Electrical Conductivity, Metal, Water; Sea Water is usually one of the highest ratings in radar attenuation, it has ions and conducts electricity, and absorbs energy. If you think of a microwave oven water boils and metal "sparkles" but safe for most dry, plastic and ceramic type, and non-metallic containers. If you think of a Faraday Cage, it's a metallic enclosure that is grounded, when microwaves hit the metal the energy is absorbed and sent to or conducted to the ground.
If you think about ground penetrating radar it is the moisture of the ground, clays, and the ferric content, ferric sand and rocks, slate with iron for example. If you think about the industry, it's about the thickness and density of concrete that contains metal, and the use of lead, a very dense metal, to shield from x-rays and radar. .. see the RADAR MICROWAVE TECHNOLOGY RESEARCH page