Constrained 3D inversion of magnetic data with structural orientation and borehole lithology: A case study in the Macheng iron deposit, Hebei, China

Geophysics ◽  
2019 ◽  
Vol 84 (2) ◽  
pp. B121-B133 ◽  
Author(s):  
Shida Sun ◽  
Chao Chen ◽  
Yiming Liu
Keyword(s):  
Remanent Magnetization ◽  
Magnetic Data ◽  
Iron Deposit ◽  
Reference Field ◽  
Total Field ◽  
Equivalent Source ◽  
Structural Orientation ◽  
Amplitude Data ◽  
Ore Bodies

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.

Assessing and ameliorating edge effects in magnetic data transformations

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>

Inversion of magnetic anomaly on rugged observation surface in the presence of strong remanent magnetization

Geophysics ◽  
2014 ◽  
Vol 79 (2) ◽  
pp. J11-J19 ◽  
Author(s):  
Shu-Ling Li ◽  
Yaoguo Li
Keyword(s):  
Remanent Magnetization ◽  
Country Rock ◽  
Magnetization Vector ◽  
Magnetic Data ◽  
Magnetization Direction ◽  
Equivalent Source ◽  
Total Magnetization ◽  
Amplitude Inversion ◽  
Amplitude Data ◽  
And Inversion

We study the inversion of magnetic data acquired over a rugged observation surface and where the buried source bodies have strong remanent magnetization that leads to unknown total magnetization directions. These factors pose significant challenges for processing and inversion of such data. To tackle the challenges from both a rugged observation surface and an unknown magnetization direction, we propose a strategy through the joint use of the equivalent source technique and 3D amplitude inversion to obtain 3D magnetization strength. We use equivalent source processing to calculate the amplitude data in the space domain because the use of the wavenumber-domain method is invalid due to large variations in the data elevation. We then carried out an amplitude inversion to generate a 3D subsurface distribution of the magnitude of the total magnetization vector. The results from a synthetic example and aeromagnetic data in Daye Mine in China showed that this approach is effective and images the magnetic units whose contact zones with the limestone country rock host the mineralization. The method is general and can be applied to a variety of cases with similar challenges.

Quantifying the error level in computed magnetic amplitude data for 3D magnetization inversion

Geophysics ◽  
2018 ◽  
Vol 83 (5) ◽  
pp. J75-J84 ◽  
Author(s):  
Camriel Coleman ◽  
Yaoguo Li
Keyword(s):  
Field Data ◽  
Regularization Parameter ◽  
Three Dimensional ◽  
Inverse Model ◽  
Magnetic Data ◽  
Total Field ◽  
Noise Estimate ◽  
Amplitude Inversion ◽  
Amplitude Data ◽  
Magnetic Amplitude

Three-dimensional inversion plays an important role in the quantitative interpretation of magnetic data in exploration problems, and magnetic amplitude data can be an effective tool in cases in which remanently magnetized materials are present. Because amplitude data are typically calculated from total-field anomaly data, the error levels must be characterized for inversions. Lack of knowledge of the error in amplitude data hinders the ability to properly estimate the data misfit associated with an inverse model and, therefore, the selection of the appropriate regularization parameter for a final model. To overcome these challenges, we have investigated the propagation of errors from total-field anomaly to amplitude data. Using parametric bootstrapping, we find that the standard deviation of the noise in amplitude data is approximately equal to that of the noise in total-field anomaly data when the amplitude data are derived from the conversion of total-field data to three orthogonal components. We then illustrate how the equivalent source method can be used to estimate the error in total-field anomaly data when needed. The obtained noise estimate can be applied to amplitude inversion to recover an optimal inverse model by applying the discrepancy principle. We test this method on synthetic and field data and determine its effectiveness.

