Total Magnetization Direction Research Articles

Determination of the magnetization direction via correlation between reduced‐to‐the‐pole magnetic anomalies and total gradient of the magnetic potential with vertical magnetization

AbstractThe total magnetization of an underground magnetic source is the vector sum of the induced magnetization and the natural remanent magnetization. The direction of the total magnetization serves as important a priori information in the inversion and processing of magnetic data. We demonstrated that the total gradient of the magnetic potential with vertical magnetization constitutes the envelope of the vertical component of the magnetic field for all directions of the Earth's field and source magnetization. The total gradient of the magnetic potential with vertical magnetization and the reduction‐to‐the‐pole field simultaneously tend to achieve maximum symmetry near the correct total magnetization direction. As a result, the total magnetization direction can be estimated by computing the correlations between the reduction‐to‐the‐pole and the total gradient of the magnetic potential with vertical magnetization. The proposed method yields accurate magnetization directions in synthetic model examples. The total gradient of the magnetic potential with vertical magnetization is less susceptible to data noise than transforms which are derived from the high‐order magnetic field derivatives or tensors. The estimation results are slightly affected by changes in the source magnetization direction. In a field example in the Weilasito region (North China), the reduction‐to‐the‐pole fields calculated using the estimated magnetization directions are well centred over the source. The proposed method obtained a more focused magnetization direction than that of a three‐dimensional magnetization vector inversion. The total gradient of the magnetic potential with vertical magnetization therefore provides a novel and accurate approach to determine the total magnetization direction from the total field anomaly in a variety of situations.

Transformations of borehole magnetic data in the frequency domain and estimation of the total magnetization direction: A case study from the Mengku iron-ore deposit, Northwest China

Borehole magnetic prospecting measures the three components of the magnetic field and is sensitive to the vertical depth of the magnetic source, which plays an important role in deep mineral exploration. Magnetic field transformations in the frequency domain constitute common and important processing for ground and airborne data but are rarely applied to borehole magnetic data. Here, we deduce component transformation and magnetization-direction transformation formulas for borehole data. It is found that the transformation factors differ from those of ground and airborne data. The new formulas depend on the position of the drillholes. The transformed data are then used to estimate the total magnetization direction by computing the correlation coefficient between the transformed components and the amplitude anomaly. The advantages of this method are that the borehole data yield a local and accurate magnetization direction and the total amplitude anomaly is computed directly from the individual anomalies in the observed components. The frequency-domain transformation is tested on the synthetic data, and the estimated directions are consistent with the true values. The processing procedure is applied to borehole data collected at the Mengku iron-ore deposit in Northwest China, where magnetic surveys from 55 boreholes have been acquired. The estimated magnetization directions yield good approximations of the results from the ground magnetic data and the physical property measurements of rock samples.

Improved MAX-MIN method to estimate total magnetization direction using Wiener filtering

Knowledge of the total magnetization direction is useful for forward modeling and reduction-to-the-pole (RTP) of magnetic anomalies. However, remanent magnetization deviates the total magnetization direction from the induced magnetization direction; therefore, it is essential to determine the correct direction of total magnetization. Theoretically, the MAX-MIN method, which searches for the position of the maximum value of the function that comprises the minimum values of the RTP field with different directions, can yield the correct direction of total magnetization. However, such estimations of direction usually indicate large errors in comparison with the results of crosscorrelation methods, which calculate the correlation coefficient between the RTP field and the magnetic magnitude transforms. Our analysis reveals that noise is the primary cause of the inaccuracy in estimation results. We have improved the MAX-MIN method using Wiener filtering based on the principle of least-mean-squares deviation. The k-means clustering algorithm clusters points that are as closely connected as possible, while ensuring that the distances between clusters are as large as possible. Therefore, the k-means clustering algorithm is used to divide the radial logarithmic power spectrum into the effective signal with low wavenumbers and the noisy signal with high wavenumbers. Then, the Wiener filtering RTP operator is constructed using the slope and intercept of the fitted straight line in the corresponding wavenumber bands. The improved MAX-MIN method is tested using magnetic anomalies of spherical and combined cuboid models with different levels of added Gaussian noise and different total magnetization directions. The improved MAX-MIN method is applied to the magnetic anomaly in the Yeshan region (eastern China) and the Taihe region (southwestern China). Results indicate that the improved MAX-MIN method based on the principle of least-mean-squares deviation could obtain reliable estimation of direction from noisy magnetic anomaly data.

