Cartographic overlay of geology on shaded total field magnetic data, Slave Craton and environs, District of Mackenzie, Northwest Territories

We have developed a case study on the use of constrained inversion of magnetic data for recovering ore bodies quantitatively in the Macheng iron deposit, China. The inversion is constrained by the structural orientation and the borehole lithology in the presence of high magnetic susceptibility and strong remanent magnetization. Either the self-demagnetization effect caused by high susceptibility or strong remanent magnetization would lead to an unknown total magnetization direction. Here, we chose inversion of amplitude data that indicate low sensitivity to the direction of magnetization of the sources when constructing the underground model of effective susceptibility. To reduce the errors that arise when treating the total-field anomaly as the projection of an anomalous field vector in the direction of the geomagnetic reference field, we develop an equivalent source technique to calculate the amplitude data from the total-field anomaly. This equivalent source technique is based on the acquisition of the total-field anomaly, which uses the total-field intensity minus the magnitude of the reference field. We first design a synthetic model from a simplified real case to test the new approach, involving the amplitude data calculation and the constrained amplitude inversion. Then, we apply this approach to the real data. The results indicate that the structural orientation and borehole susceptibility bounds are compatible with each other and are able to improve the quality of the recovered model to obtain the distribution of ore bodies quantitatively and effectively.

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

Geophysics ◽

10.1190/geo2020-0582.1 ◽

2021 ◽

pp. 1-47

Author(s):

Rukuan Xie ◽

Shengqing Xiong ◽

Shuling Duan ◽

Jinlong Wang ◽

Ping Wang ◽

...

Keyword(s):

Magnetic Anomaly ◽

Geomagnetic Field ◽

Approximation Error ◽

Magnetic Data ◽

Total Field ◽

Projection Formula ◽

Total Magnetization ◽

Vector Formula ◽

Relationship Of ◽

Anomaly Formula

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.

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Aeromagnetic total field, Stanton, Northwest Territories

10.4095/127808 ◽

1989 ◽

Author(s):

Keyword(s):

Northwest Territories ◽

Total Field

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Fast 3D magnetic inversion of a surface relief in the space domain

Geophysics ◽

10.1190/geo2018-0712.1 ◽

2019 ◽

Vol 84 (5) ◽

pp. J57-J67 ◽

Cited By ~ 2

Author(s):

Marlon C. Hidalgo-Gato ◽

Valéria C. F. Barbosa

Keyword(s):

Hessian Matrix ◽

Computational Cost ◽

Magnetization Vector ◽

Forward Modeling ◽

Magnetic Data ◽

Total Field ◽

Inversion Algorithm ◽

Sensitivity Matrix ◽

Magnetic Inversion ◽

Magnetic Basement

We have developed a fast 3D regularized magnetic inversion algorithm for depth-to-basement estimation based on an efficient way to compute the total-field anomaly produced by an arbitrary interface separating nonmagnetic sediments from a magnetic basement. We approximate the basement layer by a grid of 3D vertical prisms juxtaposed in the horizontal directions, in which the prisms’ tops represent the depths to the magnetic basement. To compute the total-field anomaly produced by the basement relief, the 3D integral of the total-field anomaly of a prism is simplified by a 1D integral along the prism thickness, which in turn is multiplied by the horizontal area of the prism. The 1D integral is calculated numerically using the Gauss-Legendre quadrature produced by dipoles located along the vertical axis passing through the prism center. This new magnetic forward modeling overcomes one of the main drawbacks of the nonlinear inverse problem for estimating the basement depths from magnetic data: the intense computational cost to calculate the total-field anomaly of prisms. The new sensitivity matrix is simpler and computationally faster than the one using classic magnetic forward modeling based on the 3D integrals of a set of prisms that parameterize the earth’s subsurface. To speed up the inversion at each iteration, we used the Gauss-Newton approximation for the Hessian matrix keeping the main diagonal only and adding the first-order Tikhonov regularization function. The large sparseness of the Hessian matrix allows us to construct and solve a linear system iteratively that is faster and demands less memory than the classic nonlinear inversion with prism-based modeling using 3D integrals. We successfully inverted the total-field anomaly of a simulated smoothing basement relief with a constant magnetization vector. Tests on field data from a portion of the Pará-Maranhão Basin, Brazil, retrieved a first depth-to-basement estimate that was geologically plausible.

