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- Volume 8, Issue 4, 2010
Near Surface Geophysics - Volume 8, Issue 4, 2010
Volume 8, Issue 4, 2010
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An integrated geophysical study of the western part of the Rochechouart‐Chassenon impact structure, Charente, France
Authors Cecilia Bobée, Myriam Schmutz, Christian Camerlynck and Henri RobainABSTRACTA multi‐method geophysical survey was carried out with the aim of reassessing the structure of the Rochechouart‐Chassenon astrobleme and the geometry of the impact‐related formations, referred to as fallout breccias, in the vicinity of Chassenon. Gravity data extracted from the BRGM InfoTerre database were reprocessed and revealed both the shape of the astrobleme and the presence of a small negative gravity anomaly close to Chassenon, due to the low density of impact‐related rocks; a 2D gravity model across the fallout deposits was provided. Geological settings, well‐logging and geophysical soundings (direct current (DC), time‐domain electromagnetic (TDEM) and frequency‐domain electromagnetic (FDEM)) were carried out on the fallout breccias at Chassenon. The 1D resistivity models based on TDEM and DC soundings, resulting from a mutually constrained inversion and coupled with 1D models of FDEM data, were constrained by borehole data and provided 2D resistivity models across the fallout deposits of Chassenon. The combination of these methods allows the description of both the impact‐related rocks geometry and the global structure of fallout breccia remains at Chassenon. The study also shows that the fallout breccia has a complex structure, consisting of thin layers of lithic breccia and suevite, the total thickness of which is larger at Chassenon than previously estimated by means of geological analysis.
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Correction of the attenuation effect of vegetation on airborne gamma‐ray spectrometry data using laser altimeter data
Authors Andreas Ahl and Gerhard BieberABSTRACTThe interpretation of airborne gamma‐ray data is considerably complicated by the attenuating effects of vegetation. In order to correct these effects we used simultaneously measured laser altimeter data to characterize the vegetation cover. To describe the attenuation effect and correct its consequences on gamma‐ray data we used a simple model of exponential absorption of monoenergetic gamma‐ray photons by an attenuating medium. The determination of the corresponding linear attenuation coefficients for potassium (K), uranium (U) and thorium (Th) is based on data from an airborne geophysical survey in Austria.
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Gravity anomalies of strike limited listric fault sources with analytically defined fault planes and arbitrary density contrast variations with depth
More LessABSTRACTA method to compute the theoretical gravity anomalies of strike limited listric fault sources is presented along with a relevant computer code, GRLSTRCFLT. The fault plane of a listric fault structure is described with a polynomial function of arbitrary degree. Further, the density contrast within the structure is presumed to be varying randomly with depth comprehending the geological settings where the detached downthrown block of a fault structure is assumed consisting of both high and low density formations. The expression for the gravity anomaly to realize forward modelling is derived in the space domain using both analytical and numerical approaches. The validity and efficacy of the method is demonstrated with a set of synthetic fault models. The gravity anomaly computed at two selected offsets across the fault structure in either case reveals the fact that the magnitude of the anomaly is dependent on the offset of the profile.
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Geophysical investigation for the rehabilitation of a flood control embankment †
Authors E. Cardarelli, M. Cercato and G. Di FilippoABSTRACTTo comply with recently published seismic regulations and environmental standards, existing dams and embankments have to be evaluated for safety control, in addition to standard maintenance and repair, which is common practice for aging structures. In either case, engineering geophysics is almost the only viable option for investigating these structures and the underlying soil as a whole. In this contribution, electrical and seismic surveys are performed on an outdated flood control embankment that is expected to be put into service again. Integration of DC resistivity results with those of various seismic prospecting methods (seismic refraction, cross‐hole S‐wave and P‐wave tomography and surface wave analysis) is found to be successful for defining a clear physical representation of the entire structure. The low‐strain elastic properties (from seismic speeds of propagation) as well as the geometrical characteristics of the internal core of the dam and of the foundation soil serve as guidance for the rehabilitation intervention.
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Tank modelling of azimuthal resistivity surveys over anisotropic bedrock with dipping overburden
Authors Kathryn A. Watson and Ron D. BarkerABSTRACTRotational or azimuthal resistivity sounding is frequently employed for determining the electrical anisotropy of the subsurface, from which the orientation of fracturing, which might give rise to the anisotropy, is interpreted. However, symmetrical 4‐electrode arrays, such as the Wenner, Schlumberger and square, are ambiguous and will also produce an anisotropy‐style signature over dipping strata or a gradational lateral change in rock resistivity. This problem may be overcome by use of a 5‐electrode offset‐Wenner array. Simple tank modelling of an anisotropic bedrock overlain by an isotropic overburden demonstrates that rotational offset‐Wenner sounding will clearly indicate whether observed anisotropy is real or whether it is merely due to the similar and ambiguous effects of a varying overburden thickness.
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Geophysical modelling of the Ebersbrunn diatreme, western Saxony, Germany
Authors Heidrun Matthes, Corinna Kroner, Thomas Jahr and Horst KämpfABSTRACTThe Ebersbrunn diatreme near Ebersbrunn, western Saxony, Germany, causes an elliptical Bouguer anomaly of about –2.5 m Gal, which covers an area of approximately 2.2 km × 2 km and an elliptical magnetic anomaly with values between –250 nT and 1000 nT about half a kilometre smaller in diameter and situated within the southern part of the Bouguer anomaly. Considering magnetic susceptibility and density values determined from core samples of the diatreme, a symmetrical, cone‐shaped body with an oval surface outcrop of 2 km × 1.5 km and a dip angle of the slopes of about 82° is modelled. It comprises a central structure with a diameter of 800–900 m and susceptibility values up to 9000*10−6 SI higher and densities up to 220 kg/m3 lower than the surrounding chimney. Additionally, susceptibility within the body increases with depth and decreases with the degree of weathering. The geometry can clearly be associated with the diatreme of a maar‐diatreme‐volcano. Regarding the petrographic information (the occurrence of pelletal lapilli as juvenile phase, the basaltoid character of the occurring ground mass of magmatic origin and 80% wall rock content) and the final geophysical model, the Ebersbrunn structure can definitely be regarded as the partially eroded remnant (diatreme and root zone) of a maar‐diatreme‐volcano. The association of earthquake swarms in intraplate regions of intersecting fractures and volcanism has led to renewed interest in the eroded maar‐diatreme‐volcano.