A new method to estimate the total magnetization direction from the magnetic anomaly: Multiple correlation

Natural remanent magnetization diverts the direction of the total magnetization vector from the earth’s magnetic field direction. It is important to determine the magnetization direction because the processing and inversion of magnetic usually need the magnetization direction as a priori information. The conventional strategy for estimating the magnetization direction is to compute the crosscorrelation between the reduced-to-the-pole (RTP) field and one magnetic magnitude transform, which is insensitive to the magnetization direction. A new theory and method are proposed that the total magnetization direction is determined by computing the multiple correlation coefficients between the RTP field and the multiple direction-insensitive magnitudes transforms, such as the total magnitude anomaly and the normalized source strength. The linear regression between the RTP field and the direction-insensitive magnitude transforms is established. Then, the correlation is computed by the RTP field and the constructed regressor. The proposed method is tested on synthetic of single and complex models, respectively. Then, the method is applied to the field data of the Yeshan region (eastern China) and the Black Hill Norite (southern Australia). The proposed multiple correlation is an extension of the conventional crosscorrelation method that provides an accurate and robust way to estimate the magnetization direction from the total field anomaly.

Application of Helbig integrals to magnetic gradient tensor multi-target detection

We extended the magnetic gradient tensor Helbig integrals (MGTHI) from classical Helbig integrals (HI), which can be perfectly adapted to the grid measurement of planar-cross magnetic gradient tensor system. With different sizes of windows, we can estimate the MGTHI by using the two-dimensional trapezoidal rule algorithm and calculate the total magnetization direction of grids, where the node with the smallest change in the magnetization direction is target horizontal position. In the targets recognition area determined by the invariant improved tilt angles, the source depth and dipole moment magnitude can be calculated with magnetic vector and tensor field data directly above the source. In experiments, the horizontal position deviation of multiple small magnets in the survey area of 2.1 m × 2.1 m and 1.2 m × 1.2 m is less than 0.08 m, and the buried depth deviation is less than 0.15 m, the positioning accuracy is improved by more than 5 times from the classical HI method.

Magnetic Structure and its Tectonic Implication Around Longmenshan Fault Zone Revealed by EMAG2v3

The formation of magnetic minerals is bound up with the tectonic evolution history, whereupon the distribution of magnetic anomalies has great meanings for regional tectonics. In this study, we use the latest global lithospheric magnetic field model EMAG2-v3, processed by various techniques including reduction to the pole (RTP), upward continuation, derivations, Euler deconvolution, estimation of total magnetization direction, and Curie point depth (CPD), to unveil the tectonics around Longmenshan fault zone (LFZ). LFZ is clearly displayed as a positive and negative anomaly transition zone in RTP anomalies and acts as a magnetic basement boundary. The Sichuan Basin (SB), located to the east of LFZ, is marked by strong magnetic basement and NE-strike banded induced positive anomalies which are associated with the Neoproterozoic magmatic activity. The banded shape, absence of radial pattern of anomalies, and existence of fossil subduction zone supports that the magnetic basement was formed in arc environment. The CPD in SB estimated by radial average power spectral is 30–51 km, which allows magnetic minerals in deep crust or even in lithospheric mantle to exhibit high magnetizations. The Songpan-Ganzi fold belt (SGFB), in contrast, is located to the west of LFZ and covered by thick and low-susceptibility Triassic deposits of flysch, manifests as weak negative anomalies caused by relatively shallow CPD and widespread remanent magnetization. Significant positive anomalies, appearing around the Manai and Rilonguan granitic massifs, indicate a strong magnetic basement beneath SGFB, which is conjectured as westward extension of the Yangtze Block at depth.

