Adolfo:
The Gemini and other similar 2-box units are good on long wires and metal pipes, and on large continuous masses of metal. However, they are not much good for finding an object the size of a hammer, or a large amount of metal in the form of pieces such as a cache of coins. When people use a 2-box unit for locating a cache, they are usually looking for a metal box, or the metal bands binding a wooden box, within which the cache is located. If the cache is really large, or is in the form of bullion bars rather than coins, then a 2-box unit may find the contents of the cache.
It is possible, though not probable, that the Gemini would detect large pieces of a bicycle (frame, wheels) at a distance of 2 meters (6 1/2 feet). It is also possible that it would detect the roof of a house 3-4 meters (10-13 feet)away if the roof is made out of metal or has metal beams. Electrical wiring and metal plumbing in and around the house may also be detected.
A cache of silver coins buried at a depth of 50 cm (for anybody north of the border reading this, 50 cm is 20 inches) should be locatable by most modern VLF and pulse machines if equipped with a large searchcoil, and if the iron mineralization is moderate to low.
Most of Mexico is volcanic and metamorphic geology, which when it weathers and disintegrates in a tropical or subtropical climate, usually gives rise to soils high in iron oxide and hydrated oxide minerals. These iron minerals can cause a large reduction in the depth capability of metal detectors.
I am not very familiar with the Garrett Deepseeker, but I believe it has the capability to find your cache with the 14 inch coil if the iron mineralization in your soil is not too much. I would recommend not using the discriminator-- use the ground balanced all-metals mode. With a little practice you can tell the difference between a deep and a shallow target, and the difference between a large and a small target. If the target is shallow and small, you don't have to dig it, but it might be something valuable so you might want to dig it anyway. If it is deep, then it is large because a small deep target won't be detected. Anything like that you will have to dig. If the target is large and shallow, you will have to dig that, too, but since it's shallow, it won't take a lot of work to dig it. In the process of digging metal other than the cache you're looking for, you may find unexpected items of value. An item with no intrinsic value may have historical value.
In searching for a cache like this, your search will have to be slow, systematic, and methodical. In the process of searching, you will find out what kinds of things you are detecting, at what depth. This will give you an indication whether or not the Garrett is likely to be able to detect the cache under your conditions. If it is not powerful enough to locate the cache, at least you will have eliminated a lot of false targets which would interfere with a search using a more powerful metal detector.
If you fail to find the cache with the Garrett, it may mean that the cache was already retreived by someone else, or that the cache was not buried in the area where you searched, or that the Garrett was unable to find it.
If you don't find the cache with the Garrett, and you believe the cache is still present, you may want to buy or borrow a more powerful metal detector. For cache locating in highly mineralized soils, the best choice will probably be a pulse induction unit with a large searchcoil, preferably one with the ability to balance out red iron oxides while not sacrificing too much sensitivity to coins of the size you believe are in the cache. The Minelab SD series would probably do the job, but they are expensive and designed primarily for locating small gold rather than caches. There are several smaller manufacturers of pulse induction units with large coils designed specifically for cache locating-- I presume that Mr. Bill and Eric Foster will be happy to tell you what they have that is most suitable.
Hope this has been helpful. If anyone who is reading this feels inclined to add to what I have said (or to contradict it) please feel free to do so.
Good luck!
--Dave Johnson
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