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- Volume 4, Issue 2, 2006
Near Surface Geophysics - Volume 4, Issue 2, 2006
Volume 4, Issue 2, 2006
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Semblance‐based topographic migration (SBTM): a method for identifying fracture zones in 3D georadar data
Authors Bjoern Heincke, Alan G. Green, Jan van der Kruk and Heike WillenbergABSTRACTSteep‐dipping fracture zones are generally difficult to delineate using traditional ground‐penetrating radar (georadar) techniques. Evidence for their presence in standard georadar images may be either completely absent or limited to diffractions and/or chaotic reflection patterns. To address this issue, we present a novel three‐dimensional (3D) migration scheme based on computations of semblance. This new approach, which accounts for undulating surface topography, emphasizes diffractors while markedly reducing the effects of specular reflectors. After demonstrating the efficiency of the technique on 3D synthetic data, we apply it to a 3D georadar data set acquired across an unstable mountain slope in the Swiss Alps. This region is characterized by rugged topography and numerous shallow‐ to steep‐dipping fracture zones and faults. Only the shallow‐ to moderate‐dipping structures are imaged as reflectors in conventionally migrated versions of the georadar data. Our semblance‐based topographic migration (SBTM) scheme produces a 3D volume containing clouds of high‐semblance values. Application of morphology image processing to these clouds reveals the presence of geologically meaningful structures, most of which are very steeply dipping (>75°). Several of the steep‐dipping structures project to open fracture zones and associated lineaments at the surface, thus demonstrating the capability of the combined SBTM and morphology procedure for mapping near‐vertical fracture zones.
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Experimental studies for monitoring water‐level by dipole‐antenna array radar fixed in the subsurface
Authors Satoshi Ebihara, Kaoru Nagoya, Noriyasu Abe and Masaru ToidaABSTRACTWe propose a method for monitoring the movement of moving targets, such as water‐level, at considerable depth, using dipole array‐type radar with the dipoles fixed at various positions in the subsurface. We report on monitoring the water‐level variation in a sand tank over several hours with the dipole‐antenna array. A dipole‐antenna with an optical modulator made of non‐metallic material is used as the receiving antenna element. The antenna has little effect on the electromagnetic field being measured, and the optical modulator does not need electrical power. The basic idea of a radar system fixed in a subsurface is to distribute the dipole‐antennae and to monitor the movement of a target, such as water distribution, over long periods of time, using scattered waves from the target. In order to demonstrate the validity of the radar system, we carried out laboratory experiments. We positioned several receiving dipole‐antennae with the optical modulators, in a small package inside the sand. We raised the height of the water‐level in the sand and measured waves scattered from the boundary between the saturated and unsaturated layers. We analysed the radar signals with the MUSIC algorithm utilizing Method of Moment analysis and monitored the 2D location of a moving scattering centre over several hours. By comparing the water‐level in a vertical borehole with the estimated positions, we found that the estimation is reasonable.
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Linking regional‐scale lineaments to local‐scale fracturing and groundwater inflow into the Päijänne water‐conveyance tunnel, Finland
Authors Annukka Lipponen and Meri‐Liisa AiroABSTRACTPotential structural or lithological controls of groundwater flow into the mostly rock‐surfaced Päijänne water‐conveyance tunnel in southern Finland were investigated by integrating geological, hydrogeological and aerogeophysical data. Fracture‐zone geometry and distribution, interpreted from processed aeromagnetic images, and topographic and geological information are compared for three different sections of the Päijänne Tunnel. Documented observations on fracturing and ground‐water inflow from inside the tunnel provide subsurface information on the fractures and their hydraulic properties. Topographic and aeromagnetic lineaments both exhibit general NE–SW and NW–SE orientations at a regional scale. Based on the aeromagnetic interpretation, bedrock fractures intersecting the tunnel are part of a regional fracture network and their evolution can be connected to the main tectonic stages in southern Finland. Their distribution is lithologically controlled and reflects the rock type and its structure. For more detailed scales, a superficial deposit relief map is a powerful tool for obtaining a more accurate location of fracture zones and for evaluating their connection to superficial deposits as possible groundwater reservoirs. Depending on the scale, integrated interpretation using both topographic and aeromagnetic data gives the best results, particularly when supported by independent verification such as the observed fracturing inside the tunnel.
