How Deep Will a Metal Detector Go?
This is, if not the most asked, one of the most frequent questions we get about metal detectors. It is not as easy of a question to answer as one might think. There are a lot of variables at play. Of course there are metal detectors that are made specifically for finding deep targets. Some can detect several meters under ground. Some, lesser detectors, are manufactured to only detect shallow objects. The detector's frequency and sensitivity are two factors. But there are many other things to consider.
For those of you who only read the first couple paragraphs of an article, I can give you a canned answer:
A typical hand held metal detector can effectively detect an object about the size of a small coin at a depth that is approximately the same distance as the size of the detectors coil.
So, for example, a detector with an 8" coil would detect a small coin sized object at 8" +/-.
However, this is not the limit to the detectors depth capacity. For instance, I can stand 10 feet away from my car and get a signal with my Garrett AT Pro that has an 11" coil on it.
Factors involved in how deep a metal detector can detect metal:
Size of Object
The size of the object is a big factor in determining how deep a metal detector will detect an object. So, you may only be able to find a coin at about a foot deep, but a large box of coins can be detected at perhaps double that depending on the detector's configuration.
Size of The Metal Detector's Coil
Size does matter. This goes for the size of the object as well as the coil. Many detector brands provide alternative coils to those that come stock with the detector. For instance, Minelab offers a 15" DD Smart Coil for the Equinox Series of metal detectors as opposed to the 11" DD coil that comes stock. Don't be fooled. When metal detecting, bigger isn't always better. When you are detecting trashy areas, larger coils give you a much larger detection pattern. Which means you will be detecting many more targets at once. This can be not only a hinderance but a nuisance. Most companies make small "sharpshooter" like Minelab's 6" DD Smartcoil coils for this very reason.
If you are searching for a very large, and perhaps very deep, target, you may want to consider a "box" style metal detector like the Garrett GTI 2500 with the Eagle Eye coil attached. These detectors can detect certain large objects several feet under ground.
Metal Detector's Frequency
The operating frequency of the metal detector is another factor that determines depth. Rule of thumb is the lower the frequency the better a metal detector is at finding deep, large objects. The higher the frequency the better a metal detector can detect small shallow targets (5 kHz-10kHz). This is why most gold prospecting metal detectors operate at higher frequencies (40 kHz-100kHz). This is why you will see most detector operating between 12 kHz and 18 kHz - frequencies that are all around useful to find a broad spectrum of objects like coins, jewelry, and relics.
Orientation of Object
In the example of a normal detector being able to detect a coin at the approximate depth of the coil's size, there are stipulations. If, for instance, the coin is tilted on its edge rather than laying flat, the detector may not detect it quite as deep since the pattern of the radio waves sent back to the detector will be read differently due to the coins orientation.
It is commonly known that detectors get better depth when the soil is moist. Here in the Midwest, the water content in our soil varies broadly depending on the season. In the hot, dry summer, you may lose a few inches whereas in the moist spring an fall, you can expect maximum depth out of your detector. Which, when you think about it may not be a bad thing. Nobody likes to dig a 12" hole in rock hard soil.
Metal Detector's Sensitivity
To get the maximum depth out of a detector the frequency should be adjusted to "high". However, when you are under normal conditions, the sensitivity should be adjusted to about 3/4 to 2/3 of the detector's maximum. Having the sensitivity set up to the maximum all of the time may result in the detector acting erratic and picking up even the smallest undesired objects like small pieces of bailing wire.
I hope this clarifies the question for most people out there. Good luck and happy hunting!
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- Joshua Turpin