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

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.

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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 ◽

10.1190/geo2014-0082.1 ◽

2015 ◽

Vol 80 (2) ◽

pp. B13-B22 ◽

Cited By ~ 23

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.

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Magnetic amplitude inversion for depth-to-basement and apparent magnetization-intensity estimates

Geophysics ◽

10.1190/geo2019-0726.1 ◽

2021 ◽

Vol 86 (1) ◽

pp. J1-J11

Author(s):

Marlon C. Hidalgo-Gato ◽

Valéria C. F. Barbosa ◽

Vanderlei C. Oliveira

Keyword(s):

Field Data ◽

Regional Scale ◽

Inversion Method ◽

Magnetization Direction ◽

Potential Field Data ◽

Amplitude Inversion ◽

Basement Rocks ◽

Amplitude Data ◽

Magnetization Intensity ◽

Magnetic Amplitude

We have developed an inversion method to recover the depth and the total magnetization intensity of the basement under a sedimentary basin using the amplitude of the magnetic anomaly vector (amplitude data). Because the amplitude data are weakly dependent on the magnetization direction, our method is suitable for interpreting areas with remanent magnetization. Our method assumes constant magnetized basement rocks overlain by nonmagnetic sediments. To overcome the inherent ambiguity of potential field data, we assume knowledge of the average depth of the basement and use it as a constraint to regularize the inversion. A sensitivity analysis with synthetic data shows the weak dependency of the magnetic amplitude inversion on the magnetization direction. Different combinations of magnetization directions recover the interface separating sediments from basement rocks. Test on field data over the Foz do Amazonas Basin, Brazil, recovers the shape of the basement relief without any knowledge about the magnetization intensity and direction. The estimated basement relief reveals a smooth basement framework with basement highs in the central part of the area. In a regional-scale perspective, the deeper and constant estimated basement relief at the northernmost limit of the area may suggest changing in crustal domains from a hyperextended continental crust to homogeneous oceanic crust.

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

Geophysics ◽

10.1190/geo2019-0231.1 ◽

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.

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

Geophysics ◽

10.1190/geo2018-0257.1 ◽

2019 ◽

Vol 84 (2) ◽

pp. B121-B133 ◽

Cited By ~ 4

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.

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Comprehensive approaches to 3D inversion of magnetic data affected by remanent magnetization

Geophysics ◽

10.1190/1.3294766 ◽

2010 ◽

Vol 75 (1) ◽

pp. L1-L11 ◽

Cited By ~ 94

Author(s):

Yaoguo Li ◽

Sarah E. Shearer ◽

Matthew M. Haney ◽

Neal Dannemiller

Keyword(s):

Mineral Exploration ◽

Three Dimensional ◽

Canadian Arctic ◽

Magnetization Vector ◽

Magnetic Data ◽

Inversion Algorithm ◽

3D Inversion ◽

Magnetization Direction ◽

Direct Inversion ◽

Amplitude Data

Three-dimensional (3D) inversion of magnetic data to recover a distribution of magnetic susceptibility has been successfully used for mineral exploration during the last decade. However, the unknown direction of magnetization has limited the use of this technique when significant remanence is present. We have developed a comprehensive methodology for solving this problem by examining two classes of approaches and have formulated a suite of methods of practical utility. The first class focuses on estimating total magnetization direction and then incorporating the resultant direction into an inversion algorithm that assumes a known direction. The second class focuses on direct inversion of the amplitude of the magnetic anomaly vector. Amplitude data depend weakly upon magnetization direction and are amenable to direct inversion for the magnitude of magnetization vector in 3D subsurface. Two sets of high-resolution aeromagnetic data acquired for diamond exploration in the Canadian Arctic are used to illustrate the methods’ usefulness.

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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

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.

