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- Volume 4, Issue 5, 2006
Near Surface Geophysics - Volume 4, Issue 5, 2006
Volume 4, Issue 5, 2006
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Imaging subsurface objects by seismic P‐wave tomography: numerical and experimental validations
More LessWe tackle the problem of characterizing the subsurface, more specifically detecting shallow buried objects, using seismic techniques. This problem is commonly encountered in civil engineering when cavities or pipes have to be identified from the surface in urban areas. Our strategy consists of processing not only first arrivals, but also later ones, and using them both in tomography and migration processes, sequentially. These two steps, which form the basis of seismic imaging, can be carried out separately provided that the incident and diffracted wavefields are separated in the data space. Tomography is implemented here as an iterative technique for reconstructing the background velocity field from the first‐arrival traveltimes. The later signals are then migrated by a Kirchhoff method implemented in the space domain. To study the reliability of this methodology, it is first applied to synthetic cases in the acoustic and elastic approximation. Both the background velocity field and the local impedance contrasts are reconstructed as defined in the predicted model. An experimental case, specifically designed for the purpose, is then considered in order to test the algorithms under real conditions. The resulting image coincides well with the predicted model when only P‐waves are generated. In the elastic mode, surface waves make P‐wave extraction difficult, so that the reconstruction remains incomplete. This is confirmed by the real data example. Finally, we demonstrate the appropriateness of the proposed method under such circumstances, provided that suitable preprocessing of data is carried out, in particular, the removal of surface waves.
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Geophysical investigation of the June 6, 1944 D‐Day invasion site at Pointe du Hoc, Normandy, France
Authors M.E. Everett, C.J. Pierce, N. Save, R.R. Warden, D.B. Dickson, R.A. Burt and J.C. BradfordA near‐surface geophysical survey at the D‐Day invasion site on top of the cliffs at Pointe du Hoc in Normandy, France, was carried out using ground‐penetrating radar, electromagnetic induction and magnetic gradiometry equipment. The subsurface targets of investigation are predominantly buried concrete and steel structures and earthworks associated with the German coastal fortifications at this strongpoint of Adolf Hitler’s Atlantic Wall. The targets are readily detectable, embedded within the vadose zone of a weakly magnetic, electrically resistive, loess soil cover. The radar and electromagnetic induction responses lend themselves to plan‐view imaging of the subsurface, while the magnetics data reveal the presence of buried magnetic bodies in a more subtle fashion. Several intriguing geophysical signatures were discovered, including what may be the buried remains of a railway turntable, ordnance fragments in the bomb craters, a buried steel‐reinforced concrete trench, and a linear chain of machine‐gun firing positions. Geophysical prospecting is shown to be a very powerful tool for historical battlefield site characterization.
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Mapping fly‐ash water pond leakage with TEM and IP data at Loy Yang coal‐mine (Australia)
Authors A. Viezzoli, J.P. Cull and D. MassieThe fly‐ash water pond at the Loy Yang power station (Latrobe Valley, Victoria, Australia) seeps and therefore presents a hazard to the local environment from its high total dissolved solids content. A transient electromagnetic survey (TEM) was completed to investigate the contaminant plume in more detail.
The contaminate plume was successfully delineated; however, multiple contiguous soundings featured late‐time negative decays not typically recorded in coincident‐loop TEM surveys. This indicates that conductivity is frequency‐dependent. These anomalous soundings suggested the presence of a shallower contaminate plume than that mapped by the boreholes and therefore further analysis was required. An induced polarization survey revealed that the area where the late‐time negative responses were recorded coincided with a shallow conductive and extremely chargeable anomaly. A simple power‐law relationship holds between the negative transients decay rates and normalized apparent chargeability. Analysis on soil samples obtained from auger holes related the polarization anomaly to clay‐, organic‐ and metal‐rich layers of fill material.
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Resistivity tomography as an aid to planning gas‐pipeline construction, Risha area, north‐east Jordan
More LessTwo‐dimensional (2D) electrical resistivity tomography measurements were conducted with the Iris Syscal R2 resistivity instrument to map the ground resistivity in advance of gas‐pipeline construction in the Risha area, north‐east Jordan. The cost of pipeline construction is strongly dependent on the ground conditions encountered. Accurate prediction of these conditions can reduce the planning risk considerably. The determination of ground conditions reduced the time and cost associated with gaining land access and permission for trenching. It is also served to detect areas of high ground conductivity due to clays or saline soils, which can create conditions in which pipeline corrosion is accelerated. Resistivity measurements were carried out utilizing a Wenner‐array with 1‐m electrode spacing along approximately 52 km of the proposed gas‐pipeline lane. The survey took six days to complete, providing a map of the resistivity of the surficial deposits and the bedrock. The results obtained, based on 2D inversion of field data and borehole information, were interpreted to determine the extent of shallow bedrock, which would require blasting, and deeper overburden, which could be trenched down to the depth necessary for the pipeline. The thickness of the overburden layer along the gas‐pipeline lane in the north‐eastern Jordanian plateau was found to be relatively thin: in the range 0.5– 8 m. The largest values are observed over topographic lows because of the tectonic uplift of the area which leads to fast erosion. Correlation between the field results obtained shows that the overburden depth is the main factor controlling the electrical resistivity of the near‐surface material.
