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[Observatoire Gravitaire Géoazur] Subsurface Geophysical Survey
Subsurface Geophysical Survey

- Landslides triggered factors analysed by time lapse electrical survey and multidimensional statistical approach
T. Lebourg, M. Hernandez, S. Zerathe, S. El Bedoui, H. Jomard and B. Fresia

Abstract
A temporal imagery of water circulation in a landslide by Electrical Resistivity Tomography (ERT) was conducted to identify spatially and temporally the coupling between rainfalls, consecutive water inflows in a sliding mass, and induced resistivity variations. This work is based on a multi-scale experimental approach applied on the “Vence” landslide (South-eastern France) which is characterised by a sandy-clay sliding mass on a marly limestone substratum mostly controlled by high rainfall events.

At the landslide scale, historical, geological and geotechnical data combined with field investigations and the interpretation of three ERT allowed the definition of a geometrical model of the landslide and the calibration of the resistivity values. On the basis of these results, a permanent time lapse ERT survey was designed on a specific part of the landslide, coupled with water level acquisition (piezometric levels) and rain fall events. Results covering a three-month period are showed. The statistical analyses of all the physical parameters measured during the three months of investigation show that the matrix of correlations highlighted strong correlations between the rainfall, piezometric elevation and the resistivity. These first results show an accurate answer resistivity/piezometric elevation that can be associated as a precursor of the reactivation of the landslide.

Keywords: Landslide; Electrical resistivity tomography; Time lapse survey; Statistical analysis; Triggering factors Engineering Geology, Volume 114, Issues 3-4, 10 August 2010, Pages 238-250, doi:10.1016/j.enggeo.2010.05.001


- Reflection–refraction seismics in the Gulf of Corinth: hints at deep structure and control of the deep marine basin
C. Clément, M. Sachpazi, P. Charvis, D. Graindorge, M. Laigle, A. Hirn and G. Zafiropoulos

Abstract
The Gulf of Corinth is a natural laboratory for the study of seismicity and crustal deformation during continental extension. Seismic profiling along its axis provides a 24-fold normal-incidence seismic reflection profile and wide-angle reflection–refraction profiles recorded by sea-bottom seismometers (OBS) and land seismometers. At wide-angle incidence, the land receivers document the Moho at 40-km depth under the western end of the Gulf north of Aigion, rising to 32-km depth under the northern coast in the east of the Gulf. Both refraction and normal-incidence reflection sections image the basement under the deep marine basin that has formed by recent extension. The depth to the base of the sedimentary basin beneath the Gulf, constrained by both methods, is no more than 2.7 km, with not, vert, similar1 km of water underlain by no more than not, vert, similar1.7 km of sediment, less than what was expected from past modeling of uplift of the south coast in the East of the Gulf. Unlike the flat sea-bottom, the basement and sedimentary interfaces show topography along this axial line. Several deeps are identified as depocenters, which suggest that this axial line is not a strike line to the basin. It appears instead to be controlled by several faults, oblique to the S60°E overall trend of the south coast of the Gulf, their more easterly strikes being consistent with the instantaneous direction of extension measured by earthquake slip vectors and by GPS.

Keywords: Gulf of Corinth; Aegean region; Seismic refraction; Reflection; Crustal structure; Rift; Basin; Extension Tectonophysics, Volume 391, Issues 1-4, 29 October 2004, Pages 97-108, doi:10.1016/j.tecto.2004.07.010


- Converted seismic wave investigation in the Gulf of Corinth from local earthquakes
D. Latorre, J. Virieux, T. Monfret and H. Lyon-Caen

Abstract
We propose a procedure for the detection of converted phases in seismograms between the P- and S-arrivals. This procedure has been successfully applied to waveforms of different stations from the 1991 passive experiment around the Aigion area (Greece). Three major converted phases have been detected at two stations on the northern Gulf of Corinth. The first phase has been interpreted as a P-to-S transmitted wave occurring at an interface located at a depth of 3 km in the northern part of the Psaromita Peninsula. The two later phases that have a more complex polarisation might come from deeper interfaces.

Keywords: Converted phase analysis; PS-transmitted waves; Gulf of Corinth
C. R. Geoscience, Volume 336, Issues 4-5, March 2004, Pages 259-267, doi:10.1016/j.crte.2003.11.014


- Rapid least-squares inversion of apparent resistivity pseudosections by a quasi-Newton method
M.H. Loke and R.D. Barker

Abstract
A fast inversion technique for the interpretation of data from resistivity tomography surveys has been developed for operation on a microcomputer. This technique is based on the smoothness-constrained least-squares method and it produces a two-dimensional subsurface model from the apparent resistivity pseudosection. In the first iteration, a homogeneous earth model is used as the starting model for which the apparent resistivity partial derivative values can be calculated analytically. For subsequent iterations, a quasi-Newton method is used to estimate the partial derivatives which reduces the computer time and memory space required by about eight and twelve times, respectively, compared to the conventional least-squares method. Tests with a variety of computer models and data from field surveys show that this technique is insensitive to random noise and converges rapidly. This technique takes about one minute to invert a single data set on an 80486DX microcomputer.
Geophysical Prospecting, Volume 44, Issue 1, January 1996, Pages 131-152, doi:10.1111/j.1365-2478.1996.tb00142.x

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