Magnetically sensitive search devices.

Magnetically sensitive search devices..

MAGNETIC-SENSITIVE SEARCH DEVICES.

To search for metals in concealed environments — underground, under water, in snow, etc., several types of metal detectors are used, differing from each other primarily in their operating principle — the sensitive element. Metal detectors can be divided into two main groups: the first group is induction (eddy current) metal detectors, the second group is magnetically sensitive metal detectors. Induction metal detectors allow you to detect any conductive objects hidden in a non-conductive or weakly conductive environment. Induction metal detectors operate on the following principle — an exciting induction coil, through which a pulsed or periodic current flows, induces eddy currents in the desired conductive object and a system of signal coils receives a signal from the induced currents in the object. According to the described principle, induction metal detectors can be considered active devices, that is, they affect the search object.

In this article, we will focus on magnetically sensitive devices, the main difference between which and induction metal detectors is that these devices can only find ferromagnetic objects. Ferromagnetic objects either have their own magnetic field or distort the Earth's homogeneous field, and in both cases, the magnitude of the magnetic field in the sensitive element zone changes its magnitude and direction. This is a sign of a ferromagnetic object. In relation to the sought object, these devices are passive, that is, they do not have any effect on the object.

The most important parameter of a metal detector is its sensitivity, i.e. the maximum detection range of the desired object. At the same time, it is practically impossible to formalize this parameter and make it uniform for all metal detectors, not only because metal detectors differ in their operating principle, but also in the design of converters and the signal processing function, as well as the variety of shapes of the desired objects and the properties of metals. For induction metal detectors, round or square plates made of various metals and of various sizes are usually used as sensitivity standards. Using these plates, it is possible to compare the distances at which these plates are detected by different induction metal detectors. This method of determining the sensitivity of magnetically sensitive metal detectors is unacceptable for the reasons that the maximum detection depth of a ferromagnetic object will depend not only on the size of this object, but also on its orientation in space and relative to the sensitive element, as well as on the degree of magnetization of the object. It is customary to indicate the depth of the object, the shape, size and material of which are known to a sufficient number of people, when describing the characteristics of search devices to specify sensitivity.

For magnetically sensitive metal detectors, sensitivity is usually designated by the value of the magnetic induction of the field that the device is capable of registering. Sensitivity is usually measured in nanoteslas (nT) 1nT = (1E-9)T.

In addition to sensitivity, a parameter such as resolution is used to determine the quality of the device. It is also measured in nanoteslas and determines the minimum difference in induction that can be recorded by the device.

In order to represent the magnitude of the magnetic field induction that is recorded by modern magnetometers, it is enough to calculate the magnitude of the magnetic field created by a conductor with a current of 1 mA at a distance of 0.1 m.

The Earth's field is approximately 35,000 nT. This is an average value — at different points on the globe it varies in the range of 35,000 — 60,000 nT. Thus, the task of searching for ferromagnetic objects is to detect, against the background of the Earth's natural field, an increase in the field caused by distortions from ferromagnetic objects.

There are several physical principles and types of magnetometric devices based on them that allow recording minimal changes in the Earth's magnetic field or distortions introduced by ferromagnetic objects. Modern magnetometers have a sensitivity of 0.01nT to 1nT, depending on the operating principle and the class of tasks being solved.

Let's consider the most common physical principles for constructing magnetometers.

The first method, which has received the widest distribution, is the method based on the nonlinear properties of ferromagnetic materials. Sensitive elements implementing this principle are called ferroprobes. A ferroprobe is an inductance coil with a nonlinear core. Permalloy wire is most often used as such a core. Fig. 1 and 2 show a drawing and a graph explaining the operating principle of the ferroprobe.

Fig. 1

Fig. 2

If an alternating current is passed through the excitation coil, which will create an alternating field with an amplitude of intensity Hm and apply a coaxial constant field of intensity Ho to the ferroprobe, then at the output of the receiving coil of the ferroprobe a voltage will appear proportional to the constant magnetic field Ho and with a doubled frequency. The appearance of a voltage of doubled frequency is due to the nonlinear characteristic of the ferroprobe core. This voltage is the signal by which the external magnetic field is judged.

The ferroprobe is a vector device, i.e. the output signal of this sensitive element depends not only on the magnitude of the external magnetic field, but also on its direction relative to the ferroprobe axis.

