How to Measure Shear-Wave Velocity (Vs)
Vs can be measured through several different approaches: surface-wave survey, refraction survey, down-hole survey, cross-hole survey, and laboratory measurement (see
illustrations below). The first two are non-destructive in-situ methods, while the others require preparation of borehole(s) and some additional pre-measurement procedures.
One formidable challenge common to all these methods, except for the surface-wave survey, is dealing with a relatively low signal-to-noise ratio (S/N). A high S/N is the most
crucial factor in all types of wave-based techniques to ensure a high accuracy in results.
On the other hand, the surface-wave method utilizes Rayleigh-type surface waves as signal waves. These are the strongest seismic waves, and until about 15 years ago
were considered to be most troublesome source-generated noise during the conventional seismic surveys for oil exploration. Because the surface wave method works on
these Rayleigh waves as signal, it always achieves the highest S/N. As a consequence, the field operation to acquire data and subsequent data analysis procedures become
extremely simple and effective in comparison to other methods, always ensuring the most reliable results. The MASW method is the most advanced one among all types of
surface-wave approaches historically developed.