How Metal Detectors Work
How Do Metal Detectors Work?
Metal detectors work by transmitting an electromagnetic field from the search coil into the ground. Any metal objects (targets) within the electromagnetic field will become energised and retransmit an electromagnetic field of their own. The detector’s search coil receives the retransmitted field and alerts the user by producing a target response. Minelab metal detectors are capable of discriminating between different target types and can be set to ignore unwanted targets.
The battery provides power to the detector.
2. Control Box
The control box contains the detector’s electronics. This is where the transmit signal is generated and the receive signal is processed and converted into a target response.
3. Search Coil
The detector’s search coil transmits the electromagnetic field into the ground and receives the return electromagnetic field from a target.
4. Transmit Electromagnetic Field (visual representation only - blue)
The transmit electromagnetic field energises targets to enable them to be detected.
A target is any metal object that can be detected by a metal detector. In this example, the detected target is treasure, which is a good (accepted) target.
6. Unwanted Target
Unwanted targets are generally ferrous (attracted to a magnet), such as nails, but can also be non-ferrous, such as bottle tops. If the metal detector is set to reject unwanted targets then a target response will not be produced for those targets.
7. Receive Electromagnetic Field (visual representation only - yellow)
The receive electromagnetic field is generated from energised targets and is received by the search coil.
8. Target Response (visual representation only - green)
When a good (accepted) target is detected the metal detector will produce an audible response, such as a beep or change in tone. Many Minelab detectors also provide a visual display of target information.
Key Detecting Concepts
The frequency of a metal detector is one of the main characteristics that determines how well targets can be detected. Generally, a single frequency detector that transmits at a high frequency will be more sensitive to small targets and a single frequency detector that transmits at low frequencies will give more depth on large targets. Minelab’s single frequency technologies are VLF and VFLEX. Minelab’s revolutionary BBS, FBS and MPS technologies transmit multiple frequencies and are therefore simultaneously sensitive to small and deep large targets.
Ground Balance is a variable setting that increases detection depth in mineralised ground. This ground may contain salts, such as in wet beach sand or fine iron particles, such as in red earth. These minerals respond to a detector’s transmit field in a similar way that a target does. Due to the much larger mass of the ground compared to a buried target, the effect of mineralisation can easily mask small targets. To correct this the Ground Balance setting removes the responding ground signals, so you clearly hear target signals and are not distracted by ground noise.
There are three main types of Ground Balance:
1. Manual Ground Balance – Manually adjust the Ground Balance setting, so the minimum amount of ground signal is heard.
2. Automatic Ground Balance – The detector automatically determines the best Ground Balance setting. This is quick, simple and more accurate than a manually set Ground Balance.
3. Tracking Ground Balance – The detector continuously adjusts the Ground Balance setting while detecting. This ensures that the Ground Balance setting is always correct.
Minelab detectors use exclusive advanced technologies for superior ground balancing capabilities that cannot be matched by any other detectors.
Discrimination is a metal detector’s ability to identify buried targets based on conductive and/or ferrous properties. By accurately identifying a buried target you can decide to dig it up or consider it as junk and continue searching. Minelab detectors produce target identification (Target ID) numbers and/or Target Tones to indicate the type of target that has been detected.
There are four main types of discrimination in Minelab detectors:
1. Variable discrimination – The simplest type of discrimination which uses a control knob to adjust the level of discrimination.
2. Iron Mask/Iron Reject – Used mostly with gold prospecting detectors to ignore iron junk.
3. Notch discrimination – Allows specific target types to be accepted or rejected.
4. Smartfind – The most advanced form of discrimination. Target IDs are plotted based on both ferrous and conductive properties on a two dimensional (2D) display. Individual segments or larger areas of the display can be shaded to reject unwanted targets. [Refer to page 29]
Detection Depth Factors
The most common question about metal detectors is ‘How deep do they go?’
The simple answer is "as deep as the diameter of the coil". So detectros with larger coils will detect deeper.
However, detection depth also depends upon detector technology and many environmental factors. A more complex answer usually starts with ‘It depends...’ The depth that a metal detector can detect a target depends on a number of factors:
A target in low mineralised ground can be detected deeper than a target in highly mineralised ground. The level of ground mineralisation has a significant influence on detection depth.
Large targets can be detected deeper than small targets.
Circular shapes like coins and rings can be detected deeper than long thin shapes like nails.
A horizontal coin (e.g. lying flat) can be detected deeper than a vertical coin (e.g. on edge).
High conductive metals (e.g. silver) can be detected deeper than low conductive metals (e.g. lead).