This property of the ferroprobe allows it to be used as a spatial orientation device relative to the Earth's field lines of force, but for constructing a search magnetometer this property is rather a disadvantage, since during the search a change in the orientation of the search device converter is inevitable. As was said above, the search for ferromagnetic objects occurs against the background of the Earth's natural field, which is five orders of magnitude greater than the field increments introduced by the search objects, therefore, to solve the problem of eliminating the influence of orientation, non-trivial design and electronic circuit techniques must be adopted.

Figure 3 shows a schematic design of the converter of the ferroprobe search device, in which the influence of orientation relative to the Earth's field lines of force is largely compensated.

Fig. 3

The converter consists of two ferroprobes connected differentially and located on the same axis and at a certain distance (base) from each other. Each ferroprobe in such a converter is called a half-probe.

Adjustment screws 1 and 2 provide mutually perpendicular displacement of the semi-probes relative to the hinge points and thus allow achieving a high degree of coaxiality of the semi-probes. Fig. 4 shows the electrical circuit of the differential ferroprobe converter.

Fig. 4.

Igen. – ferroprobe excitation current;
Usignal – voltage at the output of the measuring windings.

Usignalis a complex harmonic signal in which the information about the magnitude of the external magnetic field is carried by the difference in the amplitudes of the second harmonic from each half-probe. Since the half-probes are made identical, the output signal does not depend on the uniform field of the Earth, but is determined only by the gradient of the external field. A ferroprobe converter made according to a differential circuit (see Fig. 3, 4) is called a gradiometric or gradiometer. The converter adjustment procedure allows to exclude, to a sufficient degree for practice, the influence of the spatial orientation of the converter relative to the lines of force of the Earth's magnetic field on the output signal. In addition, the converter is structurally placed on a rotating axis so that under its own weight it always occupies a vertical position relative to the earth's surface, which is advisable for two reasons: firstly, the magnetic lines of force of the natural field are directed at an angle of 400 to the surface of the Earth and the field gradient from the distortions introduced by ferromagnetic objects will be maximum when the direction of the natural field approaches the axis of the converter, and secondly, such a natural location of the converter reduces errors from spatial vibrations of the converter inevitable during the search.

The design of the ferroprobe metal detector includes a rod with a battery power supply and an electronic unit placed on it, and a ferroprobe converter rotating on an axis perpendicular to the rod and the converter.

Fig. 5 shows how the Earth's magnetic field lines are distorted by a ferromagnetic object, which is recorded by a flux-gate device.

Fig. 5

As mentioned above, the fluxgate transducer is a vector device, i.e. the output signal of the transducer depends on the magnitude and direction of the applied field. This allows obtaining additional information about the orientation and size of a hidden ferromagnetic object. Fig. 6 shows the envelope of the transducer output signal from an extended object (pipe) underground.

Fig. 6

Using a fluxgate gradiometer, it is possible to estimate the depth of objects, for which it is necessary to draw the envelope of the signal from the object in the coordinates: U– signal level, L distance. The width of this envelope at a level of 0.5 from the maximum is approximately equal to the depth of the object. Fig. 7 shows the signal envelope with the transducer when it moves over an object hidden underground at a distance of H.

Fig. 7

The most famous serial devices in our country are the ferroprobe metal detectors of the Institute of Dr. Foerster (Germany). These are the OGF, Ferex4.021 and Ferex 4.032 models.

Below are the technical characteristics and a brief description of the Ferex4.021 and Ferex 4.032 metal detectors

The Ferex4.021 metal detector is designed to search for ferromagnetic objects underground and underwater. Common search objects are unexploded bombs, main pipelines, power cables, and wrecks of ships and aircraft that have crashed.

The device can be used for various applications depending on the set operating mode:

• Search and localization of all ferromagnetic objects.
• Search and localization of large objects with suppression of the influence of small objects.
• Search for moving objects — the influence of static objects is suppressed.
• Use as a compass.

Ferex4.021 consists of the following basic components:

• hermetic electronic unit;
• hermetically sealed battery compartment;
• hermetically sealed converter.

Depending on the practical application, there are three versions of the device, differing from each other in some changes in the basic components and additional accessories.

The Ferex4.021 metal detector allows you to detect large ferromagnetic objects at depths of up to 6 m, but larger objects can be detected at a much greater depth.

Table 1 contains test data for the Ferex 4.021 flux-gate metal detector

Table 1

The Ferex4.032 metal detector is based on the best characteristics of the previous Ferex4.021 model and differs from it:

• lighter weight;
• higher sensitivity due to a larger base between the semi-probes;
• low energy consumption;
• built-in data recording device;
• possibility of working with several converters in parallel;
• completely waterproof;
• two-year warranty period.

