ST3 3 Channel 24 bit Seismograph with Integrated 3D Sensor 2 Hz Frequency

Methodologies

M.A.S.W. (Multichannel Analysis of Surface Waves

The MASW (Multichannel Analysis of Surface Waves) technique has the objective to identify variation profiles with the depth of the speeds of volume waves (Vp and Vs). The method is based on the known relations between these speeds and the dispersion of surface (or Rayleigh) waves observed when propagating through a stratified elastic medium. The analysis can be based on signals produced with a borehole on site by acquisition device (with a ram or explosion), or on the recording of the vibrations produced by far away sources (rives, industrial activities, traffic, etc).

In the first case, we are talking about active MASW, with which it is possible to explore a few tens of metres of sub-soil, and in the second case, we are talking about passive MASW, that allows to reach greater depths, in particular conditions.

Passive MA.S.W. is used with the purpose to obtain a speed profile 1D of the elastic waves of cut S. The technique consists in the recording of the “seismic noise” in temporal windows and following study of the signal processed. It is carried out by arranging a bidimensional geophonic chain with low resonant frequency in line or in “array” (circular and irregular geometries) and measuring the environmental noise. From the F-K analysis (frequency-space) of the wave-trains, it is possible to obtain a dispersion curve of surface waves that leads to the calculation of the speed profile of the shear waves and estimate of a coverage in relation to the semispace.

Nakamura Method, HVSR, H/V

A significant part of the damages observed in destructive earthquakes all over the world is associated with the amplification of seismic waves due to the effects of the local site. The analysis of the site response is therefore essential in the evaluation of the seismic risk in areas subject to earthquakes. In order to evaluate the effects of the local site, a series of surveys must be carried out. Among the empiric methods, the method of spectrum analyses H/V on environmental vibrations is one of the most common. The method, also called “Nakamura” technique (Nakamura, 1989), was introduced by Nogoshi and Igarashi (1971) based on the initial studies of Kanai and Tanaka (1961). Since then, many researchers worldwide performed a large number of applications.

An important requirement to carry out the H/ V method consists in a fairly good knowledge of seismology combined with basic information on local geological conditions supported by geo-physical and geo-technical data. The method is generally applied in micro-zoning studies and in the analysis of the local response of specific sites.

Technical Specifications

General

• ADC technology: 24bit Delta-Sigma ADC
• Geophone number: triaxial 2Hz gephones
• Acquisition: Passive
• Dimensions: Ø14x10 cm
• Weight: 2 kg
• Export compatibility: .seg2; .csv
• In-line test: Noise analysis
• Case: IP67
• Enviromental condition: -20°C / 80°C
• Power supply: External source
  
  
  
  

Acquisition

• Dynamic range: 144 dB (dynamic); 109 dB (at 1ms sampling)
• Broadband: -3dB at 3500 Hz
• Frequency: 251Hz, 502Hz, 1008Hz
• Gain: from 0 to 42dB
• Anti aliasing: -3dB, 80% Nyquist's frequency, -80dB
• Common mode rejection: 110 dB at 50Hz
• Input signal: ±2.5V