Pyramid Geophysical Services conducted a geophysical investigation across a proposed apartment complex property in Charlotte, NC. This survey was performed to determine average shear wave velocities in the upper 100 feet of the subsurface to provide seismic data to the client for the purposes of determining a seismic site classification. The geophysical survey consisted of performing multi-channel analysis of surface waves (MASW) seismic surveys to collect 1-Dimensional shear wave velocity profiles.
MASW is a seismic method utilizing an array of geophones to record surface waves either as a result of an active controlled source, or natural and man-made background sources. The frequency and phase velocity of these waves is used to determine the shear-wave velocities of the underlying earth. MASW is useful in determining depths and thicknesses of soil or rock formations, determining depths of fill, and determining average shear wave velocity for seismic site classification (i.e. Vs30 or Vs100). Shear wave velocity (Vs) is also one of three generally accepted methods for determining soil or rock densities along with cone penetration test (CPT) and standard penetration test (SPT). This makes it unique among geophysical techniques.
Data were collected using twenty-four 4.5Hz geophones and a Geometrics 24 channel Geode seismograph. Shear wave velocities were exported and the data were assembled into a table to calculate depth-averaged velocities. This technique allows for non-invasive engineering testing to be performed at a site, thereby characterizing the subsurface down to depths of 100+ feet rapidly and providing quantitative velocity data for site classification.