Detection of magnetic objects using gradient methods

contact: doc. Ing. Antonín Platil, Ph.D. (platil(AT)fel.cvut.cz), created May 27, 2021

Measurement methods using magnetic gradient are useful in many application areas, including detection of vehicles, detection of metallic objects (arms, unexploded ordnance), perimeter protection, geomagnetic data cleaning and denoising, etc.

Much of current research worldwide is focused on advanced data processing of full-tensor magnetic data and/or higher gradient tensor data e.g. for improved target positioning estimation. Our research shall focus on improving positioning estimation accuracy and reliability.

The full tensor of magnetic (first order) gradient is described by a 3x3 matrix of individual components. However, due to symmetry, only five components are independent.

The gradient with real-world sensors is only approximated by a point measurement in a specified  distance, i.e.:

However, as we decrease d - the gradiometric base, we need sensors with very low noise. One possibility is to build the gradiometer from fluxgate sensors. Our department developed miniature sensor head with 10 fluxgates which was successfully calibrated and tested for noise (Janošek 2015a)

                 Full tensor head arrangement (left) and X-ray image (right)

Astatization of the gradiometer is important - below we can see response of gradiometer to movement in homogeneous field (i.e. no gradient - should be a flat line):  the uncorrected gradient (Gxx) - blue, and astatized gradient (red) - the reduction after using a proper numeric minimization technique (alignment of sensors, their gain) was about 150-times:  

Astatization of gradiometer in homogeneous field - before (blue) and after (red)

For an alternate solution, see https://patents.google.com/patent/EP2388608A1/ar, which is a published patent (Janošek, Ripka). An axial (dBx/dx) gradiometer can be also constructed with only one fluxgate sensor sharing multiple coils. We have developed the following “single-core” gradiometer combined with magnetometer, which has an advantage of gradiometric feedback, increasing the gradiometer stability and reducing inhomogeneous magnetic field present at the magnetic core when exposed to (measured) gradient + homogeneous fields.

Single-core fluxgate magnetometer/gradiometer (from patent EP2388608A1)

Most of the gradient components usable for UXO/object detection can be also approximated from only two sensor triplets - also the “total field gradient” (which is not a correct term) can be estimated from two coordinates - see figure below for car detection / magnetic field mapping (Novotný 2019).

Car magnetic signature characterization using an array of AMR magnetometers/gradiometers

We have been active in the development of UXO (unexploded ordnance) detectors using fluxgate sensor gradiometer (Schiebel DIMADS). The advantage of using fluxgate sensors sensitive to the DC field component (anomalies of the Earth’s field) is the  sensitivity to large, deep targets (buried WW2 bombs, i.e.), rather than shallow detection where eddy current detectors are mostly deployed (landmines) - see Fig. below.

Another studied application of DC-field magnetic gradiometers is security/perimeter monitoring, where scanning techniques can be deployed. It is then necessary to use advanced signal processing techniques in order to suppress ambient (urban) magnetic noise, and detect the (moving) target. Movement sensors and cross-correlation measurements are one of the methods which were studied in a recent security-related research of the Czech Ministry of Interior with the company URC Systems.

Detection of body-worn ferromagnetic objects for security applications (from utility model/užitný vzor CZ 33608 U1 (Čechák 2020))

“Total gradient” measurements using  flying/submersible drones

Our magnetometer suspended on a cable from a drone was used for aerial surveys in multiple locations, including Tunguska (Takac 2020). The figure below is from Australia. In this case, the magnetometer is downgraded into a scalar measurement and “gradient” is used mostly to compensate for oscillations induced by movement of the suspended instrument.

Cable-suspended magnetometer in survey operation and the field map

More information here: https://maglab.fel.cvut.cz/uncategorized/uav-mag-in-australia/ and here https://maglab.fel.cvut.cz/products/uav-mag-v-1-1/

In another arrangement, our submersible magnetometer/gradiometer was used in exploration of the Chebarkul lake meteorite site near Chelyabinsk, Russia (Kletetschka 2015)

Combined survey data from Chebarkul meteorite impact site - from (Kletetschka 2015)

Gradient arrangement is especially useful in suppression of background field variations, like in the following magnetocardiography experiment. The two sensors were arranged as a transverse, dBx/dy gradiometer (with x denoting the sensor axis), and the y-distance — or gradiometric base — was 12 cm.

