys in the southern Baltic Sea
Authors Laura Frahm, Christian Hübscher, Arne Warwel, Jonas Preine and Hendrik HusterABSTRACTIn this study, we undertake a renewed investigation of up‐bent reflections seen in seismic time sections from the Baltic Sea, Bay of Kiel. These warped reflections stretch over the entire vertical extent of the sections, from Permian to Quaternary strata, and underlie tunnel valleys. Previous studies interpreted these structures as anticlines, explaining them together with adjacent faults and disrupted strata as the consequence of ice‐load‐induced salt tectonics. This conclusion would have influenced theories on how tunnel valleys formed. However, well data from tunnel valleys in other regions supported the interpretation of the up‐bent reflections as imaging artefacts (pull‐ups). A newly acquired long‐offset, multichannel seismic data set images all strata from Base Zechstein up to the seafloor. Owing to the length of the streamer and a shallow water depth, the data display significant moveout and refracted waves, allowing the application of different quantitative methods to investigate velocities. By generating partial‐offset sections, we reveal an offset dependence in the imaging of the up‐bent structures caused by a local, near‐surface high‐velocity zone. This also explains a smoothing of the up‐bending with depth in the seismic image. A velocity model gained by a travel‐time tomography shows positive velocity anomalies in the upper strata correlating with tunnel valleys resolved in the reflection images. A pre‐stack depth migration performed with a velocity model containing a high‐velocity zone results in a seismic image almost free of the observed up‐bent reflections. High‐frequency reflection seismic data confirm this result as it shows a detailed image of a tunnel valley with a phase‐reversed bottom reflection caused by the velocity inversion at the base of the high‐velocity valley fill deposits. Hence, there is consistent evidence that all up‐bent reflections in the Bay of Kiel are imaging artefacts (pull‐ups) that formed beneath tunnel valleys. A salt tectonic control on tunnel valley evolution is, consequently, not likely. This study is the first purely seismic data‐driven study that proves high‐velocity valley fill deposits. Our findings imply that extra care must be taken when interpreting reflection undulations as tectonic features where glacial deposits are present.
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Obtaining reliable S‐wave velocity depth profile by joint inversion of geophysical data: the combination of active surface‐wave, seismic refraction and electric sounding data
Authors Mustafa Senkaya, Hakan Karsli, Laura Valentina Socco and Sebastiano FotiABSTRACTThe inversion of active surface‐wave data is highly affected by the non‐uniqueness of the solution. While a deterministic approach is generally chosen due to certain advantages, there is the risk of getting trapped in a local minima, especially when no a priori information is available about the sub‐surface geometry since the layer thickness is assumed fixed to a priori. The fixed‐layer thickness in a deterministic inversion of the active surface‐wave raises significant issues, such as the relevance of the initial model geometry (the thickness of the intermediate layer and the total depth of the initial model) and the equivalence problems. Thus, the inversion result is inherently not reliable, even in the case of the normal dispersion medium, and the result could be unachievable in challenging sub‐surface situations. These issues could be reduced by using a joint inversion approach. The present paper first presents examples of issues through four case histories in Trabzon, Turkey. Then, two joint inversion approaches based on local search are carried out to handle the issues concerning individual inversion. The first approach combines active surface‐wave data with electric sounding data and the second includes travel times from seismic refraction data. In addition, an independent inversion is carried out with a neighbourhood algorithm for a global search to compare against the joint inversion results. The joint inversion schemes clearly reduce the ambiguities of the individual inversion of the active surface‐wave data, and the dependence on the initial model regarding the layer thickness is also mitigated. Moreover, the joint inversion approach provides an estimate of the complementary model parameters, namely electrical resistivity and the compressional wave velocity. It is shown that the proposed joint inversion approaches provide consistent results with former boreholes, seismic tomographic profiles and the known geologic features of the study area even in the absence of any a priori information.
