Halo Effect & Disappearance of A Good Signal
Even if your metal detector's discrimination is set up on a lowest level, i.e. on rejection of only iron (metal of the lowest conductivity), and you are detecting at the site containing iron junk, you will encounter a phenomenon - unexpected disappearance of a good solid signal after digging the hole. In some cases, like with the White's XLT metal detector and its standard 9.5" search coil with an open center, the target's signal would disappear if the target is positioned on the dug hole's sidewall and close to the surface. Another case of disappearing signal is due to inaccurate pinpointing and described further below. But the most occuring case of signal disappearance is due to a phenomenon called "Halo Effect."
"A Solid Silver Signal's Gone Again!"
In scientific terms, the Halo Effect is a conductive increase in target size as seen by the metal detector's electromagnetic field. In simple words, the detector "reads" an iron target as a nonferrous object of high conductivity, i.e. the object made of copper or silver. The effect is caused by excessive target oxidation permeating the soil directly surrounding the target. The Halo Effect is associated with long term burial of an object or highly acidic soils.
The Halo Effect always disappears upon digging up the iron target, and if the detector's discrimination level is set up on rejecting iron, the target's signal is not heard any more. The target's signal disappears because the soil surrounding the target has been disturbed, and the metal detector does not "see" the iron object of low conductivity because of discrimination. In All Metal Mode, when iron is accepted, the "silver" or high-pitch tone of a signal changes to low-pitch tone or "iron" upon digging up the target.
Inaccurate pinpointing can easily cause a good signal's disappearance. Here is the case when after you pinpointed the coin wrong, you cut the dirt plug and pulled it out, but the coin was actually left positioned on the hole's sidewall. After you check the plug for a signal and do not get one, you get back to the hole. If you still do not see the coin on the sidewall, you continue to dig deeper and deeper after checking and rechecking the hole with the search coil.
You still do not see the coin so you decide to widen the hole by scraping the sides. Without your knowledge, the coin now drops to the bottom of the hole on its edge in loose dirt. The coin is now about nine inches deep or more and beyond your detector's ability to detect it. Feeling frustrated, you think about filling the hole and moving on.
If you do not give up easily, you take out the loose dirt, spread it on the flat surface or ground cloth and scan the dirt one last time. There in dirt is a silver coin! This scenario is the most common way many detectorists think that they have recovered coins at incredible depths. To avoid this problem and save time, I usually cut the plugs a little bit wider than required.
If you have the target that still signals at full discrimination after the hole has been dug to unusual depth, the target might be very large and nonferrous. Such targets can be aluminum soda cans, copper drain pipes, and old pots or buckets with galvanized plating. Or it could be a money cache or coin hoard. If you detect the site on which the modern junk is not present, you better keep on investigating these signals! Just remember about the Halo Effect and try to recover the targets with the least amount of ground disturbance possible.
Happy Hunting!
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