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- Volume 16, Issue 6, 2018
Near Surface Geophysics - Volume 16, Issue 6, 2018
Volume 16, Issue 6, 2018
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High‐resolution reflection seismics reveal the structure and the evolution of the Quaternary glacial Tannwald Basin
ABSTRACTOver‐deepened basins exist throughout the Alpine realm. Improving our knowledge on these basins is of high social relevance, since these areas are often well‐populated and they possess, for instance, unusual hydrological settings. Nonetheless, geophysical and sedimentological investigations of over‐deepened basins are rare. We analyse the sedimentary succession of such a basin, the Tannwald Basin, through geological interpretation of seismic reflection profiles. The basin is located approximately 60 km north of the European Alps. It was incised into Tertiary molasse sediments by the Rhine Glacier and later filled by glacial, fluvial, and lacustrine deposits of 250 m thickness. The Leibniz Institute for Applied Geophysics acquired a grid of five high‐resolution seismic reflection lines that imaged till the deepest parts of the Tannwald Basin. The seismic profiles, processed to a pre‐stack depth migration level, allow a detailed geological interpretation that is calibrated with the help of a nearby borehole. We determine the structure and the seismic facies of the sediment succession in the basin and presume the following hypothesis of the evolution of the basin: sub‐glacial erosion comprises the excavation of the over‐deepened basin as well as detachment of large fragments of molasse material. These molasse slabs were deposited within the basin in a layer of basal till that graded upwards in water‐lain till and fine‐grained deposits. During the last two glaciations, the basinal structure became buried by till sequences and glacio‐fluvial sediments.
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Estimating picking errors in near‐surface seismic data to enable their time‐lapse interpretation of hydrosystems
Authors M. Dangeard, L. Bodet, S. Pasquet, J. Thiesson, R. Guérin, D. Jougnot and L. LonguevergneABSTRACTTime‐lapse applications of seismic methods have been recently suggested in the near‐surface scale to track hydrological properties variations due to climate, water level changes, or permafrost thaw, for instance. But when it comes to traveltime tomography or surface‐wave dispersion inversion, a careful estimation of the data variability associated with the picking process must be considered prior to any time‐lapse interpretation. In this study, we propose to estimate picking errors that are due to the inherent subjectivity of human operators, using statistical analysis based on picking repeatability. Two seismic datasets were collected along the same profile under distinct hydrological conditions across a granite–micaschist contact at the Ploemeur hydrological observatory (France). Both datasets were recorded using identical equipment and acquisition parameters. A thorough statistical analysis is conducted to estimate picking uncertainties, at the 99% confidence level, for both P‐wave first arrival time and surface‐wave phase velocity. With the suggested workflow, we are able to identify 33% of the P‐wave traveltimes and 16% of the surface‐wave dispersion data, which can be considered significant enough for time‐lapse interpretations. In this selected portion of the data, point‐by‐point differences highlight important variations linked to different hydrogeological properties of the subsurface. These variations show strong contrasts with a non‐monotonous behaviour along the line, offering new insights to better constrain the dynamics of this hydrosystem.
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Estimation of hydraulic parameters using electrical resistivity tomography (ERT) and empirical laws in a semi‐confined aquifer
Authors Santiago Perdomo, Eduardo E. Kruse and Jerónimo E. AinchilABSTRACTThe estimation of hydraulic parameters is critical for the rational use of water resources and the development of reliable hydrogeological models. However, the cost of such estimation can be very high and the data are limited to the area near the pumping well. For this reason, complementary methods for estimating hydraulic conductivity and transmissivity have become increasingly important in recent years, such as the adjustment of empirical relationships between geoelectrical and hydraulic parameters. In this paper, two linear relationships were tested, combining resistivity measurements from well logging profiles and hydraulic conductivity values from pumping test data, in a semi‐confined fluvial aquifer in the province of Buenos Aires, Argentina. Furthermore, these relationships were used to obtain two‐dimensional (2D) hydraulic conductivity and transmissivity sections from electrical resistivity tomography using a high‐definition electrode array. Predicted values were compared with traditional pumping test in a near well showing very good agreement with both methods. Results showed that it would be possible to quantify the 2D variation of hydraulic parameters in aquifers and to identify high‐ or low‐productivity areas. By knowing this information in advance, it is possible to reduce the number of failures or unexpected results when drilling a well. These 2D sections also provide additional information about hydraulic parameters and their lateral variability, and can improve hydrogeological models without drilling new wells.
