On “Mapping remanent magnetization using the local phase” (J. B. Thurston, 2001, GEOPHYSICS , 66, 1082–1089)
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Abstract:
Thurston (2001) proposes a method and presents synthetic and practical results that we find difficult to understand both theoretically as well as practically.Significant remanence and demagnetization alter the intensity and orientation of total magnetization,which complicates interpretation of magnetic data.To deal with the problem,we propose a method of inverting 2D magnetization vector distributions using the 2D borehole magnetic data.We firstly invert the magnetization intensity distributions based on magnetic anomaly amplitudes of borehole magnetic data. With the known magnetization intensity distributions,subsequently,we invert the magnetization orientation distributions by fitting the magnetic component anomalies.Both magnetization intensity and orientation are solved by conjugate gradients.And a preconditioned matrix is utilized to improve the inverse quality of magnetization intensity. All synthetic examples involving significant remanence and high susceptibility demonstrate that this method is capable of accurately recovering the magnetization vector distributions. The magnetization vector distributions comprehensively include the influences of induced magnetization,remanent magnetization and self-demagnetization.Therefore,it provides an effective approach to study the ore deposits when high susceptibility and significant remanent magnetization are present.
Stoner–Wohlfarth model
Rock magnetism
Natural remanent magnetization
Intensity
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Inversion of magnetic data is complicated by the presence of remanent magnetization. To deal with this problem, we invert magnetic data for a three-component subsurface magnetization vector, as opposed to magnetic susceptibility (a scalar). The magnetization vector can be cast in a Cartesian or spherical framework. In the Cartesian formulation, the total magnetization is split into one component parallel and two components perpendicular to the earth’s field. In the spherical formulation, we invert for magnetization amplitude and the dip and azimuth of the magnetization direction. Our inversion schemes contain flexibility to obtain different types of magnetization models and allow for inclusion of geologic information regarding remanence. Allowing a vector magnetization increases the nonuniqueness of the magnetic inverse problem greatly, but additional information (e.g., knowledge of physical properties or geology) incorporated as constraints can improve the results dramatically. Commonly available information results in complicated nonlinear constraints in the Cartesian formulation. However, moving to a spherical formulation results in simple bound constraints at the expense of a now nonlinear objective function. We test our methods using synthetic and real data from scenarios involving complicated remanence (i.e., many magnetized bodies with many magnetization directions). All tests provide favorable results and our methods compare well against those of other authors.
Stoner–Wohlfarth model
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Modelling of magnetic rock properties from magnetic field observations has been an important practice in resource exploration for decades. However, the application of this practice has been limited by conventional thinking that assumes rock magnetization is dominated by induced magnetization such that magnetization direction is aligned with the geomagnetic field. Convention has also accepted that we are unable to model for magnetic remanence without a-priori knowledge of remanence direction and strength.Recent practical successes in directly modelling magnetization vector direction and strength using Magnetization Vector Inversion (MVI) have challenged these conventions, and MVI modelling is proving useful in practical exploration scenarios. The addition of new information, namely the direction and amplitude of magnetization, demands new thinking and approaches to understanding what this information means, and how to use the modelled direction of magnetization in practical situations.This paper presents a new statistical and quantitative approach to define and discriminate different magnetization domains within a full 3D MVI voxel model. Our studies show that modelled vector direction is meaningful even without prior knowledge of remanence (and other) magnetization characteristics. We also demonstrate that reasonable magnetization direction can be recovered from both weakly and strongly magnetized source rocks.
Stoner–Wohlfarth model
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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.
Stoner–Wohlfarth model
Natural remanent magnetization
Rock magnetism
Single domain
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Remanent magnetization complicates the inversion of magnetic data due to altering the strength and direction of the magnetization vector. To deal with the problem, we have developed a 2D sequential inversion method of successively recovering the distributions of magnetization intensity and susceptibility and estimating the magnetization direction. The magnetization intensity distribution is first recovered from the total magnitude anomaly that is frequently transformed from the observed total-field anomaly and is invariant with the magnetization direction. With the recovered magnetization intensity distribution, we forward model the total-field anomalies caused by sources with different magnetization directions and calculate the correlations between the observed and predicted data. The orientation when the correlations attain a peak of maximum is defined as the optimal magnetization direction. Finally, the estimated magnetization direction helps to recover the susceptibility distribution by further inverting for total-field data. This method was tested by use of synthetic data and field data of two iron-ore deposits involving significant remanence, and all tests returned favorable results. The method obtaining the magnetization intensity and susceptibility distributions and an averaged magnetization direction made full use of the amplitude and phase information of magnetic anomalies, and it was more applicable for scenarios with a homogeneous magnetization direction.
Stoner–Wohlfarth model
Intensity
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