TY - JOUR A1 - Endrun, Brigitte A1 - Ohrnberger, Matthias A1 - Savvaidis, Alexandros T1 - On the repeatability and consistency of three-component ambient vibration array measurements N2 - Ambient vibration measurements with small, temporary arrays that produce estimates of surface wave dispersion have become increasingly popular as a low-cost, non-invasive tool for site characterisation. An important requirement for these measurements to be meaningful, however, is the temporal consistency and repeatability of the resulting dispersion and spatial autocorrelation curve estimates. Data acquired within several European research projects (NERIES task JRA4, SESAME, and other multinational experiments) offer the chance to investigate the variability of the derived data products. The dataset analysed here consists of repeated array measurements, with several years of time elapsed between them. The measurements were conducted by different groups in different seasons, using different instrumentations and array layouts, at six sites in Greece and Italy. Ambient vibration amplitude spectra and locations of dominant sources vary between the two measurements at each location. Still, analysis indicates that this does not influence the derived dispersion information, which is stable in time and neither influenced by the instrumentation nor the analyst. The frequency range over which the dispersion curves and spatial autocorrelation curves can be reliably estimated depends on the array dimensions (minimum and maximum aperture) used in the specific deployment, though, and may accordingly vary between the repeated experiments. The relative contribution of Rayleigh and Love waves to the wavefield can likewise change between repeated measurements. The observed relative contribution of Rayleigh waves is generally at or below 50%, with especially low values for the rural sites. Besides, the visibility of higher modes depends on the noise wavefield conditions. The similarity of the dispersion and autocorrelation curves measured at each site indicates that the curves are stable, mainly determined by the sub-surface structure, and can thus be used to derive velocity information with depth. Differences between velocity models for the same site derived from independently determined dispersion and autocorrelation curves-as observed in other studies-are consequently not adequately explained by uncertainties in the measurement part. Y1 - 2010 UR - http://www.springerlink.com/content/111183 U6 - https://doi.org/10.1007/s10518-009-9159-9 SN - 1570-761X ER - TY - JOUR A1 - Di Giulio, Giuseppe A1 - Savvaidis, Alexandros A1 - Ohrnberger, Matthias A1 - Wathelet, Marc A1 - Cornou, Cecile A1 - Knapmeyer-Endrun, Brigitte A1 - Renalier, Florence A1 - Theodoulidis, Nikos A1 - Bard, Pierre-Yves T1 - Exploring the model space and ranking a best class of models in surface-wave dispersion inversion application at European strong-motion sites JF - Geophysics N2 - The inversion of surface-wave dispersion curve to derive shear-wave velocity profile is a very delicate process dealing with a nonunique problem, which is strongly dependent on the model space parameterization. When independent and reliable information is not available, the selection of most representative models within the ensemble produced. by the inversion is often difficult. We implemented a strategy in the inversion of dispersion curves able to investigate the influence of the parameterization of the model space and to select a "best" class of models. We analyzed surface-wave dispersion curves measured at 14 European strong..-motion sites within the NERIES EC-Project. We focused on the inversion task exploring the model space by means of four distinct pararneterization classes composed of layers progressively added over a half-space. The classes differ in the definition of the shear-wave velocity profile; we considered models with uniform velocity as well as models with increasing velocity with depth. At each site and for each model parameterization, we performed an extensive surface-wave inversion (200,100 models for five seeds) using the conditional neighborhood algorithm. We addressed the model evaluation following the corrected Akaike's information criterion (AlCc) that combines the concept of misfit to the number of degrees of freedom of the system. The misfit was computed as least-squares estimation between theoretical and observed dispersion curve. The model complexity was accounted in a penalty term by AlCc. By applying such inversion strategy on 14 strong-motion sites, we found that the best parameterization of the model space is mostly three to four layers over a half-space: where the shear-wave velocity of the uppermost layers can follow uniform or power-law dependence with depth. The shear-wave velocity profiles derived by inversion agree with shear-wave velocity profiles provided by borehole surveys at approximately 80% of the sites. Y1 - 2012 U6 - https://doi.org/10.1190/GEO2011-0116.1 SN - 