Magnetic radial inversion for 3-D source geometry estimation

SUMMARY We present a method for inverting total-field anomaly data to estimate the geometry of a uniformly magnetized 3-D geological source in the subsurface. The method assumes the total-magnetization direction is known. We approximate the source by an ensemble of vertically juxtaposed right prisms, all of them with the same total-magnetization vector and depth extent. The horizontal cross-section of each prism is a polygon defined by a given number of equi-angularly spaced vertices from 0° to 360°, whose polygon vertices are described by polar coordinates with an origin defined by a horizontal location over the top of each prism. Because our method estimates the radii of each polygon vertex we refer to it as radial inversion. The position of these vertices, the horizontal location of each prism and the depth extent of all prisms are the parameters to be estimated by solving a constrained non-linear inverse problem of minimizing a goal function. We run successive inversions for a range of tentative total-magnetization intensities and depths to the top of the shallowest prism. The estimated models producing the lowest values of the goal function form the set of candidate solutions. To obtain stabilized solutions, we impose the zeroth- and first-order Tikhonov regularizations on the shape of the prisms. The method allows estimating the geometry of both vertical and inclined sources, with a constant direction of magnetization, by using the Tikhonov regularization. Tests with synthetic data show that the method can be of utility in estimating the shape of the magnetic source even in the presence of a strong regional field. Results obtained by inverting airborne total-field anomaly data over the Anitápolis alkaline–carbonatitic complex, in southern Brazil, suggest that the emplacement of the magnetic sources was controlled by NW–SE-trending faults at depth, in accordance with known structural features at the study area.

Generalized positivity constraint on magnetic equivalent layers

It is known from the potential theory that a continuous and planar layer of dipoles can exactly reproduce the total-field anomaly produced by arbitrary 3D sources. We have proven the existence of an equivalent layer having an all-positive magnetic-moment distribution for the case in which the magnetization direction of this layer is the same as that of the true sources, regardless of whether the magnetization of the true sources is purely induced or not. By using this generalized positivity constraint, we have developed a new iterative method for estimating the total magnetization direction of 3D magnetic sources based on the equivalent-layer technique. Our method does not impose a priori information about the shape or the depth of the sources, does not require regularly spaced data, and presumes that the sources have a uniform magnetization direction. At each iteration, our method performs two steps. The first step solves a constrained linear inverse problem to estimate a positive magnetic-moment distribution over a discrete equivalent layer of dipoles. We consider that the equivalent sources are located on a plane and have a uniform and fixed magnetization direction. In the second step, we use the estimated magnetic-moment distribution and solve a nonlinear inverse problem for estimating a new magnetization direction for the dipoles. The algorithm stops when the equivalent layer yields a total-field anomaly that fits the observed data. Tests with synthetic data simulating different geologic scenarios show that the final estimated magnetization direction is close to the true one. We apply our method to field data from the Goiás alkaline province, over the Montes Claros complex, in the center of Brazil. The results suggest the presence of intrusions with remarkable remanent magnetization, in agreement with the current literature for this region.

Predicting Magnetization Directions Using Convolutional Neural Networks

AbstractMagnetic data have been widely used for understanding basin structures, mineral deposit systems, the formation history of various geological systems, and many others. Proper interpretation of magnetic data requires accurate knowledge of total magnetization directions of the source bodies in an area of study. Existing approaches for estimating magnetization directions involve either unstable data processing steps or computationally intensive processes such as 3‐D inversions. In this study, we developed a new method of automatically predicting the magnetization direction of a magnetic source body using convolutional neural networks (CNNs). CNNs have achieved great success in many other applications such as computer vision and seismic image interpretation but have not been used to extract parameters from magnetic data. We simulated many magnetic data maps with different magnetization directions from a synthetic source body, all subject to the same background field. Two CNNs were trained separately, one for predicting the inclination and the other for predicting declination. We determined the optimal CNN architectures for predicting inclinations and declinations by systematically comparing 13 different CNN architectures. In addition, we investigated the effect of having different parameters such as magnetization magnitude, source body shape, location, and depth on the performance of our predictive models. We also tested the method using field data from Black Hill Norite, Australia, and Yeshan region, China, for which prior research results are available for comparison. Our study shows that machine learning provides an effective means of automatically predicting magnetization directions based on magnetic data maps.

