Magnetic field analysis of Morro do Leme nickel deposit

Geophysics ◽  
2014 ◽  
Vol 79 (6) ◽  
pp. K1-K9 ◽  
Author(s):  
Vinicius Hector Abud Louro ◽  
Marta Silvia Maria Mantovani ◽  
Vanessa Biondo Ribeiro
Keyword(s):  
Layered Intrusion ◽  
Magnetic Anomalies ◽  
Field Analysis ◽  
Magnetic Data ◽  
Magnetic Survey ◽  
Nickel Deposit ◽  
Magnetization Direction ◽  
Magnetic Field Analysis ◽  
Western Border ◽  
High Concentrations

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.

Magnetization vector imaging for borehole magnetic data based on magnitude magnetic anomaly

Geophysics ◽  
2013 ◽  
Vol 78 (6) ◽  
pp. D429-D444 ◽  
Author(s):  
Shuang Liu ◽  
Xiangyun Hu ◽  
Tianyou Liu ◽  
Jie Feng ◽  
Wenli Gao ◽  
...  
Keyword(s):  
Conjugate Gradient Method ◽  
Conjugate Gradient ◽  
Gradient Method ◽  
Magnetization Vector ◽  
Real Data ◽  
Magnetic Anomalies ◽  
Magnetic Data ◽  
Preconditioned Conjugate Gradient ◽  
Magnetization Direction ◽  
Magnetization Intensity

Remanent magnetization and self-demagnetization change the magnitude and direction of the magnetization vector, which complicates the interpretation of magnetic data. To deal with this problem, we evaluated a method for inverting the distributions of 2D magnetization vector or effective susceptibility using 3C borehole magnetic data. The basis for this method is the fact that 2D magnitude magnetic anomalies are not sensitive to the magnetization direction. We calculated magnitude anomalies from the measured borehole magnetic data in a spatial domain. The vector distributions of magnetization were inverted methodically in two steps. The distributions of magnetization magnitude were initially solved based on magnitude magnetic anomalies using the preconditioned conjugate gradient method. The preconditioner determined by the distances between the cells and the borehole observation points greatly improved the quality of the magnetization magnitude imaging. With the calculated magnetization magnitude, the distributions of magnetization direction were computed by fitting the component anomalies secondly using the conjugate gradient method. The two-step approach made full use of the amplitude and phase anomalies of the borehole magnetic data. We studied the influence of remanence and demagnetization based on the recovered magnetization intensity and direction distributions. Finally, we tested our method using synthetic and real data from scenarios that involved high susceptibility and complicated remanence, and all tests returned favorable results.

scholarly journals Magnetic and spectrometric studies of Al-Gor area, Southwestern Sinai, Egypt

2021 ◽  
Vol 51 (3) ◽  
pp. 207-223
Author(s):  
Mostafa A. M. ZAEIMAH
Keyword(s):  
Gamma Ray ◽  
Magnetic Anomalies ◽  
Magnetic Data ◽  
Magnetic Survey ◽  
Rock Types ◽  
Ore Minerals ◽  
Um Bogma Formation ◽  
Copper Zinc ◽  
Relationship Of ◽  
Southwestern Sinai

Al-Gor area is a part of Southwestern Sinai of Egypt. It is considered as one of the most promising areas for mineralization in Egypt, being rich in many mineral deposits of: manganese, iron, copper, zinc, lead, cobalt, nickel, silver, gibbsite, and uranium. Besides, some industrial ore minerals such as kaolin and glass sand,… etc. are also found in this area. The area was studied by Gama-ray spectrometry to trace the radioactive anomalies, their concentrations and their relationship to the existing rocks, and by magnetic survey to study the relationship of radioactive anomalies and their trends with the trends of geological structures. The gamma-ray spectrometric maps show different levels over the surveyed area, which reflect contrasting radioelement contents for the exposed various rock types. The highest radiospectrometric levels are located in the northwest southeast direction and some scattered parts all over the study area. They are mainly associated with Um-Bogma Formation, bearing gibbsite. The study area possesses radiospectrometric ranging between 0.6 and 110.9 Ur as a total-count, 0.1 to 1.8 % for K, 0.1 to 99 ppm for eU and 0.1 to 23 ppm for eTh. The qualitative analyses of magnetic data show the existences of a number of different magnetic anomalies, with different amplitudes and frequencies as well as trends. From the application of spectral analyses of magnetic data, the regional and residual depths of magnetic anomalies can be computed. The first depth represents the regional (deep-seated) anomalies, at about 75 m and the residual (shallow-seated) anomalies, at about 20 m. The trends of the structures as derived from the spectrometric and ratio maps correspond to those inferred from the residual-component magnetic map, which reflects the effect of structures on the concentration of radioactive elements and, consequently mineralization.

