Three-axis Fluxgate Magnetometer Research Articles

Drone-based magnetometer prospection for archaeology

Magnetometry is one of the most efficient and successful methods of archaeological prospection. Drone-based prospecting is increasingly being used in many fields of remote sensing but with respect to magnetometry, with rather poor results. For magnetic surveys, drone prospecting comes with the problem that magnetic and mechanical disturbances originating from the aircraft decrease the quality of the measurements.Here we present the adaptation of a commercial three-axis fluxgate magnetometer setup, which can be suitable for archaeological prospection, after applying appropriate filter methods and minimising the flying altitude and speed. For approval of the system, we performed drone-based surveys in high spatial resolution (50 cm line spacing) at constant, ultra-low sensor altitudes of 45 ± 10 cm and 75 ± 10 cm. We chose an archaeological site from the roman period as survey site (3.8 ha), where high-quality ground-based caesium magnetometer data was available. This allows us to demonstrate the first detailed comparison of drone-based and ground-based magnetic survey data for archaeology. For further evaluation, the influence of the drone on the measurements is assessed, drift and sensor resolution checks are carried out, and suitable data filtering methods are evaluated.Our results show that we can detect main archaeological features such as ditches, pits, fireplaces and remnants of stone fundaments with the drone-based setup, which are the relevant structures for the majority of archaeological prospections worldwide. Further, we can prove that the data quality can significantly be increased by lowering the flight altitude and speed. We conclude that drone-based magnetometry serves above all to cover large and inaccessible areas in short time. Our findings are a first step towards the development of standards for drone-based archaeological prospection approaches.

Dynamic Compensation of a Fluxgate Magnetometer Based on a Hybrid Optimizing Algorithm

Under dynamic conditions, when the measured magnetic field is projected onto the sensitive axis of the fluxgate magnetometer, it will show strong dynamic characteristics, resulting in errors or delayed interpretation of the measured physical process. Considering the dynamic characteristics of the three-axis fluxgate magnetometer, this paper proposes a series dynamic compensator based on system identification to carry out system identification and modeling of the dynamic characteristics of the three-axis fluxgate magnetometer and proposes a system parameter calculating method based on hybrid optimization of the genetic algorithm and the simulated annealing algorithm (GA-SA). With these methods, not only can the parameter optimization process effectively avoid falling into the local optimal, but they can also greatly improve the search efficiency and shorten the time of solving the optimal solution. Thus, the full-space search ability of the algorithm is improved, and the accuracy of the dynamic magnetic measurements is improved to the maximum extent. The simulation results of the step signal show that the response time is reduced to 18.22% of the pre-compensation time. The standard signal test results of the step signal and sinusoidal signal, respectively, show that the response time of the step signal is reduced to 5.5% and, in the 0.01–10 Hz frequency band, the maximum attenuation decreases from 0.24% to 0.10%, and the phase difference is less than −0.5°. Moreover, the experimental results of the geomagnetic dynamic measurements show that the dynamic compensated data are mostly close to the real geomagnetic field values. In addition, the standard deviation errors are reduced from 95.30/277.35/108.51 nT to 3.41/2.36/2.75 nT in three geomagnetic vector components. The results of this work show the ability of dynamic measurement accuracy improvement of fluxgate magnetometers and are useful for geomagnetic dynamic measurement and related applications.

Experimental investigation on the Euler-type location and orientation algorithm using magnetic field and its gradient tensor

The magnetic source location has wide application prospects in the fields of space exploration, non-destructive testing, security monitoring and target tracking. A constrained Euler location algorithm is applied to the location of a small current-carrying multi-turn ring coil, which is the closed analytic form of the position solution. The error of position solution for the traditional Euler location algorithm is deduced analytically. An experimental device of three-axis fluxgate magnetometer (TAFM) cross-array is constructed. The interference of ambient magnetic field is filtered from the magnetic data by the digital band-pass filter, and the magnetic field of the multi-turn coil is restructured by the least-squares fitting. The Euler and constrained Euler location algorithms are compared with the numerical simulation and the experimental data, and the results show that the constrained Euler location precision of the location algorithm is better than the Euler one. And more, experimental tests verify that the relative error of position solution is less than 10% for the two location algorithms in the given distance range.

