Decomposed rock mass characterization with crosshole seismic tomography at the Heroísmo station site (Porto)

Abstract

At the Heroísmo station site (of Porto underground) there is a granitic rock mass formation named “Granito do Porto”, where the rock mass presents significant heterogeneity, with highly weathered and fractured rock zones and with frequent and large intercalations of residual (saprolitic) soil from granite. At the request of Transmetro-ACE, LNEC carried out crosshole seismic tomographies on several crosshole sections with about 30m depth in order to zone and to characterize these heterogeneities. The tomographies obtained showed heterogeneous P-wave velocity distributions with frequent velocity inversions in depth according to highly weathered and fractured rock zones detected at the boreholes and even with the core recovery rates, especially for the case of residual soil. The lowest velocity zones occur typically on the surface region, above the water level, where the decomposition, weathering and fracture density are more accentuated and where the rock mass is more uncompressed.