As we all know that the field data is our most trustful data and scientists use it for comparision with other methods. Shear wave velocity VS30 is the main parameter for estimation of seismic site-conditions. We often get differences in estimation of shear velocity using different field methods at the same sites. in comaprision, geotechnical data also leads to another error in our VS30?
In the following reference, a comparison of two field methods SASW (Spectral analysis of surface wave) and measured VS30 in boreholes (DH) are done.
Brown, T. L., Boore M. D., and Stokoe II H. K., (2002), “Comparison of Shear-Wave Slowness Profiles at 10 Strong-Motion Sites from Noninvasive SASW Measurements and Measurements Made in Boreholes”
The comparison shows that the mean error between both field methods reach 40 m/s. The maximum error in estimation of SASW and DH methods is about 80 m/s for SVA site. Both methods show difference in knowing site-class type 3 out of 10.
The main question what to do to improve accuracy in estimation of Vs30 shear velocity?
In my humble opinion the current methods for estimation of Vs are accurate enough for engineering purposes. Using solely Vs30 is not enough for site characterization and should be use with caution, especially if the stratigraphy includes significant impedance differences between layers. To improve the reliability of site characterization we could use a mix of different direct measurements (downhole test, surface profiles, H/V ratios) complemented with empiric relations from CPT, SPT, DMT. Another good thing is having measurements at as many points at a site as possible, but this is usually constrainted by budget. However with careful planning of surveys and maximum use of previously known information some very good (for engineering purposes) results are not hard to obtain.
Dear Stoyan,
Thank you for your reply. Yes, may be using mean value of various field shear velocity methods is much more trustable estimation. I do research on the use of topographic slope as indication of surface shear velocity Vs30, the results was very good and the mean velocity error is about +/-50 m/s as maximum. Once i saw that the mean estimated Vs30 error using field methods (SASW, DH) is 40 m/s, i said it is very good technique to get approximate VS30 without spending money or laboratory or even field work. The raised question here is which field value is more accurate, i found that mixing of different field techniques for estimation of Vs30 is important. Anyway, we still need further uncertainty check and future work.
Dear Mr. Ahmed,
The application of VS30 is indeed dependent on the accuracy of VS measurement. The error in VS30 as you have mentioned may or may not have effect on seismic site classification depending on which system you use. According to IBC (2012), the range of VS30 for site classes SC (360 - 760 m/s) and SD (180 - 360 m/s) is quite large. So it is difficult to say how much uncertainties in VS measurement will have an important effect.
At the same time, the term VS30 and the site characterization system used also has a profound effect on the estimating seismic site conditions. As Mr. Andreev has mentioned, the VS30 is not suitable for site classification if there is strong impedance contrast between layers, especially at shallow depths (when bedrock is shallower than 30 m). For shallow bedrock sites, when we average the VS of layers up to 30 m depth, it is possible to include stiffness of rock. This leads to higher VS30 value and assignment of stiffer site class for which the spectral acceleration will be underestimated. This is a problem because the spectral acceleration, especially at short-periods, will be higher when there is a strong impedance contrast due to bedrock. Hence, suitability of VS30 for a particular region should also be considered while determining seismic site conditions. I would suggest you to refer the following paper if you are interested more on this topic.
http://www.tandfonline.com/doi/full/10.1080/13632469.2016.1277570
Thank you.
Dear Satish,
I know well what do you mean. My major question is whether the field velocity Vs is correct enough or not since we have big error among various field work methods.
For site-class if you notice in my publication i try to use slope angle as indicator for site-class. That is why i focus on the accuracy of estimation Vs using field work.
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