Passive seismic - CEOS - Nakamura HVSR
The indirect CEOS surface wave spectral ratio method can quickly estimate the basement depth. It is also known as Nakamura-HVSR Method (Horizontal Vertical Spectral Ratio or H / V Spectral Ratio). It is a non-intrusive test, based on the collection, at defined time intervals and by means of a triaxial seismometer (N-S, E-W and Z), and subsequent analysis, of environmental micro-vibrations on the ground surface. By relating the horizontal and vertical components or spectra, a composite signal is obtained that is processed and analyzed to obtain the Resonance Frequency or Soil Fundamental Period. This parameter is of vital help to prevent the period of some structures from coinciding with the period of the ground, avoiding seismic amplifications due to resonance effect and, therefore, the consequent damage to the structure. It is used for seismic microzone studies, evaluating site effects. Additionally, from this relationship, through regression, it is possible to estimate the thickness of the sedimentary fill and, therefore, the basement depth. The method consists of recording over a prolonged period (~ 20 min), using a triaxial seismograph, on the earth's surface, the vertical and horizontal components of the environmental vibrations or micro-tremors.