|In the Multichannel Analysis of Surface Waves (MASW) technique, Surface Waves (Rayleigh Waves) are utilised to determine the elastic properties of the shallow subsurface (<15m).
In conventional refraction seismic surveys, Surface Waves carry up to two/thirds of the seismic energy but are usually considered as noise. The penetration depth of Surface Waves changes with wavelength, i.e. longer wavelengths penetrate deeper. When the elastic properties of near surface materials vary with depth, Surface Waves become dispersive, i.e. propagation velocity changes with frequency. The propagation (or phase) velocity is determined by the average elastic property of the medium within the penetration depth. Therefore the dispersive nature of Surface Waves may be used to investigate changes in elastic properties of the shallow subsurface. The MASW method employs the multi-channel recording and processing techniques which allow better waveform analysis and noise elimination.
Readings are taken using 24 low frequency vertical geophones connected via multi-core cables to a high resolution 24 channel digital seismograph using an energy source such as a hammer and plate or a buffalo gun.
MASW processing through specific software generates a shear wave velocity (Vs) profile. The shear wave velocities are converted into small strain shear modulus values (Gmax) using the formula: Small Strain Shear Modulus = (Vs)2 *density.
P-wave velocities (Vp) collected through conventional refraction seismic surveying can be combined with the shear wave velocity (Vs) data and, using the formulae in Davies & Schulteiss, 1980, calculate the following values: