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How deep can a metal detector go?

7th April 2021 Print

The maximum depth allowed by metal detectors depends on both exogenous factors (type of soil, humidity, size of objects to be detected, type of metal) and internal factors linked to the technical characteristics of the device and to the technology that it embeds (beat frequencies or pulse induction).

Because mastering real depth is one of the recurring problems for prospectors and other metal detector enthusiasts, we offer you an explanatory article on the subject to set off on an adventure in the best possible conditions!

Understand how the metal detector works

But what makes a metal detector buzz when it passes over a well-buried treasure? Why is it important to keep the detector moving? Here are the main points to know before looking at the detection depth of the device:

- A battery located in the upper part of the metal detector activates the transmitter circuit, which transmits electricity downwards through a cable in the handle to the transmitter coil located in the lower part;

- When electricity flows through the transmitting coil, it creates a magnetic field all around it;

- If you sweep the detector over a metallic object, the magnetic field passes through it from end to end;

- The magnetic field causes an electric current to circulate inside the metallic object;

- This electric current creates another magnetic field all around the object. The magnetic field passes through the receiver coil while moving above it. The magnetic field circulates electricity around the receiver coil. It causes current to rise through the receiver circuit at the top, causing a speaker or your headphones to buzz to alert you that you have found something potentially interesting.

External factors that impact the detection depth of the device

By "external factors," we mean all the variables that impact the depth and do not depend on the device itself. This is, for example, the type of soil, which is judged by its degree of aeration or compaction as well as its level of mineralization. It is also the slope of the ground. The angle can indeed affect the depth of detection because the object can be buried deeper, and the angle means that there will be more earth between the coil and the metal target.

The size of the metal object in question is also a decisive external factor. Obviously, the larger, thicker, or wider the object, the more "deeper" it can be detected. For this reason, gold nuggets, very small in size, are difficult to detect beyond a certain depth. You will be given the figures in centimeters of depth at the end of this article.

Finally, soil moisture impacts the detection depth of your metal detector. Naturally, a ground that is too wet will give you a hard time as objects will tend to sink deeper and at a higher speed, making it difficult to detect them.

Internal factors that impact the detection depth of the device

As you will have understood, these are variables related to the technical characteristics of your metal detector. The size of the coil plays a major role here. For beat frequency metal detectors, the detection depth is near or slightly greater than the coil's width. The wider the coil, the greater the depth of detection it will provide. However, too large a coil will cause you to lose sensitivity and precision. There is, therefore, a compromise to be made which depends on your objectives and your "profile" as a metal prospector.

The type of technology embedded in your metal detector will also determine the depth at which it can work. Pulse induction provides better depth than beat frequency. It is also less affected by minerals and other components found in the soil.

Finally, be aware that all metal detectors operate at different frequencies. A low frequency will be associated with a greater depth but with low sensitivity. Conversely, a high frequency will allow better sensitivity with less depth. The best metal detectors are the ones that find the best compromise between these two variables. In commercial parlance, an ultra-precise metal detector will undoubtedly have shortcomings in the variable depth of detection. We found that the Bounty Hunter TK4 Tracker IV metal detector strikes the good balance in this manner. Read more about Bounty Hunter Tracker IV in this review.

Lost coins and remains of battlefields are often much shallower than one might think.

In general, a ring or coin lost on the surface about one to two centuries ago will only descend to about 3 to 20 cm in-depth, depending on soil and weather variables. This is why the best metal detectors are specially designed to be as accurate as possible at this depth.

Some relics are often found on the surface after more than 100 years under the sun. If the depth exceeds about 30 cm, you will be less and less likely because it is very unlikely that the objects of interest have sunk any deeper unless they have been buried deliberately. In short, for the detection of valuables, the precision that the metal detector allows is a more important criterion than the depth.

Metal detector: some reference values ??to find your way around

No manufacturer can guarantee precise depth due to the many internal and external factors that come into play. But here is a rough guide so that you can get an idea of ??depths and object types:

- Small jewelry (e.g., necklaces and thin gold rings): 7 to 15 cm;

- Large jewelry: from 15 to 30 cm;

- Small coins: 10 to 15 cm;

- Large coins: from 15 to 30 cm;

- A jam jar lid: 22 to 40 cm;

- A can of coffee: 30 to 60 cm;

- A cannon shell: from 60 to 90 cm.

Getting started with a detector

Knowing how to use a metal detector is essential; otherwise, you are quickly discouraged, and you end up abandoning the detection. If you are a beginner we would recommend to check out Metal Pursuits blog post to good metal detectors for beginners

Get to know your device and learn step by step.

Know that you will only master your detector after multiple and long tests, which you will also find fascinating and which will leave you with wonderful memories.

First of all, you need to familiarize yourself with your device, buttons, and functions.

You need to know by heart all the meanings and uses of all the buttons on your detector before you even leave for the field (one button badly used, and you will come back empty-handed every time!).

Then you will have to implement them in the situation. But before going outdoors, you will practice and familiarize yourself with your detector's functions by testing in the air and not in the ground.

To do this, place your detector flat on a table and pass in front of the disk of metal objects at about 20 cm. Try coins, capsules, aluminum bottles, gold objects, or precious alloys, and take care to remember your detector's different reactions.

You will do the same by using the sensitivity and discrimination potentiometers with each of its objects to learn about these functions' settings and see your detector's signals. Ditto for the other functions or buttons of your detector.

You can then move on to the field after having assimilated the various basic manipulations.

First, check the condition of the batteries and always take extra batteries with you.

Make the appropriate settings (power on, sensitivity, and desired discrimination). Begin sweeping the ground by moving the detector disk from right to left while remaining parallel to the ground and as close to it as possible.

The sweep should describe an arc of a circle that will allow you to determine a "detection lane" of 0.8 to 1.2 meters in which you should not leave any unexplored surface.

The weakest sound signals are often the best because they reflect a fairly large depth of the detected object, which possibly means that it has been buried for a long time (the deeper the object, the greater the chance that it is not modern trash).

A good headset will allow you to increase the smoothness of your device's performance and your prospecting comfort.

Understand sensitivity standards for comparison purposes

By comparing different metal detectors' performance, you can examine individual sensitivity to specific types of metals.

In some cases, the clear winner is the metal detector with the best spherical sensitivity across all metal types when measured at the aperture center. However, in other cases, it may not be as clear as one detector outperforms the other on one type of metal and vice versa.

Minimize the risk of product recalls due to metal contamination

It is important that your metal detector offers the highest possible sensitivity. However, just as important is the system's stability and reliability to avoid false rejections and unnecessary product waste.

The targeting effect and the product effect must be understood and addressed where appropriate to ensure that you maximize your metal detection program's performance.