A Novel Linear Calibration Model for Three-Axis Fluxgate Magnetometer

With low cost, simple structure and high accuracy, the three-axis fluxgate magnetometer (TFM) has been frequently used in many fields. However, the systematic errors are a major obstacle to the performance of TFM. Based on Taylor expansion and Cholesky decomposition, a new linear error calibration model is proposed from the general error model of TFM. In our new model, the parameter matrix can be decomposed to two parts: one is completely equivalent to the parameter matrix of the traditional linear model, and the other is a matrix containing second-order terms composed of error parameters, which was neglected in the traditional linear model. Thus, compared with the traditional model, our new linear model can acquire better calibration performances. In simulations, the functional link artificial neural network algorithm is provided to solve the model parameters. Simulation results show that the new model improves the calibration effect by 32.9 to 33.8% compared with the existing linear calibration model. After that, a single three-axis fluxgate magnetometer experimental platform is set up and related physical experiment is carried out. The results further prove the advantages and the practical value of the proposed model. Compared with the reference method, the effect is improved by 79.3%. The conclusions of this paper can expand the linear calibration method of three-axis magnetometer and improve the accuracy of such methods.

Coil Optimization in a Fluxgate Magnetometer With Co₆₈.₂Fe₄.₃Si₁₂.₅B₁₅ Amorphous Wire Cores for Geomagnetic Station Observation

The coils in a three-axis fluxgate magnetometer have been investigated to enhance the noise and power performance for the use in geomagnetic stations. The turn number of the excitation and pickup coils in the fluxgate sensors is carefully optimized by measuring the equivalent magnetic noise spectral density of the magnetometers. Correspondingly, the values of the tuning and demodulation capacitors are delicately designed to match the optimized coils. The noise of the fluxgate magnetometer is 6 pT/ √Hz at 1 Hz with a power consumption lower than 360 mW. The size of the packaged magnetometer is 30 ×30 ×113 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> , and its weight is less than 80 g (without cable). The features of the fluxgate magnetometer make it a suitable instrument for geomagnetic stations and other applications.

DESMEX: A novel system development for semi-airborne electromagnetic exploration

There is a clear demand to increase detection depths in the context of raw material exploration programs. Semi-airborne electromagnetic (semi-AEM) methods can address these demands by combining the advantages of powerful transmitters deployed on the ground with efficient helicopter-borne mapping of the magnetic field response in the air. The penetration depth can exceed those of classic airborne EM systems because low frequencies and large transmitter-receiver offsets can be realized in practice. A novel system has been developed that combines high-moment horizontal electric bipole transmitters on the ground with low-noise three-axis induction coil magnetometers, a three-axis fluxgate magnetometer, and a laser gyro inertial measurement unit integrated within a helicopter-towed airborne platform. The attitude data are used to correct the time series for motional noise and subsequently to rotate into an earth-fixed reference frame. In a second processing step, and as opposed to existing semi-AEM systems, we transform the data into the frequency domain and estimate the complex-valued transfer functions between the received magnetic field components and the synchronously recorded injection current by regression analysis. This approach is similar to the procedure used in controlled-source EM. For typical source bipole moments of 20–40 kAm and for rectangular current waveforms with a fundamental frequency of approximately 10 Hz, we can estimate reliable three-component (3C) transfer functions in the frequency range from 10 to 5000 Hz over a measurement area of [Formula: see text] for a single source installation. The system has the potential to be used for focused exploration of deep targets.

Integration of an Aeromagnetic Measurement System Based on an Unmanned Aerial Vehicle Platform and Its Application in the Exploration of the Ma’anshan Magnetite Deposit

An unmanned aerial vehicle (UAV) aeromagnetic measurement system can precisely obtain large-scale data with high efficiency and low cost in regions with complex exploration conditions, which has aroused interest in the field of aerial survey. In this article, based on the unmanned helicopter platform, an aeronautical magnetic field measurement system is integrated to complete the integration of a rotor UAV, magnetometers, and data recorders. Equipped with a three-axis fluxgate magnetometer for real-time flight compensation, the compensation deviation is ±0.052 nT, which meets the aeromagnetic survey specifications. The UAV can fly autonomously with a maximum load of 45 kg at a speed of 60 km/h for 2 h, and the wind resistance level is level 7. The system has completed a field test of 111.9 km in the Ma'anshan crisis mine region of China and collected 145,495 data points. Then, the adaptive down-sampling method is used to compress the data (to 1.7% of the original data), and it is applied to the three-dimensional magnetic inversion, which successfully obtains the spatial distribution characteristics of the underground orebody, improving the inversion efficiency while ensuring the target area resolution. Six favorable magnetite prospecting sites are delineated in the survey area through inversion results, which demonstrates the potential of the developed UAV aeromagnetic system in the exploration over the areas with complex terrain.

