Effects Of Remanent Magnetization Research Articles

Localization accuracy of multiple magnets in a myokinetic control interface

Magnetic localizers have been widely investigated in the biomedical field, especially for intra-body applications, because they don’t require a free line-of-sight between the implanted magnets and the magnetic field sensors. However, while researchers have focused on narrow and specific aspects of the localization problem, no one has comprehensively searched for general design rules for accurately localizing multiple magnetic objectives. In this study, we sought to systematically analyse the effects of remanent magnetization, number of sensors, and geometrical configuration (i.e. distance among magnets—Linter-MM—and between magnets and sensors—LMM-sensor) on the accuracy of the localizer in order to unveil the basic principles of the localization problem. Specifically, through simulations validated with a physical system, we observed that the accuracy of the localization was mainly affected by a specific angle (theta = tan−1(Linter-MM / LMM-sensor)), descriptive of the system geometry. In particular, while tracking nine magnets, errors below ~ 1 mm (10% of the length of the simulated trajectory) and around 9° were obtained if θ ≥ ~ 31°. The latter proved a general rule across all tested conditions, also when the number of magnets was doubled. Our results are interesting for a whole range of biomedical engineering applications exploiting multiple-magnets tracking, such as human–machine interfaces, capsule endoscopy, ventriculostomy interventions, and endovascular catheter navigation.

Inversion of magnetic and frequency-domain electromagnetic data for investigating lithologies associated with gold mineralization in the Canadian Malartic area, Québec, Canada

Canadian Malartic is an Archean low-grade bulk tonnage native gold deposit. The deposit is mostly located in altered clastic metasedimentary rocks, mafic–ultramafic dykes, and monzodioritic porphyry intrusions. Airborne magnetic and frequency-domain electromagnetic (EM) data were inverted to reconstruct the geological units associated with the mineralization, especially the intrusive masses. The 3-D inversion of magnetic data, which used a tetrahedral mesh to a depth of 2.4 km, shows that mafic volcanic rocks and iron formation rocks extend to depth in the area, more so than diabase dykes. The magnetic inversion also shows that the diorite and monzodiorite rocks of the Lac Fournière A pluton are dipping toward the south on its northern edge at the contact with the metasedimentary rocks. The 1-D inversion of the frequency-domain EM data, for both electrical conductivity and magnetic susceptibility, is able to reconstruct geological structures to a depth of approximately 100 m, providing more details and information about these features. The intrusive masses such as diabase dykes, diorite and monzodiorite rocks, and mafic volcanic rocks are reconstructed as electrically conductive structures in the inversion results. The metasedimentary rocks are resistive, and the overburden is conductive in most of the area. The geophysical data and inversion results suggest the presence of some features (such as diabase dykes and monzodiorite rocks) that are not yet present on some parts of the geology map. A comparison of the EM-derived susceptibility and the magnetic-derived susceptibility over the iron formations can reveal the effect of remanent magnetization.

Effects of remanent magnetization on dynamic magnetomechanical and magnetic-sensing characteristics in bi-layer multiferroics

Influences of remanent magnetization on dynamic magnetomechanical mechanisms in a bi-layer asymmetric magnetoelectric (ME) laminate consisting of lead zirconate titanate and samarium iron alloy has been studied systematically, and the underlying hysteresis physics involved in dynamic magnetomechanical process as well as its magnetic-sensing characteristics was intensively characterized. To appreciate the distinct magnetostriction and ferromagnetism simultaneously in samarium iron alloy, key magnetomechanical parameters of dynamic piezomagnetic coefficient, Young's modulus and mechanical quality factors exhibit hysteresis behaviors under magnetic field application. Consequently, high sensitivity in proposed bi-payer laminate for field detection can be reached without the facilitation of additional bias field. Experimental results show that the ME output has an approximately linear correlation with the applied AC magnetic field, and the low-frequency and the detection limits at 1 kHz and 120 kHz can reach 2.3 × 10−6 T and 2.2 × 10−8 T, respectively. These unique features provide such an asymmetric ME composite great potentials for weak magnetic field detection without DC bias field.

