If you’ve reached this page you probably have a good understanding of what metal detectors do – find buried metal objects, but do you know how they find the objects? On this page we cover a basic technical background on how metal detectors work and functionality for optimal detecting.
Topics covered are:
- Basic metal detector components
- How do metal detectors work?
- Metal detector discrimination
- Search coil basics
- How do search coils work?
- What is ground balance?
Metal detectors consistent of three basic components:
- The search coil
- A power supply
- The control box
Inside the search coil there are usually two sets of copper wire coils. The metal detector passes an electrical current through one of these coils (commonly called the ‘transmit winding’) and an electromagnetic field is created. The electromagnetic field penetrates the ground beneath the machine, detecting metal objects within the field.
Metal objects conduct electricity and disturb the electromagnetic field. The alteration to the field caused by the presence of a metal object is received by the second copper wire coil in the search coil (commonly called the ‘receive winding’).
When the electromagnetic disturbance is received, it is sent to the control box where it is interpreted as a metal object and an audible signal sounds to alert the operator.
Most metal detectors have the ability to identify the type of metal object causing the disturbance in the electromagnetic field and ignore ‘unwanted’ types of metal. Detectors emit a signal to the operator when a desirable target is found.
This function is called ‘discrimination’ and is achieved by the detector testing a target for its conductivity level. Ferrous (iron-rich) targets such as nails and wire have very low conductivity. By setting your detector to an appropriate level of discrimination you can ‘tune’ it to discriminate metal targets, reducing unwanted signals from non-targeted objects.
Detectors may also react to various non-metallic minerals in the ground. This non-metallic groundmass is known as ‘mineralised ground’ and can cause the detector to emit false signals (commonly called ‘ground noise’).
To eliminate ‘ground noise’ in mineralised ground, like gold fields, the detector will require a ‘ground balance control’ that will tune-out the ‘ground noise’ and allow you to target your search for the specific metal objects you are seeking. Find out more about ground balancing below.
The effective depth of penetration of the electromagnetic pulse is dependent on a number of factors. A crucial determinant is the size and mass of the metal object. As you would expect, the detector’s effective depth is proportional to the size and mass of the metal object detected.
The search coil is a vital part of your metal detector. It is the flat, typically circular disk, which generates a magnetic field and senses metallic targets in the surrounding environment. It is located at the end of the stem and is connected to the control housing via a cable normally wound around the stem. The size and depth of the magnetic field is determined by the shape and size of the search coil. Understanding the purposes behind the various sizes and shapes of search coils will empower you with the ability to choose the best search coil for each application.
Search coils on VLF detectors generally consist of two internal sets of coiled wires, a Transmit Coil (TX) and a Receive Coil (RX) in concentric or ‘Double D’ (DD) configuration. Generally, a concentric coil will be more sensitive to small targets than a DD coil of the same size, but the DD coil will operate more smoothly in mineralised soils.
Mono-loop coils are specialist coils which can only be used with a ‘Pulse Induction’ metal detector, where the same winding acts as both TX and RX. This type of system is not possible on a Constant Wave (CW) VLF metal detector. When the detector is turned on, the TX coil generates a magnetic field in the surrounding space. When a metallic object is within this generated magnetic field, it will create a distortion in the magnetic field. The RX coil will sense this distortion and send a signal to the control housing. A search coil’s detection pattern is determined by the combination of the TX’s generated field pattern and the RX’s sensing field pattern.
The amount of minerals in the soil varies from location to location. Sometimes the composition of minerals changes within a small area. Other locations, the mineral composition in the soil will be fairly consistent over a large area. The type of minerals contained in the soil will also vary according to location. For instance, one of the more common minerals present in soil is iron, and some areas of the country have high concentrations present.
Without some method of "ground balancing", the metal detector would be detecting all the minerals present in the soil, therefore not seeing through them, and thus, small or deeper targets would not override the signals produced by the minerals. In extremely mineralized soil, the metal detector would hardly be able to detect a target laying on the surface of the soil.
Ground balancing the detector is simply, the term used to describe the function of adjusting the metal detector to ignore the minerals in the soil so that they are not detected by the metal detector.
Various manufacturers utilise different ground balancing options offered on their models such as:
- Pre-set Ground Balance
- Automatic Ground Balance
- Manual Ground Balance
- Combination of any pre-set and manual ground balancing
Manual Ground Balance
Some metal detectors employ manual ground balance technology to provide the operator with the capability of independently and manually altering ground balance to accommodate a range of varying ground mineralisation conditions to enhance the detection performance of the equipment.
Automatic Ground Balance
Automatic ground tracking refers to the capability of the detector unit to track changes in ground mineralisation and automatically adjust the ground balance to suit the varying conditions of the terrain.
This capability ensures optimal ground balance configuration and provides for full-depth detection in any terrain, eliminating the need for the operator to stop and manually adjust the detector as ground conditions and mineralogy change.
Pre-set Ground Balance
This is also called Fixed Ground Balance, it means the metal detector has factory pre-set (fixed) ground balance, set for ‘median’ soil conditions, and is not operator adjustable. Typically, entry-level detectors will have a Pre-set (Fixed) Ground Balance.
This limits optimal detection capability to a specific type of terrain or ground conditions. However, in conditions where the terrain is uniform and ground conditions do not vary greatly, detector performance is excellent, especially in less mineralised locations such as parklands, playgrounds and dry beach sand.