Modern metal detectors are an advantage over previous prospectors. Hence, it is not uncommon to find valuable gold nuggets over areas thought cleared, by using modern technology.
But, how do metal detectors actually work?
It is thought that the first metal detector was accidentally discovered in 1925 by a radio engineer who noticed that a metallic water tank would cause interference with his experiments.
Metal detectors evolved since 1925, becoming smaller, easier to use, more affordable, and more capable of detecting gold and overcome interferences or “noise”.
In the early 1970s, portable hand-held cdetectors were first developed and brought along a new era of gold prospecting and discoveries.
Metal detectors work on the principle of detecting conductable metals, however they also react to non-metallic targets such as salty water, laden sand/soils, and magnetic rocks.
The VLF (very low frequency) ground cancelling detectors work on this principle of conductivity, and can locate a metal with a conductive survface within 45 to 60cm below ground.
When an electro-magnetic field is generated by the coil in the metal detector, “eddy currents” (electrical currents) flow along the surface of a metal, which generate its own electro-magnetic field radiating in all directions.
When a metal is detected, the currents emitted are received by the search coil in the detector, and the signal is communicated to the user. This is received by the user via a speaker, headphones, or a meter.
Naturally, beside metals, any material capable of transmitting current will emit a signal. Hence, beside gold, other minerals are detected, such as:
- Iron ores
- Magnetic minerals
- Beach salts
These non-ferrous materials create a signal which disturbs the electro-magnetic field created by the metal detector.
Most VLF metal detectors offer a “TR” discriminator mode, which is 90% accurate in discriminating between non-ferrous and ferrous substances.
The TR mode measures the conductivity of the detected material and informs the user by discriminating materials between the following two groups:
- Group 1 (high conductivity materials): gold, silver, copper, lead, zinc, and nickel.
- Group 2 (low conductivity materials): iron, tin, foil-coated wrappers, and bottle tops.
However, TR mode does not cancel the effects of magnetic rocks. Therefore, the metal detector will cause an audible signal similar to that caused by metals. Salt laden sands will also cause this noise.
MULTI-PERIOD SENSING (MPS)
Multi-period sensing (MPS) was developed in 1994 by physicist Bruce Candy. It involves the use of a single coil to transmit and to receive a magnetic pulse.
The coil transmit pulses of voltage on and off, enabling the device to transmit and to receive the magnetic pulses. When the coil is on, high voltage is induced into the coil winding which generates a spike of voltage transmitted into the ground. When the coil is off, it enables it to “hear” the response.
When gold and surrounding soil are hit by the voltage pulse, Eddy currents in the soil dissipate faster than Eddy currents in the gold nugget. The metal detector can interpret this delay between the two signals received, enabling it to detect the metal.
The rate at which this on/off pulse is emitted depends largely on the soil conditions. If the soil is moderately to strongly mineralised, the detector must be setup to a higher pulse rate.
Hence, in strongly mineralised soils such as those found in the Eastern Goldfields of Western Australia, the MPS technology with variable pulse rate was a major breakthrough, creating a new gold rush era.
The development of metal detectors with MPS enabled detection of smaller metal nuggets at greater depth. Hence, previously prospected ground was revisited with great success.
EVOLUTION OF METAL DETECTORS
“Noise” has been the main limiting factor in the ability of metal detectors to detect valuable gold nuggets. Hence, different generations of metal detectors vary mainly in the ability of the devices to reduce noise.
The first generation of metal detectors (1975 to 1984) were capable of detecting large nuggets close ot the surface, but due to ground interference or “noise”, smaller pieces of gold could not be detected.
The second generation of metal detectors (1985 to 1991) had improved technology that enabled the device to reduce ground noise and increase the penetration depth.
The third generation of metal detectors began in 1995 after the invention of Multi-Period Sensing (MPS). These detectors were capable of finding gold nuggets as small as 0.3 grams, as well as locating larger nuggets at greater depths than ever before.
The fourth generation of metal detectors began in 2006 via improvements in the ability of monocoils to reduce ground interference noise on highly mineralised terrain.
The fifth generation of metal detectors begain in 2010 with further developments on the ability of coils to reduce ground interference. The fifth generation uses Pulse Induction (PI) technology to further enhance the capability of metal detectors to reduce noise.
The sixth generation of metal detectors began in 2015 with the development of Zero Voltage Transmission (ZVT) technology. ZVT has the advantage of transmitting a constant and stable magnetic field.
PULSE INDUCTION (PI) TECHNOLOGY
Pulse Induction (PI) technology may have two or even three coils working together. Detectors equipped with PI technology emit to the ground powerful but short burst (pulses) of current, each generating a short magnetic field.
When the pulse ends, the polarity of the magnetic field reverses, creating a sharp but short electrical spike (“reflected pulse”). Up to 1000 pulses per second are emitted by the metal detector.
When a pulse is emitted over a metal object, it creates an opposite magnetic field in the object, creating an “echo” which is then received by the device.
ZERO VOLTAGE TRANSMISSION (ZVT) TECHNOLOGY
ZVT technology combines the advantage of PI for ground balancing, and the advantage of CW for detecting very large nuggets.
Both ZVT and PI technologies measure a signal immediately after a rapid change in magnetic field, however ZVT transmits a constant magnetic field.
By transmitting a constant magnetic field, the coil’s reactive voltage during the receive period is zero, hence the term “Zero Voltage”.
One key advantage over PI technology is the ability to detect large nuggets. This is because PI is less sensitive than ZVT to longer time constant signals emitted by large nuggets.
Another significant advantage of the ZVT is that the stable and constant magnetic field emitted by the device ensures that the large reactive component of the soil signal is ignored. Thus, less noise is received, making it more sensitive for small nuggets.
CONCLUSION
Ultimately, there are many factors that affect the ability of a detector to find gold, including individual skills of the operator.
The ability of a detector to find gold depends on many factors such as particular detector settings, coil size and configuration, ground conditions, types and degree of ground mineralisation, interference by electromagnetic waves, nugget size and composition, and nugget shape.
However, having an understanding of the essential function of each part of a detector, the parameters which cause noise, how to minimise noise, and how metal detectors work general, will greatly improve your chances of finding gold.
Another important factor that is often overlooked is that many metal detectors, specially the ones on the lower end, have search coils which do not pick up gold nuggets or other objects at certain depth unless they are scanned immediately below the centre of the disc.
With such detectors, it is absolutely essential that every patch scanned receives at least some attention from this central, more sensitve point of the search coil.