HALO EFFECT and related ground oddities
Dave Johnson, FTP-Fisher 9 April 2012
Halo Effect
There are lots of reports by detectorists of “halo effect”. The story is usually that a buried object (for instance in
a test garden) became more detectable the longer it was in the ground; or, that a long-buried object was detected
at a depth beyond what would have been expected. The popular theory is that corrosion products create a
conductive “halo” around the metal object, increasing its effective size. Here’s my take on the subject.
Soil disturbance I believe that many reports of “halo effect” are actually a misinterpretation of soil
disturbance. When an object is buried, the act of digging and refilling the hole creates a localized soil anomaly
which causes the detected signal to either add to or subtract from the signal from the metal object. What
actually happens depends on the kind of soil and the details of how the detector processes signals, but most
often the detectability of the target is reduced. Over time the disturbed soil settles and becomes mixed with
surrounding soil by earthworm activity etc. such that the anomaly is gradually erased.
Gold Gold doesn’t corrode. Most detectorists say that they have observed that gold exhibits no halo effect.
In the case of natural gold, it is often found in an accumulation of magnetite, which may either increase or
decrease detectability depending on circumstances, but most often it will decrease detectability. The effect of
magnetite, which is disturbed upon removal of the gold from its natural setting, could lead a detectorist to
believe they’ve observed a halo effect when in fact there was none.
Iron Iron does corrode. This reduces the amount of metallic iron, which reduces the signal produced by the
iron target. However, in many soils, the corrosion products create a ring of high magnetic susceptibility rust
particles surrounding the object, and as the searchcoil sweeps over the object, the rust particles bounce the
signal around. The way signals are processed in most modern machines, the bounce from the rust particles
boosts the signal from the conductive metal at the center of the rusted mass, raising its apparent conductivity
making it harder to discriminate out. Detectorists are often puzzled that when they pull the iron target out of
the ground and air test it, it seems a rather weak target and discriminates out easily. What happened is that the
rust anomaly stayed in the dirt.
Copper (including copper alloys such as brass and bronze) In alkaline soils, copper alloys are fairly stable,
but in acid soils they corrode sometimes fairly rapidly especially in the presence of sulfur. The result is a green
ring of corrosion products around the metal piece the mass of which is reduced by corrosion. There are many
reports of presumed halo effect in relation to copper alloys, such an interpretation being encouraged by the
obviousness of the corrosion halo.
But there’s this little problem: the electrical conductivity of the corrosion halo is very low compared to metal.
Therefore we need other interpretations. Interpretation #1: the object in question was a high-conductivity
coin, for example a wheat penny. Corrosion makes it thinner, increasing its effective resistance. Although the
total signal is weakened, the resistive component of the signal which is primarily responsible for detection of
metal objects is actually increased. Interpretation #2: maybe the chemical corrosion processes also change the
iron chemistry of the soil in the corrosion halo, either increasing or decreasing their magnetic susceptibility.
This could theoretically impact detectability. ………Some experienced detectorists may say that these two
theories are inadequate to explain what they have actually observed. And that may be true. These theories just
happen to be the best ones I’ve got for now, they’re not a final word on the subject.
Silver “Big picture” fairly similar to that of copper. In most soils, silver is much more resistant to corrosion
than copper alloys. Corrosion products of silver may have higher electrical conductivity than those of copper,
but still, for a given amount of silver the electrical conductivity of the corrosion products is far lower than that
of silver in solid metal form.