Amplitude inversion of the 2D analytic signal of magnetic anomalies through the differential evolution algorithm

In this work, analytic signal amplitude (ASA) inversion of total field magnetic anomalies (TMA) has been achieved by differential evolution (DE) which is a population-based evolutionary metaheuristic algorithm. Using an elitist strategy, applicability and effectiveness of the proposed inversion algorithm have been evaluated through the anomalies due to both hypothetical model bodies and real isolated geological structures. Some parameter tuning studies relying mainly on choosing the optimum control parameters of the algorithm have also been performed to enhance the performance of the proposed metaheuristic. Since ASAs of magnetic anomalies are independent of both ambient field direction and the direction of magnetization of the causative sources in two-dimensional (2D) case, inversions of synthetic noise-free and noisy single model anomalies have produced satisfactory solutions showing the practical applicability of the algorithm. Moreover, hypothetical studies using multiple model bodies have clearly showed that DE algorithm is able to cope with complicated anomalies and some interferences from neighbouring sources. The proposed algorithm has been then used to invert a small- (120 m) and a large-scale (40 km) magnetic profile anomalies of an iron deposit (Kesikkopru-Bala, Turkey) and a deep-seated magnetized structure (Sea of Marmara, Turkey), respectively to determine depths, geometries and exact origins of the source bodies. Inversion studies have yielded geologically reasonable solutions which are also in good accordance with the results of normalized full gradient (NFG) and Euler deconvolution (EUD) techniques. Thus, we propose the use of DE for not only the amplitude inversion of 2D analytical signal of magnetic profile anomalies having induced or remanent magnetization effects but also the low-dimensional data inversions in geophysics.

Numerical evaluation of active source magnetics as a method for imaging high-resolution near-surface magnetic heterogeneity

We have evaluated a geophysical method that uses a low-frequency magnetic source to image subsurface magnetic heterogeneity. This active source approach can be used to image magnetic features at higher resolutions than the conventional passive geomagnetic method. Importantly, this frequency-domain active source approach is independent of the effects of remanent magnetization, which complicates the interpretation of geomagnetic data. We carried out forward modeling of frequency-domain electromagnetic (EM) data and we found that, at frequencies of a few hertz, the magnetostatic response due to the induced magnetization dominates the EM induction response. The result suggests that it is possible to make magnetic interpretation of low-frequency EM data without having to consider the conductivity structure and the corresponding EM induction effect. We compare the anomalous magnetic responses with magnetic noise components and find that the proposed active source magnetic (ASM) method has a depth of investigation of approximately 300 m. Free-space field and inductive noise are considered as the most important issues affecting the depth of investigation. We also determine the potential for linear interpretation of magnetic heterogeneity under 0.1 SI by showing that the low-frequency magnetic response can be approximated by a linear magnetic response. In our synthetic experiments, inversion of the ASM data shows a marked enhancement in resolution, with no effect of the remanent magnetization, in contrast to geomagnetic inversion. These results show that the ASM method is a useful geophysical tool, especially when high-resolution imaging of magnetic susceptibility is required or where strong remanent magnetization complicates the magnetic interpretation.

An approach for estimating the magnetization direction of magnetic anomalies

An approach for estimating the magnetization direction of magnetic anomalies in the presence of remanent magnetization through correlation between normalized source strength (NSS) and reduced-to-the-pole (RTP) is proposed. The observation region was divided into several calculation areas and the RTP field was transformed using different assumed values of the magnetization directions. Following this, the cross-correlation between NSS and RTP field was calculated, and it was found that the correct magnetization direction was that corresponding to the maximum cross-correlation value. The approach was tested on both simulated and real magnetic data. The results showed that the approach was effective in a variety of situations and considerably reduced the effect of remanent magnetization. Thus, the method using NSS and RTP is more effective compared to other methods such as using the total magnitude anomaly and RTP.

