Other seismic methods

ESAC (Extended Spatial Autocorrelation) passive seismic survey allows estimation of the variation of shear wave velocity (Vs) with depth, which is useful for subsurface modeling and seismic classification of soils. It is a passive technique that relies on the analysis of ambient noise recorded by a series of geophones arranged generally along two branches orthogonal to each other or with more geometrically complex configurations. The ESAC survey is particularly used in Seismic Microzonation studies because of its ability to investigate greater depths than other passive methodologies.

The M.A.S.W. method (Multichannel Analysis of Surface Waves) allows, through the acquisition of Rayleigh and Love surface waves recorded by a series of geophones along a straight spread and connected to a common multichannel seismograph, the reconstruction of a vertical velocity profile of Shear Waves (Vs). Surface waves are easily generated by a seismic source such as, for example, a beating bat. Acquisition can be done with vertical energization and the use of vertical geophones (ZVF) or with transverse energization, using horizontal geophones (THF).

Passive seismic survey HVRS is able to detect the characteristic resonant frequencies of soils. It is particularly used in Seismic Microzonation studies and for the design of earthquake-resistant buildings; in addition, the versatility of this type of survey also makes it possible to obtain the resonance frequencies of a building and then compare them with those of the soil. The technique involves making ambient noise recordings using a digital tromometer connected to a seismograph.

This method, whose name is derived from Refraction Microtremor, uses the same array as an M.A.S.W. or refraction seismic, low-frequency geophones, but, instead of recording the seismic perturbation induced by an energization, it records, for a time period of 20 to 30 seconds, microtremors.