Drone-Borne Hyperspectral and Magnetic Data Integration: Otanmäki Fe-Ti-V Deposit in Finland

The technical evolution of unmanned aerial systems (UAS) for mineral exploration advances rapidly. Recent sensor developments and improved UAS performance open new fields for research and applications in geological and geophysical exploration among others. In this study, we introduce an integrated acquisition and processing strategy for drone-borne multi-sensor surveys combining optical remote sensing and magnetic data. We deploy both fixed-wing and multicopter UAS to characterize an outcrop of the Otanmäki Fe-Ti-V deposit in central Finland. The lithology consists mainly of gabbro intrusions hosting ore bodies of magnetite-ilmenite. Large areas of the outcrop are covered by lichen and low vegetation. We use two drone-borne multi- and hyperspectral cameras operating in the visible to near-infrared parts of the electromagnetic spectrum to identify dominant geological features and the extents of ore bodies via iron-indicating proxy minerals. We apply band ratios and unsupervised and supervised image classifications on the spectral data, from which we can map surficial iron-bearing zones. We use two setups with three-axis fluxgate magnetometers deployed both by a fixed-wing and a multi-copter UAS to measure the magnetic field at various flight altitudes (15 m, 40 m, 65 m). The total magnetic intensity (TMI) computed from the individual components is used for further interpretation of ore distribution. We compare to traditional magnetic ground-based survey data to evaluate the UAS-based results. The measured anomalies and spectral data are validated and assigned to the outcropping geology and ore mineralization by performing surface spectroscopy, portable X-ray fluorescence (pXRF), magnetic susceptibility, and traditional geologic mapping. Locations of mineral zones and magnetic anomalies correlate with the established geologic map. The integrated survey strategy allowed a straightforward mapping of ore occurrences. We highlight the efficiency, spatial resolution, and reliability of UAS surveys. Acquisition time of magnetic UAS surveying surpassed ground surveying by a factor of 20 with a comparable resolution. The proposed workflow possibly facilitates surveying, particularly in areas with complicated terrain and of limited accessibility, but highlights the remaining challenges in UAS mapping.

Misalignment calibration of geomagnetic vector measurement system using parallelepiped frame rotation method

Misalignment error is one key factor influencing the measurement accuracy of geomagnetic vector measurement system, which should be calibrated with the difficulties that sensors measure different physical information and coordinates are invisible. A new misalignment calibration method by rotating a parallelepiped frame is proposed. Simulation and experiment result show the effectiveness of calibration method. The experimental system mainly contains DM-050 three-axis fluxgate magnetometer, INS (inertia navigation system), aluminium parallelepiped frame, aluminium plane base. Misalignment angles are calculated by measured data of magnetometer and INS after rotating the aluminium parallelepiped frame on aluminium plane base. After calibration, RMS error of geomagnetic north, vertical and east are reduced from 349.441nT, 392.530nT and 562.316nT to 40.130nT, 91.586nT and 141.989nT respectively.

Scalar Calibration of Aeromagnetic Data Using BPANN and LS Algorithms Based on Fixed-Wing UAV Platform

A new airborne platform named the unmanned aerial vehicle (UAV) is used to detect the geomagnetic anomaly with low magnetic interference. Nevertheless, there are steering errors of three-axis fluxgate magnetometers (TFMs) with the changes of UAV flight course. The main reasons are due to the effects of nonorthogonality, scale factors, and zero shifts. Therefore, it is quite necessary to establish calibration methods to get magnetic information of high precision. The methods of least squares (LS) and backpropagation artificial neural network (BPANN) are proposed to correct the system errors of measured data in this paper. The results show that the errors are suppressed using LS and BPANN methods. The measured errors of geomagnetic field decrease obviously after calibration. Furthermore, the BPANN method is more effective to calibrate the data error of TFMs than LS method when UAV changes its flight direction. Moreover, the spatial distributions of magnetic fields for TFMs after calibration using LS and BPANN methods are quite consistent with the distributions for optical pump magnetometers. This paper can provide a better way to improve the performance of TFMs and be widely used in the data error calibration of multiaxis sensors.

