Why it works
Soil, rock and ice all transmit some radio energy. Whenever there's a boundary between materials of different dielectric constant (a pipe in soil, a void in concrete, water under ice), some energy reflects. Map those reflections vs. depth and you get a vertical slice.
Frequency vs. depth
High frequency (1–3 GHz) gives centimetre resolution but penetrates only a metre or two. Low frequency (50–100 MHz) reaches tens of metres but resolves only large features. Glacier-sounding radars run as low as 5 MHz.
Archaeology and forensics
GPR is now standard for surveying before any major dig. Stonehenge, Vesuvius, the Cahokia mounds — all imaged before a shovel touches the ground. Forensic teams use GPR to find unmarked graves; it's faster and less invasive than digging.
Construction and infrastructure
Every drilled hole in a city sidewalk gets a GPR sweep first to find power, gas and fibre. Bridge inspectors use GPR to map rebar corrosion. The same physics, very different stakes.