PORTABLE MAGNETOMETRIC GRADIOMETER

PORTABLE MAGNETOMETRIC GRADIOMETER FOR HIDDEN FERROMAGNETIC ITEMS DETECTION 

Vladimir Tkach¹, 

Igor Parftentsev, PhD in Engineering Science, Assistant Professor¹, 

Stanislav Zvezhinsky, Doctor of Science, Professor²

This article gives a summary of results obtained during the development of a new portable magnetometric gradiometer for the detection of ferromagnetic items, that are hidden in concealment environment. 

Keywords: magnetometric gradiometer, magnetic field of the Earth, detection range, instrumentation error. 

A rightful place in the range of special concealed items detection systems is occupied by magnetic gradiometer, which detects ferromagnetic items by the recording of local changes in the main (stationary) magnetic field of the Earth [1]. Although the density of this field may reach ca. Т ≈5*10⁴nTl, its gradient is quite small (less than 10^-4nTl/m), which allows the detection of local anomalies in density gradient of the main magnetic field of the Earth, which arise from the vicinity of ferromagnetic items, that are interesting for the detection, such as: explosive ordnances in shells, explosives with ready strike elements, arms caches etc.

The main benefit of the magnetometric  method (or magnetic anomalies detection, MAD) compared to active (pulse and harmonic) metal detectors, that are widely used, is the extended range and capability to obtain good results in any kind of concealment environment and underlying terrain (soil, water, swamp, grass etc.) [1, 2]. Indeed, this method cannot be used for the detection of dielectric (e.g. explosives in plastic shells) and diamagnetic (duralumin) items, but such items are generally 'less' hazardous to people.

Magnetometric gradiometer is a system comprising three main elements [3, 4]:

• sensitive probe in rigid sealed enclosure, which uses а  base (generally 0.4…0.7 m) with two identical co-linear «spot» magnetometers, (usually ferroprobes) that are connected in opposition, for the measurement of the difference in the field density in hundreds of nTl;
• processing units with a microprocesses, which amplifies and visualize signals of the sensitive probe and minimizes instrument error, which limits the range of the gradiometer;
• non-ferromagnetic housing with integrated power sources (generally Li-ion batteries or rechargeable batteries).

Resolution (sensitivity) of the gradiometer is defined by two types of instrument error of the probe:

1)        angular ξ_ang. error, related to the non-co-linearity of  ∆ axes of sensitivity of two magnetometers in the probe;

2)        amplitude ξ_amp. error, related to the inequality of their conversion factors.

The last one can be easily eliminated by the calibration and adjustment (during the manufacturing and operation including automatic operation). Angular error is the main type of error, which is defined by the probe manufacturing process and it is predominant. For the worst case, when the sensitivity axis О of the probe (along the main dimension) is normal to Т vector of the main magnetic field of the Earth, this error may be calculated as follows:

                                                                                           (1)

and for ∆ = 1 ang. sec = = 3·10^-4 rad it corresponds to ~15 nTl, which allows for the stable detection of changes in field strength over 30 nTl only. For the best (co-linear) case of О and Т vectors the error is decreased by hundreds of times. As the vector of the main magnetic field of the Earth in Russia, Europe and the USA generally has vertical orientation, it is quite
'beneficial' to record the vertical component of its gradient, which is widely applied in modern products. Probe is mounted on the housing of the gradiometer on a swivel and oriented vertically by the gravity force. This configuration allows for the minimization of angular error by 2-3 times as compared to (1).

During the detection of items in the environment, the operator moves along the route and uses readings of the indicator (LED, LCD or arrow indicator) for the evaluation of the level of field density gradient. Noise (in any direction from zero) arises from angular (amplitude) error of the gradiometer. When the object of the detection is approached, the indicator starts showing the predominant trend (in noise fluctuation), i.e. useful signal signature, which, depending on the

orientation of item magnetization  (residual, induced magnetization) may have two main types, which are shown in Fig.1 (actual signature may be somewhat in between). Experienced operator may study the area, where stable signals are obtained and read the type of the signature to determine the approximate location, depth and even magnetic moment М of the item, which is indirectly related to its dimensions or weight [3]. When the measurement function is combined with information on the position (using GPS or GLONASS navigator) a map of the area may be drawn including the map of magnetic anomalies.