• eight sensitivity ranges from 3 nT to 10,000 nT with a resolution of 0.3 nT;
• 650 mm base;
• power supply – 4 C-type batteries;
• water protection – IP57, 95% relative humidity;
• operating temperature range from -25 to +55 0С;
• weight 4.5 kg;
• dimensions 900×500 mm.

Photo 1. Ferromex 120 ferroprobe metal detector by Unimex Handels Gmbh (Germany)

Among Russian developments, the FT-100 ferroprobe metal detector is known (photo 2). The device was developed by the company “AKA-CONTROL”, Moscow. This is a simplified version of the search device, in which the electronic unit and the converter form a single rigid structure, there is no precise adjustment of the semi-probes, and the signal level is judged by the variable-frequency sound signal. With this device, the “Ekipazh” search group discovered a tank from the Great Patriotic War in a lake at a depth of 6 m.

Photo 2. FT-100 ferroprobe metal detector

In addition to fluxgate metal detectors, the most widely used quantum devices are those based on the nuclear magnetic resonance effect and the Zeeman effect with optical pumping. They implement more fundamental physical principles and have greater sensitivity.

In the classical view, free microparticles with both magnetic and mechanical moment precess in a constant magnetic field. The precession frequency (Larmor frequency) is proportional to the magnetic field induction B. A distinction is made between quantum magnetometers with free and forced nuclear precession. In a magnetometer with free nuclear precession, an ampoule with a working substance (water or another proton-containing liquid) is placed in a receiving coil, which is included in a frequency-tunable oscillatory circuit. An auxiliary constant magnetic field, stronger than the measured one, polarizes the working substance in a direction perpendicular to the working field.

After a quick switch-off of the auxiliary magnetic field, the moments of the atomic nuclei freely precess relative to the direction of the measured field B with an exponentially decreasing amplitude over 2–3 s. In this case, an EMF with a precession frequency (Larmor) is induced in the receiving coil, which is measured by a frequency meter. The sensitivity of proton metal detectors with free precession in weak uniform fields of the order of the earth's magnetic field reaches 1 nT.

In metal detectors with optical pumping of the working substance, the frequency of the high-frequency generator is recorded when it coincides with the frequency of inverse quantum transitions between the sublevels of fine and hyperfine magnetic splitting. The moment of coincidence is observed by the resonant absorption of light energy, accompanied by scattering or refraction of light when it interacts with the atoms of the working substance. The sensitivity of such metal detectors reaches 1E-13 T.

There is also a class of superconducting magnetometers (metal detectors) based on the Josephson effect. Superconducting quantum interferometers (SQUIDs) of direct or alternating current are used as measuring transducers in such magnetometers.

In magnetometers with a direct current SQUID, the increment of the external magnetic flux is converted into an oscillating voltage at the contacts of the sensitive element: during measurement, the total number of voltage oscillations is counted during the time of flux imposition.

In magnetometers with alternating current SQUID, the oscillating function of the magnetic flux is the total inductance of the superconducting ring and, therefore, the voltage on the high-frequency oscillatory circuit connected to it. In superconducting magnetometers, a record sensitivity level of 10E-15 T at frequencies of 0-1 Hz has been achieved.

The disadvantage of SQUID magnetometers is the need to maintain superconductivity conditions in the volume of the sensitive element using liquid helium or nitrogen. This complicates the design of the device and makes it inconvenient to operate in the field.

A common disadvantage of quantum and superconducting magnetometers is their low speed compared to fluxgate magnetometers, which can lead to missing search objects during fast scanning.

Brief descriptions and technical characteristics of quantum and superconducting magnetometers are given below.

The new highly sensitive magnetometer SMARTMAG with a cesium vapor converter with optical pumping has a high resolution, long-term stability and relatively high speed. SMARTMAG is a compact and lightweight device, the converter of which is fixed on the rod, and the graphic screen with a resolution of 64×240 pixels displays a variety of graphic information about the modes and results of the search. Together with the magnetometer SMARTMAG a specially developed software system is supplied, which allows processing of magnetic profiles, mapping and selective analysis of anomalies. The software provides for the convenience and simplicity of data transfer and processing on any IBM-compatible computer.

• measurement range 20,000-100,000 nT;
• sensitivity 0.01 nT at a standard measurement frequency of 2 meas./sec;
• speed 4; 2; 1; 0.5 measurements per second;
• power supply — 2x12V batteries;
• continuous operation time — 8 hours;
• operating temperature range from -20 to +50 0С;
• weight with batteries 10 kg.

The SMARTMAG magnetometer is distributed in Russia by Aerogeotech”.

