Ground Mineral Anomalies
Conductive mineral salts usually produce broad signals which will not be mistaken for a metallic object. Common causes are concentrations of mineral fertilizer, spots where evaporation has concentrated natural mineral salts, residue from de-icing salts and urine from livestock. Unless completely dry, “cow pies” can sound off like they are metal.
In some areas “clay domes” are a feature in the soil which will typically give a broad audible response. It often happens that if you dig the clay, the signal disappears. This is usually because of the loss of electrical continuity within the clay, but may be the result of loss of alignment of microscopic magnetic particles within the clay.
In spots where there has been intense fire, such as a campfire site or where a stump was burned during land clearing, the soil minerals may be altered by oxidation so that their ground balance setting is lower than that of the surrounding soil. In such cases, search slowly and change the ground balance setting as frequently as necessary.
In some areas, electrically conductive industrial minerals such as fuel coke, slag, clinkers (left over from burning mineral fuels) or charcoal have been dumped or used as landfill. Such lumps will usually have a broad signal, not crisp like a gold nugget usually is. If you’re searching an area where there is a lot of contamination by such materials, do not dig unless a signal is crisp and repeatable.
There are rare ground conditions where the layering of magnetic and electrically conductive properties within the soil causes ground balance to try to “go backwards” when lowering the search-coil to the ground. Under these conditions manual balancing and “grab” usually won’t work. You will probably never encounter this condition, but if you do, set the ground balance to a value which seems best to you and it will probably be usable while sweeping horizontally at reduced gain. The most advanced tracking systems may acquire good balance while sweeping horizontally under this condition even if they lock out while “pumping” vertically.