Metal Detector Depth

How deep can metal detectors detect?
Marc McDermott

A metal detector can find a dime at a few inches – but can it find a chest of gold at six feet? Or, in a more likely scenario, a cast iron sewer pipe in desperate need of replacement? The answers to common questions about how deep metal detectors can detect can feel a bit unsatisfying, simply because a whole range of factors affect depth performance: target characteristics, the machine being used, soil conditions, and much more. But in general, a VLF machine can find smaller targets from about 4–12”, while some deep-seeking, very specialized detectors can find large objects or voids more than 50 feet below the surface. To examine this question in more detail, this article will look at some of the major factors that affect the depth and how they might affect one’s metal detecting experience or choice of machine.

Technology

Obviously, technology plays a big role here. Many early metal detectors only got a few inches of depth on anything but substantial targets. But times have changed considerably, and metal detectors and similar devices can detect much deeper now than even a few decades ago. We will take a look at three basic categories of detector technology to help us get a handle on questions of depth: VLF (very low frequency) and multi-frequency, pulse induction, and two-box detectors.

VLF and multi-frequency

Most of the metal detectors for hobbyists fall into this category, and VLF detectors have been manufactured for decades. However, VLF machines on the market today reach far deeper and are much better at discriminating than those of decades past. The traditional VLF detector operates on a single frequency, often chosen as part of a balancing act to make a detector as versatile as possible. But the introduction of multi-frequency metal detectors in the last several years represents a significant advancement in technology. Just as it sounds, rather than operating on one frequency, a multi-frequency machine like the Minelab Equinox operates on multiple frequencies simultaneously. This makes a good multi-frequency detector able to fill many different roles effectively, and it also makes for more depth. A quality single frequency VLF detector can detect a coin-sized target at a maximum depth of about 11 inches in ideal soil conditions. With ideal conditions and a multi-frequency machine operated by an experienced user, you might get up to 16 inches of depth on a coin-sized target.

Pulse induction

Pulse induction detectors, such as the Garrett ATX or the Minelab GPX, function quite differently than VLF detectors, and, without getting into the technical why, they achieve significantly greater depth than VLF machines. A detector like the Garrett ATX can detect a coin-sized target at approximately 18 inches and perhaps a bit more, while large targets can be detected at depths three feet or more. However, this depth comes with some drawbacks. Firstly, at some point, it becomes impractical to retrieve targets at such extreme depths with simple hand tools. Secondly, pulse induction detectors tend to be heavier than VLF machines, and they do not discriminate well. Still, their depth and good performance in saltwater make them valuable tools for beaches, gold-bearing sites, and sites without a high density of trash targets.

Two-box and deep-seeking detectors

This category takes us partially out of the realm of hobbyist detecting. One could put machines into this category that can detect at a depth of three feet to 25 feet or more. These are not meant for finding individual coins or rings but are used by treasure hunters to find caches at depth and detect voids in the ground where a pipe, tunnel, or cave might be. Some devices that fit this category, like some types of gradiometric devices and deep-seeking detectors that provide 3D imaging, are also used by archaeologists and industrial applications. But as fascinating as these machines are, they are not practical for the average detectorist. The amount of earth that must be excavated to retrieve targets at these depths is significant, and many (though not all) deep-seeking detectors come with an unsurprisingly hefty price tag. But for the right application, they can achieve extreme depth – some 3D imaging detectors claim to identify targets at depths of more than 50 feet.

Coils

Advanced detector technology is great, but it won’t do you much good without the right search coil. Generally speaking, the larger the search coil, the deeper it can find targets. But large coils are more sensitive to electromagnetic interference, can limit your ability to detect small targets and make pinpointing difficult in some cases. Coil shape can also affect depth. For example, a round, concentric coil will offer the most depth and is perhaps the most popular coil shape, but this style is more susceptible to soil mineralization than a DD coil, for example. Practically, it is important to choose a coil suited to the task at hand. If that task is going as deep as possible, a large, round coil will probably be best for your VLF or pulse induction detector, but this might vary based on how mineralized or trashy your site is.

Targets

Size, shape, and orientation can have a significant effect on a detector’s ability to see a target at depth. Finding a small target like a coin or a ring – both of which are round and thus have more detectable surface area than a more irregular shape – at a few inches is easy for most detectors, but to find the same target at 12 inches is much more difficult and takes good equipment and knowledge of one’s machine. The deep-seeking metal detectors we have discussed above don’t even take small targets like this into account and would have to have a large target like a cast iron pot, a manhole cover, or an engine block to be effective at depth. The position also matters. For example, imagine a coin lying in the ground, flat side up. At four, six, or even eight inches for a quality detector in decent conditions, this makes for a fairly easy target. However, if that coin is “on edge,” as detectorists say, what the detector will see is much different and the signal will be different and less obvious. A final aspect of your target to take into account is conductivity. Not all metals conduct electromagnetic signals equally well (this is part of how discrimination works) and that conductive or less-conductive property can have an effect on how well your detector can see it at depth.

