2Dinversion for borehole magnetic data in the presence of significant remanence and demagnetization
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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.Keywords:
Stoner–Wohlfarth model
Rock magnetism
Natural remanent magnetization
Intensity
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Following recent advances in superconducting quantum interference devices (SQUIDs), airborne full tensor magnetic gradiometry (FTMG) is emerging as a practical geophysical exploration method that is intended to recover information about remanent magnetization. In this paper, we introduce 3D regularized inversion of FTMG data that recovers the total magnetization vector in each cell of the 3D earth model. If a priori information about the susceptibility or remanent magnetization is available, the 3D inversion can be constrained to recover the remanent magnetization vector. If a priori information is not available, it is possible to recover attributes such as the amplitude, components, and angle of the magnetization vector relative to the inducing field. We present a case study for data acquired over a dike swarm in South Africa that compares our 3D FTMG inversion for magnetization with a 3D total magnetic intensity (TMI) inversion for a positively-constrained susceptibility distribution. Given the significant remanent magnetization present, the 3D FTMG inversion for magnetization recovers results that are most consistent with the known geology.
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Inversion of magnetic data is complicated by the presence of remanent magnetization. To tackle this problem we design a methodology for inverting magnetic data for subsurface magnetization, as opposed to magnetic susceptibility. Our approach contains flexibility to obtain different types of magnetization models and the inversion routine is appropriate for use on data that contains the response of material exhibiting remanence. The magnetization is split into one component parallel (or anti‐parallel), and two components perpendicular, to the Earth's field. Finding a vector magnetization greatly increases non‐uniqueness but additional information, such as bounding the parallel component of the magnetization, can help produce useful results.
Stoner–Wohlfarth model
Single domain
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A theoretical basis is given for two empirically derived methods of three‐axis alternating field (AF) demagnetization of static samples used initially by Zijderveld and Dunlop to remove “spurious” components of magnetization produced by the AF. It is shown theoretically that both methods of treating the experimental results to reveal the natural remanent magnetization (NRM) vector are precisely those which algebraically eliminate gyroremanent magnetization (GRM). By implication, the “spurious” remanences found by these and other workers are almost certainly of gyromagnetic origin and not remanences produced as a result of shortcomings in instrumentation. It is further demonstrated that by suitably treating the data obtained by either three‐axis method, GRM can be identified and used to provide information about the anisotropy of the rock.
Natural remanent magnetization
Spurious relationship
Stoner–Wohlfarth model
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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.
Stoner–Wohlfarth model
Rock magnetism
Natural remanent magnetization
Potential field
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