What Does Seismic Refraction Mean?
Seismic refraction is one of the methods of geophysics used to investigate subsurface ground conditions for trenchless operations. It utilizes seismic waves sourced from the ground surface. A seismic wave is energy transfer by way of particle motion and are of three types namely, compression wave, shear wave and surface wave.
The velocity of seismic waves depends on density elastic moduli. The data obtained by seismic refraction is processed to interpret and produce models of seismic velocity and layer thickness of subsurface ground structure. It can also measure overburden thickness where bedrock is deep.
Trenchlesspedia Explains Seismic Refraction
While conducting seismic refraction survey, low frequency seismic energy is produced by a seismic source on the surface such as a hammer plate, buffalo gun or weight drop - depending on depth of penetration required and prevalent site conditions.
As the seismic waves propagate downward, they are reflected or refracted off layers in the subsurface. 24 or 48 geophones arranged at regular intervals of 1-10 meters detect these refracted seismic waves. The output data from the geophones is compiled and processed by a seismograph. The seismic trace components include direct wave, reflected wave and critically refracted wave. Wave refraction occurs at interfaces where lower layer has greater seismic velocity than overlying layer.