Seismic Techniques
Continuous Surface-Wave System (CSWS)
Detail
The continuous surface-wave method utilises a specific type of seismic wave, known as the Rayleigh wave, in order to determine in situ shear modulus-depth profiles to depths of between 8m to 20m below surface. The velocity of a Rayleigh wave is related to the shear modulus (G) and density of the ground through which it propagates. Unlike crosshole seismic methods, which are routinely used to determine geotechnical parameters such as the shear modulus (and additionally Poisson's ratio), the CSW technique require no boreholes.
Rayleigh waves are constrained to propagate within a zone approximately 1 wavelength in depth, such that increasing the wavelength (decreasing the frequency) of the transmitted energy will result in an increase in the depth of investigation. The wavelength and phase-velocity of the Rayleigh waves generated at a particular frequency are calculated by determining the phase shift between the transmitted and measured signals at each geophone location.
Phase-velocities are measured over a range of frequencies in order to build up a dispersion spectrum for the ground below the spread. This is then inverted to determine a velocity-depth profile and finally a stiffness-depth profile.The system comprises a portable frequency-controlled vibrator and an array of low frequency geophones arranged co-linearly with the source. A laptop computer controls both the vibrator and data acquisition. Rayleigh waves are generated at frequencies of between 5Hz and 100Hz in 0.1-5Hz increments in order to build up a comprehensive stiffness-depth profile.
For further information on the Continuous Surface-Wave System see 'The Continuous Surface-Wave System: A Modern Technique for Site Investigation', Menzies, B. & Matthews, M. (1996); Special Lecture: Indian Geotechnical Conference, Madras, December 11-14th 1996.
