Ground Penetrating Radar is a very versatile geophysical method, reaching from rebars to deep ice investigations. However, like any geophysical method, it is useful to understand the limitations. For GPR the conductivity of the ground/construction is critical. If the conductivity is too high, the electromagnetic waves are efficiently subdued, energy gets lost and no information is gained from depth.
The conductivity can be a problem in areas with clayey, and silty soils, salt or brackish water (both ground water and lake/sea/river), in areas with dissolved ions (different types of ground contamination), in newly lain concrete (due to dissolved ions), newly salt sprinkled roads.
Most often a short test measurement or knowledge of the electrical conductivity can tell if GPR is a way forward or if another investigations technique should be considered.
As a rule of thumb, GPR is good for any geological medium when the resistivity is above 100 Ohm-m (conductivity below 10 mS/m). GPR will, in most cases, be ineffective in a geological medium with a resistivity lower than 50 Ohm-m (above 20 mS/m).
Note! GPR waves do not penetrate metallic objects but sometimes can give a typical signature, so called “ringing”, which can be used to separate metallic objects from non-metallic objects.