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Three-dimensional depth-to-basement modelling based on seismic and potential field data – basement configuration in the westernmost Polish Outer Carpathians

10.5194/egusphere-egu2020-7882 ◽

2020 ◽

Author(s):

Mateusz Mikołajczak ◽

Jan Barmuta ◽

Małgorzata Ponikowska ◽

Stanislaw Mazur ◽

Krzysztof Starzec

Keyword(s):

Qualitative Analysis ◽

Cross Sections ◽

Potential Field ◽

Field Data ◽

Gravity Data ◽

Three Dimensional ◽

Magnetic Data ◽

Gravity Inversion ◽

Potential Field Data ◽

Outer Carpathians

The Silesian Nappe in the westernmost part of the Polish Outer Carpathians Fold and Thrust Belt exhibits simple, almost homoclinal character. Based on the field observations, a total stratigraphic thickness of this sequence equals to at least 5400 m. On the other hand, the published maps of the sub-Carpathian basement show its top at depths no greater than 3000 m b.s.l. or even 2000 m b.s.l. in the southern part of the Silesian Nappe. Assuming no drastic thickness variations within the sedimentary sequence of the Silesian Nappe, such estimates of the basement depth are inconsistent with the known thickness of the Silesian sedimentary succession. The rationale behind our work was to resolve this inconsistency and verify the actual depth and structure of the sub-Carpathian crystalline basement along two regional cross-sections. In order to achieve this goal, a joint 2D quantitative interpretation of gravity and magnetic data was performed along these regional cross-sections. The interpretation was supported by the qualitative analysis of magnetic and gravity maps and their derivatives to recognize structural features in the sub-Carpathian basement. The study was concluded with the 3D residual gravity inversion for the top of basement. The cross-sections along with the borehole data available from the area were applied to calibrate the inversion.In the westernmost part of the Polish Outer Carpathians, the sub-Carpathian basement comprises part of the Brunovistulian Terrane. Because of great depths, the basement structure was investigated mainly by geophysical, usually non-seismic, methods. However, some deep boreholes managed to penetrate the basement that is composed of Neoproterozoic metamorphic and igneous rocks. The study area is located within the Upper Silesian block along the border between Poland and Czechia. There is a basement uplift as known mainly from boreholes, but the boundaries and architecture of this uplift are poorly recognized. Farther to the south, the top of the Neoproterozoic is buried under a thick cover of lower Palaeozoic sediments and Carpathian nappes.Our integrative study allowed to construct a three-dimensional map for the top of basement the depth of which increases from about 1000 m to over 7000 m b.s.l. in the north and south of the study area, respectively. Qualitative analysis of magnetic and gravity data revealed the presence of some &#160;basement-rooted faults delimiting the extent of the uplifted basement. The interpreted faults are oriented mainly towards NW-SE and NE-SW. Potential field data also document the correlation between the main basement steps and important thrust faults.&#160;This work has been funded by the Polish National Science Centre grant no UMO-2017/25/B/ST10/01348

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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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3D Convolution Conjugate Gradient Inversion of Potential Fields in Acoculco Geothermal Prospect, Mexico

Frontiers in Earth Science ◽

10.3389/feart.2021.759824 ◽

2022 ◽

Vol 9 ◽

Author(s):

José P. Calderón ◽

Luis A. Gallardo

Keyword(s):

Conjugate Gradient ◽

Potential Field ◽

Field Data ◽

Large Scale ◽

Three Dimensional ◽

Potential Fields ◽

Magnetic Data ◽

Geothermal System ◽

Potential Field Data ◽

Model Compression

Potential field data have long been used in geophysical exploration for archeological, mineral, and reservoir targets. For all these targets, the increased search of highly detailed three-dimensional subsurface volumes has also promoted the recollection of high-density contrast data sets. While there are several approaches to handle these large-scale inverse problems, most of them rely on either the extensive use of high-performance computing architectures or data-model compression strategies that may sacrifice some level of model resolution. We posit that the superposition and convolutional properties of the potential fields can be easily used to compress the information needed for data inversion and also to reduce significantly redundant mathematical computations. For this, we developed a convolution-based conjugate gradient 3D inversion algorithm for the most common types of potential field data. We demonstrate the performance of the algorithm using a resolution test and a synthetic experiment. We then apply our algorithm to gravity and magnetic data for a geothermal prospect in the Acoculco caldera in Mexico. The resulting three-dimensional model meaningfully determined the distribution of the existent volcanic infill in the caldera as well as the interrelation of various intrusions in the basement of the area. We propose that these intrusive bodies play an important role either as a low-permeability host of the heated fluid or as the heat source for the potential development of an enhanced geothermal system.

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