MINEX Double-D search concept for improved precise detection

First choice for basic mine detection tools

Researchers and laboratories worldwide are desperately trying to develop a 100% effective mine detector. The approaches to reaching this goal are as different as the basic technologies that form the basis of their efforts.

Ground-penetrating radar and infrared sensors get tested, as do so-called "artificial noses". While dogs and ratsare already used for mine searches, specially trained bees are on their way to enter the demining scene.

Unfortunately, at the moment, all the single and combined techniques are confronting the deminer with remarkable disadvantages. Most detection devices are only capable of analyzing a potential target based on particular characteristics. This can be the outer shape, the chemical content, certain in-built materials, and much more. Since these devices cannot combine different information sources, the fault alarm rate is very high, or certain targets are simply "invisible" for a particular technique.

In the current situation, new developments like the quadruple resonance principle or combined systems are showing promising performance on the one hand, but on the other hand, high costs, high weight, and other disadvantages like high training efforts must be accepted.

With this background, the standard hand-held metal detector still remains the first choice as a basic tool for mine-search, offering a reasonable combination of cost-efficient and effectiveness in the field.

Metal detectors can only detect metal

Metal detectors have been used for mine searches since the Second World War. Their reputation is suffering from the fact that they never detect mines; they detect the metal content within a mine. Unfortunately, the metal content of mines has shrank dramatically since World War II, a fact that forced the producers of such equipment to push its sensitivity up to a level working close to the theoretical physical limits.

By meeting the deminer’s requirements by offering high sensitivity, the modern metal detector is indicating highly problematic minimum-metal mines at sufficient depths. The better the metal detector performs, the better it will also indicate nails, fragments of exploded ammunition, and all kinds of scrap, not making any difference to a mine. This slows down the demining work remarkably.

In addition to this fact, a couple of soil types around the world are responding to the detector's signal just like a piece of metal. While the indication of "non-mine metal parts" cannot be excluded, the signals given by magnetic soil are the "New Frontier" for metal detector producers worldwide. The detector's premier task is not any more to show the highest possible sensitivity; it is the suppression of problematic soil’s signals in order to minimize the number of fault alarms. Good detectors have to represent an optimized compromise between high sensitivity and efficient soil adaptation.

Excellent ergonomics, low maintenance costs, easy handling, reliability, and low energy consumption are understood as standards for a professional metal detector.

How does a metal detector work?

The very basic idea behind the metal detector is to use the hidden object as an antenna. In order to do so, a detector produces an electromagnetic field. This field gets created automatically around a conducting wire.

The electromagnetic energy will penetrate the soil. As soon as it hits a metal object, like, for example, the firing pin of a mine, this object will work like any receiving antenna:

Due to the same physical rule, which describes the birth of an electromagnetic field around a conductor, a field will create a current inside a conductor as soon as it gets in contact with it. The hidden metal object will follow this rule, and due to the electromagnetic energy from the detector’s coil, a flow of current will arise.

In this case, the current is called "eddy-current" because, without any electric plus- or minus-pole to force it in a particular direction, it is moving through the object in circles. At this point, inside the metal object, there is the same situation as we already observed around the sending detector: current-leading metal! So it is not a surprise that the piece of metal will not only work like a receiving antenna. In addition to this, it will also work like a sending antenna, too. The eddy-current will give birth to another electromagnetic field, this time sent by the metal object.

The detector is prepared to receive the electromagnetic energy sent from the object by offering another conductor. In most cases, this job will be taken over by a receiving coil inside the search head. It is the last time the energy has to initiate a current. The detector’s electronics will interpret this current and indicate it by an audio signal or otherwise.

Despite its complexity, this process has inherent limitations. The signal's long journey, involving multiple stages such as field initiation, eddy current formation, and signal return, results in a weak final signal. In addition, different soil types pose different challenges to the detector's performance.

Coming back to the energy that gets sent out by the detector, we have to mention two different possible systems. The one described is a "continuous wave principle". Another term for it is "sinus principle". The electromagnetic energy in this system is sent continuously and can be illustrated like a sinus wave.

Another way to send the energy is to do it in a row of very short impulses. This holds two big advantages:

1. Since the sending coil between two impulses can be used to receive a signal, the producer of the equipment only has to integrate one coil, which lowers the cost.

2. The impulse signal offers a lot of information, and with sophisticated software, the evaluation of this information can suppress the influence of "aggressive" soil with good success. Modern impulse detectors also show excellent sensitivity, especially in the air.

As always in life, everything has good and bad sides. Most continuous-wave detectors, although keeping up very good sensitivity while measuring over soil, have outstanding problems suppressing the same as soon as they are dealing with aggressive soil. Good impulse detectors are able to handle this aggressive soil, but in comparison to their sensitivity in air, the performance over soil for the same target is quite reduced; furthermore, the answer signal to the short pulse, produced by a very little piece of metal (like the pin in a minimum metal mine), might be too short to be detected.

