Methods of blind backfilling to prevent surface subsidence over abanoned mines have been demonstrated in both flooded and dry workings in recent years. These systems, developed from methods to combat mine fires, involve the injection of slurries containing crushed mine refuse, fly ash, or other material available in large quantities nearby, which are not desired on the surface. A major drawback of all current backfilling methods is the difficulty of monitoring the location and behavior of the injected material underground. The expense of drilling many boreholes to permit physical confirmation of slurry distribution essentially reduces the amount of surface that can be protected, and the drilling itself has negative impacts on surface activities and installations. The objective of this program has been to develop conceptual systems for blind backfill monitoring that will require fewer boreholes and give better definition than is currently possible. Proof-of-concept tests on two recommended systems were conducted in the field.