Automatic determination of the magnetization–density ratio and magnetization inclination from the joint interpretation of 2D gravity and magnetic anomalies

Geophysics ◽  
2004 ◽  
Vol 69 (4) ◽  
pp. 938-948 ◽  
Author(s):  
Carlos Alberto Mendonça
Keyword(s):  
Vector Fields ◽  
Density Ratio ◽  
Magnetic Anomalies ◽  
Magnetic Data ◽  
Data Sets ◽  
Magnetization Direction ◽  
Gravity And Magnetic ◽  
Geophysical Surveys ◽  
Magnetization Density ◽  
Gravity And Magnetic Data

The Poisson theorem establishes a linear relationship between the gravity and magnetic potentials arising from common dense and magnetized bodies with constant magnetization–density ratio and magnetization direction. For geological formations satisfying such constraints (i.e., the Poisson conditions), this theorem provides suitable relationships between the gravity and magnetic anomalies that are useful in interpreting the related data sets. In such applications, both magnetization–density ratio (MDR) and magnetization direction can be estimated, thus helping the subsurface geological mapping from potential field data acquired on the earth's surface. However, no existing method is fully automatic, which has hampered extensive use in routine applications. Such a drawback follows the adoption of equations that, although obeying the Poisson theorem, relate particular components of the gravity and magnetic fields, thus requiring either a known magnetization direction or the implementation of iterative procedures to determine it. To allow one‐pass estimates for both MDR and magnetization direction (more precisely, its inclination projected on the plane normal to the source strike), this paper presents simple analytical solutions for these parameters by relating suitable gravity and magnetic vector fields that are derived from the gravity and magnetic data sets. Because current geophysical surveys usually provide only a single‐field component, a data processing scheme is developed to determine the required components in evaluating the desired vector fields. This is done by applying suitable linear transformations on the measured components according to well‐established filtering techniques in processing gravity and magnetic data. Except for distortions from noise, the proposed method automatically determines the MDR and the projected magnetization inclination for the underlying rocks everywhere the Poisson conditions are satisfied. Two‐dimensional sources are assumed, but no constraint upon their depth and cross‐section shape is required. Distorted estimates only appear close to the sources where at least one of the Poisson conditions is violated. In this case, the proposed technique furnishes apparent values for the rock properties. The abrupt changes of apparent values over contacts detect edges, thus facilitating the mapping of geological boundaries. The proposed technique is used to interpret two profiles across the Appalachian fold belt from the eastern portion of the State of Georgia, and the results are compared with some of the geological information available for the area.

Kimberlites and exploration geophysics

Geophysics ◽  
1979 ◽  
Vol 44 (8) ◽  
pp. 1395-1416 ◽  
Author(s):  
James C. Macnae
Keyword(s):  
Magnetic Anomaly ◽  
Magnetic Anomalies ◽  
Magnetic Data ◽  
Magnetic Survey ◽  
Emplacement Mechanism ◽  
Geophysical Technique ◽  
Geologic Model ◽  
Main Factors ◽  
Original Size ◽  
Large Survey

