How Deep Can a Metal Detector Go?
Are you curious about metal detecting, and wondering how deep a metal detector can go?
Most metal detectors can detect objects about 4-8ʺ (10 - 20 cm) deep. In ideal conditions, a mid-range metal detector can reach 12-18ʺ (30-45 cm) underground. Some specialized detectors can go as deep as 65' (20 m).
The specific depth depends on the type of detector you’re using, and the sort of object you’re trying to detect. And other factors, like the minerals in the soil.
In this article, we’ll dive into just how deep a metal detector can detect. There’s a lot of conflicting information and unclear explanations on the topic. We’re here to help make sense of it all.
First, we’ll explain how the size, shape, and orientation of the thing you’re looking for affects how deep you can detect it. Next, we’ll explore the different types of metal detectors available. We'll cover everything you need to know, from frequencies to search coils. Finally, we’ll cover how minerals in the ground can affect search depth. Let’s dig in.
The Metal Object: Your Target
Metal detectors can only detect metal objects. If you’re looking for diamonds or wood, you’re out of luck. In metal detecting parlance, any metal object that you’re searching for is a Target.
Not to be confused with the department store, this sort of target could be a lost ring or a set of keys. It might be buried treasure, property markers, or a septic tank lid. Whatever it is, it’s a metal thing that you want to find.
The more you know about your target, the better prepared you’ll be to find it. A target’s metallic composition, size, shape, and orientation affect how deep a metal detector can go.
A metal detector can detect larger targets deeper than smaller ones. This is because larger objects have more surface area, which creates more disruption in the electromagnetic (EM) field generated by a metal detector.
Circular targets like coins or rings, and flat rectangular objects like metal boxes or chests are easier to detect at greater depths, because of more detectable surface area. Long or thin shapes such as nails or wires are more difficult to detect deeper in the soil.
A target that is lying flat (horizontal) is easier to detect deeper than a vertical target. This is because there is more surface area to disrupt the EM field from the detector. A vertical target provides less surface area to work with and is thus more difficult to detect.
The type of metal you’re looking for affects how deep you can find it. You can detect metals that are highly conductive of electricity (like silver) deeper than less conductive metals like gold, lead, or stainless steel.
Next, we'll discuss the different types of metal detectors available, and how deep they can go.
Standard Metal Detectors
So what separates these metal detectors from one another when it comes to depth? Three things—the frequency they operate at, the software they’re running, and the size and shape of the detector’s search coil.
The first difference between metal detectors is the frequency they operate at. But what does that mean?
A metal detector’s operating frequency is the number of ground-penetrating electromagnetic (EM) waves it can transmit per second. We measure these frequencies in kilohertz (kHz). A 7 kHz frequency, for instance, can transmit 7,000 EM waves per second. A 40 kHz frequency, on the other hand, can transmit 40,000 waves per second.
Most metal detectors operate at a frequency between 7 kHz and 25 kHz. There are two major types of frequency technologies: single-frequency and multi-frequency.
Most entry-level metal detectors use a single-frequency technology called VLF. VLF stands for Very Low Frequency. It continuously transmits a single frequency EM wave into the ground.
Low frequencies (under 8 kHz) are best suited for deep, large, or highly conductive targets like silver and copper. High frequencies (around 40 kHz) are sensitive to small gold nuggets and other less conductive metals. But, they are unresponsive toward more conductive metals that lower frequencies can easily detect.
Some low frequencies are prone to interference from electronics and power lines. This, as we mentioned earlier, is electromagnetic interference, or EMI.
Higher-end metal detectors use a multi-frequency transmission system. Multi-frequency technology transmits multiple frequencies across the spectrum at the same time. This makes the metal detector sensitive to both small and large or deep targets at once.
Garrett’s Multi-Flex is a multi-frequency transmission and signal processing system used in its ACE Apex detector. It uses a frequency range between 5 kHz and 20 kHz.
Minelab’s Multi-IQ transmission and processing system is widely regarded as the best in the industry. Multi-IQ operates within a frequency range of 5 kHz to 40 kHz. It differs in how the detector’s software processes the signals it receives.
In addition to frequency, there are software features that affect how deep a metal detector can detect.
Ground Balance is a feature available on many metal detectors. It works to minimize interference from minerals in the ground. We’ll cover ground mineralization in depth later in this article, but let’s briefly dive in.
