Magnetic Susceptibility Distribution Research Articles

ОЦІНКА АНТРОПОГЕННОГО ЗАБРУДНЕННЯ ПОВЕРХНЕВИХ ВІДКЛАДІВ МІСТА ЧЕРНІВЦІ ЗА МАГНІТНОЮ СПРИЙНЯТЛИВІСТЮ ТА ВМІСТОМ ВАЖКИХ МЕТАЛІВ

The paper is devoted to spatial distribution of low-frequency magnetic susceptibility (χ) of surface sediments in Chernivtsi city as an informative indicator of geochemical contamination of the topsoil. The bulk content of heavy metals in urban soils and the correlations between the χ, frequency dependence of magnetic susceptibility (kfd) and the content of Cd, Mn, Cu, Ni, Pb, Zn were studied. Χ vary from 10∙10-8 to 1286∙10-8 m3/kg with a median value of 66∙10-8 m3/kg. In 20 % of samples χ exceeds the median value three times or more, indicating that the soil is contaminated with iron dust and magnetic particulate matter derived from anthropogenic aerosols. Priority contaminants of city soils are metals such as Pb and Zn, the bulk content of which exceeds the trigger concentration in the vast majority of samples. The χ revealed strong correlation with bulk content of Pb and Zn and the weaker one, but still statistically significant, with Cu and Cd. An additional criterion for the common anthropogenic origin of heavy metals and magnetic minerals in soils is the significant negative correlation coefficients between kfd and content of Zn, Pb, and Cu. So, it can be argued that the increase in Zn, Pb, and Cu content is accompanied by an increase in concentration of magnetic minerals in the multidomain state. The maps were produced, showing spatial distribution of χ and Pollution Load Index (PLI), calculated by three metals (Zn, Pb, and Cu). For 51 % of the samples, the PLI indicates no contamination, 35 % indicate moderate contamination,14 % are strongly contaminated. These results prove that surface sediment over a large area of the city require measures to reduce their toxicity. The correlation coefficient between χ and PLI makes 0.88, indicating χ as a quick indicator of geochemical soil contamination in the city of Chernivtsi.

Learned Proximal Networks for Quantitative Susceptibility Mapping.

Quantitative Susceptibility Mapping (QSM) estimates tissue magnetic susceptibility distributions from Magnetic Resonance (MR) phase measurements by solving an ill-posed dipole inversion problem. Conventional single orientation QSM methods usually employ regularization strategies to stabilize such inversion, but may suffer from streaking artifacts or over-smoothing. Multiple orientation QSM such as calculation of susceptibility through multiple orientation sampling (COSMOS) can give well-conditioned inversion and an artifact free solution but has expensive acquisition costs. On the other hand, Convolutional Neural Networks (CNN) show great potential for medical image reconstruction, albeit often with limited interpretability. Here, we present a Learned Proximal Convolutional Neural Network (LP-CNN) for solving the ill-posed QSM dipole inversion problem in an iterative proximal gradient descent fashion. This approach combines the strengths of data-driven restoration priors and the clear interpretability of iterative solvers that can take into account the physical model of dipole convolution. During training, our LP-CNN learns an implicit regularizer via its proximal, enabling the decoupling between the forward operator and the data-driven parameters in the reconstruction algorithm. More importantly, this framework is believed to be the first deep learning QSM approach that can naturally handle an arbitrary number of phase input measurements without the need for any ad-hoc rotation or re-training. We demonstrate that the LP-CNN provides state-of-the-art reconstruction results compared to both traditional and deep learning methods while allowing for more flexibility in the reconstruction process.