Gradient measurement of heart action enables suppression of background field variations visible in the graphs on right side - from (Janošek 2020)

Some relevant publications from our team:

Čechák, J.; Marek, M.; Petrucha, V.; Janošek, M., Detektor nesených feromagnetických částí, Czech Republic. Utility Model CZ 33608. 2020-01-21. (Supported ferromagnetic parts detector, Utility model CZ 33608 U1) https://isdv.upv.cz/doc/FullFiles/UtilityModels/FullDocuments/FDUM0033/uv033608.pdf

EP2388608A1 Fluxgate sensor circuit for measuring the gradient of a magnetic field https://worldwide.espacenet.com/patent/search?q=pn%3DEP2388608B1

Janošek, M.; Vyhnánek, J.; Platil, A.; Petrucha, V.: Compact Full-tensor Fluxgate Gradiometer, Journal of Electrical Engineering. 2015, 66(7/s), 146-148. ISSN 1335-3632. http://iris.elf.stuba.sk/JEEEC/data/pdf/7s_115-37.pdf

Janošek, M.; Vyhnánek, J.; Platil, A.: Compact magnetic gradiometer and its astatization, In: Procedia Engineering Special Issue Eurosensors 2015. Oxford: Elsevier Ltd, 2015. pp. 1249-1252. ISSN 1877-7058. https://www.sciencedirect.com/science/article/pii/S1877705815025114

Janošek, M.; Platil, A.; Vyhnánek, J.: Simple estimation of dipole source z-distance with compact magnetic gradiometer, In: Proceedings of 5th International Conference on Materials and Applications for Sensors and Transducers (IC-MAST2015). Bristol: IOP Institute of Physics, 2016. IOP Conference Series: Materials Science and Engineering. ISSN 1757-899X. https://iopscience.iop.org/article/10.1088/1757-899X/108/1/012025

Janošek, M.; Butta, M.; Vlk, M.; Bayer, T.: Improving Earth’s magnetic field measurements by numerical corrections of thermal drifts and man- made disturbances, Journal of Sensors. 2018, 2018 ISSN 1687-725X. https://www.hindawi.com/journals/js/2018/1804092/

Janošek, M.; Butta, M.; Dressler, M.; Saunderson, E.; Novotný, D.; Fourie, C.: 1-pT noise fluxgate magnetometer for geomagnetic measurements and unshielded magnetocardiography, IEEE Transactions on Instrumentation and Measurement. 2020, 69(5), 2552-2560. ISSN 0018-9456. https://ieeexplore.ieee.org/document/8880685

Kletetschka, G.; Vyhnánek, J.; Kawasumiova, D.; Nabelek, L.; Petrucha, V.: Localization of the Chelyabinsk Meteorite from Magnetic Field Survey and GPS Data, IEEE Sensors Journal. 2015, 2015(15), 4875-4881. ISSN 1530-437X. http://ieeexplore.ieee.org/xpl/articleDetails.jsp?arnumber=7110532&filter%3DAND%28p_IS_Number%3A4427201%29

Novotný, D.; Janošek, M.; Petrucha, V.; Pavelka, L.; Platil, A.: Vehicle's magnetic field modeling and mapping for its presence detection, In: Magnetic Frontiers 2019: Magnetic Sensors - Abstract Book. Lisbon: Instituto Superior Técnico, Technical University of Lisbon, 2019. https://mag-frontiers.sciencesconf.org/data/pages/Magnetic_Frontiers_2019_Abstract_Book_1.pdf

Takac, M., et al. "Magnetic Signature Over the Epicenter Area of Tunguska." Lunar and Planetary Science Conference. No. 2326. 2020. https://www.hou.usra.edu/meetings/lpsc2020/pdf/1911.pdf

More information also available at our website https://maglab.fel.cvut.cz/

Some interesting publications by others:

Nara T, Suzuki S and Ando S 2006 A closed-form formula for magnetic dipole localization by measurement of its magnetic field and spatial gradients IEEE Transactions on Magnetics 42, 3291-93

Huiqiang Zhi et al.: A novel magnetic dipole inversion method based on tensor geometric invariants, AIP Advances 10, 045131 (2020); https://doi.org/10.1063/5.0003898

Qingzhu Li et al.: Magnetic object positioning method based on tensor spacial invariant relations, Meas. Sci. Technol. 31 (2020) 115015 (12pp) https://doi.org/10.1088/1361-6501/ab8dfe

Yin Gang et al.: Magnetic dipole localization based on magnetic gradient tensor data at a single point, Journal of Applied Remote Sensing, Vol. 8, 083596, 2014

Yangyi Sui et al.: Multiple-Order Magnetic Gradient Tensors for Localization of a Magnetic Dipole, IEEE MAGNETICS LETTERS, Volume 8 (2017), 6506605