While these two geophysical techniques sound similar, there are distinct differences between seismic refraction and seismic reflection. A project’s goals, location, and site conditions will typically determine which is (or whether both are) the best option.

Seismic refraction is commonly limited to mapping bedrock depths and rippabilities at depths less than 100 feet, and is generally applicable only where the seismic velocities of layers increase with depth. Where higher-velocity (e.g., stiff clay) layers may overlie lower-velocity material (e.g., sand or gravel), this technique may not detect those deeper layers.

Seismic reflection is generally applicable to depths greater than 80 to 100 feet, and is widely used on land to map a variety of deep features (e.g., stratigraphy, faults). This technique is also well suited to marine applications (e.g., lakes, rivers, oceans) where the inability of water to transmit shear waves makes collection of high-quality reflection data possible, even at very shallow depths that would be impractical or impossible on land.

In situations where both shallow and deep information is needed, both seismic refraction and reflection will be required. Where both techniques could be applied (i.e., the “overlap” zone), seismic reflection generally has better resolution, but is considerably more expensive due to more complex data processing requirements.