Seismic Refraction

The seismic refraction method is based on the measurement of the travel time of seismic waves refracted at the interfaces between subsurface layers of different velocity.

Seismic energy is provided by a source ('shot') located on the surface. Energy radiates out from the shot point, either travelling directly through the upper layer (direct arrivals), or travelling down to and then laterally along higher velocity layers (refracted arrivals) before returning to the surface. This energy is detected on surface using a linear array of geophones. Observation of the travel-times of the refracted signals provides information on the depth profile of the refractor.

Seismic Refraction surveys are commonly carried out to determine depth to bedrock and other features.  ClearView Geophysics uses hammer sources or its specially designed Seis-Gun.  The gun uses in-house powder packed 12-gauge shells.  A hole is augered into the ground to a safe depth for each shot.

firstarrivals

ClearView also carries out MASW (Multichannel Analysis of Surface Waves) surveys for shear wave velocity or 'stiffness' results.  Seismic readings are carried out in 1D with several off-spread shots or in 2D with moving shot and receiver setups.  Specialized software is used to analyze the dispersion curves to produce depth inversion models of the shear wave velocities.  In short, MASW analyzes the propagation of measured surface waves and then deduces the shear wave velocities (Vs) that is responsible for the propagation pattern of the surface waves.

Seismic refraction analyses of the same MASW seismic survey data can be used to determine compression wave velocities (Vp).  The travel-time/distance from the first-arrival shock source graph has different slopes depending upon the velocity that the shock wave travels through the ground.  Note that refraction will not work if a “slow” layer (e.g., sand) is located below a “fast” layer (e.g., clay).  The Shear Modulus can be calculated using the MASW deduced shear wave velocity (Vs) and corresponding mass density, which in turn can be approximated from the compression wave velocity (Vp).  The Vs and Vp values can then be used to calculate Poisson’s Ratio.

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