GEOTECHINCAL ENGINEERING

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, reflection  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. Reliable measurements of P-wave and S-wave velocities can also be obtained by completing downhole seismic surveys within boreholes. While downhole seismic testing of this type has lower vertical resolution than crosshole approaches that measure travel times between holes, the analysis is straightforward, less prone to error, and the field surveys do not require a second borehole to be drilled at significant additional expense. Furthermore, for most infrastructure projects, engineers typically only require an estimate of average soil and/or bedrock velocities. The seismic data can also be used to determine estimates for soil and bedrock elastic modules that would otherwise be derived from lab testing of borehole samples. In particular, Poisson’s ratio can be calculated directly from the P-wave and S-wave velocity measurements, which can then be compared with laboratory derived estimates.