0016-8033 VL - 77 IS - 3 SP - B147 EP - B166 PB - Society of Exploration Geophysicists CY - Tulsa ER - TY - JOUR A1 - Hobiger, M. A1 - Cornou, C. A1 - Wathelet, M. A1 - Di Giulio, G. A1 - Knapmeyer-Endrun, B. A1 - Renalier, F. A1 - Bard, Pierre-Yves A1 - Savvaidis, Alexandros A1 - Hailemikael, S. A1 - Le Bihan, N. A1 - Ohrnberger, Matthias A1 - Theodoulidis, N. T1 - Ground structure imaging by inversions of Rayleigh wave ellipticity sensitivity analysis and application to European strong-motion sites JF - Geophysical journal international N2 - The knowledge of the local soil structure is important for the assessment of seismic hazards. A widespread, but time-consuming technique to retrieve the parameters of the local underground is the drilling of boreholes. Another way to obtain the shear wave velocity profile at a given location is the inversion of surface wave dispersion curves. To ensure a good resolution for both superficial and deeper layers, the used dispersion curves need to cover a wide frequency range. This wide frequency range can be obtained using several arrays of seismic sensors or a single array comprising a large number of sensors. Consequently, these measurements are time-consuming. A simpler alternative is provided by the use of the ellipticity of Rayleigh waves. The frequency dependence of the ellipticity is tightly linked to the shear wave velocity profile. Furthermore, it can be measured using a single seismic sensor. As soil structures obtained by scaling of a given model exhibit the same ellipticity curve, any inversion of the ellipticity curve alone will be ambiguous. Therefore, additional measurements which fix the absolute value of the shear wave velocity profile at some points have to be included in the inversion process. Small-scale spatial autocorrelation measurements or MASW measurements can provide the needed data. Using a theoretical soil structure, we show which parts of the ellipticity curve have to be included in the inversion process to get a reliable result and which parts can be omitted. Furthermore, the use of autocorrelation or high-frequency dispersion curves will be highlighted. The resulting guidelines for inversions including ellipticity data are then applied to real data measurements collected at 14 different sites during the European NERIES project. It is found that the results are in good agreement with dispersion curve measurements. Furthermore, the method can help in identifying the mode of Rayleigh waves in dispersion curve measurements. KW - Inverse theory KW - Surface waves and free oscillations KW - Site effects KW - Computational seismology KW - Wave propagation Y1 - 2013 U6 - https://doi.org/10.1093/gji/ggs005 SN - 0956-540X VL - 192 IS - 1 SP - 207 EP - 229 PB - Oxford Univ. Press CY - Oxford ER - TY - JOUR A1 - Hannemann, Katrin A1 - Papazachos, Costas A1 - Ohrnberger, Matthias A1 - Savvaidis, Alexandros A1 - Anthymidis, Marios A1 - Lontsi, Agostiny Marrios T1 - Three-dimensional shallow structure from high-frequency ambient noise tomography: New results for the Mygdonia basin-Euroseistest area, northern Greece JF - Journal of geophysical research : Solid earth N2 - We examine the use of ambient noise cross-correlation tomography for shallow site characterization using a modified two-step approach. Initially, we extract Rayleigh wave traveltimes from correlation traces of vertical component seismic recordings from a local network installed in Mygdonia basin, northern Greece. The obtained Rayleigh wave traveltimes show significant spatial variability, as well as distance and frequency dependence due to the 3-D structure of the area, dispersion, and anelastic attenuation effects. The traveltime data sets are inverted through a surface wave tomography approach to determine group velocity maps for each frequency. The proposed tomographic inversion involves the use of approximate Fresnel volumes and interfrequency smoothing constraints to stabilize the results. In the last step, we determine a final 3-D velocity model using a node-based Monte Carlo 1-D dispersion curve inversion. The reliability of the final 3-D velocity model is examined by spatial and depth resolution analysis, as well as by inversion for different model parameterizations. The obtained results are in very good agreement with previous findings from seismic and other geophysical methods. The new 3-D VS model provides additional structural constraints for the shallow sediments and bedrock structure of the northern Mygdonia basin up to the depth of similar to 200-250 m. Present work results suggest that the migration of ambient tomography techniques from large scales (tens or hundreds of km) to local scales (few hundred meters) is possible but cannot be used as a black box technique for 3-D modeling and detailed geotechnical site characterization. Y1 - 2014 U6 - https://doi.org/10.1002/2013JB010914 SN - 2169-9313 SN - 2169-9356 VL - 119 IS - 6 SP - 4979 EP - 4999 PB - American Geophysical Union CY - Washington ER -