This paper discusses geophysical prospecting techniques for kimberlite pipes, a major source of diamonds. A simple geologic model based on descriptions by Hawthorne (1975) and Nixon (1973) is given, and the varied geophysical responses obtained over kimberlite pipes are interpreted in terms of this model. The three main factors controlling these responses are the original size and inhomogeneity of the pipe, the depth of erosion, and the degree of weathering. Kimberlite pipes are roughly elliptical in surface exposure in most cases, with a “carrot shaped” extension at depth. The unweathered portion of the pipe generally contains a few percent magnetite, and this in most cases produces a clearly detectable magnetic anomaly. The presence of deep weathering may alter the magnetite in the top of the pipe to nonmagnetic oxides of iron, thus resulting in an increased depth to the magnetic source. If this depth is large, the magnetic response may not be large enough to detect the kimberlite in the presence of noise and the effect of other structures. In addition, if little erosion has taken place since emplacement, kimberlitic sediments known as epiclastic kimberlite will be present to considerable depths in the pipe, and this may also lead to the absence of a clear magnetic anomaly. In one large survey in South Africa, electromagnetic (EM) techniques have proven to be remarkably effective in detecting the presence of weathered clays or epiclastic kimberlite contained within the pipes. All pipes discovered during this survey had unmistakable EM signatures, while five out of eight had very small magnetic anomalies which would not likely have been selected as potential targets on the basis of magnetic data alone. These examples would indicate that in any area where deep weathering is expected, an EM survey is essential in combination with a magnetic survey if reconnaissance is to be based on airborne geophysical techniques. Due to the emplacement mechanism of kimberlite, considerable inhomogeneity within a pipe may be present, leading to significant variation in the response of any geophysical technique to one pipe, with resultant interpretation difficulties. Although this is not a limitation in the discovery of new pipes, it does make their geophysical delimitation difficult.

Variations in magnetic properties of Unit 10, Eastern Layered Intrusion, Isle of Rum, Scotland: implications for patterns of high temperature hydrothermal alteration

1995 ◽  
Vol 86 (2) ◽  
pp. 91-112 ◽  
Author(s):  
J. Housden ◽  
W. O'Reilly ◽  
S. J. Day
Keyword(s):  
Magnetic Properties ◽  
Fluid Flow ◽  
High Temperature ◽  
Hydrothermal Alteration ◽  
Hydrothermal Fluid ◽  
Layered Intrusion ◽  
Magnetic Data ◽  
Magnetic Studies ◽  
Alteration Minerals ◽  
High Concentrations

ABSTRACTAn in situ magnetic susceptibility survey of Unit 10 of the Eastern Layered Intrusion of the Isle of Rum, in a line perpendicular to the strike, was carried out as a guide to selecting sampling sites for subsequent laboratory magnetic studies. These laboratory studies indicate that the dominant magnetic phase is magnetite. An effective particle size of the magnetite was derived from the magnetic data: it was found that high concentrations (∼0·3%) of magnetite in the perioditites were achieved by the presence of fewer but larger particles. The regions of lower magnetite concentration (∼0·01%), which are mainly in the plagioclase-rich rocks, contain more abundant but smaller particles.The variations in magnetic properties correlate with the abundance and inferred temperatures of formation of hydrothermal alteration minerals in the rocks. Petrographic observations indicate temperatures of alteration of 500–800°C in the olivine-rich peridodites in the lower part of the Unit, but of the order of 300°C in the plagioclase-rich rocks at the top of the Unit.These relationships between magnetic mineralogy and hydrothermal alteration suggest that the magnetite was produced by olivine oxidation during hydrothermal alteration. It is proposed that variations in the magnetic properties of layered cumulate rocks may be used to map out variations in the temperature and intensity of hydrothermal fluid flow. The variations in the Unit 10 rocks studied are interpreted as indicating control of high-temperature hydrothermal fluid flow through them by contrast in permeability between brittle peridotites and quasiplastic plagioclase-rich rocks.

