How Deep Can a Metal Detector Go?August 9, 2019 - 11:24 PM
One of the most common questions we get from our customers is: "how deep can a metal detector go?" The short answer is that it depends on several factors. These include ground mineralization, target attributes, and the shape, size, and type of search coil.
In this article, we’ll start by explaining exactly what ground mineralization means. Then, we'll look at target attributes that affect how deep a metal detector can reach. Lastly, we'll discuss the different sizes, shapes, and configurations of search coils, and the specific use cases for them.
Let’s start with soil. Soil is made of sand, animal and plant matter, and trace minerals like zinc, magnesium and iron. These minerals can be attracted to a magnet, or, in the case of metal detecting, cause a disruption in the magnetic field generated by a detector’s search coil. Metal detectors are designed to detect certain minerals (iron, nickel, aluminum, copper, gold, silver, etc.) with a magnetic field. High levels of trace minerals (mineralization) in the soil interfere with this detection and can affect how deep a metal detector reaches.
As the concentration of trace minerals in the ground (mineralization) increases, the depth at which a metal detector can detect a target (any metal object) decreases. This is due to interference in the magnetic field. High levels of mineralization in the ground can also cause a metal detector to produce false signals—when a detector beeps in response to a metal object that isn’t there.
What causes ground mineralization?
Ground mineralization is a naturally-occurring process. New soils, such as those found in parks and gardens, have low levels of mineralization. Over hundreds and thousands of years, rainfall causes iron and other trace mineral particles in the soil to migrate to the surface. These particles slowly accumulate, and over time yield higher levels of ground mineralization.
Is there a way to determine ground mineralization where I live?
While soil composition varies greatly from place to place, you can roughly estimate the levels of ground mineralization where you live by looking at the color of the soil and referencing geological survey maps. You can recognize soils with high levels of ground mineralization by their color. Ranging from purple-red to reddish-brown, iron-rich ground is found all over the world.
In the southeastern and southwestern United States, its called Red Clay soil. It's also found in southern and eastern India, Australia, South America, central and southern Africa, and elsewhere. In 2007, the United States Geological Survey (USGS) released a report of trace mineral concentrations in soils within the contiguous United States, which includes maps that illustrate the surface concentrations of many trace minerals.
Now, we'll look at the target attributes that can affect metal detector search depth.
A Target is any metal object that you’re searching for with a metal detector. It could be a ring, coins, keys, skis, or buried treasure. Knowing your target’s size, shape, and orientation can help you estimate just how deep a metal detector can go.
Larger targets can be detected deeper than smaller targets. A greater amount of surface area creates more disruption in the magnetic field generated by a metal detector.
Target Shape and Orientation
The shape and orientation of a target can affect how deep a metal detector can find it. 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.
However, if the coins or rings are on their edge in the ground, rather than lying flat, they’re just as hard to find. Next, we'll discuss the different types of search coils.
A metal detector’s search coil consists of two sets of coiled wires. One set, the Transmit Coil, generates a magnetic field, while 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 a variety of sizes, shapes and configurations of search coils, each designed for different targets and levels of mineralization.
Search Coil Sizes
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. However, there’s a point at which the magnetic field generated is so large that the detector won’t detect small objects. For coins, that threshold is a search coil greater than 14-15” (35-38cm) in diameter.
Small Search Coils
Small-sized search coils measure 4-7” (10-17cm) and are ideal for metal detecting in tight spaces. They're also useful in areas with a lot of junk, like abandoned factories or trash heaps. Small coils generate concentrated magnetic fields, and are therefore best for locating very small objects.
Medium Search Coils
Medium-sized search coils measure 8-9” (20-22cm) and are the most useful for targets like coins, rings, and other small things. They create a magnetic field that's about 8" wide and deep, making for an all-around versatile search area. Because of this, medium coils are standard with most metal detector models.
Large Search Coils
Large-sized search coils measuring 10-15” (25-38cm) provide the greatest detection depth and coverage area. But, they make locating small targets a challenge. These coils work well in relic hunting.
Search Coil Shapes
Metal detector search coils come in three major shapes: circles, ellipses, and rectangles. Here, we'll discuss the pros and cons of each coil shape. Circular coils are the most sensitive in non-mineralized soils, and can detect slightly deeper than an elliptical search coil. Elliptical search coils are easier to maneuver in small spaces. Their longer shape provides wider coverage area, but they can’t detect as deeply as circular coils. Rectangular coils are for 2-box metal detectors, which are built to find large targets as deep as 4’ (1.2m)
Search Coil Configurations
The most widely available search coil configurations include concentric coils, Double-D coils, and 2-Box coils.
Concentric search coils consist of a larger coil that generates a magnetic field (the Transmit coil), and a smaller coil that receives signals (the Receive coil). These coils are the most versatile, able to detect wide areas, deep, and symmetrically. A downside is that they are very susceptible to interference from high levels of ground mineralization.
Double-D search coils consist of two coils arranged in overlapping “D” shapes. These coils are useful in highly mineralized areas, as they offset much of the ground's mineralization in a unique way. Double-D coils generate two magnetic fields: a narrow and deep positive detection field, and a wide negative detection field that effectively cancels out the ground interference.
2-box metal detectors are unlike other kinds of coil configurations. In this system, a pair of search coils exist as two separate coil units. This setup effectively creates a 3-4’ (1-1.25m) diameter search coil capable of detecting large, deeply buried targets like caches and relics. However, it's unable to detect targets smaller than 3” (7.5cm).