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Microgravity survey in urban environment

The new underground railway M2

A new underground railway M2 is planned to be constructed within the next decade in Lausanne. The geology involved constitutes alpine molassic bedrock, an overlaying quaternary glacial fill and an overburden of loose soil. The choice of the corridor had to consider the exact thickness of these geological units. The free availability of a very large number of mechanical logging data was an opportunity for testing a microgravity survey along the 6 km Metro corridor. During two campaigns, summer 2002 and 2003, we measured over 200 microgravity stations. In such urban environment we had to correct carefully the terrain, the buildings, the caves related and the facility network. The distance from nearby constructions and the resolution of gravity measures determined whether corrections had to be made for these volumes or could be omitted. We calculated, with an in house program, the effect of different buildings to choose architectural styles with similar gravimetric responses. We will then use a GIS to group the buildings by typology. The basements voids were included in the digital elevation model to take account of the complexity of the Lausanne morphology. Along the tunnel axe our gravity interpretation show and excellent correlation between the engineering section, obtained by drillings and the one by microgravimetric interpretation. Thus this geophysical tool permits a good underground auscultation without a big amount of mechanical drills.

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