Do you mean Ground Penetrating Radar? This is typically limited to very shallow depths though - eg. archaeological investigations, police forensics (finding graves), and civil engineering site investigations. The latter can go to tens of meters and would be used for planning foundations, excavations, etc (I know of an example where it was used for site characterization for a planned rowing lake). The depth limit is typically due to attenuation in brackish groundwaters. Ice applications can typically go further due to the lack of conducting liquid water.
Or are you thinking of Reflection ('Active') Seismic Surveys, which work on a similar principle but using sound waves. Sound waves can travel to the far side of the Earth (cf. large earthquake 'P' waves), but reflection surveys are typically looking at the upper crust (few km) and rarely go beyond the base of the crust (aka the 'Moho') which is a strong seismic reflector. Reflection seismic surveys are widely used in the oil business.