How Deep Can A Metal Detector Go ?

Are you interested in the world of metal detection and have questions about the depth capabilities of metal detectors?

Most metal detectors are able to detect objects at depths of approximately 4 to 8 inches (10 to 20 cm).

In ideal conditions, a mid-range metal detector can reach depths of 12 to 18 inches (30 to 45 cm) below the surface.

Specialized detectors can reach a depth of 65 feet (20 meters).

However, it is important to note that the specific depth achieved depends on the type of metal detector used and the characteristics of the object you are trying to locate, as well as factors such as the mineral composition of the soil.

In this article, we aim to clarify the issue of the extent to which a metal detector can penetrate the ground.

The topic can be confusing with conflicting information and unclear explanations, but we’re here to help.

We’ll start by explaining how the size, shape, and orientation of the target object affect detection depth.

Next, we’ll delve into the different types of metal detectors available, covering key aspects such as operating frequencies and search coil designs.

Finally, we will discuss how the presence of metals in the soil can affect the depth at which a metal detector can locate objects.

Let’s get started.

Metal object: Your target

It is necessary to understand that metal detectors are designed to detect exclusively metal objects. If you are looking for non-metallic items such as diamonds or wood, a metal detector will be of no use. In the world of metal detection, any metal object you are trying to locate is referred to as a “target.”

To be clear, the term “target” in metal detecting does not refer to a retail store. Instead, they include a wide range of possibilities, such as a lost ring, a set of keys, buried treasure, signs of ownership, or even a septic tank lid. Basically, a “target” is any metal item you wish to find.

The more information you have about your target, including its metal composition, size, shape, and orientation, the better prepared you will be to determine how deep a metal detector can effectively detect it.

Target size:

Larger targets can be detected at greater depths than smaller targets because they have a larger surface area, which disrupts the electromagnetic field generated by the metal detector.

Target Shape:

Round targets such as coins or rings, and flat and rectangular objects such as metal boxes or boxes, are easier to detect at greater depths due to their increased detectable surface area.

In contrast, long or thin objects such as nails or wires are difficult to detect when buried deeply.

Target Orientation:

Flat (horizontal) targets can be detected more easily at greater depths than vertically oriented targets.

This is because flat targets provide a larger surface area to interfere with the metal detector’s electromagnetic field, while vertical targets have less surface area to work with, making them more difficult to detect.

Target composition:

The type of metal you are searching for also affects the detection depth.

Metals with high electrical conductivity, such as silver, can be detected at greater depths compared to less conductive metals such as gold, lead, or stainless steel.

These factors, along with the capabilities of the metal detector itself, combine to determine how deep you can effectively locate desired metal targets.

What distinguishes metal detectors from one another in terms of depth capabilities are primarily three factors: the operating frequency, the software they utilize, and the size and shape of their search coil.

Operating Frequencies:

The first key difference among metal detectors lies in their operating frequencies.

But what exactly does this mean?

A metal detector’s operating frequency represents the number of electromagnetic (EM) waves it emits into the ground per second, measured in kilohertz (kHz).

For instance, a 7 kHz frequency can emit 7,000 EM waves per second, while a 40 kHz frequency can transmit 40,000 waves per second.

Most metal detectors operate within the frequency range of 7 kHz to 25 kHz. There are two primary types of frequency technologies: single-frequency and multi-frequency.

– Single-Frequency:

Many entry-level metal detectors employ a single-frequency technology known as Very Low Frequency (VLF). VLF continuously emits a single-frequency EM wave into the ground. Lower frequencies, such as those under 8 kHz, are best suited for detecting deep, large, or highly conductive targets like silver and copper.

In contrast, higher frequencies, typically around 40 kHz, are more sensitive to small gold nuggets and other less conductive metals but may not respond well to highly conductive metals that lower frequencies can easily detect.

Some low frequencies may be susceptible to interference from electronic devices and power lines, which is referred to as electromagnetic interference (EMI).

– Multi-Frequency:

Advanced metal detectors employ multi-frequency transmission systems.

This technology simultaneously transmits multiple frequencies across the spectrum, making the metal detector sensitive to both small and large, or deep targets at the same time.

For example, Garrett’s Multi-Flex system, used in its ACE Apex detector, operates within a frequency range of 5 kHz to 20 kHz.

