Using airborne vector magnetic data to calculate the projection of magnetic anomaly vector onto normal geomagnetic field

The total-field magnetic anomaly [Formula: see text] is an approximation of the projection [Formula: see text] of the magnetic anomaly vector [Formula: see text] onto the normal geomagnetic field [Formula: see text]. However, for highly magnetic sources, the approximation error of [Formula: see text] cannot be ignored. To reduce the error, we have developed a method for calculating [Formula: see text] by using airborne vector magnetic data based on the vector relationship of geomagnetic field [Formula: see text]. The calculation uses the magnitude of the vectors [Formula: see text], [Formula: see text], and [Formula: see text] through a simple approach. To ensure that each magnitude has the same level, we normalize the magnitude of [Formula: see text] using the total-field magnetic data measured by the scalar magnetic sensor. The method is applied to the measured airborne vector magnetic data at the Qixin area of the East Tianshan Mountains in China. The results indicate that the calculated [Formula: see text] has high precision and can distinguish the approximation error less than 3.5 nT. We also analyze the characteristics of the approximation error that are caused by the effects of different total magnetization inclinations. These error characteristics are used to predict the total magnetization inclination of a 2D magnetic source based on the measured airborne vector magnetic data.

Gaussian Local Phase Approximation in a Cylindrical Tissue Model

In NMR or MRI, the measured signal is a function of the accumulated magnetization phase inside the measurement voxel, which itself depends on microstructural tissue parameters. Usually the phase distribution is assumed to be Gaussian and higher-order moments are neglected. Under this assumption, only the x-component of the total magnetization can be described correctly, and information about the local magnetization and the y-component of the total magnetization is lost. The Gaussian Local Phase (GLP) approximation overcomes these limitations by considering the distribution of the local phase in terms of a cumulant expansion. We derive the cumulants for a cylindrical muscle tissue model and show that an efficient numerical implementation of these terms is possible by writing their definitions as matrix differential equations. We demonstrate that the GLP approximation with two cumulants included has a better fit to the true magnetization than all the other options considered. It is able to capture both oscillatory and dampening behavior for different diffusion strengths. In addition, the introduced method can possibly be extended for models for which no explicit analytical solution for the magnetization behavior exists, such as spherical magnetic perturbers.

A paradigm shift in magnetic data interpretation: Increased value through magnetization inversions

Magnetic data are sensitive to both the induced magnetization in rock units caused by the present earth's magnetic field and the remanent magnetization acquired by rock units in past geologic time. Susceptibility is a direct indicator of the magnetic mineral content, whereas remanent magnetization carries information about the formation process and subsequent structural movement of geologic units. The ability to recover and use total magnetization, defined as the vectorial sum of the induced and remanent magnetization, therefore enables us to take full advantage of magnetic data. The exploration geophysics community has achieved significant advances in inverting magnetic data affected by remanent magnetization. It is now feasible to invert any magnetic data set for total magnetization. We provide an overview of the state of the art in magnetization inversion and demonstrate the informational value of inverted magnetization through a set of case studies from mineral exploration problems. We focus on the methods that recover either the magnitude of the total magnetization or the total magnetization vector itself.

