Multi-Channel Analysis of Surface Wave (MASW)
Black Geotechnical in conjunction with Terra Australis Geophysica Pty Ltd have developed this technology for use in the Australian construction industry.
Multi-Channel Analysis of Surface Waves is a non-destructive and non-invasive method of geotechnical investigation. Its applications are varied and to date the typical applications have been:
- Depth of bedrock and rippability;
- Providing stiffness parameters (Young's Modulus) for settlement calculation;
- Defining extent of weak compressible clay;
- Defining buried channels;
- Measuring the stiffness of High Energy Impact Compacted fill to provide geotechnical certification;
The application of MASW is not limited to the above as Black Geotechnical continues to develop new applications such as the detection of sinkholes and investigation of existing road pavements.
Generally the MASW can provide a more complete global view of the underground soil conditions than is possible with traditional testing methods such as drilling.
The Multi-Channel Analysis of Surface Wave (MASW) test can provide a 2-D stiffness profile over whatever investigation length and depth is desired. For the test, an impact source causes ground waves to disperse through the area of interest. The geophones, placed at known locations, record the vertical movement vs time of the ground perpendicular to the impact source and a shot record is produced.
This shot record is then processed using a Fast Fourier Transform after which, dispersion curves can be picked representing the fundamental Rayleigh waves. The dispersion curves are then used in an inversion analysis that provides 1D stiffness profiles. Multiple 1D stiffness profiles can be used to produce a 2D stiffness profile of the survey line (as shown below). The stiffness is typically represented as shear wave velocity but with the use of an assumed Poisson's Ratio and Bulk Density the Shear Modulus and Young's Modulus can be calculated. The MASW differs from more conventional seismic methods such as refraction and reflection in that it locates weak layers beneath strong layers.