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Assessing and ameliorating edge effects in magnetic data transformations

10.5194/egusphere-egu2020-5320 ◽

2020 ◽

Author(s):

Peter Lelièvre ◽

Dominique Fournier ◽

Sean Walker ◽

Nicholas Williams ◽

Colin Farquharson

Keyword(s):

Edge Effects ◽

Mineral Exploration ◽

Magnetization Vector ◽

Magnetic Data ◽

Separation Procedure ◽

Magnetic Intensity ◽

Source Inversion ◽

Total Field ◽

Equivalent Source ◽

Amplitude Data

<p>Reduction to pole and other transformations of total field magnetic intensity data are often challenging to perform at low magnetic latitudes, when remanence exists, and when large topographic relief exists. Several studies have suggested use of inversion-based equivalent source methods for performing such transformations under those complicating factors. However, there has been little assessment of the importance of erroneous edge effects that occur when fundamental assumptions underlying the transformation procedures are broken. In this work we propose a transformation procedure that utilizes magnetization vector inversion, inversion-based regional field separation, and equivalent source inversion on unstructured meshes. We investigated whether edge effects in transformations could be reduced by performing a regional separation procedure prior to equivalent source inversion. We applied our proposed procedure to the transformation of total field magnetic intensity to magnetic amplitude data, using a complicated synthetic example based on a real geological scenario from mineral exploration. While the procedure performed acceptably on this test example, the results could be improved. We pose many questions regarding the various choices and control parameters used throughout the procedure, but we leave the investigation of those questions to future work.</p>

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A comparison of methods for approximating the vertical gradient of one‐dimensional magnetic field data

Geophysics ◽

10.1190/1.1442221 ◽

1986 ◽

Vol 51 (9) ◽

pp. 1725-1735 ◽

Cited By ~ 5

Author(s):

J. W. Paine

Keyword(s):

Magnetic Field ◽

Fourier Transform ◽

Field Data ◽

Total Error ◽

Vertical Gradient ◽

Magnetic Data ◽

Total Field ◽

One Dimensional ◽

Convolution Filter ◽

Principal Factors

The vertical gradient of a one‐dimensional magnetic field is known to be a useful aid in interpretation of magnetic data. When the vertical gradient is required but has not been measured, it is necessary to approximate the gradient using the available total‐field data. An approximation is possible because a relationship between the total field and the vertical gradient can be established using Fourier analysis. After reviewing the theoretical basis of this relationship, a number of methods for approximating the vertical gradient are derived. These methods fall into two broad categories: methods based on the discrete Fourier transform, and methods based on discrete convolution filters. There are a number of choices necessary in designing such methods, each of which will affect the accuracy of the computed values in differing, and sometimes conflicting, ways. A comparison of the spatial and spectral accuracy of the methods derived here shows that it is possible to construct a filter which maintains a reasonable balance between the various components of the total error. Further, the structure of this filter is such that it is also computationally more efficient than methods based on fast Fourier transform techniques. The spacing and width of the convolution filter are identified as the principal factors which influence the accuracy and efficiency of the method presented here, and recommendations are made on suitable choices for these parameters.

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Stepped inversion of magnetic data

Geophysics ◽

10.1190/1.2711661 ◽

2007 ◽

Vol 72 (3) ◽

pp. L21-L30 ◽

Cited By ~ 3

Author(s):

Soraya Lozada Tuma ◽

Carlos Alberto Mendonça

Keyword(s):

Magnetic Anomaly ◽

Shape Function ◽

Source Parameters ◽

Real Data ◽

Magnetic Data ◽

Total Field ◽

Magnetization Direction ◽

Magnetization Intensity ◽

Magnetic Inversion ◽

Total Gradient

We present a three-step magnetic inversion procedure in which invariant quantities with respect to source parameters are inverted sequentially to give (1) shape cross section, (2) magnetization intensity, and (3) magnetization direction for a 2D (elongated) magnetic source. The quantity first inverted (called here the shape function) is obtained from the ratio of the gradient intensity of the total-field anomaly to the intensity of the anomalous vector field. For homogenous sources, the shape function is invariant with source magnetization and allows reconstruction of the source geometry by attributing an arbitrary magnetization to trial solutions. Once determined, the source shape is fixed and magnetization intensity is estimated by fitting the total gradient of the total-field anomaly (equivalent to the amplitude of the analytic signal of magnetic anomaly). Finally, the source shape and magnetization intensity are fixed and the magnetization direction is determined by fitting the magnetic anomaly. As suggested by numerical modeling and real data application, stepped inversion allows checking whether causative sources are homogeneous. This is possible because the shape function from inhomogeneous sources can be fitted by homogeneous models, but a model obtained in this way fits neither the total gradient of the magnetic anomaly nor the magnetic anomaly itself. Such a criterion seems effective in recognizing strongly inhomogeneous sources. Stepped inversion is tested with numerical experiments, and is used to model a magnetic anomaly from intrusive basic rocks from the Paraná Basin, Brazil.

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