Sedimentation process in Kuala Gigieng Coast, Aceh Besar based on magnetic and gravity surveys

The study of the sedimentation process in coastal can be used to investigate historical settlements and paleo disaster in the area. We have conducted magnetic and gravity survey to model the sedimentation process in Kuala Gigieng area, Aceh Besar, one of the important harbors during the Aceh Sultanate in 16th. The magnetic and gravity methods are able to map structures based on magnetic susceptibility and density distribution below the surface, respectively. The total magnetic field data were measured by the magnetic method, while the gravity method measured gravitational acceleration data. The data were acquired in an area of 1858 x 725 m2 with stations spacing between 70 -100 meters in grid. The standard magnetic and gravity correction was applied to the data, so that the total magnetic field anomaly and Bouguer anomaly are obtained which reflect the conditions the subsurface. The anomaly map obtained shows the pattern of maximum and minimum values, which the patterns were the same as the topography of the measurement area. To get a clearer sediment model, gravity data has been carried out 2D forward modeling with cross sections cutting from coast to land. The 2D model illustrates the existence of 3 layers in the measurement area, sequentially consisting of alluvial and clay, sandstone, and tuff sandstone sediments. The alluvial deposits in the first layer form a repetitive pattern from the land side to the coast; the pattern was formed by coastal activity. The sequence of sediment layers shows the age of the sediment, with the oldest layers formed far from the coast and the youngest layers formed near the coast.

3D magnetic amplitude inversion in the presence of self-demagnetization and remanent magnetization

We have developed a general 3D amplitude inversion algorithm for magnetic data in the presence of self-demagnetization and remanent magnetization. The algorithm uses a nonlinear conjugate gradient (NLCG) scheme to invert the amplitude of the magnetic anomaly vector within a partial differential equation framework. Three quantities— the amplitude of the anomalous magnetic field, the analytic signal, and the normalized source strength, defined as the amplitudes of magnetic data that are weakly dependent on the magnetization direction — are inverted to recover the 3D distribution of the subsurface magnetic susceptibility. Numerical experiments indicate that our NLCG amplitude inversion algorithm has a rapid convergence rate that provides a reasonable inversion solution in the absence of knowing the total magnetization direction. High-resolution aeromagnetic data collected from the Pea Ridge iron oxide-apatite-rare earth element deposit, southeast Missouri, USA, are used to illustrate the efficacy of our amplitude inversion algorithm. This algorithm is generally applicable for tackling the large-scale inversion problem in the presence of self-demagnetization and remanent magnetization.

Distribution and dispersal of surface sediments on the continental margins of India

Abstract The vast area, varying climate regimes, diverse source terrain lithology of the mainland India generate complex weathering and erosional patterns and leads to diverse sediment distribution and dispersal on the surrounding continental margins of India. Thus, to delineate the sources, distribution and sediment dispersal pathways around India, 254 sediment cores from the eastern and western continental margins of India were investigated for physical properties using non-destructive, whole core logging technique combined with grain-size analyses. The distribution of magnetic susceptibility, gamma ray density and grain-size traces the sources in the Indian landmass which contribute sediments to the margins and demarcates their extent of influence. The results from the magnetic susceptibility distribution indicate that the dominant influx of modern high susceptibility sediment into the margins of India originates from the weathering of Deccan Basalts and its subsequent transportation by Narmada and Tapti Rivers in the west coast and Krishna and Godavari Rivers in the east coast. Though the Himalayan sources are the second largest supplier of magnetic minerals to the shelf, their contribution, when compared to the Deccan sediments is not significant. The lack of present-day sedimentation in the outer shelf regions of margins of India is evident from the gamma ray density distribution and is confirmed by the discrete sample grain-size analysis. Gamma ray density variations in the margins follow the same pattern as the grain size variations and demarcate the boundary between inner shelf modern clayey sediments and the outer shelf relict carbonate sands. Regional distribution maps of physical properties and grain-size for both the margins of India are presented and geographical limits of each provenance are defined, thus providing a comprehensive picture of sediment distribution and dispersal patterns in the area and highlights the use of magnetic susceptibility as a powerful tool in deciphering the source to sink pathways of sediments.