Recovering the total magnetization direction using a novel stable reduction to the pole filter (RTP)

Interpretation of magnetic data commonly assumes that the total magnetization direction is parallel to the local geomagnetic field direction, leading to incorrect results when remanent magnetization is significant. This is a particular problem when applying interpretive filters that require knowledge of the direction of total magnetization. Here I present a novel frequency-domain, reduction-to-pole filter that is numerically stable for low latitudes and incorporates the influence of remanent magnetization. The operator is implemented through a differential algorithm that iteratively estimates the total magnetization direction. At each iteration, the code compares the ability of thirty pairs of inclination and declination values to minimize an objective function. After each comparison, the two pairs yielding the worst result are replaced by new values until the objective function is minimized. To improve results, two different methods were implemented to obtain a first guess for the inclination and declination: Helbig's integrals and comparison method. This first guess is used as reference to compose the first set of thirty pairs considered to calculate the RTP. A third possibility was also implemented, which uses thirty random pairs for the first interaction. Fisher's statistics are applied to the final population of estimated directions in order to determine the best mean direction. The new RTP filter is numerically stable and independent of a priori information about the source susceptibility, top depth, and geometry. The algorithm was tested on magnetic anomalies from several synthetic models and on real magnetic data and showed remarkable stability even in challenging settings.

The IDQ curve: A tool for evaluating the direction of remanent magnetization from magnetic anomalies

Natural remanent magnetization acts as a record of the previous orientations of the earth’s magnetic field, and it is an important feature when studying geologic phenomena. The so-called IDQ curve is used to describe the relationship between the inclination ( I) and declination ( D) of remanent magnetization and the Köenigsberger ratio ( Q). Here, we construct the IDQ curve using data on ground and airborne magnetic anomalies. The curve is devised using modified approaches for estimating the total magnetization direction, e.g., identifying the maximal position of minimal reduced-to-the-pole fields or identifying correlations between total and vertical reduced-to-the-pole field gradients. The method is tested using synthetic data, and the results indicate that the IDQ curve can provide valuable information on the remanent magnetization direction based on available data on the Köenigsberger ratio. Then, the method is used to interpret field data from the Yeshan region in eastern China, where ground anomalies have been produced by igneous rocks, including diorite and basalt, which occur along with magnetite and hematite ore bodies. The IDQ curves for 24 subanomalies are constructed, and these curves indicate two main distribution clusters of remanent magnetization directions corresponding to different structural units of magma intrusion and help identify the lithologies of the magnetic sources in areas covered by Quaternary sediments. The estimated remanent magnetization directions for Cenozoic basalt are consistent with measurements made in paleomagnetism studies. The synthetic and field data indicate that the IDQ curve can be used to efficiently estimate the remanent magnetization direction from a magnetic anomaly, which could help with our understanding of geologic processes in an area.

Improving the crosscorrelation method to estimate the total magnetization direction vector of isolated sources: A space-domain approach for unstable inclination values

Knowledge of the total magnetization direction of geologic sources is valuable for interpretation of magnetic anomalies. Although the magnetization direction of causative sources is assumed to be induced by the ambient magnetic field, the presence of remanence should not be neglected. An existing method of correlating total and vertical gradients of the reduced-to-the-pole (RTP) anomaly estimates the total magnetization direction well. However, due to the numerical instability of RTP transformation in the Fourier domain, an assumption should be considered for dealing with inclination values at approximately 0°. We have adopted an extension to the standard crosscorrelation method for estimating the total magnetization direction vector, computing the RTP anomaly by means of the classic equivalent layer technique for low inclination values. Additionally, an ideal number of equivalent sources within the layer is considered for reducing the computational demands. To investigate the relevant aspects of the adopted method, two simple synthetic scenarios are presented. First, a magnetic anomaly produced by a homogeneous and isolated vertical dike is considered. This test illustrates the good performance of the adopted approach, finding the true magnetization direction, even for low inclination values. In the second synthetic test, a long-wavelength component is added to the previous magnetic total-field anomaly. In this case, the method adopted here fails to estimate a reliable magnetization direction vector, showing weak performance for strong interfering magnetic anomalies. On the real data example, the application tests an isolated total-field anomaly of the Carajás Mineral Province, in northern Brazil, where the inclination of the ambient magnetic field is close to zero. The obtained results indicate weak remanence in the estimated total magnetization direction vector, which would never be reached in the standard formulation of the crosscorrelation technique.