One-step calibration of magnetic gradient tensor system with nonlinear least square method

Due to technological limitations, the accuracy of magnetic gradient tensor system is tightly affected by different scale factors, bias of each axis, non-orthogonality between axes and misalignment errors between magnetometers. In order to obtain precise measurement, calibration of the magnetic gradient tensor system is quite essential. A mathematical model containing error parameters of magnetic gradient tensor system is established, and one-step calibration method based on nonlinear least square method is proposed. The effectiveness and convergence of the proposed algorithm is proved by simulation and experiments. In experiments, a tri-axial nonmagnetic rotation platform, a proton magnetometer and a cross magnetic gradient tensor system containing four three-axis fluxgate magnetometers are used. Experimental results show that the proposed method can decrease the measurement errors of magnetic gradient tensor system greatly.

Improvement of magnetometer calibration using Levenberg-Marquardt algorithm

The algorithm, the model, and the measurement strategy are important for the calibration of three-axis magnetometers. A new calibration model with clear physical meaning is proposed, and scale factors, offsets, and nonorthogonal angles are directly illustrated. One limitation of traditional iteration calibration algorithms is the influence of the initial parameters. In this paper, the Levenberg–Marquardt algorithm is proposed to solve the troublesome procedure of initial parameter selection, so that it can improve the calibration performance of three-axis magnetometers. The validity of this method is proved by simulation, and the estimated parameters are found to be close to the prearranged parameters. The experimental system mainly consists of a three-axis fluxgate magnetometer (DM-050), two-dimensional nonmagnetic rotation equipment, and a proton magnetometer. The magnetic field intensity is obtained by the proton magnetometer. Experimental results show that the root-mean-square error is reduced from 84.177 to 4.076 nT. In addition, the influence of the initial parameters and measurement strategy is analyzed. Experimental results show that the Levenberg–Marquardt algorithm is not sensitive to the initial parameters and the 36 static measurements strategy is more reliable than the rotation measurement strategy. © 2014 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.

Improvement of vector compensation method for vehicle magnetic distortion field

Magnetic distortions such as eddy-current field and low frequency magnetic field have not been considered in vector compensation methods. A new compensation method is proposed to suppress these magnetic distortions and improve compensation performance, in which the magnetic distortions related to measurement vectors and time are considered. The experimental system mainly consists of a three-axis fluxgate magnetometer (DM-050), an underwater vehicle and a proton magnetometer, in which the scalar value of magnetic field is obtained with the proton magnetometer and considered to be the true value. Comparing with traditional compensation methods, experimental results show that the magnetic distortions can be further reduced by two times. After compensation, error intensity and RMS error are reduced from 11684.013nT and 7794.604nT to 16.219nT and 5.907nT respectively. It suggests an effective way to improve the compensation performance of magnetic distortions.

A New Calibration Method of Three Axis Magnetometer With Nonlinearity Suppression

Nonlinearity is a prominent limitation to the calibration performance of vector magnetometers. A new calibration model is proposed to suppress the nonlinearity of magnetometers and improve calibration performance, in which the nonlinearity coefficients of scale factors are considered. The experimental system mainly consists of a three-axis fluxgate magnetometer (MAG3300), a 2D nonmagnetic rotation equipment, and a proton magnetometer (CZM-3), in which the nonmagnetic rotation equipment is used to change the position of three-axis fluxgate magnetometer, and the scalar value of magnetic field is obtained with the proton magnetometer and considered to be the true value. The principle of this new calibration method is analyzed, and the calibration procedures are introduced. Experimental results show that after nonlinearity suppression, the root-mean-square error of calibration can be reduced by three times. It suggests an effective way to improve the calibration performance of three-axis magnetometers.

Calibration strategy and generality test of three-axis magnetometers

Calibration strategy will influence the calibration performance of three-axis magnetometers. In this paper, three different calibration strategies (symmetrical calibration, orthogonal calibration and random calibration) are introduced and verified by simulation and experiments. The experimental system mainly consists of three-axis fluxgate magnetometer (MAG3300), 2D nonmagnetic rotation equipment and proton magnetometer. The scalar value of magnetic field was obtained with proton magnetometer and considered to be the true value. Calibration performance of three different calibration strategies were analyzed and compared. Experimental results show that after calibration the scalar RMS error has dropped 88.5%, 85.5% and 87.6% respectively. To check the generality of the estimated error parameters, another site was chosen to be a validation position, and the calibration performance of three calibration strategies were tested and compared. After calibration, the scalar RMS error has dropped 83.2%, 78.9% and 78.8% respectively. Thus the symmetrical calibration is considered to be the best calibration strategy among three calibration strategies.