Susceptibility inversion of near-field magnetic sources and its application

Susceptibility inversion of near-field magnetic sources is of great application significance in the field of magnetic target detections, such as mine clearances and buried pipeline detections. This paper proposes a method of three-dimensional inversion imaging of magnetic sources in near-field with unknown shape, position and magnetization state. The method transforms the total field norm anomaly ΔT on the observation surface into the total field vector anomaly ||Ta|| and then constructs an objective function aiming at minimizing the deviations between the measured and predicted magnetic anomalies. The magnetic dipole equivalence principle is utilized to calculate the kernel matrix. The Gauss-Newton and conjugate gradient algorithms are utilized together to solve the inversion optimization process. The range of the magnetic susceptibility is constrained by using nonlinear transformations. Simulations and measurement experiments demonstrate the proposed method is capable to realize 3D inversions of curved pipes, intersecting pipes, separated magnetic sources, and “L” shaped steel pipes under near-field conditions. Through the actual magnetic field measurement experiments and the implementation of the proposed optimization inversion algorithm, accurate inversion of the magnetic susceptibility distribution and magnetic imaging of the “L” shaped steel tube was realized, testifying that the proposed method is feasible to be applied to the detection of buried pipelines.

Anthropogenic traces in bottom sediments of Chukchi Sea

This paper presents results of a high-resolution study of bottom sediments from the central and northern parts of the Chukchi Sea. Magnetic susceptibility and grain size distribution within the sediments indicate calm depositional conditions of mainly lithogenic material. The biogenic material is dominated by SiO2bio (up to 15%) and to a much lesser extent by Corg (up to 2%) and Ntot (up to 0.25%). The distinct increase of organic material within the uppermost parts of the cores causes a decrease of magnetic susceptibilty and reflects an increase of biological productivity of the Chukchi Sea, triggered by global warming of the climate.Fallout activity peaks of 137Cs, measured in the core taken in the central area of the Chukchi Sea, are assumed to represent the fall-out of the accidents of the Chernobyl nuclear power plant in 1986 and of the Fukushima nuclear power plant in 2011. The two fallout peaks of 137Cs in the sediments of the northern area correspond to the maximum of the atmospheric tests of nuclear weapons in 1963 and to the accident at Chernobyl nuclear power plant in 1986. Rates of recent sedimentation, dated by 137Cs fall-out peaks and by additional measurements of 210Pb are estimated to be 2–2.7 mm a−1 in the central part and 0.9–1.3 mm a−1 in the northern part of the Chukchi Sea.

Towards magnetometric characterization of soil pollution with rare-earth elements in industrial areas of Upper Silesian Industrial Area, Southern Poland

The paper presents systematic study concentrations of selected rare-earth elements, namely La and Ce in soils of highly industrialized regions using geochemical and magnetometric measurements as well as geostatistical methods. Soil magnetometry was used to determine if the concentrations of La and Ce in soil could be a result of anthropogenic pollution or natural soil properties. Results of analyses revealed that the highest concentrations of La and Ce were observed near a waste heap of the plant producing and processing batteries, and in the region of the Jizera Mountains, natural REE-rich minerals are very common. It was also found that the lowest concentrations of La and Ce in soil were observed in forested areas of where the dominant type of pollution sources was associated with the metallurgical industry. Distributions of magnetic susceptibility in soil profiles collected in areas with predominant industrial influence showed visible peak in topsoil, what confirmed anthropogenic origin of La and Ce in soil. In areas where large volumes of wastes were deposited in past, industrial activity, vertical distributions of soil magnetic susceptibility showed also secondary, strong peak in subsoil. Thus, the results reveal that magnetometric measurements in soil profile might be useful as supplementary method for analyzing of concentrations of rare-earth elements. Contrary, magnetometric measurements performed on the soil surface were poorly correlated with a concentration of La and Ce in soil, because they have weaker magnetic properties than typical fly ashes emitted by industry.

