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 (MDW) allow to determine the parameter Vs30, these are based on determining the travel times of the surface waves (Type Rayleigh), 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.
Passive methods are based on the measurement of environmental vibrations, which are known as microtremors (generated by human activity) and microseisms (generated by natural sources), so they do not require an impulse generating source, one of these is the method ReMi (Refraction of Microtremors) where shear wave velocity profiles are calculated using natural ground vibrations (Figure 1).
The ReMi technique is a linear passive method that uses ambient noise and surface waves to generate shear wave velocity profiles from depths up to 100 m, thus not requiring an active pulse generating source. This, like the MASW method, takes advantage of the dispersive nature of Rayleigh waves, which allows the recording of different frequencies in the wave train traveling at different speeds. However, this method has a limitation due to the interaction between the geometry of the line array and the direction of wave propagation (figure 2a and 2b), which can generate measurement errors that are summarized in reading errors and underestimation of speeds of transmission. waves.
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.
Figure 1: Plot of velocity versus frequency and dispersion curve for linear passive method (ReMi).
Figure 2: a) Plane and perpendicular wavefront propagation scheme for linear geophone array (favorable case). b) Plane and oblique wave front propagation scheme for linear geophone array (unfavorable case) (image taken from the application manual of geophysical techniques based on surface waves for determination of the Vs30 parameter, Construction Institute, 2015)