Abstract

Traveltime seismic tomography used to aid subway tunnel drilling in Barcelona, Spain, provides a detailed characterization of the shallow subsurface, including a complex network of faults and dikes. We encountered many difficulties while working in this urban setting, such as cultural noise and the inability to deploy instruments along much of the tunnel trace because of the street layout. Furthermore, the shallow subsurface is highly heterogeneous (because of gas and water lines, building foundations, and the sewage system), obscuring deeper structures. We acquired seismic data using a 10-s vibroseis sweep that provided relatively good signal-to-noise (S/N) ratio. First-arrival traveltime tomography proved to be a key tool to differentiate rock vol-umes characterized by different physical properties — in particular, the contact between the weathered upper layer and the more competent rock below. We interpreted the seismic results using surface geologic observations and borehole cores. Low seismic velocities (6001200m/s) characterize the shallow part of the subsurface, representing quaternary sediments and weathered granite. Several high-velocity anomalies (up to 5500m/s) are interpreted as porphyritic dikes and are surrounded by low-velocity anomalies that correspond with fault systems that cut and displace the dikes. The tomographic velocity models also provide a detailed image of an important Miocene fault that intersects the tunnel trace. These results provide insights important for the drilling process.

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