Seismic Surveys

Seismic methods are the most commonly conducted geophysical surveys for engineering investigations. In seismic surveying, acoustic waves are propagated through the Earth’s interior and the travel times are measured of waves that return to the surface after refraction or reflection at geological boundaries within the ground. These travel times can be converted into depth values and, hence, the distribution of subsurface interfaces of geological interest may be systematically mapped.

When a source is discharged on the ground surface, two different classification of waves travel through the subsurface. One classification is considered as body waves. They travel as spherical front waves in a uniform media and are affected very little by the free surface boundary. Acoustic velocities in subsurface materials are related to engineering properties of the medium in which they are propagating. Those engineering properties are elastic constants and density of the medium.

The second classification is surface wave which is restricted to the region near the free surface. Surface waves have larger amplitudes, but decay exponentially with distance form the source. Surface waves are the most destructive seismic waves when an earthquake happens due to their large amplitude and longer duration.

The multichannel analysis of surface waves method (MASW) is a nondestructive seismic method and is used for different purposes such as finding depth to top of bedrock, mapping soil layers, and estimating rippability, etc. once the seismic data is processed a shear wave velocity (Vs) of the subsurface can be obtained (Figure below). For more information on MASW method more .

The MASW data can be acquired relatively quickly, because the sensors (geophones) do not need to be coupled to the ground. This method is very useful in areas inaccessible to drill rigs (paved roadways, beneath bridges, on steep slopes, etc.). Depths of investigation can typically reach to 120 feet.

MASW Applications:

  • Mapping variable depth to bedrock
  • Identifying depth to some sub-bedrock interfaces
  • Mapping soil layers
  • Estimating rippability
  • Estimating shear-wave velocities

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