Application of 3D magnetic amplitude inversion to iron oxide-copper-gold deposits at low magnetic latitudes: A case study from Carajás Mineral Province, Brazil

Geophysics ◽  
2015 ◽  
Vol 80 (2) ◽  
pp. B13-B22 ◽  
Author(s):  
Marcelo Leão-Santos ◽  
Yaoguo Li ◽  
Roberto Moraes
Keyword(s):  
Iron Oxide ◽  
Gold Deposits ◽  
Magnetic Data ◽  
Magnetic Latitude ◽  
Amplitude Inversion ◽  
Carajás Mineral Province ◽  
Amplitude Data ◽  
Copper Gold ◽  
Magnetic Amplitude

Strong hydrothermal alteration modifies rock physical properties in iron oxide-copper-gold deposits (IOCGs) and may result in characteristic signatures detectable in geophysical surveys. Magnetic data are commonly used in characterizing orebodies, and 3D inversions are often used to assist in interpretations. In areas with strong remanence and self-demagnetization, the total magnetization can have directions different from the inducing field direction. This deviation precludes the use of traditional inversion methods. Magnetic amplitude inversion offers one solution to this challenge because the amplitude data are weakly dependent on the magnetization direction. In addition, the low magnetic latitude also imposes difficulty in amplitude data calculation due to the instability in the component conversion in the wavenumber domain. To formulate a practical approach, we present a case study on applying the magnetic amplitude inversion to the Furnas southeast IOCG deposit at the low magnetic latitude in Carajás Mineral Province, Brazil, and demonstrate that the approach can reliably recover an interpretable distribution of effective magnetic susceptibility and identify massive magnetite from hydrothermal alterations associated with the high-grade ore.

A New Method of Cross-Correlation by Magnetic Dipole for Estimating Magnetization Direction under the Influence of Remanent Magnetization

2014 ◽  
Vol 644-650 ◽  
pp. 3459-3462 ◽  
Author(s):  
Lei Shi ◽  
Liang Hui Guo ◽  
Feng Yi Guo
Keyword(s):  
Correlation Coefficient ◽  
Magnetic Dipole ◽  
Remanent Magnetization ◽  
Cross Correlation ◽  
New Method ◽  
Magnetic Data ◽  
Dipole Source ◽  
Total Field ◽  
Magnetization Direction ◽  
Total Magnetization

Processing and interpretation of magnetic data usually require information of total magnetization direction. However, under the effects of remanent magnetization, total magnetization direction is different from induced magnetization direction, which makes data processing and interpretation complexity. In this paper, we present a new method by cross-correlation of magnetic dipole source for determination of magnetization direction from relatively isolated and approximate equiaxial-shape magnetic total field anomaly. This method calculates cross-correlation coefficient between observed magnetic total field anomaly and theoretical magnetic total field anomaly caused by a magnetic dipole source, by using a set of varying parameters of positions and total magnetization direction of dipole source for trial and error. The corresponding magnetization direction of maximum correlation coefficient is regarded as estimated total magnetization direction. Test on synthetic data indicates that this method reliably and effectively estimates the magnetization direction from relatively isolated and approximate equiaxial-shape magnetic total field anomaly.

Constraining magnetic amplitude inversion with magnetotelluric data to image volcanic units: A case study

Geophysics ◽  
2020 ◽  
Vol 85 (3) ◽  
pp. B63-B75
Author(s):  
Kaijun Xu ◽  
Yaoguo Li
Keyword(s):  
Structural Information ◽  
Geophysical Methods ◽  
Magnetic Data ◽  
Structural Constraints ◽  
Structural Constraint ◽  
Magnetotelluric Data ◽  
Amplitude Inversion ◽  
Amplitude Data ◽  
Magnetic Amplitude

We present a case study on imaging volcanic units in gas exploration by constraining magnetic amplitude inversions using magnetotelluric (MT) sounding data at sparse locations. Magnetic data can be effective in mapping volcanic units because they have remanent magnetization and significant susceptibility contrast with surrounding rocks. Although magnetic data can identify the lateral distribution of volcanic units, they often have difficulties in defining the depth extent. For this reason, additional structural constraints from other geophysical methods can often help improve the vertical resolution. Among the independent geophysical methods, MT data can provide the needed structural information at a low cost. We have investigated an approach to combine a set of sparse MT soundings with magnetic amplitude data to image the distribution of volcanics in a basin environment. We first use a blocky 1D MT inversion based on Ekblom norm to obtain the structural constraint, and then we perform a constrained 3D magnetic amplitude inversion to recover the distribution of effective susceptibility by incorporating the structural information from MT soundings. We determine that even a small number of MT stations (e.g., 20) in a [Formula: see text] area is sufficient to drastically improve the magnetic amplitude inversion. Our results indicate that magnetic amplitude inversion with structural constraint from MT soundings form a practical and cost-effective means to map the lateral and vertical distribution of volcanics.

