In GPR method an electromagnetic pulse produced by a trasmitter antenna is sent into the soil.      Every  time   the  pulse  hits  an  object  having  sufficient   physical  dimensions and characterised by electromagnetic  properties   different  from those of the subsoil, a partial reflection is generated and recorded by the same trasmitter  antenna which is pulled along the profile. So doing a radargram, where the time delays  and  amplitudes  of the returning reflected pulses are recorded, is generated.
Frequency content of the electromagnetic pulse,  nature  of  the   subsoil  (conductivity and dielectric constant)  dimensions  and   depth  of  the  archaeological  structures  govern   the amplitude and  shape  of  the  echo  and  the   way  in  which  the  echo  can  be  seen on the radargram. Broadly speaking good result are obtained on dry soils, hidding structures like cavities, tombs, foundations. GPR is the only geophysical method able to give good results inside the buildings (churches, castles) or when the area to survey is covered by streets or squares.