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Gravity investigation of buried Pleistocene subglacial valleys
More LessBuried Pleistocene subglacial valleys are important geological structures in northern Germany and are used extensively for groundwater production. Details of their occurrence, their structure, and the hydraulic properties of their sediments are not well known. In recent years, seismic and geoelectric methods, including airborne‐electromagnetics, have been applied in reconnaissance surveys. This paper summarizes the results of gravity surveys conducted across four Pleistocene valleys in northern Germany. Each buried valley causes a significant gravity anomaly. The observed anomalies in the residual gravity field range from −0.5 mGal to 0.7 mGal. The Ellerbek valley in Schleswig‐Holstein is the only buried valley causing a negative anomaly. Its infill is dominated by sandy sediments. Two other Pleistocene valleys in Schleswig‐Holstein, the Trave valley and the Curau valley, are filled with boulder clay. Both cause positive anomalies in the residual gravity field. In the area of the Bremerhaven‐Cuxhaven valley in Lower Saxony, a 4‐km2 survey area shows a quite different situation. Strong negative anomalies caused by shallow silt/clay layers are superimposed on positive anomalies caused by the valley sediments, resulting in a complicated pattern of gravity anomalies. The horizontal gravity gradient proved to be appropriate for locating the edges of the valleys.
The quality of the gravity results was confirmed by the seismic data. Quantitative interpretations of the residual gravity anomalies, which give deeper insight into the structure of the valley fill, often suffer from a lack of information about the rock density. This lack of information can be partly compensated for by converting seismic velocities into densities, although in some cases the normally used velocity‐density formulae yield unrealistic density values. Nevertheless, the additional information obtained from gravity surveys can support the seismic interpretation or provide information in areas where other geophysical methods fail.
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Integrated study of the sinkhole development site on the Western shores of the Dead Sea using geophysical methods
Authors Michael Ezersky, Ilan Bruner, Shemer Keydar, Pini Trachtman and Michael RybakovThis paper presents a combination of high‐resolution seismic diffraction, reflection, refraction, continuous vertical electric sounding (CVES), ground‐penetrating radar (GPR) and microgravity methods to study the structure and properties of the shallow subsurface at the sinkhole development site in the Nahal Hever southern area (on the shore of the Dead Sea) in Israel. These methods have different resolutions and penetration depths. The methods are complementary to each other and reveal useful information about the subsurface. Each one investigates certain characteristics, but when combined they provide us with new information about the subsurface. The seismic refraction method is utilized to map salt layers and to detect dissolved zones. The analysis of all of the seismic refraction data obtained at the Dead Sea area, verified by the information gained from a borehole, has shown that the P‐wave velocity of the salt layer varies from 2900 to 4250 m/s. The velocity of 2900 m/s was used as the minimum velocity criterion for salt‐layer identification at this site.
A low‐velocity zone (VP < 2800 m/s) revealed within a high‐velocity area ( m/s) is interpreted as a dissolved salt or non‐saline unit. The site is characterized by two significant geological features. The first feature is a water‐filled cave, detected recently by a borehole in the depth interval 23–28.5 m, and the second feature is a depression at the surface. Geophysical results show that the depression site is presently characterized as a heavily perturbed zone in both lateral and vertical directions. Soil decompaction in the subsurface is clearly identified by the CVES method as a high‐resistivity anomaly. This decompaction is assumed to be associated with the voids and fractures occurring down to a depth of 100–120 m (by diffraction data) and in the uppermost subsurface (by GPR data). The zone containing the cave is characterized by a lateral (funnel) and vertical (faults) distortion of the subsurface structure (by seismic reflection data) and has a faint diffraction anomaly in the shallow subsurface. A slightly increased resistivity anomaly appears in the depth interval 6–12 m (from CVES data). Drilling data reveal this interval as a decompaction zone. No GPR anomalies were detected at the buried cave site. The microgravity survey at the buried void site in 1999 detected an anomaly that is very similar to the funnel‐shaped one.
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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)