Apparent magnetization mapping in the presence of strong remanent magnetization: The space-domain inversion approach

Apparent magnetization mapping is a technique to estimate magnetization distribution in the subsurface magnetic layer from the observed magnetic data, of benefit in identifying lithologic units and delineating magnetic geologic boundaries. The conventional approaches for apparent magnetization mapping usually neglect effects of remanence, resulting in large geologic deviation and the occurrence of negative magnetization when the magnetic layer contains strong remanent magnetization. We have developed a space-domain inversion approach for apparent magnetization mapping based on the amplitude of magnetic anomaly (AMA), the analytic signal (AS), and the normalized source strength (NSS) to reduce effects of remanent magnetization. The AMA, AS, and NSS are three common quantities insensitive or weakly sensitive to the remanence transformed from the magnetic total field anomaly or components. The magnetic layer underground is first divided into a regular grid of vertical rectangular prisms, each having a cross-sectional area of one grid square and a uniform magnetization. Then, an iterative algorithm is adopted to invert each quantity of the AMA, AS, and NSS to obtain an optimum value of magnetization of each prism in the magnetic layer. The inversion approach permits the top and bottom surfaces of the magnetic layer to be constant or variable in depth, and requires no prior information of magnetization directions. Our tests on the synthetic and real data from the metallic ores area in the southern margin of North China have proved the feasibility and robustness of the presented inversion approach. All of the AMA, AS, and NSS inversions produced nonnegative magnetization distribution in the magnetic layer. Also, the AS and NSS inversions produced a better resolution of magnetization distribution than that of the AMA.

Three-dimensional cross-gradient joint inversion of gravity and normalized magnetic source strength data in the presence of remanent magnetization

Three-dimensional cross-gradient joint inversion of gravity and magnetic data has the potential to acquire improved density and magnetization distribution information. This method usually adopts the commonly held assumption that remanent magnetization can be ignored and all anomalies present are the result of induced magnetization. Accordingly, this method might fail to produce accurate results where significant remanent magnetization is present. In such a case, the simplification brings about unwanted and unknown deviations in the inverted magnetization model. Furthermore, because of the information transfer mechanism of the joint inversion framework, the inverted density results may also be influenced by the effect of remanent magnetization. The normalized magnetic source strength (NSS) is a transformed quantity that is insensitive to the magnetization direction. Thus, it has been applied in the standard magnetic inversion scheme to mitigate the remanence effects, especially in the case of varying remanence directions. In this paper, NSS data were employed along with gravity data for three-dimensional cross-gradient joint inversion, which can significantly reduce the remanence effects and enhance the reliability of both density and magnetization models. Meanwhile, depth-weightings and bound constraints were also incorporated in this joint algorithm to improve the inversion quality. Synthetic and field examples show that the proposed combination of cross-gradient constraints and the NSS transform produce better results in terms of the data resolution, compatibility, and reliability than that of separate inversions and that of joint inversions with the total magnetization intensity (TMI) data. Thus, this method was found to be very useful and is recommended for applications in the presence of strong remanent magnetization.

Three-dimensional correlation imaging for total amplitude magnetic anomaly and normalized source strength in the presence of strong remanent magnetization

We present the 3D correlation imaging approach for the total magnitude magnetic anomaly and the normalized source strength data for reducing effects of strong remanent magnetization. We divide the subsurface space into a 3D regular grid and then calculate the cross correlation between the observed total magnitude magnetic anomaly or normalized source strength and the theoretical total magnitude magnetic anomaly or normalized source strength at each grid node due to a magnetic dipole. The resultant correlation coefficients are used to describe the equivalent magnetic dipole distribution underground in a probabilistic sense. The two approaches were tested both on the synthetic magnetic data and the real magnetic data from a metallic deposit area in the middle-lower reaches of the Yangtze River, China. The results show that the two approaches can considerably reduce effects of remanent magnetization and delineate magnetic sources in the subsurface, and that the approach for the normalized source strength is less sensitive to strong remanent magnetization than that of the total magnitude magnetic anomaly.

