The seismic wave
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.