HR: 14:35h
AN: NS53A-03    [Abstracts]
TI: Ssismic Methodologies Applied To The Characterization Of Fractured Rock Massifs: Case Studies
AU: * Marti, D
EM: marti@geol.queensu.ca
AF: David Marti, Dept. Geological Sciences and Geological Engineering, Miller Hall, Kingston, ONT K7L 3N6, Canada
AU: Carbonell, R
EM: rcarbo@ija.csic.es
AF: Ramon Carbonell, Dept. Geofisica i Tectonica, Institute of Earth Sciences "Jaume Almera" - CSIC, Barcelona, 08028, Spain
AU: Flecha, I
EM: iflecha@ija.csic.es
AF: Isaac Flecha, Dept. Geofisica i Tectonica, Institute of Earth Sciences "Jaume Almera" - CSIC, Barcelona, 08028, Spain
AU: Palomeras, I
EM: ipalomeras@ija.csic.es
class='hr'> AU: Font-Capo, J
EM: jordi.font-capo@upc.edu
AU: Vazquez-Sune, E
EM: enric.vazquez-sune@upc.edu
AU: Perez-Estaun, A
EM: andrs@ija.csic.es
AB: The detailed characterization of fractured media in the shallow subsurface is becoming important. The detailed knowledge of the fracture network is mandatory in any hydrogeological model to constrain the potential pathways for water circulation. The geophysical methodolgies can provide a detailed image of the fractured rock and also the 3D distribution of physical properties. Two case studies are discussed in this work. The characterization of fractures in a waste disposal site and, the design and construction of a subway tunnel. In the first case, a multiseismic experiment was carried out in an old abandoned uranium mine. 2D and 3D seismic experiments including VSP, surface seismic reflection and travel time seismic tomography provided a 3D image of the internal structure of a granitic massif for hydrogeological studies of the preferred paths for the migration of contaminants. The tectonic stability of the site was also addressed by means of seismic measurements. The joint interpretation of all the available data enabled the interpretation of the low velocity anomalies in the 3D seismic tomography image as the fragile fractures and the alteration associated to them. A 3D image of the geometry of the heterogeneous weathered surface layer was also obtained. This surface is controlled by the complex network of faults and dykes observed in the area. The second case study involves 2D and 3D seismic experiments to aid the horizontal drilling of tunnels for a new subway line in Barcelona (Spain). Seismic data acquisition in a densely populated city is very difficult. The street layout determines the geometry of the acquisition experiments. The instrumentation can not always be located on the surface projection of the tunnel trace, therefore, pseudo 3D acquisition is required, deploying the instrumentation were it is possible. Furthermore, the shallow subsurface features extremely heterogeneous "weathered" layer of variable thickness (building fundations, sewage system, water supply conductions, etc), and the background noise is very high (car traffic, electricity lines, etc). Relatively old cities also lack a detailed geological control of the subsurface. Travel time seismic tomography provided a detailed characterization of the shallow subsurface including the 2D and 3D distribution of the physical properties and the reconstruction of the network of fractures (faults and dykes). The velocity models also imaged an important Miocene fault, quite different to the previous geological section derived from the core interpretation.
DE: 5104 Fracture and flow
DE: 7270 Tomography (6982, 8180)
SC: Near-Surface Geophysics [NS]
MN: 2007 Joint Assembly