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.