Ground mineralization can produce false signals. This is when a metal detector beeps as if a target is in the ground, but it’s just detecting particles of iron or salt in the soil.
To mitigate this, Ground Balancing features mute the signals being received from the minerals in the ground. This leaves behind only signals from actual targets.
Does ground balancing affect depth?
The effects of ground mineralization can mask small targets below the surface. By using ground balancing, you can detect jewelry or coins hidden in highly mineralized soils.
Discrimination refers to a metal detector’s ability to accurately differentiate metal objects. This is based on their electrical conductivity and/or magnetic properties.
Some metals, like silver, are highly conductive of electricity. These can be separated on the basis of their conductivity relative to less conductive metals like gold or steel. Other metals, like iron, are magnetic, and can be identified that way.
Does discrimination affect depth?
When you discriminate against unwanted targets, like nails or screws, you’re able to detect deeper targets like coins, hidden below the others.
A search coil is the round(ish) thing at the end of a metal detector’s shaft. As its name may suggest, it consists of two sets of coiled wires. One set, the Transmit Coil, generates an electromagnetic (EM) field. The other set, the Receive Coil, detects disturbances in that field.
These disturbances indicate that a metal object may be in the ground. There are many sizes, shapes and configurations of search coils. Each is designed for different targets, search areas, and levels of mineralization.
Search Coil Sizes
The size of the search coil determines how much area a metal detector can cover. When sweeping the coil across the ground, a medium-sized search coil can cover 2' to 3' (0.61 to 0.91 m) every 3 to 4 seconds.
How deep can a search coil detect?
As a general rule, the detection depth of a search coil is equal to its diameter.
The larger the coil, the deeper it can detect a target. But there’s a point at which the EM field generated is so large that the detector won’t detect small objects. For coins and rings, that threshold is a search coil more than 14-15ʺ (35 - 38 cm) in diameter.
Small search coils (also called sniper coils) measure 4 - 7ʺ (10 - 18 cm) and can detect almost as deep as standard 9 - 10ʺ (22 - 25 cm) coils. As we noted in an earlier section, that’s 6 to 8ʺ (15 - 20 cm) deep.
These small coils are useful in “trashy areas” like homestead sites and garbage-strewn parks. They generate concentrated EM fields, and are best for locating tiny objects like earrings or gold nuggets. And, small coils are less susceptible to electromagnetic interference (EMI) from power lines, cell phones, microwaves, and other electronics.
Medium Search Coils are standard with most metal detectors. They tend to be 9 - 10ʺ (22 - 25 cm) in diameter, and create a search field that’s about 8ʺ (20 cm) wide and deep.
Medium-sized search coils work well in a variety of search areas. They're suited to common targets like coins, rings, and other jewelry.
Large Search Coils have diameters of 10 - 15ʺ (25 - 38 cm) and provide the greatest detection depth and coverage area.
They’re ideal for relic hunting or prospecting in remote areas. But there are trade-offs. Most notably, they make detecting small targets a challenge. The large EM field these coils generate can make the metal detector “blind” to earrings, tiny gold nuggets, and thin jewelry. And, they're more susceptible to EMI than other coil sizes.
Search Coil Shapes
Metal detector search coils come in two major shapes: circles and ellipses.
Circular search coils are the most common shape.
They provide a balance of stability, coverage area, and precision in a variety of soil types. Circular coils can detect a bit deeper than elliptical coils.
Elliptical search coils are narrow and easier to maneuver in small spaces.
Their longer shape provides the same amount of coverage area as circular coils, but they can’t detect quite as deep.
Search Coil Configurations
The most common types of search coils are concentric coils, double-d coils, and monoloop coils.
Concentric Coils consist of a larger outer coil that generates an electromagnetic field (the transmit coil), and a smaller inner coil that receives signals (the receive coil).
Combined, these coils penetrate the ground with a cone-shaped search field. Concentric search coils are the most precise, but are more susceptible to EMI and minerals in the ground.
Double-D Coils have the transmit and receive coils arranged in overlapping “D” shapes.
This configuration provides signal stability, reduces interference from EMI and ground mineralization, and the ability to detect at greater depths than concentric coils.
Double-D coils generate two search fields to offset ground mineralization. The first is a narrow and deep positive detection field. The second is a wide negative detection field that works to cancel out much of the ground interference.