Estimating the Magnetization Direction of Sources through Correlation between Reduced-to-Pole Anomaly and Normalized Source Strength

Under the effects of remanent magnetization, total magnetization direction is different from geomagnetic field direction, which makes magnetic data processing and interpretation complexity. In this paper, we present a new approach for estimating the total magnetization direction of sources via cross-correlation between the reduced-to-pole anomaly and the normalized source strength (who is less sensitive to remanent magnetization). The geomagnetic field direction is used to calculated the normalized source strength, while various assumed total magnetization directions are used to calculated the RTP anomalies. The maximum correlation between the RTP anomalies and the normalized corresponds to the estimated total magnetization direction. Test on synthetic data showed that the new approach is simple and effective.

A New Method of Cross-Correlation by Magnetic Dipole for Estimating Magnetization Direction under the Influence of Remanent Magnetization

Processing and interpretation of magnetic data usually require information of total magnetization direction. However, under the effects of remanent magnetization, total magnetization direction is different from induced magnetization direction, which makes data processing and interpretation complexity. In this paper, we present a new method by cross-correlation of magnetic dipole source for determination of magnetization direction from relatively isolated and approximate equiaxial-shape magnetic total field anomaly. This method calculates cross-correlation coefficient between observed magnetic total field anomaly and theoretical magnetic total field anomaly caused by a magnetic dipole source, by using a set of varying parameters of positions and total magnetization direction of dipole source for trial and error. The corresponding magnetization direction of maximum correlation coefficient is regarded as estimated total magnetization direction. Test on synthetic data indicates that this method reliably and effectively estimates the magnetization direction from relatively isolated and approximate equiaxial-shape magnetic total field anomaly.

The Paradox of Scale: Reconciling magnetic anomalies with rock magnetic properties for cost-effective mineral exploration

Targeting magnetic anomalies is a common practice in the mineral industry. However, it is uncommon for anomalies to be reconciled with their causative lithologies after a hole has been drilled. Furthermore, the effects of remanent magnetization are seldom considered, even though they are likely to be significant. This study explores how timely rock magnetic property measurements coupled with magnetic field modelling can be used to explain the anomaly whilst drilling is underway, thus saving critical exploration expense. The Rover 3 anomaly, near Tennant Creek in the Northern territory, Australia, was inverted using three different source geometries; an ellipsoid, a plunging elliptic pipe, and a plunging polygonal pipe, all with assumed homogeneous magnetization. All the modelled bodies have resultant magnetization vectors with moderate inclinations down to the southeast, and the modelled tops of the bodies are in the range of 190–240 m below surface. Analyses of remanent magnetization in the intersected lithologies found the primary causative lithology to be a rhyodacitic unit with only moderate magnetic susceptibility (K < 0.01 SI) and strong remanent magnetization (Koenigsberger ratio (Q) > 10), also directed down to the southeast, with moderate inclination. Some samples of the mafic units near the top of the volcanic pile also displayed a large component of remanent magnetization. However, much of it was found to be “soft” low coercivity remanence, carried by multidomain magnetite. This detailed knowledge of the rock properties was used to calculate the relative contribution of each unit by separating both the remanent magnetization and induced magnetization into X, Y, and Z vector components, attributing the resultant components to specified thicknesses of rock, weighting the contribution according to its distance from the observation point and adding the resultants for each layer. This analysis determined a bulk magnetization vector oriented at moderate inclination down to the southeast. In order to reconcile the measured properties with the observed anomaly, we constructed a model in ModelVision Pro TM using stratigraphic units defined by drilling, with measured magnetization vectors attributed to each layer. The shape and lateral extent of the layers are unknown, but using only simple elliptical prisms the modelled anomaly matched the actual anomaly to within 10% rms, illustrating that the anomaly could be reconciled with the rock property measurements. In this case, if the remanent magnetization had been measured on site during drilling, it may have been possible to recognise that the anomaly was due to remanence by the time drilling had reached a depth of approximately 400 m. This may have resulted in a saving of approximately two weeks and the significant cost associated with drilling a further 350 m.

