Determination of shallow shear wave velocity profiles using ambient by djh75337

VIEWS: 24 PAGES: 1

									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.

								
To top