PRINCIPLES

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.

RESULTS

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:

• Poisson’s ratio
•  Youngs Modulus
•  Bulk Modulus