Seismic Refraction

A common approach to non-intrusive imaging of the subsurface is the seismic refraction method.

Applications for this method include the following:

  • General geologic structure
  • Faulting or other geological hazards
  • Road and tunnel construction
  • Mining applications
  • Bedrock mapping and delineation
  • Rock rippability and quality studies
  • Water table depths
  • Other geotechnical and civil engineering applications requiring 2D site characteristics

The near surface geology is sounded and illuminated with seismic waves. Gehrig, Inc. currently generates these waves with a sledge hammer and is measured with 10 Hz geophones connected to 48-channel seismograph. The seismic tomography method allows the imaging of seismic wave velocity distribution in the subsurface based on these first breaks and the recording geometry. The method iteratively improves a subsurface velocity model and models how seismic waves propagate through the model. The inversion computation is complete once the modeled first breaks optimally fit the measured and picked first breaks. Based on the resulting P-wave seismic velocity model, the interpreter then can make an educated guess at subsurface features, such as local thickness of the soil or sediment layer overlying a rock stratum. Seismic body wave velocity (P-wave) is a physical material property. Each material has a typical range of seismic velocities. Soil or loose sediment will typically have a low seismic velocity compared to hard rock, which will have a high seismic velocity.

In the provided example, loose sediments are impact overlying a shallow, but undulating, limestone bedrock. Undulation of the limestone unit is likely attributed to spatially variable weathering characteristics of the bedrock unit possibly related to localized faulting.