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- Volume 5, Issue 3, 2007
Near Surface Geophysics - Volume 5, Issue 3, 2007
Volume 5, Issue 3, 2007
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The effect of a dipping layer on the first‐arrival traveltime from zero‐offset and fixed‐offset borehole radar
Authors Dale Rucker and Ty P.A. FerréThe effect of a dipping layer on the first‐arrival traveltime from single‐offset profiling with borehole radar is investigated, including both zero‐offset and fixed‐offset profiling. In our study, offset refers to the vertical distance between bistatic radar antennae. Using forward modelling of electromagnetic wave travel, the traveltime of zero‐offset and fixed‐offset profiles through a dipping layer is compared to those of flat‐lying layers. The model considers three distinct raypaths: direct, critically refracted and cross‐dip refracted. Whereas critical refraction only occurs in a layer of low propagation velocity relative to an adjacent high‐velocity layer, cross‐dip refraction can occur in any velocity structure. The forward model demonstrated that the slope of the traveltime through the cross‐dip portion of the profile is approximately half of that in the critically refracted portion. To obtain the electromagnetic wave propagation velocities above and below the dip, only one profile is necessary. However, to invert for the dip angle and position, two profiles with different offsets must be considered together.
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Retrieving self‐potential anomalies in a complex volcanic environment: an SP/elevation gradient approach
More LessThe problem of differentiating self‐potential (SP) anomalies from different origins on volcanoes is addressed using the SP/elevation gradient. Surveys on volcanoes classically show the presence of a purely hydrogeological zone on the flanks and a hydrothermal zone in the central area. The hydrogeological zone is characterized by negative SP/elevation gradients, ranging typically between 0 and −2 mV/m, whilst gradients vary in a broader range in the hydrothermal zone. Simple models show how these gradients will be perturbed in the presence of perched water tables, lateral variations of physical properties (e.g. resistivity) or by positive anomalies associated with upward hydrothermal circulations of fluids. The SP/elevation gradient is much more sensitive to the perturbations than the SP and, therefore, it is proposed that the gradient is used to enhance the anomalies. Using digital SP and topographic grids, the gradient can be computed in 2D and a regional SP map of the Piton de la Fournaise volcano (La Réunion, Indian Ocean) is used to illustrate the method. Anomalies are easily identified in the active central area, but are difficult to discern in other areas. Conversely, the anomaly zones are clearly shown on the gradient map as well as the different types of terrain (i.e. with different SP/elevation gradient values) in the hydrogeological zone. In the summit area, where the data coverage is dense, a comparison of the gradient information as a function of the SP map resolution is carried out to illustrate the sensitivity of the method to detect small‐scale structures. As a technique, the gradient approach appears to be very promising for the qualitative interpretation of SP surveys in volcanic areas as well as in other environments.
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Improving the processing of vibroseis data for very shallow high‐resolution measurements
More LessVery shallow seismic surveys, i.e. 20–100 m target depth, require a large bandwidth of high frequencies to ensure detailed resolution. A survey with a small hydraulic vibrator across young glacial structures in northern Germany that gave unsatisfactory results was repeated with monitored vibrator motions. This enabled us to use the groundforce signal as correlation operator and to apply sweep deconvolution.
The application of the groundforce signal resulted in a distinct improvement compared to correlation using the reference sweep, whatever the data compression technique used. This is due to harmonics that show high amplitudes at high frequencies and contribute considerably to signal energy. Sweep deconvolution works at least as well as correlation. Due to the minimum‐phase nature of sweep deconvolved data, this technique, if combined with a minimum‐phase deconvolution, gave the best results. The geological target, consisting of thin interstadials embedded in a sequence of sands deposited during the last glaciation, could be imaged much more clearly than before.
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An experiment of non‐invasive characterization of the vadose zone via water injection and cross‐hole time‐lapse geophysical monitoring
Authors Rita Deiana, Giorgio Cassiani, Andreas Kemna, Alberto Villa, Vittorio Bruno and Andrea BaglianiThe characterization of the vadose zone, i.e. the part of the subsurface above the water table, is a challenging task. This zone is difficult to access with direct methods without causing major disturbance to the natural in‐situ conditions. Hence the increasing use of geophysical methods capable of imaging the water presence in the vadose zone, such as ground‐penetrating radar (GPR) and electrical resistivity tomography (ERT). This type of monitoring can be applied both to processes of natural infiltration and to artificial injection (tracer) tests, by collecting multiple data sets through time (time‐lapse mode). We present the results of a water‐injection experiment conducted at a test site in Gorgonzola, east of Milan (Italy). The site is characterized by Quaternary sand and gravel sediments that house an extensive unconfined aquifer, potentially subject to pollution from industrial and agricultural sources. ERT and GPR profiles were acquired in 2D cross‐hole configuration and time‐lapse mode over a period of several days preceding and following the injection of 3.5 m3 of fresh water in a purpose‐excavated trench. A 3D model of the water‐infiltration experiment was calibrated against the time‐lapse cross‐hole data, particularly focusing on the ability of the model to reproduce the vertical motion of the centre of mass of the injected water as imaged by GPR and ERT. This model calibration provided an estimate of the isotropic hydraulic conductivity of the sediments in the range of 5–10 m/d. However, all isotropic models overpredict the measured excess of moisture content, caused by water injection, as imaged by GPR. The calibration of anisotropic models for the vertical hydraulic conductivity, with the horizontal hydraulic conductivity determined by direct measurement, also leads to a good fit of the sinking of the centre of mass, with a better mass balance in comparison with field data. The information derived from the experiment is key to a quantitative assessment of aquifer vulnerability to pollutants infiltrating from the surface.