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Hydrogeophysical characterization of shallow light non‐aqueous phase liquid contamination at a karst aquifer
Authors Jafar Abu Rajab, Ali El‐Naqa and Mohammed Al‐QinnaABSTRACTTwo wells at an industrial facility were recently removed from service due to signs of long‐term hydrocarbon contamination. Groundwater monitoring at the polluted wells documented the presence of a massive body of light non‐aqueous phase liquid materials that were accumulated at a depth of ∼50 m in the water table. We have used transient electromagnetic and electrical resistivity tomography to characterize the ground at the light non‐aqueous phase liquid contaminated wells. Additionally, analyses of hydrochemical samples and calculations of hydrogeological parameters have been implemented to interpret and describe the geophysical signatures of several anomalous hydrocarbon contaminations.
Resistivity models, which are based on the three‐dimensional inversion of transient electromagnetic data and the two‐dimensional inversion of electrical resistivity tomography lines conducted at the source of leaks and contaminated wells, confirmed the presence of a hyper‐conductive zone (i.e. 0.5 Ω⋅m), 15 m deep and 20 m thick, that contained a mixture of light non‐aqueous phase liquid, hot water, and chemical acids. In addition, resistivity levels up to hundreds of ohm metre can be attributed to near‐surface cavernous structures. Furthermore, the resistivity models can resolve a 30‐m‐thick conductive layer underlying cavernous structures, yielding resistivity values ranging from 0.78 to 5 Ω⋅m. The ease of light non‐aqueous phase liquid migration is promoted due to zones of cavernous structures acting as preferential flow pathways down to the water table, where light non‐aqueous phase liquid materials follow the hydraulic gradient of the limestone aquifer, which has a calculated porosity of 20%. Chemical analysis of groundwater samples at contaminated well sites shows the depletion of sulphate and nitrate concentrations and an increase in bicarbonate at locations where the highest hydraulic conductivities are observed. These findings facilitates the construction of a conceptual model for contamination which is used to better understand the evolution of light non‐aqueous phase liquid pollution at the study area.
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Interpretation attributes derived from airborne electromagnetic inversion models using the continuous wavelet transform
More LessABSTRACTPlanning, contracting, data acquisition and processing plus the inverter's quality assessment and inversion of a regional airborne electromagnetic survey may take some months, while the interpretation of the results is a considerably more complex and comprehensive process. Most often an interpretation necessitates additional data that are time consuming to collect and complicated to integrate into an overall model, for example borehole logs, borehole core samples, water chemistry, surface vegetation, satellite imagery plus all existing geological background knowledge. Interpretation basically has to do with identifying categories and finding boundaries between them so that depths, thicknesses and a whole range of other model attributes can be estimated, qualitatively and/or quantitatively. I present two methods using the continuous wavelet transform of finding attributes intended to assist the interpreter: one finds layer boundaries in the smooth multi‐layer models that are most often used in the inversion of large airborne electromagnetic data sets, and the other finds the natural categories of the model parameter. Naturally, being based on the subsurface conductivity distribution, the boundaries and categories suggested are useful only to the extent that they coincide with geological/hydrogeological boundaries and categories – which is for the interpreter to decide.
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Imaging shallow gas migration pathways in a mud‐volcano province using an autonomous underwater vehicle (Malta Plateau, Mediterranean Sea)
Authors A. Savini, S. Pinson, A. Bistacchi, G. Etiope and C.W. HollandABSTRACTData acquired by an autonomous underwater vehicle (AUV) towing a source (1600–3500 Hz) and a horizontal array of hydrophones have been analysed to image discrete, isolated or even a small cluster of scatterers within the sediment, to determine shallow migration paths of hydrocarbons in a mud volcano system of the Malta Plateau. An algorithm based on a semblance function was applied to the acoustic data to highlight scatterers rather than interface reflections. The resulting scatterer map, obtained along the AUV track, generated a pseudo‐three‐dimensional coverage of the study area, with a horizontal and vertical resolution of roughly 3–5 m and 5–10 m, respectively. This map was combined with high‐resolution bathymetric and backscattering seafloor maps obtained from previous explorations. This integrated dataset provides new evidence for the role of fault zones as a preferential path for gas/fluid migration and reveals the intermittent activity of seeping gas. The data show, in particular, that gas bubble slugs, i.e. discontinuous gas columns, rise through Plio‐Quaternary sediments along a complex system of conduits terminating at the surface into quiescent mud volcanoes. The gas flux is facilitated by the regional stress field that results in dilatant conditions on the mapped fault zones.
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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)