Exchange bias, and coercivity investigations in hematite nanoparticles

<abstract> <p>Hematite nanoparticles of average size of 20 nm were synthesized using sol-gel method and the structural characterisations were conducted using XRD and TEM. The XRD profile revealed the coexistence of small fraction of maghemite phase along with the main hematite phase. Magnetization versus applied field (M-H) measurements were performed between −5 and 5 T and respectively in the temperatures 2, 10, 30, 50, 70,100,150,200, and 300 K under zero field and 1, 2, 3, 4 T field cooling. At all field-cooling values, the coercivity was found to display a weak temperatures dependence below 150 K and a strong increase above 150 K reaching the largest value of 3352 Oe at 300 K for the field-cooling value of 3 T. Horizontal and vertical hysteresis loop shifts were observed at all temperatures in both the zero-field and field-cooled states. In the field-cooled state, both loop shifts where found to have significant and nonmonotonic field-cooling dependences. However, because saturation magnetization was not attained in all measurements our calculations were based on the minor hysteresis loops. M-H measurements were performed between −9 and 9 T at room temperature under zero field cooling and 1, 2, 3, 4, 5, 6 T field cooling. Saturation magnetization was not attained, and the loops displayed loop shifts similar to those for the ±5 T sweeping field. The highest coercivity value of 4400 Oe is observed for the 6 T field cooled MH loop. The ferromagnetic (FM) contribution towards the total magnetization was separated from the total magnetization and hysteresis loops displayed both horizontal and vertical shifts. The novel results of the temperature and field dependence of exchange bias were attributed mainly to the magnetic exchange coupling between the different magnetic phases (mainly the FM) and the spin-glass-like regions.</p> </abstract>

Improving the crosscorrelation method to estimate the total magnetization direction vector of isolated sources: A space-domain approach for unstable inclination values

Knowledge of the total magnetization direction of geologic sources is valuable for interpretation of magnetic anomalies. Although the magnetization direction of causative sources is assumed to be induced by the ambient magnetic field, the presence of remanence should not be neglected. An existing method of correlating total and vertical gradients of the reduced-to-the-pole (RTP) anomaly estimates the total magnetization direction well. However, due to the numerical instability of RTP transformation in the Fourier domain, an assumption should be considered for dealing with inclination values at approximately 0°. We have adopted an extension to the standard crosscorrelation method for estimating the total magnetization direction vector, computing the RTP anomaly by means of the classic equivalent layer technique for low inclination values. Additionally, an ideal number of equivalent sources within the layer is considered for reducing the computational demands. To investigate the relevant aspects of the adopted method, two simple synthetic scenarios are presented. First, a magnetic anomaly produced by a homogeneous and isolated vertical dike is considered. This test illustrates the good performance of the adopted approach, finding the true magnetization direction, even for low inclination values. In the second synthetic test, a long-wavelength component is added to the previous magnetic total-field anomaly. In this case, the method adopted here fails to estimate a reliable magnetization direction vector, showing weak performance for strong interfering magnetic anomalies. On the real data example, the application tests an isolated total-field anomaly of the Carajás Mineral Province, in northern Brazil, where the inclination of the ambient magnetic field is close to zero. The obtained results indicate weak remanence in the estimated total magnetization direction vector, which would never be reached in the standard formulation of the crosscorrelation technique.

EFFECT OF Mn AND Ti ADDITION ON THE CRYSTALLOGRAPHIC STRUCTURE AND MAGNETIC PROPERTIES OF SrFe12O19

The synthesis and characterization of composition SrFe12-(x+y)MnxTiyO19 (x = y and x ≠ y) compound by using solid state reaction have been performed. The raw materials were SrCO3, Fe2O3, MnCO3, and TiO2. The mixed powder was compacted at 5000 psi into pellets and sintered at 1050°C in the air at atmosphere pressure for 15 hours and furnace cooling. The refinement results of x-ray diffraction pattern show that the doping composition (x = y) was a single phase while the doping composition (x ≠ y) was multi phase. We concluded that effect of substitution upon magnetic properties revealed that total magnetization, remanence and coercivity changed with substitution due to preferential site occupancy of substituted Mn2+ and Ti4+ ions. The coercivity decreases with increase in Mn and Ti concentration. This effect is related with Fe3+ magnetic moment changes after they have already substituted Mn2+ and Ti4+ ions. Since the coercivity and total magnetization may be controlled by substitution while maintaining resistive properties, making this material useful for microwave absorber.

Interdependence of shape and magnetic properties in Al-nanoparticles doped with Ni and Pt

By means of ab initio molecular dynamics, we investigate the magnetic behaviour of Al, AlNi and AlPt nanoparticles of 19 atoms. New geometrical shapes are detected and a depression of the total magnetization is associated with geometrical reconstructions where the fivefold symmetry character is lost, independently of the chemical doping.