Classification of building sites based on average shear wave velocity to a depth of 30 m (Vs30) is a widely used approach to seismic hazard assessment. The National Building Code of Canada 2015 (NBCC 2015) recommends five seismic site classes (A through E), which can be determined based on the estimated Vs30 value. 

 

Several seismic methods can be applied to estimate average shear wave velocity at a particular building site, of which multichannel analysis of surface waves (MASW) and seismic refraction are the most commonly used. Both are non-intrusive techniques and thus provide a time and cost-effective solution for seismic site class determination. The MASW is employed at sites with thick overburden; at rock sites and sites with shallow overburden, employing the shear wave refraction method ensures obtaining accurate velocity values for bedrock and providing reliable site classification.

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Seismic testing in a borehole using vertical seismic profiling (VSP) technique, also known as downhole seismic testing, is applied when a higher accuracy of measurement or greater investigation depth than surface seismic methods can provide is required. It is also carried out in dense urban areas where site conditions do not allow performing a surface test. This survey requires a borehole drilled typically to a depth of 30 m below the proposed building foundation.

 

Methods:

• MASW

• Vertical seismic profiling (VSP) / Downhole seismic

• Seismic refraction

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