used to find/map heavy metal (gold, silver, nickel, chrome, copper, etc) strikes along geologic contacts or boundaries or ore masses. This can be an initial step in mineral prospecting before drilling cores to further establish feasibility of mining.
They are like the metal detectors used by hobbyists in finding old coins or gold nuggets or whatever.
One can use them to map heavy mineralization.
For example, geological contacts may form along the edges of fault lines marked by serpentine strikes. The contacts across the landscape may not be apparent at the surface but there may be native copper, nickle, etc. where the serpentine on fault lines meets the meta-sedimentary rock. One can map the heavy mineralized areas with these tools.
I would imagine this is similar to what another poster mentioned about utility people apparently dowsing to find metal pipes before digging.
I do not know enough about this particular as to make a conclusion as to whether a fraud.