Determination of shallow shear wave velocity profiles using ambient vibrations at selected sites in Greece geotechnical site description: DCFIT SRMAXDCFIT Silty Clay till 7 m Silty Sand and Sandy Clay till 54 m Volvi Graben Clayey Silt and Marly Clay till 80 m Silty Clay with sand and gravels till 125 m F. Scherbaum1), M. Ohrnberger1), A. Savvaidis2), A. Panou 2,3), N. Theodulidis2), G. Richter1) Sandy Clay or Silty Clay with gravesl till 185 m Euroseis Test Site Weathered Schist till 196 m Schist till 200 m Geotechnical bedrock at 125 m, geological bedrock 1) Institute of Geosciences, University of Potsdam, POB 601553, Potsdam, D14415 Germany at 195 m. 2) Institute of Engineering Seismology and Earthquake Engineering (ITSAK), POB 53, Thessaloniki, GR55102 Greece 3) Geophysical Laboratory, Aristotle University, Thessaloniki, Thessaloniki, GR54124 Greece OTE geotechnical site description: DCFIT SRMAXDCFIT Introduction Artificial debits till 6 m Clay Sand and gravels till 16 m Pebbles(Conglomerates) of green schist (basement in the city) with sand till 19 m geotechnical site description: Sandy clay with limestone till 22 m Sandy clay with green graevls till 30 m AGO The use of ambient vibrations for the determination of subsurface shear wave Bedrock AT 85 m. Artificial debits till 9 m Sandy Clay with gravels and marl till 26 m velocity profiles is increasingly gaining popularity as a low cost alternative to Marly clay till 28 m Sandy Clay with gravels till 30 m elaborate geotechnical site investigations. It has recently been demonstrated for the Bedrock at 35 m. DCFIT SRMAXDCFIT Lower Rhine Embayment in Germany that robust constraints on both the shear wave velocity profile and the depth to the first impedance jump can be obtained if single station H/V spectral ratios are jointly inverted together with dispersion curves obtained from array analysis (Scherbaum et al., 2002). In order to further explore the general applicability of this approach, we have performed array measurements of ambient vibrations in the Volvi graben in northern Greece (Euroseis test site), at six different locations within the city of Thessaloniki, and on the island of Lefkas. At all these locations, the subsurface structures are well known and shear wave velocity profiles have been determined by independent geophysical and geotechnical surveys (Raptakis et al., 1998, Chavez- Garcia et al, 1998; Anastasiadis et al., 2001; Dimitriou et al. 2001). Figure Legend Here we present first results of the comparison of site models obtained from ambient vibrations with the existing structural models. The uppermost two panels in each plot show the statistics of the PRO geotechnical site description: maxima of the fk-spectra obtained from the dispersion analysis Reference "rock site". (top: azimuth, below: slowness). Displayed below are the DCFIT average spectral energies in the Z (R), N(G), and E(B) component DCFIT SRMAXDCFIT Volvi graben of all stations for each array. The two panels following below geotechnical site description: Volvi graben show the H/V spectral ratios for the individual array stations as LEF LEP Clayey Mud from 0 till 12 m Marl from 12 till 50 m geotechnical site description: well as the overall average (lower panel). Superimposed are the Bedrock at 195 m. Thessaloniki theoretical ellipticities based on the geotechnical models. DCFIT Thessaloniki Brown Clay with gravels till 17 m Marly Clay with gravels till 21 m The panels to the right show the resulting models obtained Sandy Clay till 23 m Lefkas Sandy Clay with gravels and marl till 28 m from the inversion, the corresponding fit of the dispersion Sandy Clay with gravels till 30 m Geotechnical info give bedrock at 100m, curves and the spectral ratio fits. In some cases, SH wave geological rock at 180 m. transmission response functions are displayed as well. Green lines: minimum misfit DC, red lines: minimum misfit ellipticity, blue lines: geotechnical model. DCFIT and SRMAXFIT: 10 best models obtained from a) the inversion of dispersion curves and b) the combination of City of dispersion curves and observed H/V peak frequency, Thessaloniki respectively. Geographical location of Signatures in slowness/frequency plots measurement sites Lefkas and array positions. open black circles: mean values. filled red circles: mean values of best 1% semblance values. References green circles: median values together with upper and lower quartile. black lines: Rayleigh wave dispersion curves for fundamental and Anastasiadis A., Raptakis D., Pitilakis K., Thessaloniki's Detailed first two higher modes based on the geotechnical model. Microzoning: Subsurface Structure as Basis for Site Response Analysis. Pure and Apllied Geophysics, 158, 2597-2633, 2001. light blue lines: constant v/f curves for 3 x aperture (l/3 rule), KAL VIL TIF 2 x mean station distance, and 2 x minimum Chavez-Garcia F.J., Raptakis, D., Makra K., Pitilakis K., The Importance station distance ("aliasing curves") of the Reference Station in modelling site effects up to larger geotechnical site description: frequencies. The case of the EUROSEISTEST. 12th European DCFIT SRMAXDCFIT DCFIT SRMAXDCFIT geotechnical site description: Lutuceous Sand till 7 m Conference on Earthquake Engineering, paper reference 589, 1998. geotechnical site description: Clay till 13 m DCFIT SRMAXDCFIT Silty Sand with gravels till 5 m Lutuceous Sand with a lot of gravels till 21 m Clay from 0 till 6 m Lutuceous Sand till 30 m Organic Mud till 7 m Dimitriou P, Thedoulidis N, Hatzidimitriou P., Anastasiadis A., Conclusions Lutuceous Sand 22 m alternations of Marl and Sand till 17 m Bedrock not reached. Sand with gravels till 24 m Sediment non-linearity and attenuation of seismic waves: a study of Sand and Clay till 19 m and then (Vs profile is linear starting from 250 m/s at the Clay till 31 m, which is the end of the top reaching 375 m/s at 30 m depth) Brown Clay till 30 m accelerograms from Lefkas, western Greece. Soil Dynamics and boreholle. Bedrock not reached. Geotechnical info give bedrock at 150 m, Earthquake Engineering, 21, 63-73, 2001. Geotechnical info give bedrock at 170 m, rock rock at 240 m. at 260 m. Raptakis D., Theodoulidis N., and Pitilakis K., Standard Spectal Ratio - The comparison of shear velocity models obtained from inversion of ambient and horizontal to vertical spectral ratio techniques: Data analysis of vibration recordings and those obtained by independent geotechnical surveys shows the Euroseistest strong motion array in Volvi-Thessaloniki (Greece). Earthquake Spectra, Vol 14 No 1, pp, 203-224, 1998. fits ranging from excellent to fair. In essentially all situations, the model derived from the dispersion curve alone shows a very good agreement with the very shallow Scherbaum, F., Hinzen, K-G., and Ohrnberger, M., Determination of geotechnical velocity model. shallow shear velocity profiles in the Cologne/Germany area using ambient vibrations, Geophys. J. Int., in press, 2002. - For the determination of the depth to the first strong impedance contrast, however, additional constraints (e. g. identifiable peaks in the H/V spectrum) are needed. Acknowledgments - In all cases, even for the "rock site" Profitis (PRO), the inversion of dispersion curves We gratefully acknowledge the technical support and the obtained from ambient vibration recordings seems to provide fair estimates of vs . organisation of the field experiment by Daniel Vollmer. We are 30 indebted to the GFZ Potsdam for letting us use their DGPS equipment. Matthias Ohrnberger was funded by EU- Grant No. EVG1- - The influence of array size and aperture on the "quality" of resulting dispersion curve CT-2000-00026. measurements suggest an adaptive deployment strategy.
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