ΑΜ radio detector

[taxma1981]

I will tell you a story from a machine that found gold in 1990.
I was told that there was a radio receiver on the ground somewhere ,another man with a small transmitter was walking opposite the receiver at a distance of 100-200 meter,
maybe there are two transmitters I'm not sure....the transmitter moves around the receiver and as the time passes it approaches the receiver,when something is found directly in the receiver and transmitter then the signal is reduced and a red light is on....in this way a box of gold was found, can a technician tell us a few words about this method?
https://ibb.co/SR5hzvh