GSM-19 is a modern proton magnetometer (GEM System, Canada), with the ability to configure units and converters to solve various problems.

GSM-19 can be used as a pedestrian magnetometer, which allows for almost continuous data collection along a survey route. Data is recorded at discrete time intervals as the device moves along the route. At each survey picket, the operator presses the button to bind the obtained data to reference coordinates.

To speed up obtaining a two-dimensional topology of magnetic anomalies, it is possible to bind the magnetometer to reference coordinates determined using a real-time differential GPS system and a navigation option. The accuracy of determining coordinates is within 1 meter.

GSM-19 can be used for shallow and deep sea surveys. In the first case, the magnetometer converter is placed in a sealed gondola and lowered underwater on a cable up to 100 m long, in the second case, the entire device with the converter is lowered underwater in a sealed gondola designed for immersion up to 300 m, and the device is controlled and data is collected via RS-232.

The sensitivity of GSM-19 reaches 0.02 nT, the range is 20,000 — 100,000 nT.

The memory capacity of the device, depending on the configuration, ranges from 6,000 to 700,000 measurements.

The portable cesium magnetometer-gradiometer G-858 provides high search speed with high sensitivity. The device easily detects a barrel at a depth of 6 meters. Due to the high speed, it is possible to search at a fast pace, covering an area 10 times larger than using previous quantum magnetometers, and using a horizontal gradiometer, it is possible to cover the search area with 50% time savings.

• working substance – non-radioactive Cesium 133;
• registration range 17000 – 20000 nT;
• sensitivity: 0.05 nT at a cycle rate of 0.1 s;
• orientation error ± 1 nT;
• temperature drift 0.05 nT per 10 C;
• memory for 8 hours of search at the maximum sampling frequency.

The portable quantum magnetometer MM-60M1 (Russia) has high sensitivity, speed and stability of operation in non-uniform fields with a gradient of up to 2000 nT/m.

The three-chamber design of the cesium magnetosensitive unit, in comparison with single-chamber devices, allows to expand the working angular zone by 3-4 while simultaneously reducing orientation errors by 1.5-2 times.

• Measurement range 20,000 — 100,000 nT;
• Resolution 0.01 nT;
• Time of one measurement 0.1 s;
• Memory capacity 7500 measurements;
• Operating temperature range from -10 to +500 C
• Power consumption 10 W;
• Overall dimensions:

• magnetic sensitive unit: 132×157*1000 mm;
• console: 100x200x230 mm;
• power supply 125x220x174 mm

• Weight:

• magnetic sensitive unit 2 kg;
• console 3 kg;
• power supply 4 kg.

Quantum potassium magnetometer MKK-01 (Russia, GOI)

The magnetometer is a radio spectrometer that automatically adjusts to the frequency of a specific transition in the electron paramagnetic resonance (EPR) spectrum of potassium vapor polarized in the ground state by optical pumping. The magnetometer belongs to the class of self-generating devices, being a quantum generator of a harmonic signal, the frequency of which is related by a known dependence to the field induction modulus. In the first approximation (with an accuracy better than 1%), this is a linear dependence with a proportionality coefficient of 7 Hz/nT.

• measurement range 10,000 — 80,000 nT;
• systematic error, including orientation less than 0.1 nT;
• operating temperature range from +15 to +350 C;
• supply voltage 24 V;
• power consumption 30 W.

SQUID magnetometer was developed at Novosibirsk State Technical University. It has the highest sensitivity among the magnetometers described above: 2E-13 T. At the same time, the condition of superconductivity of the sensitive element is provided by liquid nitrogen, which is much more profitable from the point of view of operating costs than cooling with liquid helium. Nevertheless, such a magnetometer is inconvenient for field conditions due to the need to have a reserve of liquid nitrogen, which inevitably evaporates from the working volume of the converter.

The above characteristics of quantum and fluxgate magnetometers indicate a higher sensitivity of quantum devices. It is sensitivity that is the determining parameter when choosing equipment for searching. This is especially important when it is necessary to survey huge areas of the seabed at great depths. However, the search task often has boundary conditions for the search depth or there is an opportunity to bring the converter closer to the level of the probable occurrence of the object, for example, when searching from the water surface on an extension cable. In this case, the determining factor is the low price, simplicity and reliability of the search device. Fluxgate devices have just these undoubted advantages. In addition, the working substance of quantum devices has a short service life and is often unsafe for the environment, which increases the cost of operation. All this explains the use of fluxgate search devices in the Armed Forces of various countries and the emergence of new modifications of devices with fluxgate converters.