Imaging the Shallow Subsurface for a New Underground Subway Line in Barcelona
Abstract
A detailed characterization of the shallow subsurface by seismic techniques has been used to aid horizontal drilling of tunnels for a new subway line in Barcelona (Spain). Seismic data acquisition in densely populated cities is very difficult. The street layout determines the geometry of the seismic data acquisition experiments. The instrumentation (source and receivers) 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 foundations, 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. The seismic data was acquired using as source a 8 s long Vibroseis sweep which provided relatively good S/N ratio. The shallow subsurface featured an extremely heterogeneous weathered layer characterized by very low seismic velocities (1000-1300 m/s) of variable thickness that made difficult to interpret the seismic reflection images at target depth. The first-arrival seismic tomography revealed as a key tool to clearly differentiate the different rock volumes characterized by different physical properties, especially the contact between the weathered layer and the more competent rock. Several high velocity anomalies (up to 5500 m/s) were observed at tunnel depth that were correlated with sub vertical porphyric dykes. This were surrounded by low velocity anomalies that correspond to fault systems that cut and displace dykes. The tomographic velocity models also provide a new image of an important Miocene fault, quite different to the previous geological section derived from the core interpretation.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2006
- Bibcode:
- 2006AGUFMNS22A..03M
- Keywords:
-
- 0910 Data processing;
- 0925 Magnetic and electrical methods (5109);
- 0933 Remote sensing;
- 0935 Seismic methods (3025;
- 7294);
- 3025 Marine seismics (0935;
- 7294)