Inthe Earth, vibrations travel as compressional (primary or P) waves, or as shear (secondary or S) waves Sound is an example of a P-wave, with the motion of particles parallel to the direction of wave propagation..   For S-waves, particle motion is perpendicular to propagation – like a loop of slack flicked down a rope. P-waves are called primary because, in any material, they travel faster than S-waves.  For any material, the seismic velocities V_p and V_s are physical properties that depend upon the density and elastic properties of that material. In terms of common engineering properties, V_p is related to rippability or ease of excavation, while V_s is related to bearing capacity or stiffness (and SPT blow counts or N-value).  The V_s of materials beneath a site dominates the Earth’s local response to dynamic loading (e.g. from a wind tower or turbine) or earthquake vibrations.

The International Building Code (IBC) specifies the weighted average V_s in the top 30 meters or 100 feet (called V₃₀ or V₁₀₀) as one criterion for determining Site Class. On a construction project, a lax site classification can entail serious safety and liability concerns, while an overly conservative site classification can have significant cost consequences. Therefore, accurate pre-design site classification is critical.

Another key difference between P- and S-waves is that S-waves cannot travel in a fluid. That is, any propagation of vibration or seismic waves through a fluid (air, water, etc.) occurs only as P-waves. Therefore, water content affects V_p, but V_s is immune to saturation effects, making S-wave surveys particularly sensitive to highly porous materials or cavities. The ability to see differences in soil stiffness, even at saturation makes, S-wave studies useful for evaluating earthen dams and levees, and for determining liquefaction potential.

There are many geophysical methods for determining 1–dimensional V_s vs. depth (and thus V₁₀₀ ).  Some are necessarily invasive (requiring instruments in one or more boreholes). Others are non-invasive – involving only surface equipment and measurements.  The non-invasive surface methods can be further subdivided into active (i.e. requiring a specific seismic source) or passive (i.e. employing whatever ambient vibrations may be present from traffic, industry, ocean waves, trees swaying in the wind, etc.). The available methods are listed and illustrated on the accompanying page.

Invasive methods usually require boreholes that are cased and grouted to provide good coupling between seismic sources and receivers and the formation, and thus tend to require more effort/cost. Non-invasive methods are easier to deploy than borehole methods, but generally cannot provide the thin-bed detection of invasive methods. However, the borehole methods sample only a very small area – which can be misleading on a large site with lateral variations in V_s.  Luckily, numerous studies have shown generally good agreement between the various methods.

Choice of a seismic shear wave method depends on specific site conditions and requirements.  The following table summarizes the advantages and disadvantages of each, with sketches depicting the physics and procedure.

Advantages Disadvantages Surface Methods sample large areas poor thin bed detection   Refraction simple field equipment and procedures blind to low velocity zones, low signal to noise ratio   Reflection good detection of low velocity zones ineffective at very shallow depths (<50 to 100 feet), low signal to noise ratio   Surface Wave Spectral Analysis simple field equipment and procedures, active source methods good at shallow depths, passive methods good for larger depths, good signal to noise ratio relatively low resolution Borehole Methods good thin bed detection, accurate velocities require specially-constructed boreholes, sample only the immediate vicinity of the borehole   ASTM Crosshole (D4428/D4428M-84) most accurate velocities requires three specially-constructed holes   Sonic Logging best thin bed detection signal frequency is higher than those that affect engineering behaviour, requires specialized logging rig   Suspension Logging signal frequency consistent with engineering behavior requires specialized logging rig   Vertical Seismic Profiling simple field equipment and procedure     Sonic Cone Penetrometer   requires specialized direct push rig