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3D resistivity imaging from an archaeological site in south‐western Anatolia, Turkey: a case study
Authors M.G. Drahor, G. Göktürkler, M.A. Berge, T.Ö. Kurtulmuş and N. TunaA large‐scale resistivity imaging survey was performed in the acropolis area of Archaic Cnidos, south‐western Turkey. This survey was a part of the geophysical studies conducted between 1999 and 2004. Two‐dimensional resistivity data were acquired along a number of parallel lines using a pole–pole array. The data was processed using a 3D inversion algorithm based on a robust technique. We also applied shaded‐relief processing to enhance the representation of the images of apparent‐resistivity data and inversion results. In addition, the inverted resistivity data were visualized by a volumetric representation technique to display both the horizontal and the vertical extents of the archaeological structures. The inversion results revealed that a rectangular gridding pattern and a dense structuring existed in the depth range 0.35–1.5 m in the acropolis. Moreover, the bedrock was the base of the archaeological structures in the area. Based on the resistivity survey, four test excavations were carried out in various localities in the acropolis in 2004. These excavations yielded results supporting those obtained by the resistivity inversion. This indicated that large‐scale 3D resistivity imaging can be a useful tool in archaeological prospection.
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Array GPR investigation of the cathedral of Uppsala
Authors J. Gustafsson and M. AlkarpWe present an example of a so‐called array GPR investigation outside the Cathedral of Uppsala, Sweden. The aim of the investigation was to reveal historically interesting features in surroundings where excavations are not allowed. In the investigation, 17 different GPR antennae of the same frequency were used to obtain measurements in 16 parallel profiles simultaneously. When several separate transmitter and receiver antennae are combined into one single antenna array unit, exactly positioned parallel profiles are obtained, resulting in a seamless high‐resolution 3D picture of the subsurface. Processing the radar data into the resulting images involves several steps, such as aligning traces, removing static shifts and matching the mean response. Radar data are merged with geometry data from a total station (used to track the position of the antenna array) and then gridded and migrated. In the Uppsala case presented here, the resulting pictures in the form of time slices gave archaeologists very valuable help in understanding the subsurface and mapping historical anomalies. The findings indicate former paths and early medieval streets, among other features.
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Using seismic traveltime tomography in geoarchaeological exploration: an application at the site of Chatby cemeteries in Alexandria, Egypt
Authors L. Polymenakos and St. P. PapamarinopoulosWe apply seismic traveltime tomography in an investigation of the subsurface of a geoarchaeological site at Chatby cemeteries (Alexandria, Egypt). We are interested in obtaining information about the geological structure and the remains of man‐made structures. Energy sources are placed at the surface and recorders are distributed vertically in an underground location. Traveltime data are inverted with a three‐dimensional tomographic algorithm. Inversion results are evaluated and interpreted with the aid of independent geophysical and archaeological information, and various tests are carried out with different initial velocity models and synthetic traveltimes.
The inversion results show good correlation with the estimated geological structure, in particular at deeper levels, as suggested by independent geophysical and archaeological information and later confirmed by excavation. The depth and variations of the bedrock surface are satisfactorily imaged. Indications of possible man‐made structures are presented and discussed. Shallow velocity features are evaluated and interpreted using tests with synthetic traveltimes. The main goal of imaging the deeper structure was successful.
As inversion tests with different initial velocity models indicate, the subsurface structure and the complexity of data information in this particular case are better handled when using a single‐layer model than when using an average two‐layer model. The single‐layer model allows for a more versatile velocity variation and raypath distribution, while the two‐layer model leads to an inversion result that is considerably influenced by the initial velocity distribution. Enhancement of particular aspects of the single‐layer inversion result can be derived with the use of an initial velocity model, that permits a gradual increase of velocity with depth. The gradient model improves the raypath distribution and limits velocity artefacts.
The resulting images can assist in the assessment of the archaeological potential of a site, particularly in excavation design, in a non‐destructive manner. They can be also of great value in other applications, for example in engineering works, when detailed information on the subsurface is sought in areas of restricted access.
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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)