Instrumentation for geognostic investigation
The MASW (Multichannel Analysis of Surface Waves) technique has the objective to identify variation profiles with the depth of the speeds of volume waves (Vp and Vs). The method is based on the known relations between these speeds and the dispersion of surface (or Rayleigh) waves observed when propagating through a stratified elastic medium. The analysis can be based on signals produced with a borehole on site by acquisition device (with a ram or explosion), or on the recording of the vibrations produced by far away sources (rives, industrial activities, traffic, etc).
In the first case, we are talking about active MASW, with which it is possible to explore a few tens of metres of sub-soil, and in the second case, we are talking about passive MASW, that allows to reach greater depths, in particular conditions.
Passive MA.S.W. is used with the purpose to obtain a speed profile 1D of the elastic waves of cut S. The technique consists in the recording of the “seismic noise” in temporal windows and following study of the signal processed. It is carried out by arranging a bidimensional geophonic chain with low resonant frequency in line or in “array” (circular and irregular geometries) and measuring the environmental noise. From the F-K analysis (frequency-space) of the wave-trains, it is possible to obtain a dispersion curve of surface waves that leads to the calculation of the speed profile of the shear waves and estimate of a coverage in relation to the semispace.