Environmental conditions and ground balance

This is a big one. Soil conditions and composition can vary drastically from one place to another, sometimes even in areas in close proximity to each other. For example, a black sandy beach will have a very different composition than red clay soil or muddy silt. Ideally, a metal detector will be able to function properly in all of these environments. A detector does this by accounting for the composition of that soil (often termed its mineralization) and making appropriate adjustments. This function is called ground balance. Some detectors do this automatically without any input from the user, while others have the option of manual or automatic adjustment. But whatever type of ground balance your detector may have, properly accounting for the soil conditions at your site is essential for good depth. If you are not properly ground balanced, you will often get false signals or inaccurate readings, especially at the edge of your detector’s depth capability.

Ground conditions aren’t all bad news, though. Some environmental factors can work in a detectorist’s favor when it comes to depth. For example, damp, sandy soils are some of the best for metal detecting and will often help a machine to perform to its limits. Another environmental advantage for land hunting is wet weather. If you have previously fallen flat at a site that should be promising, or it seems that there are targets at the edge of or beyond the depth that your machine is hitting, wait for a good, soaking rain. This can increase the conductivity of the soil and help your machine hit those deeper targets. 

Settings and frequency

Your detector’s ability to find targets at depth not only depends on the technology it uses or the prevailing environmental conditions but also the settings that you choose. Having already discussed ground balance in the previous section, we’ll focus here on sensitivity and discrimination. Your detector’s sensitivity will determine how “hot” your detector is running or, more technically, how reactive it is to the signals it is receiving. A VLF machine might detect a coin-sized target no deeper than two or three inches below ground at the lowest sensitivity setting. In comparison, the same detector might be able to ferret out a coin-sized target at ten inches at the highest sensitivity setting. Discrimination settings work a bit differently depending on the machine being used. Discrimination is a detector’s ability to differentiate between different levels of electromagnetic conductivity in targets, which for the user translates to being able to dig less junk (at least sometimes) and find more desirable targets. However, discrimination has a notable drawback concerning depth. As a general rule, you’re going to lose a little bit of depth with discrimination, even on modern machines. Running all-metal mode is the best way to get around this, but if you are looking for very deep targets using discrimination, make sure you’re using a good swinging technique (coil parallel to the soil!) to maximize performance.

Operating frequency is a more behind-the-scenes aspect of the metal detecting experience and is often something that cannot be adjusted, but it does affect depth performance. Why? Simply put, different frequency ranges have different strengths and weaknesses. Detectors operating at lower frequencies will be more sensitive to highly conductive metals like silver and copper, detect large objects at greater depth, and perform better in the water. Higher frequency machines are better for mid-to lower-range conductors like gold, platinum, and lead and are also more sensitive to small targets like gold flakes and nuggets. A detector set up for gold detecting will probably lose some depth when detecting high conductors as a result.

Electromagnetic interference (EMI)

EMI can make for a frustrating metal detecting experience, but the reality is that there is no shortage of sources of electromagnetic interference in the modern world: Wi-Fi, cell phones, power lines, and much more. Unfortunately, significant EMI will almost always cause a loss of depth for one of a couple of reasons. Firstly, the most common fix for instability caused by EMI is to lower one’s sensitivity, which will reduce the detector’s ability to find deep targets. Secondly, a detector that is left in an unstable state won’t interpret target data well, meaning that deep targets will be missed. If a site has promise, it is probably worth hitting it at different times of the day to try to avoid the worst of the EMI.

FAQ

How deep can a pulse induction metal detector work?

It depends on the size of the target and the detector, but about 18 inches for a coin-sized target and three feet or more for large targets.

How deep can a gold detector detect?

A high-end gold detector like the Minelab GPZ 7000 can reportedly detect small nuggets at depths of up to three feet.

What is the deepest metal detector for gold?

The Minelab GPZ 7000 is probably the deepest metal detector for small gold, but it comes with a prohibitively high price tag for most. Detectors like the Nokta Gold Kruzer and the Garrett AT Gold are more budget-friendly options.

How do you increase the depth of a metal detector?

To maximize the depth you’re getting out of your detector, be sure to have good swinging technique, hit sites when there has been a good, soaking rain, and, if possible, invest in a larger coil.

What type of detector is best for things like property markers and septic tanks?

The best tool for this would be a magnetic locator. They are specifically for detecting ferrous objects like iron and steel and can detect large metal objects up to depths of about 15 feet for large targets.

Final thoughts

The common question of how deep a metal detector will detect is much more complicated than it might seem at first glance. This article is far from exhaustive, but it has made clear a number of the factors affecting depth and some general guidelines on how to maximize depth capability. But one of the most important things in getting down to those deep targets is understanding what your detector is telling you, which only comes with time and practice. So get your coil to the soil and dig deep!