Here are some Metal Detector Projects that are quite interesting for the purpose of education and experiments in the Art of electronics. This site may be as old as Y2K, but the url / domain may have changed.
Metal Detector Projects
If you are interested in the theory behind the mag I can highly recommend the book The Magnetics Measurement Handbook by J.M. Janicke. Besides a good coverage of magnetics theory, it contains much of what you need to build a useful fluxgate Magnetometer, plus other devices.
Traditional Geophysical instruments include ground-penetrating radar, earth resistivity meter, infrared, and sonar, to name a few. Currently, earth resistivity projects are the most practical for the do-it-yourselfer, although some people have been tinkering with side scan sonar.
Detection of Gold
Just the Jeohunter really works and directly detects Gold - see below screen. And even gold-worth 9-15cm diameter cannon-balls get detected as "Gold" (golden ring & cannonball analyses has identical values), but thats because of the huge metal mass and special shape - alot MDs have "problems" in such situation and perhaps only a magnetometer really works here. btw. there is no difference if you search with Ground Balance minus 200, zero or plus 200, the gold detection always remains the same.
There are historical records of DIY Enthusiasts having created what is known as the "Donkey Kong Gold Hunter". This Gold Detector uses a Nintendo Unit after many Mods.