3D sparse inversion of magnetic amplitude data when strong remanence exists

Three-dimensional inversion for susceptibility distributions is a common approach for quantitative interpretation of magnetic data. However, this approach will fail when strong remanence exists because the total magnetization direction is unknown. Magnetic amplitude inversion can reduce remanence effects and thus improve reconstructed results. In this paper, we propose a sparse magnetic amplitude inversion method which minimizes an L0-like-norm of model parameters subject to bound constraints. By using the iteratively reweighed least squares technique, the bound-constrained L0-like-norm sparse inversion is transformed to a sequence of bound-constrained nonlinear least squares subproblems. To deal with the bound constraints, we use a logarithm barrier algorithm to solve each subproblem. Compared with the classical L2-norm inversion method, the proposed sparse method utilizes the known physical property information to produce binary results characterized by sharp boundaries. This method is tested on synthetic data produced by a dipping dyke model and a field data set acquired in Australia.

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.

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

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.

Three‐dimensional inversion of full magnetic gradient tensor data based on hybrid regularization method

ABSTRACTRapid developments in SQUID‐based technology make it possible for geophysical exploration to direct measure, inverse and interpret magnetic gradient tensor data. This contribution introduces a novel three‐dimensional hybrid regularization method for inversion of magnetic gradient tensor data, which is based on the minimum support functional and total variation functional. Compared to the existing stabilizers, for example, the minimum support stabilizer, the minimum gradient support stabilizer or the total variation stabilizer, our proposed hybrid stabilizer, in association with boundary penalization, improves the revision result greatly, including higher spatial and depth resolution, more clear boundaries, more highlighted images and more evident structure depiction. Moreover, suitable selection of model parameter λ will further improve the image quality of the recovered model. We verify our proposed hybrid method with various synthetic magnetic models. Experiment results prove that this method gives more accurate results, exhibiting advantages of less computational costs even when less prior information of magnetic sources are provided. Comparison of results with different types of magnetic data with and without remanence indicates that our inversion algorithm can obtain more detailed information on the source structure based on rational estimation of total magnetization direction. Finally, we present a case study for inverting SQUID‐based magnetic tensor data acquired at Da Hinggan Mountains area, inner Mongolia, China. The result also certifies that the method is reliable and efficient for real cases.

Improved total magnetization direction determination by correlation of the normalized source strength derivative and the reduced-to-pole fields

The knowledge of total magnetization (magnitude and direction) makes it easier to interpret magnetic anomalies. We have developed a simple crosscorrelation-based method to determine the total magnetization direction of a magnetic source from the vertical derivative of normalized source strength (dNSS) and the reduced-to-pole (RTP) magnetic fields. For most source types, the spread of the dNSS field (or its half-width) is similar to that of the RTP field computed with the correct total magnetization direction, and, thus, the comparison results in a more meaningful correlation coefficient than other functions used in the literature. We have determined the utility of our method using several compact source types (i.e., sphere, dike, horizontal sheet, vertical and horizontal cylinders, and prism). Moreover, the existing methods for determining the direction can be unstable at low latitudes due to noise amplification. A filter that isolates the main features of the anomaly of interest, when applied to both the fields being correlated, improves the performance of the method. We also implement a stabilizing amplitude threshold filter that made the method stable at low latitudes. Model tests indicate that our method estimates the total magnetization directions accurately for low inclinations of total magnetization and inducing field directions. We applied the method to estimate the total magnetization direction of magnetic anomalies in the north and central part of the Goiás Alkaline Province in central Brazil. The RTP fields from the total magnetization directions derived from our method meet the expectations of anomaly symmetry and centering on the outcrops or the edges of the alkaline intrusive bodies. In addition, we found that the resulting magnetic and gravity models of the Goiás Alkaline intrusives were consistent with the geologic model of inverted conical diatremes.

