Geoscience Reference
In-Depth Information
their corresponding geological age. The first system was formed in the initial (older)
Promygdonian basin during the Quaternary and consists of four successive formations
such as conglomerates, sandstones, silt/sand sediments, and red clay beds. The second
onewasformedinthelatter(younger)MygdonianbasinduringtheNeogeneandconsists
mainlyofmixturesof sands,silts,andclays.Bedrock depth insidethebasin ranges from
about 200 to 420m. Bedrock is deeper in the western part of the basin rather than the
eastern part
. At the northern and the east-southern boundaries of the
basin, a rather fractured and weathered outcrop rock appears at the free surface giving
the opportunity to use it as reference in site response studies. Water table ranges from
2 to3m (at thecentre) down to18to20m(atthe edges).
(
∼
200-300m
)
Geotechnical and geophysical conditions.
Among the principal goals of Euroseistest
project was the accurate assessment of thickness, stiffness, and attenuation of the soil
materials (Pitilakis et al., 1999; Kudo et al., 2002). All in-situ (drillings, sampling, SPT
and CPT) and laboratory (cyclic triaxial and resonant column tests, and other classi-
cal laboratory tests) geotechnical surveys together with geophysical (seismic prospect-
ing, PS logging, electrical soundings and tomographies, aeromagnetic and gravitometric
measurements) surveys were used. The entire area of the basin was covered to derive
both dynamic properties and geometry of the main geologic formations, in terms of site
response requirements. For this reason, an important campaign of field measurements
was carried out mainly in twostages.
In the first stage, the geophysical campaign concerned the NS cross-section and was
mainly conducted in the period 1993-1997 using seismic methods such as surface
wave inversion (SWI), P and SH refraction (REF), crosshole and downhole (CH, DH)
tests, in order to get both geometry and properties of a large volume of soil forma-
tions. Few electrical and magnetic soundings were also performed. In addition to the
seismic prospecting, a detailed geotechnical survey was performed including drillings,
sampling, ground water table measurements, SPT and CPT, and cyclic triaxial and
resonant column (RC) tests. Additional boreholes were drilled for the installation of
downhole3Daccelerometers,onepore-pressuresensor,andforspecialexperimentsusing
underground explosions.
In the second stage, during the period 2001-2004, additional CPT tests were performed
atmorethan20sitesalongtheEWdirectiontogetherwithlongscalerefractionmeasure-
ments. These experiments covered a distance of about 20km using forward and reverse
long seismic profiles. Underground big explosions inside boreholes were used as seis-
mic sources. The receivers were 3D accelerographs and seismographs installed at inter-
distancesbetween60and130m.Atthebordersofthebasin,alargenumberofelectrical
tomographies of about 1.0km each and soundings were also performed in order to get
information about the shape of the edges. To get a systematic and complete image of the
characteristics of the basin's structure, a large number (more than 30) of well distributed
ambientnoisemeasurements(SPACmethod)werealsoperformedinordertogetvertical
shear-wavevelocityVsprofiles(downtothebedrocktopsurface).Forallsoilformations,
