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Services: Shear Wave Profiling

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    Several methods can be used to measure the variation in shear wave velocity with depth at a site. The method chosen depends primarily on how the shear wave profile will be used, but also on the depth of investigation, dimensions of the area of investigation, logistical limitations, project budget and sources of seismic noise in the investigation area. Common reasons for shear wave profiling are stratigraphic delineation in unconsolidated sediments, depth to bedrock where the groundwater table is shallow, and for geotechnical purposes such as liquefaction assessment and calculation of the average shear wave velocity to 30 meters (100 feet) below ground surface (Vs30) for earthquake design ground motion determinations. Spectrum commonly utilizes the following methods for shear wave profiling:

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  • Shear Wave Seismic Refraction
  • ReMi Method
  • Downhole Seismic  
  • METHODS IN ACTION


    Shear Wave Profiling – LOS ANGELES COUNTY, CA





    • The refraction microtremor (ReMi) technique, along with SASW and MASW methods, has significant advantages over traditional downhole and crosshole methods for shear wave velocity profiling. A ReMi investigation was conducted in Los Angeles County, California to provide a shear wave velocity profile for assessment of earthquake site response for geotechnical building design (in accordance with CBC 2007 and IBC 2006). Lithology at the Property consisted of fill and alluvium overlying sandstone bedrock.

    • Spectrum established one 345-foot linear array of 24 vertical geophones using a Seistronix RAS-24 signal enhancement seismograph, 4.5-Hz vertical geophones for low frequency readings and 10-Hz vertical geophones for high frequency readings. Ambient and active-source noise records were recorded: ambient noise was generated by air and vehicle traffic and active-source noise was generated by driving along the line, running along the line, or swinging a 20-lb sledgehammer. In addition, p-wave refraction data were collected to provide a constraint to the ReMi interpretation. Noise records were processed using the SeisOpt® ReMi™ software. First, spectral analysis of the noise records identifies the fundamental mode dispersion curve of the Rayleigh wave. This curve is then picked and a one-dimensional shear wave velocity model to 100 feet is generated by the user that would give rise to the selected dispersion curve.



    • Shear wave velocities at the site were found to range between 550 feet per second and 3500 feet per second. Depth to bedrock was found to be approximately 75 feet below ground surface, as confirmed by the p-wave refraction data
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