A seismic refraction survey, also called seismic refraction tomography (SRT), images the ground using the speed of seismic waves. It is commonly used to find depth to bedrock and to judge how hard the ground will be to excavate.
How it works
Seismic waves generally travel faster in stiffer material. A source at the surface sends out P-waves, and where a deeper layer is faster, some waves bend along it and return to the surface. By timing these first arrivals along a line of geophones, a velocity section of the ground can be built. In India, IS 15681 covers seismic refraction work.
Field procedure
- Lay out a straight line of geophones at equal spacing.
- Survey the position and level of each geophone.
- Generate energy at several points along the line, commonly with a sledgehammer on a steel plate.
- Record the waves on a multichannel seismograph.
- Pick first arrivals and invert them into a velocity model.
What it is used for
- Depth to bedrock and thickness of overburden
- Rippability assessment for excavation and road cuts
- Weak or fractured zones along tunnels, dams and alignments
- Landslide studies, to help define the slip mass
Limitations
- It works best when velocity increases with depth. A slow layer under a fast one can be hidden.
- The usable depth depends on the line length and energy source.
- Traffic and machinery noise can affect data quality.
SRT, MASW or ERT?
SRT uses P-wave velocity and suits depth to rock. MASW gives shear wave velocity for seismic design. ERT maps resistivity, which helps with water and clay. They are often combined, and boreholes are used to confirm the results.
FAQ
How deep can a seismic refraction survey reach?
As a rough guide, about a quarter to a third of the line length, depending on ground and energy source.
Does it replace boreholes?
No. It covers the ground between boreholes, and boreholes calibrate its interpretation.
What is rippability?
It describes whether rock can be excavated by ripping or needs blasting. Seismic velocity is one input to that decision.
Talk to our team
Need depth to rock across your site? Call +91 98718 04288, email contact@geowingseng.com or see our geophysical survey service.