Minelab’s Multi-IQ system, considered one of the industry’s best, operates across a frequency range from 5 kHz to 40 kHz.

The difference lies in how the detector’s software processes the received signals.

Software Features:

Apart from frequency, the software features incorporated into a metal detector also play a role in its depth-detection capabilities.

Ground Balance:

Many metal detectors feature a Ground Balance function designed to minimize interference caused by minerals in the soil.

Ground mineralization and its impact on metal detection will be covered in detail later in this article, but briefly, it can lead to false signals when the detector mistakes iron or salt particles in the soil for a target.

Ground Balancing helps suppress signals from ground minerals, allowing only actual target signals to be detected.

Ground Balancing is beneficial for detecting small targets in highly mineralized soils.

Discrimination:

Discrimination pertains to a metal detector’s ability to accurately differentiate between various metal objects based on their electrical conductivity and/or magnetic properties.

For instance, highly conductive metals like silver can be distinguished based on their conductivity, in contrast to less conductive metals like gold or steel. Other metals, such as iron, can be identified based on their magnetic properties.
The search coil, which is located at the end of a metal detector’s shaft, is a fundamental component of the device.

It consists of two sets of coiled wires: the Transmit Coil and the Receive Coil.

The Transmit Coil generates an electromagnetic (EM) field, while the Receive Coil detects disturbances within that field, which may indicate the presence of a metal object.

These coils come in various sizes, shapes, and configurations, each tailored for specific purposes, target types, search areas, and levels of mineralization.

Search Coil Sizes:

The size of the search coil directly influences the area a metal detector can cover.

During a sweep, a medium-sized search coil can cover approximately 2 to 3 feet (0.61 to 0.91 meters) every 3 to 4 seconds.

In general, the detection depth of a search coil corresponds to its diameter.

– Large Search Coils:

These coils, with diameters ranging from 10 to 15 inches (25 to 38 cm), can detect targets at greater depths and cover larger areas.

They are particularly suited for relic hunting or prospecting in remote areas.

However, they may struggle to detect small objects like earrings or thin jewelry due to their large electromagnetic fields and are more susceptible to electromagnetic interference (EMI).

– Medium Search Coils:

Standard for most metal detectors, these coils are typically 9 to 10 inches (22 to 25 cm) in diameter, providing a balance of detection depth and coverage area.

They work well for common targets like coins, rings, and jewelry.

– Small Search Coils :

(Sniper Coils): Measuring 4 to 7 inches (10 to 18 cm) in diameter, these compact coils can detect almost as deep as standard coils, typically reaching depths of 6 to 8 inches (15 to 20 cm).

They are highly effective in “trashy areas” with abundant metal debris and are excellent for locating small objects such as earrings or gold nuggets.

Additionally, small coils are less prone to electromagnetic interference from electronic devices.

Search Coil Shapes

Metal detector search coils come in two primary shapes:

– Circular Search Coils:

The most common shape, circular coils offer stability, coverage area, and precision across various soil types.

They generally have slightly better depth capabilities than elliptical coils.

– Elliptical Search Coils:

These coils have a narrower, elongated shape, making them more maneuverable in tight spaces.

While they cover the same area as circular coils, they may not detect objects as deep.

Search Coil Configurations:

Search coils are available in different configurations, including concentric coils, double-D coils, and monoloop coils.

– Concentric Coils:

These coils consist of a larger outer coil (the transmit coil) and a smaller inner coil (the receive coil).

Together, they produce a cone-shaped search field.

Concentric coils offer precision but are more susceptible to electromagnetic interference and ground mineralization.

– Double-D Coils:

These coils have transmit and receive coils arranged in overlapping “D” shapes.

This configuration provides signal stability, reduces interference from EMI and ground minerals, and enhances depth detection.

Double-D coils create two search fields to offset ground mineralization:

a narrow, deep positive detection field and a wide negative detection field to counteract ground interference.

– Monoloop Coils:

Monoloop coils appear similar to concentric coils but have a single coiling that both transmits and receives signals.

This design allows for greater penetration of the ground.

However, they are more susceptible to interference from mineralized soils, and the increased depth comes at the cost of greater sensitivity to ground conditions.

These various search coil sizes, shapes, and configurations are selected based on the specific requirements of the metal detecting task, the target type, and the environmental conditions.