Ground-penetrating Radar (GPR) is a geophysical exploration technique using radar waves reflected back from subsurface discontinuities to depths of a few tens of metres.

This produces an output similar to a seismic reflection seismogram, but resulting from changes in the dielectric constant of subsurface layers rather than their acoustic impedance.

In other words, GPR uses radar waves: the way in which certain rocks reflect the waves back give an indication of what is down there.
GPR is used alongside seismic reflection to produce a slightly different image. The two images are combined to give a more thorough analysis of the underlying structure.