NW–SE appears to be the most common strike trend among the measured fractures linked with water inflow. In the magnetic data, this orientation is displayed (1) as swarms of short, faint signatures indicating brittle, shallow features, and (2) as extended, broad, linear magnetic gradients denoting block boundaries. Their lengths, of tens to hundreds of kilometres, suggest that they may also be deep‐reaching. The coincidence and the parallel orientation of these two magnetic features of different scales suggest their genetic relationship: the faulted block boundaries were reactivated at later tectonic stages, resulting in brittle fracturing along the same earlier structural weakness zones. Many of the locations where water‐conducting fracturing occurs, or where groundwater inflow has been measured at a larger scale, are associated with intersecting or individual topographically interpreted fracture zones, especially NW–SE trending ones.
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Multiple‐gradient array measurements for multichannel 2D resistivity imaging
Authors Torleif Dahlin and Bing ZhouABSTRACTTwo‐dimensional resistivity imaging using multiple gradient, Wenner and dipole‐dipole electrode arrays was carried out at two field sites in Sweden and one in Nicaragua, with the objective of confirming the practical applicability of results obtained with numerical modelling. The results support earlier numerical modelling studies that concluded that the gradient array, using multiple current electrode combinations, has resolution as good as or better than the commonly used Wenner array. The array behaved well in terms of sensitivity to noise at the test sites, and the results obtained generally agree with dipole‐dipole array results, although the latter at two of the sites gave resistivities that differed significantly from the other arrays in the deeper parts of the inverted models. A formula proposed for pseudosection plotting works well for data quality assessment, where it can be an advantage to make separate plots for each m‐factor (the distance between the midpoints of the current and potential electrode pairs) or for each a‐spacing. The gradient array is well suited for multichannel data acquisition, and can significantly increase the speed of data acquisition in the field and at the same time give higher data density, but it is also an attractive option for single‐channel data acquisition. The Wenner array, on the other hand, is not suitable for measuring in more than one channel. Compared to the dipole‐dipole array, it offers lower sensitivity to noise which may be a major advantage in real data acquisition, and the remote electrode needed for the pole‐dipole array is avoided, which is often a significant advantage for field logistics.
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Identification of buried valleys using the BGR helicopter‐borne geophysical system
Authors D.G. Eberle and B. SiemonABSTRACTA study of four airborne electromagnetic data sets collected during the past decade by the BGR helicopter‐borne geophysical system over various geological settings shows that such data sets indicate not only layered horizontal geological features but also vertically and laterally confined structures, such as buried valleys. Indicative geological markers comprise conductive cover layers, conductive fills in resistive bedrock, and incisions in a conductive host filled with resistive materials. In cases where the valleys are incised into a magnetic basement and refilled with non‐magnetic material, the magnetic data records can also be used for identification and thickness estimates of the infill.
From a hydrogeological point of view, buried valleys are becoming increasingly important as they host groundwater reserves, which are in many cases big enough to satisfy the future demand for fresh water. This paper describes two examples from the North‐West German lowlands and another two from the Namib coastal plains, where the Kuiseb and the Omaruru ephemeral rivers reach the shore of the South Atlantic Ocean.
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Multi‐electrode measurements at Thai Binh dikes (Vietnam)
Authors Andreas Weller, Tran Canh, Katrin Breede and Nguyen Trong VuABSTRACTThe province of Thai Binh is located in the delta of the Red River in Vietnam where an extensive system of river and sea dikes protects the population and infrastructure from flooding in the rainy season. The integrity of the river dikes is affected by a diversity of problems. Importantly, various termite species dig their nests in the dikes causing water leakage. Leakage can also occur near sluices and passages.
Geoelectrical multi‐electrode profiling has been applied to locate defects in the dikes. Surveys carried out on several dikes demonstrated that a combined half‐Wenner configuration yields high‐resolution images in which termite nests are indicated by resistive anomalies. Petrophysical investigations of soil samples and density logs yield additional information on the dike material. Elsewhere, imaging surveys confirmed the subsurface extent of a buried sluice structure in the dike body.
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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)