Positive‐contrast susceptibility imaging based on first‐order primal‐dual optimization

To achieve faster reconstruction and better imaging quality of positive-contrast MRI based on the susceptibility mapping by incorporating a primal-dual (PD) formulation. The susceptibility-based positive contrast MR technique was applied to estimate arbitrary magnetic susceptibility distributions of the metallic devices using a kernel deconvolution algorithm with a regularized minimization. The regularized positive-contrast inversion problem and its PD formulation were derived. The visualization of the positive contrast and convergence behavior of the PD algorithm were compared with those of the nonlinear conjugate gradient algorithm, fast iterative soft-thresholding algorithm, and alternating direction method of multipliers. These methods were tested and validated on computer simulations and phantom experiments. The PD approach could provide a faster reconstruction time compared with other methods. Experimental results showed that the PD algorithm could achieve comparable or even better visualization and accuracy of the metallic interventional devices in positive-contrast imaging with different SNRs and orientations to the B0 field. A susceptibility-based positive-contrast imaging technique by PD algorithm was proposed. The PD approach has more superior performance than other algorithms in terms of reconstruction time and accuracy for imaging the metallic interventional devices.

Quantitative characterization of the regulation of iron metabolism in glioblastoma stem-like cells using magnetophoresis.

This study focuses on different iron regulation mechanisms of glioblastoma (GBM) cancer stem-like cells (CSCs) and non-stem tumor cells (NSTCs) using multiple approaches: cell viability, density, and magnetophoresis. GBM CSCs and NSTCs were exposed to elevated iron concentration, and their magnetic susceptibility was measured using single cell magnetophoresis (SCM), which tracks the magnetic and settling velocities of thousands of individual cells passing through the magnetic field with a constant energy gradient. Our results consistently demonstrate that GBM NSTCs have higher magnetic susceptibility distribution at increased iron concentration compared with CSCs, and we speculate that it is because CSCs have the ability to store a high amount of iron in ferritin, whereas the free iron ions inside the NSTCs lead to higher magnetic susceptibility and reduced cell viability and growth. Further, their difference in magnetic susceptibility has led us to pursue a separate experiment using a quadrupole magnetic separator (QMS), a novel microfluidic device that uses a concentric channel and permanent magnets in a special configuration to separate samples based on their magnetic susceptibilities. GBM CSCs and NSTCs were exposed to elevated iron concentration, stained with two different trackers, mixed and introduced into QMS; subsequently, the separated fractions were analyzed by fluorescent microscopy. The separation results portray a successful label-less magnetic separation of the two populations.

Distribution of Magnetic Susceptibility of Natural Iron Sand in the Sarmi Coast Area

Iron sand, a raw source of magnetic materials, is known to exist in a particular area in Indonesia. However, researchers mostly focused on the iron sand source from the western coast of Sumatera and the southern coast of Java while the northern coast of Papua had not been yet identified. In this study, iron sand samples were collected from the coastal area of Sarmi located on the northern coast of the Papua Province between these geographic positions 1°47′52.30″S - 138°40′27.60″E and 1°57′57.50″S - 138°57′7.60″E. The results show that samples from Tor River estuary had the highest mass-specific magnetic susceptibility values, varying approximately from 3000 to 4000 × 10−8 m3/kg. Contrarily, samples from the Verkame area had a much lower mass-specific magnetic susceptibility, varying approximately from 200 to 600 × 10−8 m3/kg unexpectedly. The mineral and magnetic structure confirms the presence of iron oxide minerals in iron sands from the Tor River estuary.

