An example of what archaeologists do NOT use:

As “handy” as a metal detector might be, it’s not really the tool of choice for archaeologists in searching for new sites. Other than finding some coins and old forgotten beer cans, it does not have a whole lot of use in the field of archaeology (at least not that I am aware of).

Rudimentary metal detectors are based on the idea that electric current in wire loops creates magnetic field. When the detector is on top of a metallic object, the magnetic field is affected (basically; sparing you the physics mumbo-jumbo). Thus one could pick out the presence of metal by detecting the change. The same idea is used in turning lanes and freeway on-ramps in the U.S. If you look carefully, you could see some square or circular saw cuts in the pavement, typically about 4 or 5 feet in diameter. They are connected to wires running current (thus creating a magnetic field). When a car goes over, it detects the change of the magnetic field. This is the reason that motorcycles and bicycles do not set off the turning signals sometimes. They just do not have enough metal to cause the detector to go off.

Anyway, there are other types of more sophisticated metal detectors, concentrating on pinpointing hydrogen in the metal as opposed to the magnetic properties. These detectors are more widely used for detecting unexploded mines.

Good metal detectors do their job at finding discarded metal near the surface, but they are poor if you are interested in objects that are deeper in the ground. Most of them are just not sensitive enough to be able to pick up very small signals from metallic objects deeper underground.