Integrated geologic and geophysical interpretation of the Buraco da Velha copper deposit (Rondônia, Brazil): A basis for exploring in related environments

The Buraco da Velha copper deposit lies at the northern limit of the Parecis Basin in the Colorado Graben, Rondônia, Brazil. New geophysical data indicate the presence of a magnetic source below and to the north of the Buraco da Velha deposit, where it corresponds with high gamma-ray U, Th, and K counts. We have studied the source of the magnetic anomaly and delineated it through derivative transforms, Euler deconvolution, MaxiMin total magnetization direction analysis, staged inverse modeling, Th/K ratio, and radiometric ternary imaging. The magnetic anomaly is elongated in the east–west direction and measures [Formula: see text], and the top of the source is estimated to lie at depths mostly between 50 and 100 m. Based on the magnetic model, we indirectly estimate a Jurassic to Cretaceous age, which is compatible with the 180–80 Ma range dated for the copper mineralization. Gamma-ray data are consistent with the hydrothermal alteration in the sedimentary cover and with the presence of an intrusion in the subsurface. We suggest that the intrusion of the magnetic body generated the necessary thermal energy to mix an already oxidized brine and sulfide-bearing fluids at the border of the Parecis Basin, leading to copper deposition and providing a potential analog for similar environments of mineral deposits elsewhere, such as in the Kupferschiefer deposit (Poland) and the Zambia Copper Belt (Zambia).

Estimation of the total magnetization direction of approximately spherical bodies

Abstract. We have developed a fast total-field anomaly inversion to estimate the magnetization direction of multiple sources with approximately spherical shapes and known centres. Our method is an overdetermined inverse problem that can be applied to interpret multiple sources with different but homogeneous magnetization directions. It requires neither the prior computation of any transformation-like reduction to the pole nor the use of regularly spaced data on a horizontal grid. The method contains flexibility to be implemented as a linear or non-linear inverse problem, which results, respectively, in a least-squares or robust estimate of the components of the magnetization vector of the sources. Applications to synthetic data show the robustness of our method against interfering anomalies and errors in the location of the sources' centre. Besides, we show the feasibility of applying the upward continuation to interpret non-spherical sources. Applications to field data over the Goiás alkaline province (GAP), Brazil, show the good performance of our method in estimating geologically meaningful magnetization directions. The results obtained for a region of the GAP, near to the alkaline complex of Diorama, suggest the presence of non-outcropping sources marked by strong remanent magnetization with inclination and declination close to −70.35 and −19.81°, respectively. This estimated magnetization direction leads to predominantly positive reduced-to-the-pole anomalies, even for other region of the GAP, in the alkaline complex of Montes Claros de Goiás. These results show that the non-outcropping sources near to the alkaline complex of Diorama have almost the same magnetization direction of those ones in the alkaline complex of Montes Claros de Goiás, strongly suggesting that these sources have been emplaced in the crust within almost the same geological time interval.

Magnetic field analysis of Morro do Leme nickel deposit

The Morro do Leme laterite nickel deposit lies inside the western border of the Parecis Basin (Brazil). This deposit is characterized by high concentrations of lateritic Ni (about 1.8%) and anomalous contents of Pd, Au, Cu, Na, Co, Zn, and Pt in a peridotite and dunite layered intrusion. Besides the existence of geochemical and drilling data, the 3D distribution in the subsurface of this layered intrusion is still unknown. An airborne magnetic survey revealed three east–west elongated magnetic anomalies, characterized by a significant remanent magnetization. The sources of these anomalies were delimitated laterally and had their depths estimated between 90 and 150 m, using techniques that use derivatives. Further, the total magnetization direction was obtained from a distortion analysis of the magnetic anomalies. All these data were united in an initial model for the 3D inversion of the magnetic data. The total and induced magnetization directions were attributed to the inverted model of 0.12 (SI) susceptibility, allowing indirect estimation of the remanence. The model, defined by the depth, the inversion, and the remanence estimates, linked the intrusion to analogue events in the Rondonian-San Ignácio Province. The results indicated that to explore for laterite Ni, the best locations are the southern part of the main anomaly and in the cover above the two smaller anomalies, whereas to explore for Pd, Au, Cu, Na, Co, Zn, and/or Pt, the indicated region is the central portion of the main anomaly.