Seismic prospecting today is essential in any engineering project for the analysis of site effects and the evaluation of seismic risk. In Chile, the measurement of the average speed of the shear wave in the 30 m surface (Vs30) is used for the seismic classification of soils, according to the NCh433, NCh2745, NCh2369, DS No. 61, Road Manual, Vol 3 standards. The methods based on the dispersion of surface waves (MSW) allow determining the Vs30 parameter, these are based on determining the travel times of surface waves (Rayleigh Type), from a seismic source to a series of sensors or geophones. These methods deliver, for each prospected sector, an estimate of the profile of shear wave velocities versus depth.
This type of geophysical techniques allows determining the speed of propagation of the wave in the medium between the sensors, by measuring the time and distance of travel. The speed is controlled by the physical properties of each material and they are correlated to various mechanical properties of each one.
Surface wave (MSW) based methods can be classified into active methods and passive methods. In active methods, surface waves are generated by a controlled dynamic source that generates a disturbance on the surface and are recorded by geophones. An example of these is the MASW (Multichannel Analysis of Surface Waves) technique, which is based on the measurement of surface waves. Rayleigh and its dispersive character when passing through a stratified medium.
Using the MASW method it is possible to determine the Vs30 of different types of soils. This method is mainly based on an active type measurement, where the source that generates the disturbances corresponds to an impulsive source. As its name implies, it consists of a multi-channel arrangement of at least 12 geophones that are arranged linearly, equidistant from each other. The analysis of the shear waves produced in the subsoil allows the obtaining of the surface wave dispersion curve (Rayleigh), and the subsequent obtaining of parameters, by means of a geophysical exploration team and specialized software, which analyze and interpret the results obtained (figure 1a and 1b). This allows to determine the profile of the shear wave velocity, to subsequently carry out the classification of the soil.
In some cases this method may be insufficient for a more accurate analysis at the desired depth. These can occur in situations with a lot of environmental noise or when the soil under study has rigid strata at a shallow depth. For these situations, it is recommended to complement the use of MASW with a passive method such as ReMi.
Some applications of these techniques are:
- Detection of the rocky substrate and its mechanical state, degree of compaction, etc.
- Geotechnically characterize soils.
- Determine modulus of deformability of materials (in conjunction with seismic refraction method with Vp measurement)
- Seismically characterize the soil.
- Determination of parameter Vs30.
- Determination of the depth to the basement.
- Seismic risk assessment.
- Determine thicknesses of sedimentary deposits or anthropogenic soils.
- a) Velocity versus frequency plot and dispersion curve for MASW method. b) Misfit graph for theoretical and ground seismic model.