Q: How deep will a metal detector/locator go?
A: This is the question most frequently asked. Unfortunately, it has no absolute answers. The following variables, in addition to your own detector's capabilities, all have an effect on the answer.
1. Conductive properties of the soil: Heavily mineralized soil will tend to reduce the penetration power of your detector. Soil mineralization may vary greatly, and you may have to re-ground balance your detector to adjust for soil mineralization; unless, of course, your detector has automatic ground balancing. In this case, you may need to decrease sensitivity and/or increase discrimination on models like the Fisher 1266-X, which has extraordinary sensitivity.
2. The length of time an object is buried: Various chemicals in the soil have a corrosive action on metal. Some metals corrode faster than others. A modern zinc penny is attacked by these soil chemicals quite easily, whereas the action on copper and silver is much less, and corrosive action on gold is hardly noticeable, if at all. As these chemicals eat away at the metal, oxidation (rust) takes place, which is absorbed into the surrounding soil. This causes the soil to become more conductive, which in turn makes the metallic object appear larger than it actually is and easier to detect. This is known as the "halo effect."
3. The size of an object: The larger the metallic object, the easier and deeper it can be detected. For example, a bucket can be detected much easier than a single coin. The more surface area seen from above, the deeper the metallic object will be detected.
4. The shape of an object: Every metallic object reradiate's at least part of the signal transmitted by your metal detector. In this way, objects function like additional antennas, and consequently their shape becomes important. Ring or loop-shaped objects lying flat, on or under the ground, produce the best results; flat or dish-shaped objects are similarly easy to detect. Rod-shaped items, especially when scanned end-on, are very difficult to detect unless they're made of iron and you're using a ferromagnetic detector, such as the Fisher FX-3.
5. The degree of magnetization: With ferromagnetic locators, such as the FX-3, the degree of magnetization has a strong influence on depth. A magnet, for example, can be detected at much greater depth than an equivalent mass of iron. The more magnetization an object has, the deeper it can be sensed by a ferromagnetic metal detector.