Seismic Refraction Surface Surveys

Seismic refraction is a geophysical technique used to determine the thickness of underlying geologic strata, depth to the water table, soft and dense soils, groundwater and bedrock surfaces. The principle of seismic refraction is founded on the fact that seismic waves travel through different types of earth materials such as dry (unsaturated) soils, wet (saturated) soils and bedrock at different velocities. The denser the material, the faster the waves travel.

These seismic waves can be generated by hitting a metal plate with a hammer, dropping a heavy weight, or by the detonation of buried explosive charges. The velocity of seismic waves is governed by the density and elasticity of subsurface materials. Here, elasticity refers to a material's ability to be distorted by a seismic wave, and then return to its original form after the seismic wave has passed.

Types of waves that can be created include compressional waves, shear waves, and surface waves. The arrival of a seismic wave is detected by geophones, or motion sensitive earth sensors, which are placed firmly in the ground, or on the ground for measuring surface waves in certain situations.

A geophone for surface wave surveys,

outfitted for use on pavement

An array of geophone

set up for surface wave

surveys on pavement.