The
seismic wave or
energy returned to a
receiver from a seismic source after
reflection according to
Snell's Law at a
discontinuity where there is a contrast in
acoustic impedance.
Basically, this usually means putting a steel plate on the ground, and hitting it with a sledgehammer. The resulting seismic wave is reflected back to a receiver nearby (usually in front of the plate) and the results interpreted. Different rocks reflect seismic waves in different ways: some let them through entirely, others reflect them partially.