Figure 1 – Main signatures of useful signals during the detection of ferromagnetic items 

The detection range R of ferromagnetic items with magnetic moment М, may approximately be calculated using the following equations [2, 3]:

(2)

where , m; а – base, m (Fig. 1); М – magnetic dipole moment of the item, Аm2; В – recorded maximum of useful signal, nTl (or resolution). For example, for a 155 mm shell М can be taken as ≈ 1 Аm² [3], and for the signal level В = 5 nTl (background noise level up to 2 nTl) and typical base of the gradiometer а = 0.65 m, results in: R ≈ 2.1 m.

According to [4] the best magnetometric gradiometer are developed and manufactured by Inst. Dr. Foerster (Germany) and good products are manufactured by Ebinger (Germany), Geoscan Research (UK), Schonted Instr. (USA) etc. FEREX  4.032, that is manufactured by Inst. Dr. Foerster, is used by NATO as MK-26 Mod. 1 system. It has been setting the technological standard for this kind of special equipment for over 10 years. The main advantage of the product is a unique probe, which provides angular error less than 5·10^-5 rad for the whole service life in field conditions (with no need for regular calibration). This system has higher sensitivity than any other known product. Further advantages of FEREX 4.032 system are user friendly control and mapping interface, several operating modes (including the combined operation with GPS-navigator) and opportunity for simultaneous operation of several detectors in a vehicle-mounted detection system as well as several types of probes with typical (0.65 m) low sensitivity.

Over 10 years ago several types of gradiometers (e.g. “MBI-P”, “FT-601”) were developed in Russia, but they are far behind the best international products, mainly due to low sensitivity, small choice of options and high complexity of servicing during their operation [4]. At the same time the level of hazard in both military and demining activities sectors, which require such special equipment, has not decreased recently. Therefore the development of a new local magnetometric gradiometer with specifications in line with the front edge international level, is vital.

ZAO “Gruppa Zashchity JUTTA” [5] has launched the research and development initiative for the manufacturing of a local portable magnetometric gradiometer. The table below shows performance specifications of a pilot gradiometer OSS, which were confirmed during the preliminary test. Generally these specifications are in line with the international level of development of such special equipment.

Table – General performance specifications of OSS magnetometric gradiometer 

OSS gradiometer provides audible and visual alarm in case if the level of the recorded magnetometric signal В exceeds the pre-set detection threshold. It has 8 sub-ranges for sensitivity adjustment: from 3 nTl to 10⁴ nTl. OSS gradiometer has four operating modes:

1) dynamic (during the movement of the operator)  differential mode, i.e. “preliminary” detection of ferromagnetic items at lower sensitivity;

2) static (when the operator is in stationary position) differential mode, i.e. item position identification and evaluation on location at higher sensitivity;

3) static selective mode, i.e. detection and position identification for relatively big items, which are located at significant depth and attenuation of the influence of small items at small depth (so called spatial filter mode);

4) static absolute, i.e. activation of a single (lower) magnetometer for magnetic anomalies mapping.

Four types of probes will be provided for the gradiometer: with 40 cm; 65 cm (typical) and 140 cm bases (higher sensitivity) for operation in deep water. General view of the pilot gradiometer is shown in Fig. 2.

Figure 2 – General view of OSS magnetometric gradiometer: а) transportation and storage mode; b) duty mode

Conclusion

In general the new domestic portable magnetometric gradiometer, which is being designed by ZAO “Gruppa Zashchity JUTTA” for ferromagnetic items detection in concealment environment complies with state-of-the-art technologies. It is designed to fill the relevant niche in the range of special equipment. Functionality and interface of the unit will be further improved based on results of field tests and requests of customers.