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- Volume 1, Issue 2, 2003
Near Surface Geophysics - Volume 1, Issue 2, 2003
Volume 1, Issue 2, 2003
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Assessing reliability of 2D resistivity imaging in mountain permafrost studies using the depth of investigation index method
Authors L. Marescot, M.H. Loke, D. Chapellier, R. Delaloye, C. Lambiel and E. ReynardABSTRACT2D electrical resistivity tomography has been applied within a mountain permafrost environment to assist in ice location. In the context of climate change, a warming process could partially thaw this permafrost and thereby increase the risk of slope instabilities. The extent and location of permafrost are therefore of considerable interest to civil engineers.
The most challenging aspect of resistivity surveys on mountain permafrost concerns the surface layer, which mainly consists of large blocks with air voids. To overcome the very poor electrical contact, long steel stakes and sponges soaked in salt water are used as electrodes. Nevertheless, only a weak current can be injected. Another challenging aspect is the high resistivity contrast between frozen and unfrozen material, which makes inversion and interpretation difficult and problematic. In order to assess whether features at depth, indicated by the data, are real or are artefacts of the inversion process, a special inversion algorithm was applied to process depth of investigation (DOI) index maps. This method carries out two inversions of the same data set using different values of the reference resistivity. The two inversions give the same resistivity values in areas where the data contain information about the resistivity of the subsurface. On the other hand, the final result depends on the reference resistivity in areas where the data do not constrain the model.
As can be deduced from field data from the Swiss Alps and the Jura Mountains, this methodology prevents over‐interpretations or misinterpretations of inversion results in mountain permafrost studies. From the DOI calculations, it is evident that little reliable information on the bedrock under the massive ice can be obtained and that the resistivity within the high resistivity zones cannot be determined accurately. The DOI map also helps to explain the occurrence of erratic and non‐geological structures at depth and indicates to what depth an inverted resistivity profile can provide results.
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Quantitative interpretation of self‐potential anomalies in hydrogeological exploration of volcanic areas: a new approach
Authors Gui‐Bin Zhang and Maurice AubertABSTRACTHydrogeological exploration of volcanic areas has shown that there is a correlation between the range of a negative self‐potential (SP) anomaly and the thickness of the unsaturated zone measured in boreholes. Based on this correlation, it is assumed that the SP anomaly is created in an unsaturated medium through which meteoric water moves downwards, and thus an SP anomaly in a polarized medium has been derived analytically. This SP formula can be used in an inverse technique in the wavenumber domain, in which inversion of the SP anomaly is used to calculate the location and depth of ground water. Theoretical models and field examples demonstrate that this method can be used to explore ground water in volcanic areas.
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Geophysical Study of an Landslide in Northern Sicily
Authors P. Cosentino, R. Martorana, M. Perniciaro and L.M. TerranovaABSTRACTThe San Fratello area in the Nebrodi Mountains (northern Sicily) is a region of high instability. It has suffered many devastating occurrences, which have hit and destroyed the village of San Fratello. At present the area is still subject to a high landslide risk.
A series of geophysical surveys have been carried out with the aim of determining the thickness and dimensions of the landslide body as well as some tectonic features, in order to gain insight into the evolution of the landslide. The following geophysical techniques have been used: borehole seismic tomography, i.e. down‐hole tomographic seismic soundings (DH TSS), time‐domain electromagnetic (TDEM) soundings and georadar profiling (GPR). An additional objective was to compare the efficiency of the geophysical methods used in real geological, morphological and logistic situations.
The data obtained using seismic tomography outlined the shape and the thickness of the body of the landslide in the scar and track area, as well as some mechanical characteristics of the formations involved in the movement. TDEM soundings provided a little additional information on the upper 5–10 m. Georadar profiles supplied some information about the locations of some lateral lithological changes.
Finally the information obtained with the various geophysical techniques was used to provide an outline of the history of the area.
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MCS data processing for the interpretation of ancient ice sheet movements
By E. StucchiABSTRACTThe paper deals with the analysis and reprocessing of data from a 30‐km section of the IT88A‐01 seismic line acquired in the Ross Sea Basin during the Italian 1988 PNRA project (National Program for Antarctic Research). The purpose of this study was to achieve an improved data quality for a better geophysical and geological interpretation of the area. Field data were subjected to a processing sequence aimed at enhancing the S/N ratio and at recovering reliable acoustic impedances. Acoustic impedance recovery calls for the estimation of both the velocity field and the pseudo‐acoustic impedance in the wavelet frequency band. At the end of the study, a time‐migrated stack section and a time impedance map were produced. The migrated section shows, quite clearly, an event previously hidden by a strong sea‐bottom multiple; furthermore reflections that can be related to proximal debris left by ice expansion and regression are now visible. The impedance map shows impedance inversion along the whole length of the processed section, possibly denoting different sedimentary conditions due to ice‐sheet loading and unloading in the Ross Sea.
The processing sequence described and the recovery of the absolute acoustic impedance as outlined here could be adopted to determine, by means of seismic reflection, the position and movement of the ice sheet in the remote past.
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Time‐frequency representation of strong‐motion records for damage appraisal: examples from the Düzce earthquake (Turkey)
Authors Ahmet T. Basokur, Unal Dikmen and O. Engin TokgozABSTRACTThe Düzce (Turkey) earthquake, which struck on 12th November 1999, caused serious damage and loss of life in the town of Düzce, 8 km from the Düzce fault. However, the strong‐motion records indicate that the peak acceleration values were almost twice as high in the town of Bolu, 34 km from the epicentre, compared with those in Düzce. In the earthquake‐affected region, the peak acceleration values are not well correlated with the damage to buildings. For this reason, the use of Fourier spectra leads to some confusion when comparing the quality of the civil engineering work carried out in Düzce and in Bolu. Fourier spectra provide a complete description of amplitudes versus frequency, but they do not contain information about the dynamic characteristics of waveforms. This difficulty may be overcome by considering the time‐duration effect of an earthquake, which is the main cause of damage to buildings. The Stockwell transform of acceleration records offers a means of representing the variation of spectral components with time. This makes possible an unambiguous visualization of the time‐duration effect of an earthquake and consequently an easy correlation of strong‐motion records with damage to structures.
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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)