Multi-channel Analysis of Surface Waves (MASW) is a very useful method for investigating shallow geological structure and the relative shear strength of subsurface materials. MASW is a fast method of evaluating near-surface profile because the entire range of investigation depth is covered by one or a few generation of ground roll without changing receiver configuration.
How It Works?
Surface waves have dispersion property that body-waves lack. This property is that different wavelength has different penetration depth and, therefore, propagates with different velocity. Therefore, by analyzing the dispersion of surface waves, one can obtain near-surface velocity profile. It is the shear (S)-wave velocity (Vs) profile that is obtained by analyzing ground roll.
The entire procedure of the analysis consists of three steps: acquisition of dispersive ground roll data, construction of the dispersion curve (dispersion analysis), and back calculation (inversion) of S-wave velocities from the constructed dispersion curve.
In Multi-Channel Analysis of Surface Waves (MASW), the field configuration is almost the same as seismic refraction survey. However, a few parameters need to put in consideration. Sledge hammer is a good choice as a source, where it is more practical compared to the others. Meanwhile, low-frequency geophones (e.g., 4.5Hz) are always recommended for this survey.
Then, the field geometry parameters also need to put in consideration to yield best results. For examples, it is recommended to have:
Source offset (x1) = 5m; Receiver spacing (dx)=1m; and Receiver spread (D) = 30m.
Then, the data are acquiring in a roll-along mode.
Types of Surface Waves
There are two type of surface waves
1) Rayleigh waves
2) Love waves
Rayleigh waves also known as ground roll.
In Multi-channel Analysis of Surface Waves (MASW), Rayleigh waves (ground roll) are used in the analysis.
Advanced processing software is used to process the data. Basically, after field geometry is encoded, the next step is to process for dispersion information. This is also called as dispersion analysis. Dispersion analysis involved generation of dispersion-image called overtone and extraction of dispersion curves from it. The final step is inversion analysis. Here, either 1-D or 2-D shear velocity (Vs) profile is obtained.