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PASSIVE SEISMIC METHODS

H/V SPECTRAL RATIO TECHNIQUE

Depending on the natural or anthropic origin (i.e. low frequency microseisms or higher frequency microtremor), characteristics of noise is different in both spectral and temporal domains. For this kind of investigations, a portable short period station is required for noise acquisition.

Among the empirical methods, H/V spectral ratio on ambient vibrations is probably one of the most common approaches for local site effect estimation. The method is also called “Nogoshi-Nakamura technique”. The results of this method are clear and simple in case of horizontally layered structures with large impedance contrasts (> 4-5) and become more fuzzy a) for decreasing contrasts and b) for increasing underground interface slopes.

Typical Targets

  • Estimation of ground fundamental frequency,
  • Quantitative information of soil depth/velocity

Benefits of H/V spectral ratio

  • Cost effectiveness,
  • Fast data acquisition and processing.

ARRAY MICROTREMOR TECHNIQUE

The array microtremor technique typically uses 4 or more geophones/seismometers arranged in a two-dimensional array. The most common arrays are linear, triangular, circular, semi-circular and L-shaped arrays.

The method utilizes dispersive property of surface waves collected by passive recording of background surface waves (noises) such as the vibrations generated by passing vehicles, airplanes or trains, as well as added noise created by initiating impacts at the ground surface. The surface waves are recorded using a seismic system consisting of geophones, cables and a seismograph.

Shear wave velocity profiles are constructed by analyzing surface wave phase velocities and frequencies, and performing inversion modeling.

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