Geophones output data as time traces which are compiled and processed by the seismograph. The basic components of a seismic trace are the direct wave, the reflected wave and the critically refracted wave. Wave refraction occurs at interfaces in the ground where the seismic velocity of the lower layer is greater than the velocity of the overlying layer. This condition normally applies in near surface site investigations where soil or fill overlies bedrock.
At geophone positions close to the seismic source, the first seismic wave arrivals are direct waves. However, beyond a critical distance from the source, the first arrivals change to refracted waves due to the faster relative velocity of the refracted waves. Interpretation procedures involve the accurate measurement of first arrivals from the time traces recorded at each geophone position.
Interpretation techniques are applied to the first arrival times to calculate the seismic velocities of the layers and the depths of individual refracting interfaces. The interfaces are correlated with real physical boundaries in the ground, such as the soil-bedrock interface and other lithological boundaries, to produce a model of the subsurface ground structure. The final interpretation is presented in a format that is easily understood by engineers.