Attenuation

The loss of signal strength as it travels through the ground. Attenuation is the main limit on ground penetrating radar: in high-conductivity soils the radar pulse weakens so quickly that usable depth can fall to a foot or two.

Wet clay is the classic problem soil, along with saline ground, saturated fill, and heavily reinforced concrete. All of them absorb radar energy rather than reflecting it back, so the useful depth of a GPR survey is a property of the site rather than of the equipment. Electromagnetic locating is affected differently — its signal weakens with distance along the line and is degraded by coupling onto neighbouring metal rather than by soil conductivity. A locating company that has worked your area can usually say in advance what radar depth to expect.