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Vector P‐ and S‐wave pre‐stack reverse‐time migration for tunnel spaces
Authors Shuai Luo, Lu Guangyin, Ziqiang Zhu and Chengzhi XiaABSTRACTMigration is considered to be a key step in the data processing of the seismic advancement predictions of tunnels, and it directly affects the final interpretations. Therefore, with the goal of addressing the limitations of the current acoustic reverse‐time migration processes, as well as deepening the understanding of the interactions between different wave modes in traditional elastic reverse‐time migration, a vector P‐ and S‐wave reverse‐time migration method is proposed in this study. Consideration is given to the actual tunnel spaces, and forward calculations and reverse‐time extrapolations of the wavefields are carried out based on the first‐order velocity‐stress elastic wave equation of P‐ and S‐wave separation and a high‐order staggered‐grid algorithm. The imaging conditions of a source‐normalized cross‐correlation are used to realize the imaging. Then, a Laplace filtering method is adopted to suppress the low‐frequency artefacts. Numerical simulations of the weak interlayer and karst cave models show that the proposed method had the ability to more effectively suppress noise and achieve high‐precision imaging of tunnel spaces, when compared with using horizontal or vertical components to directly obtain the images. The positions and shapes within the tunnel spaces can be accurately imaged regardless of whether they are lithologic interfaces or local geological bodies. This is found to improve the resolution and accuracy of the tunnel advancement predictions. Finally, the proposed method is successfully applied to the real data processing, and the effects are found to be satisfactory.
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Matrix‐free crosshole elliptical‐anisotropy tomography: parametrization analysis and ground‐penetrating radar applications in carbonates
ABSTRACTA novel traveltime tomographic approach is applied to anisotropic media, limited to 2D geometry at present. A general anisotropic Eikonal solver based on a discontinuous Galerkin method is combined with an efficient adjoint formulation for multiparameter least‐squares inversion. This new approach is tested considering synthetic crosshole ground‐penetrating radar data. The configuration of the ground‐penetrating radar survey is inspired by a real experiment done on layered carbonate media disturbed by the presence of a deep gallery, which induces a localized high‐electromagnetic contrast. This made it possible to define a well‐adapted general workflow in this context. We notably show that, under the elliptical anisotropic assumption, the parametrization based on vertical and horizontal velocities provides less biased results than those obtained by considering the vertical velocity and the relevant Thomsen parameter . The initial vertical and horizontal velocity models are identical and built from an isotropic inversion. The presence of the high‐contrast gallery generates a weak diffraction pattern, which is taken into account in our tomography approach. It also creates potential artefacts due to the model discretization, which are mitigated by a model regularization term within the definition of the misfit function. This general workflow is then applied to the real experiment dataset. The vertical and horizontal velocity images provide similar structures as those previously obtained by isotropic full waveform inversion, complemented by an image of a rather weak elliptical anisotropy.
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Ground‐penetrating radar reflections and their archaeological significances at two ancient necropolis tombs in Kharga Oasis, Egypt
Authors Hamza A. Ibrahim and Mohamed O. EbraheemABSTRACTThis study presents results of ground‐penetrating radar surveys conducted for the exploration of unexcavated tombs at the Kharga Oasis. These surveys were carried out on two ancient tombs. The first site, located in Labakha, is already excavated, while the second site, located in Gebel El‐Siwa, is still unexcavated. The goal of this study was to investigate the possibility of the existence of unexcavated tombs in Labakha, as well as the depth, extension and constituents of an unexcavated tomb in Gebel El‐Siwa. Ground‐penetrating radar measurements were carried out using a single‐channel system and the common offset approach. Based on the results obtained from our measurements using 200‐ and 400‐MHz antennae in Labakha, a detailed survey was carried out on the unexcavated tomb in Gebel El‐Siwa using a cart‐mounted 400‐MHz antenna system. 2D ground‐penetrating radar profiles were acquired and subsequently processed and interpreted. Isolated reflections could be identified. Lateral continuity of the reflections could be observed only through comparative analyses of the adjacent scans. By doing so, the remnants of the two tombs became easier to correlate. Two ground‐penetrating radar anomalies could be detected in the archaeological site of Gebel El‐Siwa; these anomalies are probably caused by a nearby tomb. By correlating the obtained data with the archaeological knowledge of the discoveries made in the surrounding areas, it was possible to interpret the linear reflectors as to be caused by the tomb. This study has been useful in providing archaeologists with information on the expression of this type of ancient Egyptian necropolis tombs at Kharga.
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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)