Monoloop Coils look similar to concentric search coils. Instead of multiple wire coilings, they have a single coiling that both transmits and receives signals. This configuration penetrates the ground deeper than most concentric coils. However, the trade-off for that increased depth is more interference from mineralized soils.
Specialized Metal Detectors
Aside from standard metal detectors, there are other types of specialized metal detectors for industrial and professional uses.
A magnetic locator is a type of metal detector that only detects iron and steel. They’re used for finding property markers, cast iron pipes, water valve covers, septic tank lids, and other buried iron things.
How Deep Can It Detect?
Magnetic locators can detect iron objects between 4ʺ (10 cm) to 16ʹ (4.8 m) deep, depending on the object’s size.
Unlike a metal detector with a single search coil, two-box detectors use a pair of rectangular search coils. This setup can detect large, deep targets like caches and relics. The trade-off for that depth is that they’re unable to detect objects smaller than 3ʺ (7.5 cm).
How Deep Can It Detect?
Two-box detectors can go as deep as 20ʹ (6.1 m). This depends on the size of the search target and minerals in the soil.
3D Ground Scanner
A 3D ground scanner is a special type of deep-seeking metal detector. It can locate buried tunnels, artifacts, treasures, and subsurface anomalies.
Archaeologists survey historic settlements, ruins, and battlefields, without disturbing the ground. Gold prospectors use these scanners to search for mineral deposits before digging. Treasure hunters search for gold, silver, bronze, and other metal artifacts. And other people use these to locate tunnels, foundations, and hidden chambers.
How Deep Can It Detect?
A 3D ground scanner like the OKM eXP 6000 can detect objects up to 65ʹ (20 m) below the surface.
Radio Frequency Locators
A radio frequency locator is a tool used to locate buried plastic pipes. Like those for irrigation or plumbing. A self-contained radio transmitter called a sonde is attached to the end of a fish tape and fed into the pipe. The sonde transmits a specific frequency that the locator receives, allowing the pipe to be traced.
How deep can it detect?
Depending on the type of sonde used, a radio frequency locator can detect a sonde up to 9ʹ (2.75 m) below the surface.
We’ve name-dropped ground mineralization a few times so far, but what is it?
Let’s start with soil. Soil consists of sand, animal and plant matter, and trace minerals like zinc, magnesium and iron. High levels of these trace minerals in the soil is known as ground mineralization.
Soils with high levels of mineralization interfere with the EM field generated by a metal detector. This interference affects how deep you can reach.
As mineralization levels increase, the depth at which you can detect a metal object decreases. Metal detectors without ground balancing and discrimination features to mitigate ground mineralization are of little use in the most mineralized areas.
And, more mineralized ground increases the likelihood of false signals. This is when a detector beeps as though a metal object is there, but it's just picking up the trace minerals in the soil.
What Causes Ground Mineralization?
Ground mineralization is a natural process. New soils, such as those found in parks and gardens, have low mineral levels. Over time, rainfall causes iron and other mineral particles deep in the soil to migrate to the surface. These accumulate, yielding higher levels of ground mineralization.
How Mineralized is the Ground Where I Live?
Soil composition varies greatly from place to place. But you can estimate the levels of ground mineralization where you live by looking at the color of the soil. Ranging from purple-red to reddish-brown, iron-rich ground can be found all over the world.
In the southeastern and southwestern United States, it’s called Red Clay soil. It's found in southern and eastern India, Australia, South America, and central and southern Africa. In 2007, the United States Geological Survey (USGS) released a report of trace mineral concentrations in soils within the contiguous United States. The report includes maps that illustrate the surface concentrations of many of these minerals.
Most metal detectors can reach 6-8ʺ (15 - 20 cm) deep, on average. Some specialty metal detectors can go up to 65ʹ (20 m) deep. But, the specific depth you can reach depends on other factors, like those included below.
- Frequency your metal detector operates at.
- Features like discrimination and ground-balancing.
- Size, shape, and configuration of your detector’s search coil.
- Target size, shape, orientation, and metallic composition.
- Levels of mineralization in your soil.
How deep have you gotten with your metal detector? Tell us about it in the comments below.
About the Author
Gary Iverson is a staff writer at Metro Metal Detectors covering all things metal and metal-adjacent.