The Late Eocene and Late Miocene fronts of the Atlas Belt in eastern Maghreb: integration in the geodynamic evolution of the Mediterranean Domain

Regional cross sections at the scale of the eastern Maghreb based on subsurface and field data allow presenting the structural styles related to the Middle-Late Eocene compressional events. The structural cross sections depict that the Late Eocene front of the Atlas Belt extends far through the Northern Africa plate margin comparatively to the Late Miocene front cropping out in the Eastern Tunisian Atlas. The sections allow proposing a new subsurface front for the Atlas belt encompassing a large part of the Pelagian-Sirt basins. The consequences of this particular configuration are discussed at the scale of the south Tethys margin and replaced in the frame of the geodynamic evolution of the Mediterranean Domain.     

Automatic change detection of buildings in urban environment from very high spatial resolution images using existing geodatabase and prior knowledge

The updating of geodatabases (GDB) in urban environments is a difficult and expensive task. It may be facilitated by an automatic change detection method. Several methods have been developed for medium and low spatial resolution images. This study proposes a new method for change detection of buildings in urban environments from very high spatial resolution images (VHSR) and using existing digital cartographic data. The proposed methodology is composed of several stages. The existing knowledge on the buildings and the other urban objects are first modelled and saved in a knowledge base. Some change detection rules are defined at this stage. Then, the image is segmented. The parameters of segmentation are computed thanks to the integration between the image and the geodatabase. Thereafter, the segmented image is analyzed using the knowledge base to localize the segments where the change of building is likely to occur. The change detection rules are then applied on these segments to identify the segments that represent the changes of buildings. These changes represent the updates of buildings to be added to the geodatabase. The data used in this research concern the city of Sherbrooke (Quebec, Canada) and the city of Rabat (Morocco). For Sherbrooke, we used an Ikonos image acquired in October 2006 and a GDB at the scale of 1:20,000. For Rabat, a QuickBird image acquired in August 2006 has been used with a GDB at the scale of 1:10,000. The rate of good detection is 90%. The proposed method presents some limitations on the detection of the exact contours of the buildings. It could be improved by including a shape post-analysis of detected buildings. The proposed method could be integrated into a cartographic update process or as a method for the quality assessment of a geodatabase. It could be also be used to identify illegal building work or to monitor urban growth.     

Techniques for mode separation in Rayleigh wave testing

The spectral analysis of surface wave (SASW) developed in the early eighties has opened the way to the use of surface waves for the definition of shear wave velocity profiles in soil deposits or pavement structures without the need of any borings or intrusion. The SASW testing procedure was designed to minimize the contribution of higher modes and thus assumes that the Rayleigh waves which propagate at the surface belong only to the fundamental mode. Several studies have however demonstrated that, in some conditions, higher Rayleigh modes can contribute significantly to the dispersion curve. Different tests configurations exist today to deal with Rayleigh mode problem by the use of an array of receivers. In spite of that, the SASW configuration remains attractive due to the limited number of receivers, as well as, the Rayleigh modes contributing in SASW records configuration can be identified by multiple-filter technique and isolated using time-variable filters. The proposed techniques are first validated by simulated records and then applied to SASW records obtained in the field. The study confirms that higher modes can participate and even dominate in SASW records. An important contribution of higher Rayleigh modes can also exist, even if the shear wave velocity increases regularly with depth. The higher Rayleigh modes can significantly affect the accuracy of the shear wave velocity profile if they are not properly identified and separated. A multi-mode inversion process is shown to be important to have an accurate soil characterization.     

Basin structure and water quality of the Oligocene aquifer in the Sebkhet El Behira basin (central Tunisia)

ABSTRACT

The Sebkhet El Behira–Garaat El Majdoul multilayer aquifer system is composed of Mio-Plio-Quaternary and Oligocene groundwater exploited in Sidi Bouzid and Kairouan governorates. Annual withdrawal volumes from El Behira Oligocene groundwater were about 0.37 hm³ in 2005 and 0.36 hm³ in 2008. The present study of the Oligocene reservoir in the Sebkhet El Behira–Garaat El Majdoul basin is based on various data including 2D seismic sections, petroleum wells, field geological cross-sections and geochemical analysis. The gathered data allowed us to establish the tectonic framework and to define its influence on the structure of the aquifers seated in the deep Oligocene sandstone reservoirs. Three Oligocene sub-basin aquifers are defined showing different depth, thickness and petrophysical characteristics: lower salinity (<1.5 g/L) and higher porosity (30%). Two field sections in the northern and in the southern parts of Cherahil anticline exhibit the presence of four to five sandstone levels. The anticlines limit the Sebkhet El Behira–Garaat El Majdoul basin. In addition, the morphostructural configuration controls the piezometry of underground flows in the Oligocene and Mio-Plio-Quaternary unconfined aquifers.

Editor D. Koutsoyiannis; Associate editor M. Besbes     

DIRDOP: a directivity approach to determining the seismic rupture velocity vector

Directivity effects are a characteristic of seismic source finiteness and are a consequence of the rupture spread in preferential directions. These effects are manifested through seismic spectral deviations as a function of the observation location. The directivity by Doppler effect method permits estimation of the directions and rupture velocities, beginning from the duration of common pulses, which are identified in waveforms or relative source time functions. The general model of directivity that supports the method presented here is a Doppler analysis based on a kinematic source model of rupture (Haskell, Bull Seismol Soc Am 54:1811–1841, 1964) and a structural medium with spherical symmetry. To evaluate its performance, we subjected the method to a series of tests with synthetic data obtained from ten typical seismic ruptures. The experimental conditions studied correspond with scenarios of simple and complex, unilaterally and bilaterally extended ruptures with different mechanisms and datasets with different levels of azimuthal coverage. The obtained results generally agree with the expected values. We also present four real case studies, applying the method to the following earthquakes: Arequipa, Peru (M _w = 8.4, June 23, 2001); Denali, AK, USA (M _w = 7.8; November 3, 2002); Zemmouri–Boumerdes, Algeria (M _w = 6.8, May 21, 2003); and Sumatra, Indonesia (M _w = 9.3, December 26, 2004). The results obtained from the dataset of the four earthquakes agreed, in general, with the values presented by other authors using different methods and data.