Maybe they're measuring changes in earth etheric field?
http://www.biofieldsciences.com/Human-energy-field.htm
Buried object could cause local changes in it.
At least we can separate field of a living stuff (plants, rocks,..) from "non-living" stuff (coins) using life energy meter?
Some instruments for measuring:
http://www.orgonelab.org/cgi-bin/sho...e=ylemeter.htm
http://www.orgonelab.org/lemeter.htm
http://www.ambro.hu/borze/egely/index2.htm
Perhaps using a biological sensor such as petrovoltaic rock could be very good for that:
http://www.soteria.com/petro/
Or scalar field:
http://jnaudin.free.fr/html/sclxmtr.htm
Or time shift anomalies for at least ores:
http://www.ctglabs.com/tsd1.htm
http://www.intalek.com/Index/Project.../SmartSPOT.htm