Mitigating remanent magnetization effects in magnetic data using the normalized source strength

We have developed an approach for the interpretation of magnetic field data that can be used when measured anomalies are affected by significant remanent magnetization components. The method deals with remanent effects by using the normalized source strength (NSS), a quantity calculated from the eigenvectors of the magnetic gradient tensor. The NSS is minimally affected by the direction of remanent magnetization present and compares well with other transformations of the magnetic field that are used for the same purpose. It therefore offers a way of inverting magnetic data containing the effects of remanent magnetizations, particularly when these are unknown and are possibly varying within a given data set. We use a standard 3D inversion algorithm to invert NSS data from an area where varying remanence directions are apparent, resulting in a more reliable image of the subsurface magnetization distribution than possible using the observed magnetic field data directly.

Frequency and damping adaptation of a TMD with controlled MR damper

This paper describes the new concept of a semi-active tuned mass damper with magnetorheological damper (MR-STMD). The real-time controlled MR damper force emulates controlled damping and a superimposed controllable stiffness force to augment or diminish the force of the passive spring stiffness which enables us to control the MR-STMD natural frequency. Both the damping and natural frequency are tuned according to Den Hartog’s formulae to the actual dominant frequency of the main structure irrespective of whether it is a resonance or a forced frequency. The MR-STMD is experimentally validated on the Empa bridge with a 15.6 m main span for different added masses to shift its resonance frequency −12.2% and +10.4% away from its nominal value. The experimental results are compared to those obtained when the MR-STMD is operated as a passive TMD that is precisely tuned to the nominal bridge. The comparison shows that the MR-STMD outperforms the TMD both in the tuned and all de-tuned cases by up to 63%. Simulations of the MR-STMD concept point out that the proposed semi-active control algorithm is most suitable for MR-STMDs due to the small amount of clipped active forces. A sensitivity analysis demonstrates that the real MR-STMD could be even more powerful if the force tracking errors in the MR damper force due to the current driver and MR fluid dynamics and remanent magnetization effects could be further reduced. The MR-STMD under consideration represents the prototype of the 12 MR-STMDs that have been running on the Volgograd Bridge since late fall 2011.

Graphical interactive generation of gravity and magnetic fields

This paper presents a MATLAB ® - based geopotential field generator called GamField that constructs and visualizes subsurface sources in 3-D space and computes their gravity and magnetic effects. GamField also computes anomaly gradients and remanent magnetization effects. The user inputs Cartesian prisms along with their physical properties to fabricate subsurface sources. Examples illustrating the utility of GamField for synthetic anomaly generation of gravity and magnetic fields are shown. ftp://ftp.ingv.it/pub/alessandro.pignatelli/Pignatelli

Effect of Core Magnetization on Frequency Response Analysis (FRA) of Power Transformers

This paper presents how the frequency response analysis (FRA) measurements on a transformer can be influenced by magnetization condition of the core. Measurements were performed on two transformers at different magnetization levels to show effects of remanent magnetization in the core due to removal of 3-supply, of relaxation demagnetization and of dc flux on the frequency response of winding impedance. The most important observations were that any sudden change of excitation field (applying or removing magnetization current) yields a slowly varying magnetic relaxation, which causes the impedance to change with time due to magnetic viscosity. Results of diagnostic measurements of transformer winding impedance can therefore depend on the level of remanent magnetic flux appearing in the core limbs after switching-off a transformer. These observations lead to a conclusion that, for avoiding the effects of core magnetization, diagnostic FRA measurements on power transformers should not be performed directly after disconnection from network.

