The Past & Future of Metal Detecting
The Past & Future of Metal Detecting
As early as the late 1800’s some of the first metal detectors were created to help detect bullets in soldiers, including one lodged in President Andrew Garfield.
A gentleman named Gerhard Fischer was messing around with radio navigation when he discovered that metal objects distorted radio frequencies. He later converted this idea into a search coil that would send the frequencies into the ground.
At the same time, another gentleman named Shirl Herr invented a hand-held metal detector that could go up to 8-feet and was used to recover lost artifacts and wreckage.
Later, a Polish inventor Stanislaw Kosacki designed a metal detector specifically for finding land mines. Kept secret for years, this detector was quite successful, and was even used in the invasion of Normandy.
In the 50’s you hear about Whites, Garrett, and later others who made lighter machines (albeit still heavier than today's detectors), with battery packs.
(Photo) Some of Bob's antique metal detectors.
These lighter machines functioned on BFI, or Beat Frequency Induction, which required movement of the coil to detect, and pulsed electromagnetic frequencies into the ground.
These early detectors were very simple, and could even be built at home. However, they had no discrimination, and had to be very precisely tuned to find either minerals, or conductive objects.
Ground mineralization has magnetic properties, and can drive a detectorist crazy. BFI’s would be able to be tuned to see either the mineralization, or the conductive target, but tuning them was a headache. The one benefit of these machines is that they could ignore larger ferrous items.
(Photo) Early antique Whites elaborate control boxes.
Discrimination of Metal Objects
A phase response is the relationship between the input and output of the detector, what's coming in, and what it's sending out. Every metal object has a different phase response when exposed to an alternating current.
Lower frequencies represent longer waves, and penetrate deeper into the ground, and are more sensitive to higher conductors like silver copper and brass. While higher frequencies create shorter waves, and are more sensitive to lower conductors like iron, gold, nickel, aluminum, and generally smaller objects.
Lower frequencies would be anywhere from 3-15kHz, whereas higher frequencies range from 20kHz and higher.
Shifting the phase response can reveal more information about a target's conductivity, and its conductivity in comparison with other targets, including whether it’s ground mineralization or an actual target. Phase response is a sort of discrimination in other words.
Unfortunately, metals like aluminum, nickel, and gold all have a similar phase response, making them very difficult to discriminate.
(Photo Below) Vacuum Tubes existed before solid state transistors, microphones, and computers. Below shows a vacuum tube at the bottom of the early Whites metal detector.
VLF, Very Low Frequency Metal Detectors
VLF, or Very Low Frequency detectors blew up in the 1990’s. Various companies started engineering better coils that could handle the ground, and avoid search coil drift effects.
In a huge advance, VLF’s worked with a continuous wave, and could eliminate the ground noise with ground balancing. This was a huge advance in handling heavily mineralized ground sites where gold is found.
Unlike earlier detector models, VLF’s got fooled by larger iron targets like nails. However, eventually iron discrimination was integrated, but only to a certain depth extent. So detectorist's still have problems with reliable target identification at depth.
Now VLF detectors offer a huge array of settings, including sensitivity adjustment, discrimination, GPS, WiFi, sweep/recovery speed, threshold adjustment, and target volume pitch and tones.
Displays called VDI (Visual Display Indicator) viewed on a screen on the main control box of the detector allow users to see the depth of their target, a number that corresponds to the conductivity of a target-- including ferrous vs. non-ferrous. Some advanced detectors can display multiple targets animated on the screen at once, each color-coated to represent their appropriate conductivity.
PI’s, Pulse Induction Metal Detectors
Pulse induction metal detectors don’t use uniform alternating currents, but instead magnetize the ground with a short pulse of current. In other words, PI’s alternate between transmitting and receiving in short eddy currents. These eddy currents momentarily “light up” a target, and, depending on the target’s conductivity, that eddy current will be stronger or weaker. This allows PI’s to ignore heavy ground mineralization that has weak conductivity.
(VIdeo) Garrett ATX Pulse Induction Detector in action.
Untitled from Keph Sherin on Vimeo.
This technology helped penetrate the most difficult black sand and heavy iron mineralization with its ability to ignore it. Unfortunately PI’s don’t have any discrimination, with the exception of the Garrett ATX, which uses iron discrimination most likely by alternating a circuit switch when the user enables the iron discrimination feature.
Multi-Frequency VLF Metal Detectors
The introduction of multiple frequencies used at once in VLF’s allowed for better target identification by the detectors processor, and better ability to ignore ground mineralization and go deeper. The king of this group would undoubtedly be the Minelab CTX 3030.
The Future of Metal Detectors
Further innovation has brought forth detectors that can switch between frequencies, and some in which the user can switch between multiple and single frequencies. These include detectors in the Nokta Makro Kruzer series, the Minelab Equinox series, the XP Deus series, and the Garrett Ace Apex to name a few.
These detectors offer the ability to quickly adjust to environments which resonate better with specific frequencies depending on the ground mineralization characteristics. So instead of having to carry around an arsenal of single frequency metal detectors, a user can have one powerful and versatile multi-purpose detector.
Now companies have begun to experiment with hybrid PI/VLF detectors, which would boast the ability to discriminate like a VLF and penetrate heavy mineralization like a PI while still detecting lower conductor targets. So far, we have not seen ample discrimination features on these hybrids, and the amount of ancient and modern trash that needs to be addressed in gold country begs for a discrimination feature.
A few years ago, Minelab came out with the GPZ 7000, which utilizes an entirely new kind of technology they call zero voltage, that uses a huge magnet that flips back and forth between polarities.
Some companies have begun to integrate smartphones into metal detecting. Smartphones processors are astronomically more powerful than processors on current metal detectors and could be used to create incredible algorithms to better detectors discrimination.
Some smartphone apps are offering 3-D augmented reality on a cell phone screen to show exactly where you have detected, and target locations and conductivities represented by colored cones.
It is of the writer's opinion that engineers haven’t even scratched the surface of utilizing the sheer power of smartphone processors, and that stepping outside the box, there is still a lot of improvement and innovation that can happen with the tools that are at our disposal.