Introduction

Multi-Channel-Analysis-of-Surface-Waves (MASW) utilizes the dispersive nature of surface waves to produce a surface wave velocity cross-section of the subsurface. Long wavelengths travels deeper into the subsurface than shorter wavelengths and travels thus at different velocities. By generating short and longer wavelengths, one can thus sample the subsurface to different depths. The velocities of different wavelengths can be determined by calculating the phase difference between two receivers for each wavelength generated. Using a multi-channel system with 24 or more receivers a cross-section of surface wave phase velocities as a function of depth and chainage can be produced.

Surface wave velocities are close related to shear wave velocities by Poisson’s ratio (are slightly less than shear wave velocities). Using inversion methods, the surface wave phase velocities can be inverted to shear wave velocities. MASW can thus be used to map shear wave velocities as a function of depth and chainage, allowing for calculation of the stiffness (maximum shear modulus Gmax) of a site.

Operation

A multi-channel seismograph with an impact source like a sledgehammer is used. Shots are generated with a fairly tight spacing between shots (typically 2 x the geophone separation) to allow for essential coverage of all the depths of interest. For deeper penetration (for which low frequencies are needed) a source like a weight drop can be used. Data is acquired in a profile mode (similar to seismic reflection) and shear wave velocity cross-sections are produced as a final product.

MASW is an extremely attractive method as surface waves are easy to generate. It is therefore suitable in areas of high seismic background noise (such as industrial and built-up areas) which a technique like seismic refraction cannot be used. It is a quantitative method which can be used to determine the maximum shear modulus, Gmax, of a site.

Applications

  • Mapping bedrock topography
  • Mapping of low velocity/density zones
  • Mapping of fracture zones in bedrock
  • Soil and rock shear wave velocity measurements

MASW Overtone analysis displaying phase velocity as a function of frequency
Inverted 1D shear wave velocity profile where velocities are calculated with debth

Innovative Low Frequency Drop Weight Source for MASW Investigation