Spatial variability of soil magnetic susceptibility under different scales: a case study of Xiangtan

The characteristics of soil magnetic parameters can reflect the natural environment evolution and the human activities influence. By systematically sampling and measuring the sediments and onshore soils in representative water of Xiangtan City, spatial estimation was carried out by Kriging Spatial Interpolation with the help of SPSS, ArcGIS, Surfer, and other statistical and spatial analysis software. The results showed that: (1) Under the city scale, the spatial variation of soil magnetic susceptibility was large and ranged from 1.167×10−8 m3 kg−1 to 165.93×10−8 m3 kg−1; (2) the soil magnetic susceptibility varies greatly under different land-use patterns, which may be related to the accumulation and absorption of magnetic substances in the soil environments. On the functional area scale, human activities on soil magnetic susceptibility were relatively large, which might be closely related to agricultural activities and domestic waste discharge; (3) the distribution of soil magnetic susceptibility was mainly affected by various factors; (4) the spatial autocorrelation distance decreased from the city scale to the functional area scale. Structural elements, such as sedimentation, topographic gradient, animals and plants, have a more significant influence on the spatial distribution of soil magnetic susceptibility.

Magnetic Susceptibility Estimation for Magnetostatics

This paper presents a parameter estimation method to determine the linear behavior of an object constructed of thin plates. Based on the magnetostatic field equations, an integral equation is derived that fully determines the induced magnetization, whenever the spatial magnetic susceptibility distribution is known. This forward problem is used as an underlying physical model for the parameter estimation method. Using near-field magnetic measurements around a thin plate, the parameter estimation yields a distribution of the magnetic susceptibility. Furthermore, a sensitivity analysis is performed to understand the behavior of this parameter estimation method.

ІНФОРМАТИВНІСТЬ МАГНІТНИХ МЕТОДІВ ПРИ МОНІТОРИНГУ ПРИРОДНО-ТЕХНОГЕННИХ ПРОЦЕСІВ, ЯКІ ПОВ'ЯЗАНІ З ОБ'ЄКТАМИ КРИТИЧНОЇ ІНФРАСТРУКТУРИ

Presented are the results of research studies with the attracting of the GIS and geophysical methods for solving the problems of monitoring of dangerous natural and man-made processes at the areas of the critical infrastructure objects location. The possible ways of the magnetic methods applied for the critical infrastructure objects monitoring were considered. The natural and man-made processes associated with the critical infrastructure were defined. Among them are the objects that can influence the changes in soils, atmospheric air composition, as well as near-surface geological horizons. The most important objects are the energy enterprises, chemical industry, transport network, and landslide areas. The "Rzhyshchev" section and the "Grebeni" site were used as testing objects of the landslide critical infrastructure. According to the results of processing of cosmoimages and measurements of lateral and vertical distributions of magnetic susceptibility of soils and near-surface geological horizons, the classification of territory risks has been carried out. The territory of the cliff on the slopes of the Dnieper is the most potentially dangerous. Anthropogenically impacted natural and man-made territory of the field road and private economy has received an average category of risks. The most risky part of the site are the elements of the landscape, which are located on the land with a minimum slope, the absence of anthropogenic loading, agricultural land and virgin land. Magnetic methods proved to be rapid, effective and low-cost technology for the study of natural and man-made processes associated with objects of critical infrastructure. The integration with other geophysical, soil science methods, and GIS technologies are required.

Effects of Contour Plowing by Rotary Cultivator on Vertical Redistribution of Soil Organic Carbon

Simulated tillage and magnetic tracing techniques were applied to study the effects of contour plowing by rotary cultivator on vertical migration of soil organic carbon (SOC) on a steep hillslope in purple soil, revealing the relationship between SOC vertical and downslope movements. Contour plowing by rotary cultivator played an important role on downslope transfers of the soil, resulting in an obvious transformation of soil profiles with tillage depth at the summit position. The depth distribution of soil magnetic susceptibility showed that tillage brought about the vertical downward migration of the soil magnetic tracer. The intense tillage led to the vertically downward transfer of SOC, but vertical redistribution patterns differed from one another at different slope positions. Due to upward migration of parent rock debris and downslope transport of soil, SOC concentrations in the surface soil layer (0-10 cm) was reduced significantly at the summit position. For the backslope and footslope positions, SOC concentrations at 0-20 cm was reduced remarkably due to mixture effects of tillage and tillage erosion. SOC in the surface layer (0-5 cm) was decreased slightly at the toeslope position, but that at 5-20 cm increased significantly. This study indicated that the vertical movement of soil interacted with the downslope movement, which resulted in significant variation of SOC from the summit to toeslope positions.

