tech brief

The Spectral Analysis of Surface Waves (SASW) method is applied to soil sites to measure the in-place shear wave velocity profile of soil and rock without requiring a borehole. In general, the method uses the dispersive characteristics of surface waves to determine the variation of the shear wave velocity (stiffness) of layered systems with depth. Once the shear wave velocity profiles are determined, shear and Young’s moduli of the materials can be estimated through the use of simple mathematical equations. The shear wave velocity profiles (shear wave velocity versus depth) are determined from the experimental dispersion curves (surface wave velocity versus wavelength) obtained from SASW measurements through a process called forward modeling or through an inversion process. The SASW method can be performed on any material provided there is an accessible surface for receiver attachments and source impacting.

The SASW method has been applied to soil sites with measurement depths exceeding 150 feet in depth. It should be noted that the method requires an accessible area on the surface with a length equal to or greater than the measurement depth required. The method can applied on both bare ground as well as paved surfaces.

NOTE: SASW measurements are accurate to within 5% for the determination of the thickness and stiffness of the top layer in a pavement system. Correlation between SASW and Crosshole Seismic [CS] tests on soil sites showed that the values from both tests typically compare within a 10% difference.

Applications of SASW, but are not limited to the following:

  • Determination of pavement system profiles including the surface layer, base and subgrade materials
  • Determination of soil velocity profiles needed for earthquake and dynamic loading analysis
  • Determination of abutment depths of bridges
  • Condition assessments of concrete liners in tunnels, and other structural concrete conditions

Material Tested: Concrete, Asphalt, Soil, Rock, Masonry, and Wood