Near-Surface Seismic Refraction Surveys
Seismic refraction surveys are most often used to estimate overburden thickness or depth to bedrock. The figure below shows a simplified plot of the survey geometry for a refraction survey over a two-layer earth and the travel-time versus distance plot for the idealized p-wave fields generated by an energy source. Refracted wavefronts are normally much easier to observe in near-surface surveys than reflected waves. Reflected waves from near-surface layers are best observed in the 'optimum window' but are seldom observed with a refraction array. Reflection surveys normally have a closer geophone spacing than that used in refraction.
Figure 1. P-wave fields generated by a refraction survey on a two-layer earth.
Figure 2. This figure shows the field geometry for a near-surface reversed-spread refraction survey.
Figure 3. A photograph of a refraction survey. The geophone separation in the photo is 1.0-m and a 12-gauge 'buffalo gun' was used as the energy source.
Figure 4. A winter seismic survey in progress.
Figure 5. A view of the refracted events can be observed immediately in the field on a 24-channel seismograph display.