Optimized MR Simulation Protocol for Framed Stereotactic Radiosurgery at 3T

Magnetic resonance imaging is often the primary imaging modality for framed stereotactic radiosurgery (SRS). However, eddy currents, chemical shift, gradient nonlinearities, and the magnetic susceptibility distribution of the patient can all contribute to geometric distortions that may affect the accuracy of SRS if not properly controlled. The goal of this work was to establish an optimized MR simulation protocol for framed SRS at 3T, and assess the severity of any residual geometric distortions. Imaging was performed on a 3T scanner using an SRS phantom (CIRS, model 603A). A titanium Leksell head frame was secured to the phantom. The fiducial box was then affixed to the head frame. The combined, phantom, head frame, and fiducial box assembly were then immobilized in treatment position using a holder of our design. The combination of two flexible phase-array coils and the integrated spine RF coil was used for signal reception. Magnetic field mapping was performed using a dual echo GRE sequence. Different shimming modes (tune up, standard, standard + local shim, standard + brain shim, advanced, advanced + local shim, advanced + brain shim) were applied prior to acquisition of each field map. In addition, 3D T1 FLASH images were acquired with different gradient slew rate modes (whisper, normal, fast) and readout bandwidths. Vendor-provided 3D gradient nonlinearity distortion correction was applied to all images prior to them being sent to a commercial deformable registration algorithm system for analysis. Large magnetic field variations were observed with tune up and standard shim modes. The advanced + brain shim mode resulted in smallest magnetic field variations over the brain. Shimming over the brain did not affect the geometric accuracy of the fiducial markers. The low slew rate whisper gradient mode reduced eddy current-induced distortion by 1.5mm compared to normal gradient mode. Reducing the readout bandwidth from 980 Hz/pixel to 440 Hz/pixel reduced eddy current-induced distortion further by 2mm and increased SNR, while minimizing the severity of chemical shift. Eddy currents induced by the gradients in the titanium frame can result in geometric distortions of fiducial that exceed 1mm in regions of the fiducial box within 2cm of the SRS frame. We established an optimized MR simulation protocol for framed SRS at 3T. Our recommendations include: 1) use of low slew rate gradient modes to reduce eddy current induced distortions, 2) reduce readout bandwidths to 440 Hz/pixel as a tradeoff between eddy current induced distortions and water-fat shift, 3) scan with the prescription volume centered at isocenter, 4) perform advanced high-order shimming with shim volume localized to brain, 5) confirm A-P phase encode direction to avoid flow artifacts, 6) apply 3D gradient nonlinearity distortion correction prior to using images for treatment planning, 7) avoid registering images in treatment planning system using fiducials within 2cm of the SRS frame.

Quantitative susceptibility mapping (QSM) with an extended physical model for MRI frequency contrast in the brain: a proof-of-concept of quantitative susceptibility and residual (QUASAR) mapping.

Quantitative susceptibility mapping (QSM) aims to calculate the tissue's magnetic susceptibility distribution from its perturbing effect on the MRI static main magnetic field. The method is increasingly being applied to study iron and myelin in clinical and preclinical settings. However, recent experimental and theoretical findings have challenged the fundamental theoretical assumptions that form the basis of current numerical implementations of QSM algorithms. The present work introduces a new class of susceptibility mapping algorithms, termed quantitative susceptibility and residual mapping (QUASAR), which takes into account frequency contributions not related to the spatial variation of bulk magnetic susceptibility in the Lorentz sphere model. We present a simple proof-of-concept QUASAR algorithm that, unlike most of the QSM algorithms currently used widely, results in an improved anatomical accuracy of the susceptibility distribution without any a priori assumptions about the susceptibility distribution during the field-to-source inversion. The algorithm was evaluated both in silico and in vivo in the preclinical setting. Our preliminary application of QUASAR in rodents provides the first in vivo evidence that the susceptibility-field model traditionally used in the QSM field cannot fully explain the frequency contrast in brain tissues. Only when an additional local frequency contribution is added to the physical model can the frequency contrast in the brain be related properly to the underlying anatomy.

