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GEOLOGY

GENERALITY

On uncomplicated sites with layer-cake geology, a reliable geological site model can be produced from a few sparse boreholes. With increasing geological complexity, a greater density of boreholes is required to properly characterize the site, resulting in increased environmental impacts. Geophysical methods have thus become attractive for site characterization projects because they can fill in the gaps between boreholes without disturbing the ground and at a much lower cost than for an expanded geological drilling program. Moreover, geophysical surveys can be conducted prior to a drilling program, with the goal of optimizing locations for subsequent boreholes.

APPROACH

Seismic methods are most commonly utilized for geotechnical site assessments because seismic velocity measurements can be used to infer soil and rock strength. Seismic refraction and multichannel analysis of surface wave (MASW) techniques are complementary and can provide independent measurements of compressional (P-wave) and shear (S-wave) velocities using the same survey design. For both methods, data collection is achieved using a linear array of geophones and a seismic source. While the seismic refraction method involves measuring the travel-times of the first-arriving P-waves, the MASW method is based on analysing the frequency-dispersion characteristics of the Rayleigh waves, which can be generated by the same seismic source  for a detailed explanation of the method.

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