scholarly journals Synthetic Case Study: Discrimination of Unexploded Ordnance (UXO) and non-UXO Sources with varying Remanent Magnetization Strength using Magnetic Data

2021 ◽  
Author(s):  
Mark David Wigh ◽  
Thomas Mejer Hansen ◽  
Arne Døssing
Keyword(s):  
Magnetic Moment ◽  
Remanent Magnetization ◽  
Prior Information ◽  
Forward Model ◽  
Magnetic Data ◽  
Unexploded Ordnance ◽  
The North ◽  
Discrimination Rate ◽  
High Discrimination

Summary We investigate if it is theoretically possible to discriminate between unexploded ordnance (UXO) and non-UXO sources by modelling the magnetic dipole moment for ferrous objects of different shapes and sizes. This is carried out by approximating the volumetric demagnetization factors of rectangular prisms, representing shapes similar to a long rod or flat steel plate. By modelling different UXO as prolate spheroids the demagnetization factors can be determined which can be compared with the magnetic response of a prism. The inversion is carried out in a probabilistic framework, where the UXO forward model and the non-UXO forward model are assigned individual prior models in terms of shape, size, orientation and remanent magnetization of the object. 95 independent realizations of the prism prior model are generated to make 95 synthetic anomalies exemplifying non-UXO objects, which are inverted for using the UXO model. It is investigated if an identical magnetic moment can be produced between the two models and how well resolved the magnetic moment is in terms of the measured anomaly. The case study is carried out in two steps where we first have little prior information of expected UXO properties and another where a UXO prior is introduced with expected values of aspect ratio and size of 24 different UXO, that are often encountered in the North Sea. With no prior information of expected UXO, discrimination is at many times implausible, unless elongated rod prism objects are considered, where the magnetic moment often can not be reproduced by a spheroid. Introducing the UXO prior we achieve a much better discrimination rate when using the list of expected UXO properties. By using the UXO prior we can account for a much higher remanent magnetization allowed in the prior, and still achieve high discrimination capabilities in comparison to a case with no UXO prior.

Magnetic vector inversion using  XYZ measured by fluxgate magnetometer in UAV 

2021 ◽  
Author(s):  
Arto Karinen
Keyword(s):  
Magnetic Field ◽  
Remanent Magnetization ◽  
Magnetization Vector ◽  
Field Direction ◽  
Magnetic Data ◽  
Total Field ◽  
Main Field ◽  
Vector Inversion ◽  
The Magnetic Field ◽  
Measured Magnetic Field

<p>Traditionally, the inversion of magnetic data assumes the magnetization of the local geology to run parallel to the Earth’s internal magnetic field that is usually modelled using International Geomagnetic Reference Field (IGRF). Assuming the magnetization parallel to the main field, only the total (scalar) magnetic data are the sufficient input for the inversion of source susceptibility.</p><p>Local magnetization may alter from the main field direction in areas of remanent magnetization. Recently, magnetization vector inversion (MVI) using the total field has become an important tool trying to distinguish magnetic data affected by remanenence. Total field as a scalar field exclude all information of the direction of the internal magnetization and more information is required to reveal any remanent magnetization from the main field direction.  Compared to total field using the 3-component XYZ vector magnetic measurements provide more information of the source.  More measurements increase the unambiguous nature of data and may reveal the areas of possible remanence. </p><p>To measure XYZ vector magnetic field we use fluxgate 3-component magnetometer with rigid installation on a fixed-wing UAV. With the help of accurate inertial measurement units the measured magnetic field can be determined in the direction of fixed coordinate system. The components of the measured magnetic field rotated into the geographical coordinate represent the magnetic field at survey area.</p><p>UAV survey provided the data as the input for the inversions. We made the inversion separately for both susceptibility and magnetization vector. Susceptibility inversion means inversion of induced magnetization, i.e., a single component of magnetization parallel to the main field direction. Magnetization vector inversion, however, resolves all three components of magnetization, which may or may not include remanent magnetization in addition to induced one.</p><p>The benefits from utilizing XYZ components of the magnetic field with magnetization inversion seem promising in finding remanenence magnetization.</p><p> </p><p> </p>

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