Fast 3D forward modeling of the magnetic field and gradient tensor on an undulated surface

Magnetic field gradient tensor technique provides abundant data for delicate inversion of subsurface magnetic susceptibility distribution. Large scale magnetic data inversion imaging requires high speed and accuracy for forward modeling. For arbitrarily distributed susceptibility data on an undulated surface, we propose a fast 3D forward modeling method in the wavenumber domain based on (1) the wavenumber-domain expression of the prism combination model and the Gauss–FFT algorithm and (2) cubic spline interpolation. We apply the proposed 3D forward modeling method to synthetic data and use weighting coefficients in the wavenumber domain to improve the modeling for multiple observation surfaces, and also demonstrate the accuracy and efficiency of the proposed method.

Geophysical helicopter-based magnetic methods for locating wells

We studied the problem of determining accurately the location of abandoned and sometimes undocumented wells and the challenging and increasingly important task related to subsurface reservoir integrity and regional economic development. We reviewed a variety of semiquantitative methods based on geophysical workflows, and we tested these with airborne magnetic data collected at two field sites. Our main conclusion is that airborne magnetic surveys represent a high-value tool to aid in the accurate determination of abandoned well locations and characteristics. At one site, two surveys were collected at slightly different altitudes to compare workflow robustness and allow the observed vertical magnetic gradient to be included in well detection workflows. We also investigated using focal zone anomaly statistics (using the magnetic field intensity and its first and second horizontal derivatives), analytic signal, tilt derivative, and calculated vertical gradient. In addition, we used a 3D inversion of a small subset of data to investigate the successful recovery of well-related magnetic susceptibility distribution and estimate subsurface well topology. The recovered magnetic susceptibility volume showed distinctive vertically elongated objects that correspond to known wells. Maximum likelihood estimation and confidence calculations were then applied to these data sets and indicated that high-confidence well locations could be determined and characterized using such airborne total magnetic data.

Topographic correction of magnetic data on rugged topography with application to Río Blanco-Los Bronces and El Teniente porphyry copper districts, Southern Andes, Chile

Airborne magnetic surveys over rugged topography generate noticeable magnetic signatures that are likely equivalent in amplitude with the signal of geological interest. Synthetic models demonstrate that this magnetic terrain effect has the same wavelength as the topography and cannot be overcome by drape flying. The magnetic terrain effect amplifies negative and positive magnetic signals over steep valleys and ridges, respectively. These magnetic artefacts may induce incorrect geological interpretation of magnetic features. In order to remove these spurious signals, we develop a semiquantitative methodology based on 3D magnetic modelling of the topographic effect. Observed total magnetic field is then corrected by subtracting the synthetic field related to the topographic effect. The key element in this approach is the appropriate estimate of the magnetisation associated with the topography, which is especially difficult to determine in areas characterised by rugged terrain. We estimate the magnetic signal related to the topographic effect by: (1) filtering the magnetic data based on the wavelength band in which magnetic and topographic data show maximum coherency, (2) inverting the filtered magnetic data in order to obtain a model of the magnetic susceptibility distribution associated with the topography, and finally (3) calculating the magnetic signal response of the topography-related susceptibility model. We successfully tested this approach in the Río Blanco-Los Bronces and El Teniente porphyry copper districts (Andes of Central Chile), which are characterised by rugged topography and the presence of highly magnetised volcanic rocks. Validation is achieved by comparing the magnetic response over zones with a good geological and petrophysical knowledge. In these examples, the topography-corrected magnetic data show the distribution of geological units and susceptibility better than the non-corrected magnetic data.