Strong remanent magnetization in pyrrhotite: A structurally controlled example from the Paleoproterozoic Tanami orogenic gold province, northern Australia

Rocks associated with an orogenic gold system in the Paleoproterozoic Granites/Tanami Inlier of northern Australia exhibit strong natural remanent magnetism, identifiable in regional aeromagnetic data. Petrographical analysis indicates the two dominant magnetic minerals in the rocks of this region are multidomain magnetite and monoclinic pyrrhotite. Chemical analysis using an electron microprobe has determined that the magnetite and pyrrhotite are stoichiometrically pure, without significant elemental substitution. Alternating field demagnetization, Curie Temperature, and hysteresis analysis indicates that the remanently magnetized phase is monoclinic pyrrhotite with low magnetic susceptibility. In contrast, multidomain magnetites lack remanence and have high magnetic susceptibilities. Pyrrhotite is preserved in the rock mass along with other sulfide minerals as millimeter-scale veins parallel to regionally developed, penetrative mylonitic shear bands (c-planes) formed during sulfide remobilization in the waning, cooling stages of hydrothermal Au deposition. In contrast, magnetite grains exhibit porphyroblastic textures, consistent with formation during prograde to peak metamorphism, indicating formation prior to pyrrhotite, earlier in the orogenic cycle. Because of the low Curie Temperature ( ca. 325 °C) of pyrrhotite, it is therefore likely that remanent magnetization effects observed in aeromagnetic images of the Tanami Inlier, are associated with structures intimately related to the late stages of Au mineralization during retrograde greenschist metamorphism after the peak thermal point of the orogenic cycle.

Identifying block rotations from remanent magnetization effect: Example from northern Central Turkey

Abstract One of the difficulties in interpreting magnetic anomaly data is the distortion caused by rock body magnetization. There have been several successful studies carried out on the estimation of body magnetization based on the correlation of the Bouguer and magnetic anomaly data. A number of other significant research projects have aimed at identifying the effect of remanent magnetization in magnetic data only. In this paper, we present a threedimensional method for determining the body magnetization direction by means of comparing the amplitude of the analytic signal with the horizontal gradient of pseudogravity of magnetic anomaly using the correlation coefficient equation. The method has been successfully applied to test cases and is applied here to the aeromagnetic anomalies located in northern Central Turkey. Anomalies displaying individual polarities were separated out and correlated with themselves using the correlation coefficient equation. Counter-clockwise rotations in the range of 3° and 174° were determined. Clockwise rotations were determined in the range of 19° and 153°. The inclination of the body magnetization directions for one of the blocks was estimated to be 68.4°, while all other blocks had a low inclination angle of body magnetization in the range of 32° to 57°. These results may imply that these blocks gained their magnetizations when Turkey was at low latitudes. Possible buried faults are also identified. The results reported here comply with the new mobilistic theory.

Effect of remanent magnetization on exchange biasing in [Pd/Co]×5/FeMn films

The effect of remanent magnetization, which is defined as the initial state of magnetization before FeMn pinning, on exchange biasing in [Pd/Co]×5/FeMn multilayers with perpendicular magnetic anisotropy was investigated. As the Co thickness increases from 0.23 to 0.55 nm in [Pd(0.6 nm)/Co]×5 multilayers, the remanent magnetization M rm values of as-deposited samples decrease from maximum magnetization M s to almost zero due to the weakened perpendicular anisotropy. After the [Pd/Co]×5 multilayers were pinned by FeMn layer, the magnetization curve and perpendicular exchange-biasing field H eb become strongly related to the remanent magnetization.

Effect of Remanent Magnetization of Ferromagnetic Thin Film on I-V Characteristics of MOS Transistor : I-V Characteristics of Circular and Opposite Patterned Ferromagnetic Thin Film Across Gate Area

Effect of Remanent Magnetization of Ferromagnetic Thin Film on I-V Characteristics of MOS Transistor : I-V Characteristics of Circular and Opposite Patterned Ferromagnetic Thin Film Across Gate Area