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 (MOS) allow to determine the parameter Vs30, these are based on determining the travel times of the 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.
On the other hand, 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 a source of impulse generation, two of These methods are called SPAC (Spatial Autocorrelations) and F-K 2D (Frequency-Number of Waves) where shear wave velocity profiles are calculated using natural ground vibrations.
In areas of high environmental noise, it is recommended to complement the measurement with a symmetric two-dimensional array (figure 1), since these reduce the errors generated by the direction of propagation of the seismic waves. For the two-dimensional arrays, the F-k 2D and SPAC method is used, which consists of measuring microtremors using an array of sensors, this allows the determination of the dispersive characteristics of a site without considering the directions of propagation of the waves, obtaining the profile shear wave velocity (Vs). Circular arrays also allow low frequency waves to be recorded, with which profiles of the shear wave velocity (Vs) can be obtained at great depths. Furthermore, it is possible to detect relatively soft strata underlying the stiffer strata, which is not possible with other conventional geophysical exploration methods.
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: Example of circular and linear two-dimensional array of geophones.
(Image extracted from the manual for the application of geophysical techniques based on surface waves for the determination of the Vs30 parameter, Construction Institute, 2015).