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Field layout of a tiltmeter array to monitor micro‐deformation induced by pumping through a horizontal collector well
Authors H.C. Chen, H.‐J. Kümpel and C.M. KrawczykABSTRACTPump‐induced tilt deformation in the subsurface has repeatedly been observed as a hydromechanical, poroelastic response to changes in pump rates in the vicinity of fluid‐producing wells. Within the Fuhrberg waterworks area, north of Hannover, Germany, groundwater is withdrawn from 22 m depth through a system of horizontal wells out of a sandy, unconfined aquifer. To measure the pump‐induced tilt, a specific field layout was designed and a unique data set recorded. The monitoring experiment was set in place by installing 12 borehole tiltmeters at depths of approximately 2 m below surface. We monitored micro‐deformation signals in situ for up to two years, so that long‐term, short‐term and event‐type tilt signals could be identified. During the study, the pump rate of the producing well system varied from 50–400 m3/h in an irregular but controlled manner. The field layout of the tiltmeter array allowed to trace both the overall signature of the induced micro‐deformations around the well system as well as the tilt responses at individual tiltmeter positions. For a variation of the pump rate by 100 m3/h we found the average amplitude of the induced signal to be around . There is a general trend for the induced tilt signals to build up in a concentric way around the central pit. Nevertheless, there are significant deviations from the expected tilt response, namely of order ±10° in azimuth or in amplitude per 100 m3/h change in pump rate for roughly half of all the 42 tiltmeter sites monitored. The deviating tilt azimuths are believed to reflect the presence of heterogeneities in the glacio‐fluviatile sediments at this location. While influence of the local topography is of minor importance, as well as noise from meteorological and hydrological causes, effects due to the nearby vegetation can mask the pump‐induced tilt signal by up to 30%. Here, the distance of the observation points away from larger trees is essential, rather than the overall amount of vegetation.
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Imaging a shallow aquitard with seismic reflection data in Memphis, Tennessee, USA. Part I: source comparison, walk‐away tests and the plus‐minus method
Authors Jiandang Ge, Maria Beatrice Magnani and Brian WaldronABSTRACTTwo walk‐away tests were conducted at two sites in Memphis, Tennessee, USA. One site is representative of an urban setting (road median) while the other represents a rural setting (metropolitan park). Three P‐wave sources, a 7.5 kg sledgehammer, a 20 kg weight‐drop and a 12‐gauge shotgun, were tested. Analysis of the data collected indicates that the seismic data recorded from the shotgun source possess the strongest energy, the highest dominant frequency, the broadest frequency band and the least amount of ground roll energy. The source repeatability was also studied by observing the first cycle of each seismic source, showing that the shotgun can generate the most repeatable source wavelets. None of the data recorded from the three sources show significant seismic energy above 100 Hz due to seismic wave attenuation. The loess in the rural site exacerbated the attenuation and resulted in a much lower peak frequency (43.7 Hz), which is nearly half of the peak frequency recorded at the urban site (85.3 Hz). Since attenuation can be a big factor in shallow reflection surveying, we recommend that site attenuation be considered before a reflection survey is performed in the Memphis area and a reflection survey be conducted outside of the loess blanketed area when possible. Since the final goal of the survey is to search for aquitard breaches, Hagedoorns’ plus‐minus method was applied to the walk‐away data set to map the first refractor, the top of the aquitard. One depression from the obtained structure was interpreted as a paleochannel, indicating that river channel erosion may be one of the causes for the formation of aquitard breaches.
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Imaging a shallow aquitard with seismic reflection data in Memphis, Tennessee, USA. Part II: data analysis, interpretation and traveltime tomography
Authors Jiandang Ge, Maria Beatrice Magnani and Brian WaldronABSTRACTWe collected P‐wave seismic reflection data to image suspected breaches in the confining unit (aquitard) above the Memphis aquifer in Memphis, Tennessee, USA, where previous studies of water quality have suggested potential contamination. A 1‐km‐long reflection line was acquired across depressions of the water table that have been interpreted to reflect a breach in the aquitard. Although raw data are dominated by surface waves, after frequency filtering, shot static correction and filtering, consistent reflections can be observed. Integration of constant velocity analysis (CVS) and super gather semblance analysis was used to determine the RMS velocity field. The stack section shows a set of shallow reflectors interpreted to correspond to the top and bottom of the confining unit. The aquitard thins by about half from the north end to the south end along the reflection line. The continuity of the top and bottom of the aquitard was disrupted by a few faults interpreted along the line. None of these faults juxtapose the surficial aquifer against the Memphis aquifer but it is possible that the faults themselves may act as a hydraulic connection between the surficial aquifer and the Memphis aquifer and thus serve as pathways for a potential leakage. The refraction tomography technique was applied to the first arrival data and it revealed three depressions that are interpreted as paleochannels on the upper part of the confining unit, which is consistent with the background geology of the area. These inferred paleochannels may suggest that ancient river channel erosion may contribute to the process responsible for the formation of aquitard breaches in this area.
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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)