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Seismic Method

06 November, 2024

Purpose: Seismic surveys are essential for mapping subsurface structures, especially depth to bedrock and soil layering. Seismic refraction and MASW (Multichannel Analysis of Surface Waves) methods are commonly applied to determine material properties like stiffness, shear wave velocity, and compression wave velocity, making them valuable for geotechnical, construction, and environmental projects.

Applications:

Depth to Bedrock and Structural Mapping: Seismic refraction pinpoints bedrock depth and identifies subsurface features, providing key data for construction and land use planning.

Material Properties: MASW surveys offer insights into shear wave velocity and material stiffness, essential for assessing ground stability and load-bearing properties.

Technical Process: Using tools like the Geometrics Geode and Seis-Gun or hammer sources, seismic waves are generated and recorded by geophones or hydrophones, depending on the survey environment. Data analysis, including travel-time graphs and wave velocity calculations, reveals compression and shear wave velocities (Vp and Vs), which help determine subsurface characteristics like Poisson’s Ratio and Shear Modulus.

Key Uses: Seismic surveys provide high-resolution data for bedrock mapping, soil stability assessments, and detailed geological characterization, crucial in both exploration and engineering applications.


 

  • We complete seismic Refraction surveys to determine depth to bedrock and other features.  ClearView Geophysics uses hammer sources or its specially designed Seis-Gun.
  • MASW (Multichannel Analysis of Surface Waves) surveys determine shear wave velocity and ‘stiffness’ results.
  • We use the Geometrics Geode with 4.5 Hz or 14 Hz geophones for land surveys, and DHA-7 hydrophones for borehole and water-borne surveys.
  • Seismic refraction analyses of the same MASW seismic survey data determines 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).  We calculate the Shear Modulus using the MASW deduced shear wave velocity (Vs) and corresponding mass density. Mass density approximates from calculations using the compression wave velocity (Vp).  Vs and Vp values are factors to calculate Poisson’s Ratio.
  • IXRefraX and ParkSEIS software edit, analyze and present the Refraction and MASW geophysical method results respectively.