Up to a certain degree, this can be handled by raising the power output, but sending out high-energy impulses consumes more battery power on the one hand, and on the other hand, the strong impulses might trigger mines, which are constructed to react to those.

How does the MINEX 4.610 overcome these challenges?

The MINEX has been constructed with the goal of combining the advantages of both systems by excluding as many of the disadvantages as possible. The basically high sensitivity of a continuous-wave system had to be modified in order to enable sufficient ground control. This has been realized by adding a second frequency. The sending coil of the MINEX search head is continuously emitting two frequencies in parallel.

The choice of the particular frequencies has been based on the fact that different metals react more or less "friendly" at different frequencies. A system that is optimized for the search of aluminum objects does not necessarily find steel with the same efficiency. By combining two reasonable frequencies within the possible spectrum, all types of metal will be indicated with approximately the same sensitivity. The in-built electronics are performing a continuous comparison of the answer signal on both frequencies.

The MINEX mainly uses a change in the imaginary component for signal evaluation; this change is forced, for example, by steel. As a result, the steel object will be indicated, whether it's in saltwater or magnetic surrounding not. There is no doubt that for this system, too, sooner or later a "physical hurdle" becomes effective. The deminer meets this hurdle as soon as he moves within a terrain with highly inhomogeneous magnetic soil.

"Inhomogeneous" simply describes the fact that within the soil, a lot of particular concentrations of "magnetic" soil are distributed, of which each single one holds its particular characteristic signal. In this situation, a general soil signal characteristic of a whole area cannot be found, and therefore it cannot be used to be filtered. Operating a simple push-button will help the MINEX adjust to this situation, and this will solve the problem again up to a certain degree. Some magnetic stones are so aggressive that they will still lead to a fault alarm because the detector is indicating them just like a piece of metal. There’s one last way to "filter" them:

Knowing the type of mine in a particular area gives the deminer the chance to check it out beforehand. If he is able to detect this type of mine with a reduced sensitivity, equipped with this knowledge, he can reduce the fault alarm rate additionally by doing so.

In addition to the mentioned features, the MINEX has a third coil system inside its search head. (In fact, we are not talking about a coil; we are talking about a multi-layer printed board system that combines the highest precision with high mechanical peak-load capabilities.)

The three coils (we will stay with this expression since it is still commonly used) are responsible for the search head’s characteristic shape.

The outer coil is the sending one, while there are two receiving coils.

A very common mistake is to mix up the MINEX 2-frequency system and the two coils. It has to be underlined that each single receiving coil is continuously receiving the answer signal of both frequencies.

In fact, there are four signals to be handled by the detector, with two frequencies being received by each of the receiving coils. These receiving coils are placed in a so-called differential arrangement. If a signal affects both with the same intensity, no indication will be given. As soon as the influence of a metal object on one coil is stronger than on the other, an audible alarm will be produced.

Each of the coils is equipped with its own particular characteristic sound.

The deminer can hear if the right or left half of the search head is closer to the object. By passing an object with the search head, the sound will switch. This allows easy pinpointing as well as a clear separation of objects. Furthermore, the search along big metal objects like rails or fences is possible.

What are the advantages of the MINEX Double-D search head coil?

The presence of one mine may interfere with the location of another mine nearby, especially if they are buried at different depths or orientations. A Double-D search head has two overlapping D-shaped coils that create a more precise and deeper search field compared to other types of search heads. This type of search head can detect multiple metallic objects, including landmines, that are close to each other.

If one coil of the double-D search head reaches saturation, it means that it has reached its maximum detection capacity and may not be able to detect smaller objects. In such a case, the other coil can still be used to search for small objects for example near the fence.

How does the MINEX 4.610 help mitigate risks?

The MINEX 4.610, with its double-D search coil, works with active magneto-inductive methods (EMI). These methods are well suited for the detection of metals. The detection performance depends on the transmitting and receiving parameters as well as on the type of metal and the local ground conditions, as the method is based on the conductivity and magnetic permeability properties of the metal. A search coil generates magnetic fields that propagate through the ground. When metal parts are touched by this magnetic field, eddy currents are formed, which in turn trigger a secondary magnetic field. The effects of this field are detected and evaluated by the metal detector's receiving coil. At the same time, the interfering signals generated on the ground must be compensated.

Due to the MINEX dual-frequency system, sensitivity and ground control are being improved. By emitting two frequencies simultaneously, the MINEX provides comprehensive metal detection. The three coils enable the detector to discriminate between different metal objects based on their shape and orientation. Its electronics compare the response signals from both frequencies, effectively filtering out interference.

Leading to higher sensitivity and better pinpointing abilities than a traditional metal detector without a dual-frequency system. The received signals are evaluated and trigger acoustic or optical warnings so that the metal part can be located. Its user interface is designed to be intuitive, facilitating ease of use for demining teams.