Drifting FADs are not tethered to the bottom and can be man made, or natural objects such as logs.

​

Moored FADs occupy a fixed location and attach to the sea bottom using a weight such as a concrete block. A rope made of floating synthetics such as polypropylene attaches to the mooring and in turn attaches to a buoy. The buoy can float at the surface (lasting 3–4 years) or lie subsurface to avoid detection and surface hazards such as weather and ship traffic.

 

Subsurface FADs last longer (5–6 years) due to less wear and tear, but can be harder to locate. In some cases the upper section of rope is made from heavier-than-water metal chain so that if the buoy detaches from the rope, the rope sinks and thereby avoids damage to passing ships who no longer use the buoy to avoid getting tangled in the rope.

​

Smart FADs include sonar and GPS capabilities so that the operator can remotely contact it via satellite to determine the population under the FAD.

​

Source: Wikipedia