COMPARISON OF SHEAR-WAVE VELOCITY-DEPTH PROFILES FROM DOWNHOLE AND SURFACE SEISMIC EXPERIMENTS
We conducted shallow seismic survey at 10 strong-motion stations in Turkey and estimated shear-wave velocities down to a depth of 30 m using the method of multichannel analysis of surface waves (MASW).At each station site, we also drilled a borehole with casing down to 30-m depth and determined a downhole shear-wave velocity-depth profile. We then compared the shear-wave velocity-depth profiles estimated by the MASW and determined by the downhole seismic survey. We also had the opportunity to compare both sets of profiles with the estimates from the spectral analysis of surface waves (SASW)previously reported. An integrated data acquisition system that includes an accelerated impact source with a 50-kg weight, a 48-channel receiver cable with 2-m geophone interval and 4.5-Hz vertical geophones, and two 24-channel, 24-bit Geode recording units was used to acquire the shallow seismic data. We acquired two 48-channel seismic records at each of the station sites using the common-spread recording geometry. By applying Rayleigh-wave inversion to the surface waves isolated from the shot records, we estimated an S-wave velocity-depth profile for the site. For the downhole seismic survey, we used a threecomponent 14-Hz borehole geophone and a shear-wave impact source at the surface, and recorded data at 1-m depth intervals. We find that there exists a 10-15% difference between the velocities determined by the surface-wave and downhole seismic methods --- consistent with the previously reported results. The surface-wave method yields a spatially averaged velocity-depth profile along the line traverse coincident with the geophone spread, whereas the downhole seismic method yields a velocity-depth profile that is pertinent to the borehole location, only. As such, the downhole measurements can be adversely affected by the local borehole conditions. We believe that the spatially averaged velocities estimated by the surface-wave methods may be more desirable for site-specific characterization to determine geotechnical earthquake engineering parameters.
Rayleigh-wave inversion MASW SASW borehole seismic
O.Yilmaz M.Eser Abdullah Sandikkaya S.Akkar S.Bakir T.Yilmaz
Anatolian Geophysical,Istanbul,Turkey Department of Civil Engineering,Middle East Technical University,Ankara,Turkey
国际会议
14th World Conference on Earthquake Engineering(第十四届国际地震工程会议)
北京
英文
2008-10-12（万方平台首次上网日期，不代表论文的发表时间）