Three-dimensional inversion of magnetic data in the simultaneous presence of significant remanent magnetization and self-demagnetization: example from Daye iron-ore deposit, Hubei province, China
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Natural remanent magnetization and self-demagnetization on high-susceptibility bodies are two important factors affecting magnetic data inversion. We propose a framework for the inversion and interpretation of magnetic anomalies, in which significant remanent magnetization and self-demagnetization are present simultaneously. The framework is based on the assumptions that the external applied field and internal self-demagnetization field are uniform and the deflection of self-demagnetization in the total magnetization direction is negligible. First, the magnetization vector distributions are obtained from magnetic data by estimating the magnetization direction, then inverting for the magnetization intensity distribution, using the inferred magnetization direction as a constraint. Based on a priori information about the Koenigsberger ratio derived from petrophysical measurements, the direction and intensity of the remanent magnetization are obtained. The self-demagnetization factor is then computed using the finite volume method. Finally, the true-susceptibility distribution is achieved by correcting for the self-demagnetization effect. The method is first applied to synthetic magnetic data produced by a prism-shaped source model that has significant remanent magnetization and high susceptibility. In a case study of the Daye iron-ore deposit, Hubei province, China, the true susceptibility and remanent magnetization are reconstructed. The remanence direction information reveals that local geological activities such as synclines and faults lead to changes in the remanence directions at different local deposits.Keywords:
Natural remanent magnetization
Rock magnetism
Stoner–Wohlfarth model
Single domain
We have investigated basic properties of transition remanent magnetization of natural magnetite in granite samples collected from the Minnesota River Valley, North America. Transition remanence was imparted during cooling and/or warming through the Verwey transition around 120 K. Depending on magnetic field conditions during cooling and warming, three types of transition remanences have been categorized: (1) TrRM, acquired during a cycle of field cooling and field warming; (2) TrWRM, acquired during zero-field cooling and field warming and (3) TrCRM, imparted during field cooling and zero-field warming. These remanences fulfil basic laws of remanent magnetization: (1) directions of the transition remanences are parallel to direction of the applied field (the law of parallelism), (2) intensities of the transition remanences are proportional to the applied field intensity (the law of proportionality) and (3) sum of the partial transition remanences is equal to the total transitional remanence, that is, TrRM = TrWRM + TrCRM. In addition, the ratio of TrRM to TRMLTD (the demagnetized component of thermoremanent magnetization by low-temperature demagnetization) shows a nearly constant value of ∼0.34. This relationship might reflect differences in equilibrium magnetic domain state at low and high temperature.
Natural remanent magnetization
Thermoremanent magnetization
Rock magnetism
Stoner–Wohlfarth model
Charge ordering
Single domain
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Natural remanent magnetization and self-demagnetization on high-susceptibility bodies are two important factors affecting magnetic data inversion. We propose a framework for the inversion and interpretation of magnetic anomalies, in which significant remanent magnetization and self-demagnetization are present simultaneously. The framework is based on the assumptions that the external applied field and internal self-demagnetization field are uniform and the deflection of self-demagnetization in the total magnetization direction is negligible. First, the magnetization vector distributions are obtained from magnetic data by estimating the magnetization direction, then inverting for the magnetization intensity distribution, using the inferred magnetization direction as a constraint. Based on a priori information about the Koenigsberger ratio derived from petrophysical measurements, the direction and intensity of the remanent magnetization are obtained. The self-demagnetization factor is then computed using the finite volume method. Finally, the true-susceptibility distribution is achieved by correcting for the self-demagnetization effect. The method is first applied to synthetic magnetic data produced by a prism-shaped source model that has significant remanent magnetization and high susceptibility. In a case study of the Daye iron-ore deposit, Hubei province, China, the true susceptibility and remanent magnetization are reconstructed. The remanence direction information reveals that local geological activities such as synclines and faults lead to changes in the remanence directions at different local deposits.
Natural remanent magnetization
Rock magnetism
Stoner–Wohlfarth model
Single domain
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Abstract Development in instrumentation and technology now allows for mapping of magnetic anomalies, caused by spatial variations in magnetization in the source material, from the mineral to the crustal anomalies scale. High‐resolution magnetic mapping techniques allow for accurate investigation of the magnetization in natural rock samples and particularly of their remanence carriers, which can record geologically meaningful information. Multidomain magnetic grains are expected to retain a remanence that is susceptible to change by exposure to magnetic fields or by changes in temperature. Although this makes multidomain grains less reliable remanence carriers for paleomagnetic studies, their magnetization contributes to rocks bulk magnetization and to induced anomalies in the Earth crust. Here, we investigate the fine‐scale magnetization of a sample that exhibits a multidomain behavior. We used a scanning magnetic microscope, equipped with a room temperature magnetic tunnel junction sensor, to map the magnetic field over a petrographic thin section of the sample containing large magnetite grains (>100 μm) surrounded by serpentine and carbonate. We modeled the fine‐scale remanent magnetization of the magnetite by inverting the magnetic scans data acquired in near‐field‐free conditions. We applied a multistep inversion, a priori tested on a synthetic model, with a controlled range on the intensity of the magnetization. Modeling results on the study sample suggest homogenously magnetized regions within the magnetite grains with variable remanent magnetization intensities and directions coherent with the multidomain behavior inferred from bulk measurements.
Natural remanent magnetization
Rock magnetism
Stoner–Wohlfarth model
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Rock magnetism
Magnetism
Natural remanent magnetization
Magnetocrystalline anisotropy
Stoner–Wohlfarth model
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The Horseshoe Range is a banded iron formation (BIF) in the Southern Capricorn Orogen, WA and is associated with a large horseshoe-shaped positive magnetic anomaly. Electron microscope mineralogy identified ubiquitous goethite and magnetite/hematite. This study focussed on measuring the magnetic properties of the rocks at Horseshoe Range in order to accurately predict their geophysical responses when buried beneath cover.Remanent magnetisation intensities of the rocks were high (up to 1300 A/m) and vectors measured in the rocks were oriented predominately downward which typically result in negative anomalies which is inconsistent with the observed anomaly. Due to the positive nature of the magnetic anomaly and the ability to accurately model the response without remanent magnetisation it appears that the high intensity remanent magnetisations may be volumetrically insignificant and likely limited to the near surface. The remanence may be caused by near surface formation of maghemite during bushfires and/or induced by lightning strikes.The BIF can be modelled using a single homogenous layer with a susceptibility of 0.8 SI. However, this is not geologically consistent with BIFs which typically display variable iron-oxide mineralogy and associated petrophysical properties. One way to more accurately model BIFs is to use the first vertical derivative as the model input. Using this approach, a 4 layer model was generated which matched the anomaly to an RMS of Modelled susceptibilities ranged from 0.01 - 0.55 SI which are consistent with the measured properties. However, this model did not take into account the measured high intensity downward magnetisation vectors.
Maghemite
Natural remanent magnetization
Rock magnetism
Petrophysics
Anomaly (physics)
Magnetic mineralogy
Banded iron formation
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Thermoremanent magnetization
Natural remanent magnetization
Rock magnetism
Single domain
Archaeomagnetic dating
Magnetic mineralogy
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