orecastAnyone sweeping a coil across a field relies on their machine. But a large part of what does or doesn't come through as a signal is decided before the detector even gets involved – by the soil itself. Our new episode looks at what "mineralization" actually means, why it has nothing to do with ore, and how much it can affect search depth.
What mineralization actually is
Mineralization refers to magnetizable iron oxides in the soil – magnetite, maghemite, hematite. The ground itself then responds like a target, often far louder than whatever you're actually searching for: according to Minelab's modeling, the signal from strongly mineralized soil can be orders of magnitude stronger than that of a deep coin. The target signal itself also drops off extremely steeply with distance – twelve percent more depth already halves the signal, and going from 40 to 59 centimeters of distance can make it ten times weaker with an 11-inch coil. That's one of the most solid numeric facts on this whole topic.
Red earth, basalt, black sand, brick rubble, slag, and old hearths add further interference, and hot rocks can mimic a metal signal where there isn't one. Ground balancing – manual, automatic, or tracking – subtracts the soil's own response and, in mineralized terrain, brings back depth and quiet. It won't help against rubble or hot rocks, though. The displayed conductivity (target ID) also gets shifted by the soil; deep non-ferrous finds can end up reading like iron. Reliable discrimination usually only works close to the surface.
Handle centimeter figures with care
Fixed numbers like "large coin down to 25 centimeters" circulate constantly. These are experience-based estimates from the detecting trade, not measurements – and they depend heavily on soil type. On strongly mineralized ground, such as at some Roman sites, Minelab notes that depths for a small bronze coin rarely exceeded eight centimeters. Minelab itself officially states there's "no specific answer" to give. That's exactly why the video speaks of trade experience rather than fixed numbers for "the" detector.
The depth readout on a device is also calibrated for coin-sized objects: large objects tend to appear too shallow, small ones too deep. Salty, wet sand – for instance at the coast – is a particular challenge for single-frequency machines; multi-frequency or pulse induction units cope better here. Pulse induction only measures after the field is switched off, making it less sensitive to mineralization, though it's worse at distinguishing between metals.
What happens after the signal
Whether a find leaves the ground in any condition worth having depends above all on soil chemistry. At the Harzhorn site, where more than two thousand iron finds were examined, pH was the strongest indicator of preservation. Damp, aerated fine sand turned out to be especially destructive – sand doesn't automatically preserve finds, if anything it's the opposite. The most damaging condition is alternating wet and dry; permanently wet, low-oxygen environments like peat or lake sediment tend to preserve finds best. Chlorides in the soil accumulate especially in iron and, once a find is brought up into the air, can trigger complete disintegration.
How these soil types and their properties are distributed across a region can be traced on the orecast map – a useful starting point for judging a detecting spot not just by access, but by what's actually underfoot.