Identifying remanent magnetization effects in magnetic data

Geophysics ◽  
1993 ◽  
Vol 58 (5) ◽  
pp. 653-659 ◽  
Author(s):  
Walter R. Roest ◽  
Mark Pilkington
Keyword(s):  
Magnetic Anomaly ◽  
Remanent Magnetization ◽  
Close Agreement ◽  
Magnetic Anomalies ◽  
Analytic Signal ◽  
Magnetic Data ◽  
Horizontal Gradient ◽  
Impact Structure ◽  
Magnetization Direction ◽  
Additional Constraints

Remanent magnetization can have a significant influence on the shape of magnetic anomalies in areas that are generally characterized by induced magnetization. Since modeling of magnetic anomalies is nonunique, additional constraints on the direction of magnetization are useful. A method is proposed here to study the possible contribution of remanent magnetization to a particular anomaly, by comparing two functions that are calculated directly from the observations: (1) the amplitude of the analytic signal, and (2) the horizontal gradient of pseudogravity. From the amplitude and relative position of maxima in these derived quantities, we infer the deviation of the magnetization direction from that of the ambient field. The approach is applied to the magnetic anomaly in the center of the Manicouagan impact structure (Canada). Our results, based only on the magnetic anomaly observations, are in close agreement with constraints on the direction of remanent magnetization from rock samples.

Magnetic investigations of the junction between Wilson and Bowers terranes (northern Victoria Land, Antarctica)

1995 ◽  
Vol 7 (2) ◽  
pp. 149-157 ◽  
Author(s):  
Emanuele Bozzo ◽  
Giorgio Caneva ◽  
Giovanni Capponi ◽  
Alessandro Colla
Keyword(s):  
Volcanic Rocks ◽  
Magnetic Anomalies ◽  
Magnetic Data ◽  
Magnetic Survey ◽  
High Susceptibility ◽  
Victoria Land ◽  
Magnetic Signature

A magnetic survey was carried out in the area between Lady Newnes Bay and Evans Névé (northern Victoria Land), to ascertain whether the contact between the Wilson and the Bowers terranes could be identified remotely. The survey consisted of three ground and 12 helicopter-borne profiles. The method was calibrated on the southernmost profiles, which cover a well-exposed section of the contact between the Wilson and Bowers terranes. The northern profiles were located in an area where the contact is poorly constrained by outcrops, so that it could be tested whether the junction displays a magnetic signature. The magnetic data and the 2.5-D modeling of three selected profiles indicate that no easily recognizable magnetic signature defines this contact. The main features of the area are magnetic anomalies probably controlled by the “Meander Intrusives” and the McMurdo volcanic rocks, both characterized by high susceptibility values. If an anomaly related to the contact exists, then it is probably masked by these stronger anomalies.

Airborne geophysical surveys in Greenland in 1998

1999 ◽  
pp. 34-38 ◽  
Author(s):  
Thorkild M. Rasmussen ◽  
Leif Thorning
Keyword(s):  
High Resolution ◽  
Geophysical Data ◽  
Digital Data ◽  
Geological Survey ◽  
Magnetic Data ◽  
Magnetic Survey ◽  
Original Series ◽  
Geophysical Surveys ◽  
The Government

NOTE: This article was published in a former series of GEUS Bulletin. Please use the original series name when citing this article, for example: Rasmussen, T. M., & Thorning, L. (1999). Airborne geophysical surveys in Greenland in 1998. Geology of Greenland Survey Bulletin, 183, 34-38. https://doi.org/10.34194/ggub.v183.5202 _______________ Airborne geophysical surveying in Greenland during 1998 consisted of a magnetic project referred to as ‘Aeromag 1998’ and a combined electromagnetic and magnetic project referred to as ‘AEM Greenland 1998’. The Government of Greenland financed both with administration managed by the Geological Survey of Denmark and Greenland (GEUS). With the completion of the two projects, approximately 305 000 line km of regional high-resolution magnetic data and approximately 75 000 line km of detailed multiparameter data (electromagnetic, magnetic and partly radiometric) are now available from government financed projects. Figure 1 shows the location of the surveyed areas with highresolution geophysical data together with the area selected for a magnetic survey in 1999. Completion of the two projects was marked by the release of data on 1 March, 1999. The data are included in the geoscientific databases at the Survey for public use; digital data and maps may be purchased from the Survey.

Sign in / Sign up

Export Citation Format

Share Document