The technique was originally proposed by Nogoshi and Igarashi (1971), and wide-spread by Nakamura (1989). It consists in estimating the ratio between the Fourier amplitude spectra of the horizontal (H) to vertical (V) components of the ambient noise vibrations recorded at one single station.This method relies on the Earth’s own seismic noise, rather than an active seismic source. We can measure vibrations from low frequency noise caused by wind, waves, and anthropogenic activities using hand portable seismometers.
For each acquisition, the top soil is completely removed, It is made a flat surface where the 3D sensor is carefully placed. The horizontal sensors are oriented according to the magnetic north. The verticality is controlled using a special bubble contained on the instrument body. In order to facilitate the cleaning of the signal, the computer and any other source of noise potentially present in the equipment, were placed away from the 3D Gemini.
Apparatus data acquisition system used for the execution of passive seismic survey is constituted by: 3D geophone PASI mod. GEMINI2. The instrument is equipped with three sensors (velocimeters) that simultaneously detect seismic signals from three orthogonal directions: NS; EW and UD (vertical), turning them into electrical impulses. It is also equipped with a leveling system, bubbleand directories direction.
We can then analyse the recorded vibration data in the frequency domain and use the spectral ratio of the horizontal and vertical measured components (H/V) to determine thicknesses of lower shear wave velocity layers overlying faster ones. This is especially useful for determining the cover/regolith thickness, and also for finding palaoechannels. This technique is very inexpensive, fast to acquire and yield good results given by favourable geological terrains.
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