Seismie reflection technique, based on the common midpoint method, commonly gives good results to image long smooth subsurface features. Irregular geological features, and particularly objects with small size compared to the main wavelength and/or the geophone spread, are not or badly imaged on the zero offset section. Nevertheless, both theoretical and field experiments show that, for high resolution seismics, such small objects locally affect the wavefronts. Our goal is to analyze these wavefront-perturbations and to extract informations conceming the disturbing objects. Metric size cavities located at lower depth have been chosen as example of small objects, allowing us to do field acquisitions on a weIl known target. This study is based on two approaches: one consists of modeling (ray tracing and wave equation modeling) in order to determine the possible phases, delay time and/or amplitude anomaly for different situations. The other one is based on the processing of real data acquired on several situations (man-made cavity, karstic cave). We present here our first observations obtained on both real and synthetic data. The first real dataset have been acquired above a weIl known man-made cavity (Belbèze caves, Ariège) digged for limestone exploitation purpose. Good results have also been obtained on a karstic area (Moulis, Ariège) where an unknown cave has been detected by integrated geophysical surveys, including high resolution seismics.
Abstract : The Jiroft area, located in a semi-arid zone along the Halil Rud in southeastern Iran, seems to have been an important cultural center during the 3rd millennium ВС. This paper presents the preliminary results of the first two years (2004 and 2005) of a geoarchae- ological program carried out at the invitation of Prof. Madjidzadeh. The Quaternary geomorphological dynamics in the valley are studied, and we present the preliminary results of a geophysical prospection (electro-magnetism, seismic refraction, radar) carried out in January 2005. Climatic aridity, seismicity and large, sudden floods are the most important risks for human societies in the Jiroft basin. Artesian wells and abundant phreatic water are of primary importance for human settlements even though the water is salty. The reconstruction ofpaleoalluvialfans and fossil riverbeds under several meters ofHolocene deposits, should come first in our attempt to understand the paleogeography of the valley.
A022 High resolution seismic experiment within the low-noise underground laboratory (LSBB) of Rustrel (Provence). Abstract (1) (1) (2) (3) GUY SÉNÉCHAL DOMINIQUE ROUSSET ERWAN GLOAGUEN AND THOMAS LEBOURG (1) FRE 2639 – Imagerie Géophysique Pau – IPRA Université de Pau et des Pays de l'Adour - 64000 Pau – France (2) Dép. Des Génies Civil Géologiques et des Mines – École Polytechnique de Montréal - Montréal C.P.6079 – Canada 1 (3) UMR 6526 - Géosciences Azur – Sophia Antipolis 06560 Valbonne – France Several high resolution seismic experiments have been performed along underground galleries located 200 meters below the surface.
To study the attributes of reflected radar waves, a 3-D ground‐penetrating radar (GPR) data set was recorded, processed with ProMAX 3-D seismic processing software, and analyzed with Stratimagic 3-D seismic interpretation software. The zone studied was an old glacial valley (Vallée d’Ossau, western Pyrenees) where fluvio‐glacial deposits (8–10 m thick) overlay a karstic limestone bedrock. Reflections were picked to 250 ns (9 m depth with velocity υ = 0.07 m/ns). Analysis of the isochron map of the bedrock surface reflector showed north‐south undulations, corresponding to the displacement of the old glacier, together with sharp discontinuities, interpreted as local faults or karstic zones. Amplitude variations along the reflector were interpreted as being from water saturation and the nearby north‐south alignment of springs at the surface. In the fluvio‐glacial deposits, paths of old channels were also imaged using amplitude analysis. The radar‐wave amplitude attenuation was also characterized within the deposits by analyzing the average absolute amplitude for different time windows. The strong amplitude attenuations were interpreted as the result of water‐saturated zones. As a conclusion, we correlated the surface karstic springs with bedrock structures and followed the water path across the glacial sedimentary filling. The recognized features were very consistent with a 2-D dc resistivity profile made through the GPR data block.
Le glacier rocheux des Quatre Termes, situe a 2 300 m dans le secteur de la Mongie (Bigorre), a fait l'objet d'une etude au radar geologique. Cette methode geophysique a pour but de contribuer a la connaissance de la structure interne de la forme en proposant des images de la subsurface. Ces images ont montre deux elements. (1) Le glacier rocheux est compose de differentes unites sedimentologiques chevauchantes qui traduisent une mise en place longue, complexe et polyphasee de la forme. (2) Plusieurs elements dans les profils radar permettent de poser l'hypothese de la presence de noyaux de glace au sein du glacier rocheux. Cette information permettrait de considerer a la baisse la limite inferieure du pergelisol sporadique dans la chaine pyreneenne.
We present a processing sequence that attempts to balance geometrical and amplitude analyses in order to recover the maximum information from deep seismic reflection data. The approach, which is guided by the interpretation of other deep geophysical data sets (magnetotellurics, refraction), is applied to Lithoprobe seismic reflection line 28 across the central and northern Abitibi belt. We show, in particular, how amplitude analyses help to quantify the depth of penetration of seismic energy as well as the crustal reflectivity. Apparent lateral variations of deep structures (e.g., the Moho) can be directly related to the high levels of noise that limit the signal penetration depth. We propose a geological model that satisfies all deep geophysical constraints. In this model, the mid crust south of Casa-Berardi tectonic zone consists of imbricated volcanic–plutonic and sedimentary lithologies, which are probably comparable to the mid-crustal section of the Kapuskasing structural zone, and in this paper are referred to as "the Abitibi plate." The lithologies are characterized by high reflectivity, while north of Casa-Berardi tectonic zone the mid crust is dominantly Opatica plutonic lithologies, of lower reflectivity. In this scenario, supracrustal rocks of the Abitibi belt overlie the Opatica plutonic belt, whereas the Abitibi plate extends beneath the Opatica plutonic belt. The boundary between the Opatica plutonic belt and the Abitibi plate is a northward-dipping décollement extending from mid crust in the south to lower crust in the north. The Casa-Berardi tectonic zone appears to be a crustal boundary affecting upper and middle crust down to 20 km, between northern polycyclic terranes and southern monocyclic ones. The uniformity of the lower crust suggests that its formation was decoupled from that of the intermediate to upper crust.