Calibration of a fluxgate magnetometer array and its application in magnetic object localization

The magnetometer array is effective for magnetic object detection and localization. Calibration is important to improve the accuracy of the magnetometer array. A magnetic sensor array built with four three-axis DM-050 fluxgate magnetometers is designed, which is connected by a cross aluminum frame. In order to improve the accuracy of the magnetometer array, a calibration process is presented. The calibration process includes magnetometer calibration, coordinate transformation and misalignment calibration. The calibration system consists of a magnetic sensor array, a GSM-19T proton magnetometer, a two-dimensional nonmagnetic rotation platform, a 12 V-dc portable power device and two portable computers. After magnetometer calibration, the RMS error has been decreased from an original value of 125.559 nT to a final value of 1.711 nT (a factor of 74). After alignment, the RMS error of misalignment has been decreased from 1322.3 to 6.0 nT (a factor of 220). Then, the calibrated array deployed on the nonmagnetic rotation platform is used for ferromagnetic object localization. Experimental results show that the estimated errors of X, Y and Z axes are −0.049 m, 0.008 m and 0.025 m, respectively. Thus, the magnetometer array is effective for magnetic object detection and localization in three dimensions.

Calibration of three-axis fluxgate magnetometers with nonlinear least square method

The accuracy of three-axis magnetometers is limited by different scales, bias of each axis and nonorthogonality between axes, which is usually lower than that of scalar magnetometers. In this paper, the nonlinear least square method is proposed to calibrate three-axis magnetometers. The validity of this method was proved by simulation, in which the estimated parameters of the error model are close to prearranged parameters. In experiment, a three-axis fluxgate magnetometer (DM-050), a two dimensional nonmagnetic rotation equipment and a proton magnetometer (GSM-19T) were used. The scalar value of magnetic field was obtained by proton magnetometer and considered to be the true value. The calibration performance of unscented Kalman filter (UKF), two-step algorithm and nonlinear least square were compared. Experimental results show that the error average and standard deviation of nonlinear least square are the least among the three methods. After calibration, the average of scalar error is reduced from −76.2nT to −0.00093nT and the standard deviation is reduced from 10.832nT to 4.298nT. The results suggest an effective way for the calibration of three-axis fluxgate magnetometers.

Localization and magnetic moment estimation of a ferromagnetic target by simulated annealing

In many applications, the detection of a visually obscured magnetic target is followed by the characterization of the target, i.e. localization and magnetic moment estimation. Effective target characterization may reduce the detection false alarm rate as well as direct the searcher toward the target. We address the characterization of a static magnetic target by a three-axis fluxgate magnetometer installed on a stabilized mobile platform. The magnetometer readings are contaminated by magnetic noise, which results in a low signal-to-noise ratio. We formulate the problem as an over-determined nonlinear equation set using a magnetic dipole model for the target and use simulated annealing (SA) in order to rapidly find a good approximation to the global optimum of this equation set. Computer simulations demonstrate high accuracy of the SA method in localizing the target and estimating its magnetic moment in the presence of high-level noise. The high accuracy of the SA method is also exemplified in tests employing real-world magnetic signals. In addition to its high accuracy, the SA method is very rapid, making it appropriate for real-time practical applications.

Spatiotemporal evaluation of human colon motility using three-axis fluxgates and magnetic markers

An alternative method to study the mechanical activity of the human colon in fasting and postprandial states is presented. The method is based on measurements of the magnetic fields produced by a magnetic marker, a small cylindrical NdBFe magnet, when it was ingested by the subjects. A portable magnetic probe, consisting of two digital three-axis fluxgate magnetometers, arranged in a first-order electronic gradiometer, was implemented for this research. Measurements were taken in 16 healthy male subjects. Contractile activity frequency measurements were taken along the colon length, including the ascending, transverse and descending sections, as well as the rectal sigmoidal section. Values for the contractile activity frequency of 2-5 cycles min(-1) were measured. The set-up is simple, low-cost and suitable for use in an unshielded environment.

Magnetic background noise cancellation in real-world environments

Measurements of the magnetic field noise and the spatial correlation of the noise were made in a typical laboratory environment using two three-axis fluxgate magnetometers. The magnitude of the magnetic field noise was found to be approximately 100 pT/Hz at 10 Hz with a correlation of 90% at a separation of 1 m between the two sensors. The correlation was significantly reduced from noise induced eddy currents near large metallic surfaces.