So you want a metal detector that will find an old coin four feet deep? Some might laugh at such a question, but I have spoken with a number of green metal detectorists who want a metal detector that will go a minimum of three to four feet. This is not a dumb question, but one that merely stems from a misunderstanding of technological limitations, site conditions, and sink rate encountered when metal detecting.
While there are a number of specialty metal detectors capable of detecting at these depths, the majority are limited to the six to twelve inches range.
The most basic reason for this is that the majority of detectorists will not need more than a foot of depth as many finds are still relatively shallow. Even if a park or a private lawn contained old coins buried under four feet of soil, it is unreasonable to dig a four foot hole in a manicured lawn.
Another reason for the depth limitations is the technological tradeoffs required to attain deeper depths. For example, deep seeking metal detectors that utilize pulse induction principles can penetrate mineralized ground deeper than the average metal detector.
However, pulse induction metal detectors have very rudimentary discrimination features at best. What this means is that they may go deep, but it will be more difficult to tell the difference between a good item and a trash item.
When was the last time that you dug a four foot hole with a hand trowel or small shovel? Do you really want to spend time digging craters to find a bunch of trash and only few good relics or coins? Remember that the trash to good item ratio can be quite high. For this reason, deep metal detectors are typically used at locations where trash is minimal (some gold fields) or where digging large holes is easy and non-destructive (beaches).
One final reason for limited detection depth is due to differing ground conditions. Some soils are considered mineralized, in that they contain more particles of decomposed iron-bearing rocks and sand than do other areas. These metallic minerals negatively affect a metal detector’s signal which can lead to instability and false signals that hide good signals due to excessive beeping.
By decreasing the sensitivity the metal detector becomes more stable, but depth is lost. This is one reason for the varying depth results of detectorists around the world as some ground conditions allow for greater detection depth than others.
As you consider purchasing a metal detector, keep in mind that as a general rule, entry level metal detectors will likely be limited to around six inches of depth which is adequate for modern parks, tot lots, ball fields, and lawns. Mid-range metal detectors should provide six to nine inches of depth under normal ground conditions. High-end metal detectors usually provide as much as fifteen inches under favorable conditions and pulse induction detectors can go deeper still.