The study of the magnetic anomaly variation A and its relationship with iron ores

<p>Magnetic surveys are commonly used for solving variety of geotechnical and geological challenges in offshore areas, jointly with a set of other geophysical methods. The most popular technique employed is hydromagnetic surveying with towed magnetometers. One of the most significant challenges encountered during processing of the magnetic data is related to temporal variations of the Earth's magnetic field. Accounting for diurnal magnetic field variations is often done by carrying out differential hydromagnetic surveys, a technique developed in the 1980-s. It is based on simultaneous measurements of the magnetic field using two sensors towed behind the vessel with a given separation. This technique allows to calculate along-course gradient which is free of magnetic field temporal variations. This measurement system resembles a gradiometer, with the distance between two sensors being referred to as the base of the gradiometer. It is possible to calculate anomalous magnetic field by integrating obtained magnetic field gradient. Studies have shown that accuracy of its reconstruction decreases with increasing base of the gradiometer. This becomes most significant when distance between the sensors and sources of magnetic field anomalies is small. This situation occur when the survey area is located in shallow water (i.e. for shallow marine, river or lake surveys).</p><p>An approach for deriving magnetic anomalies and accounting for diurnal variations in differential hydromagnetic surveys based on the frequency (spectral) representation of the measurements was proposed in 1987 [Melikhov, 1987]. This approach utilizes the fact that it is possible to reconstruct the spectrum of magnetic field anomalies along the vessel course from the spectra of measured signals from the first S<sub>1</sub>(ω) and second S<sub>2</sub>(ω) sensors. Assuming that the sensors are located at the same depth, it can be achieved via the following transform:</p><p><img src="https://contentmanager.copernicus.org/fileStorageProxy.php?f=gepj.3d3911bac60061487501161/sdaolpUECMynit/12UGE&app=m&a=0&c=ff23bad5ed5181be02f7ef7ab5e8d6e4&ct=x&pn=gepj.elif&d=1" alt="" width="192" height="43"></p><p>where ω - spatial frequency, <em>l</em> - base of the gradiometer, and <em>i</em> - imaginary unit. Assuming that at a single moment in time magnetic field variations equally affect both sensors, resulting Fourier spectrum T(ω) will correspond the spectrum of anomalous magnetic field, free of the magnetic variations. It should be noted that, similar to the along-course gradient integration approach, anomalous magnetic field is restored to a certain accuracy level.</p><p>Estimates made on model examples showed that accuracy of the field reconstruction using this method is comparable to the accuracy levels of modern marine magnetic surveys (±1-3 nT). It could be noted that for gradiometer bases comparable or larger than depths to magnetic anomaly sources, errors of the field reconstruction are significantly lower for the spectral transformation-based approach compared to along-course gradient integration.</p><p>

Recently rock magnetism is contributing to solve many geological problems. There are several types of remanent magnetization. In case of igneous rocks, magnetism arises from thermo-remanent magnetization. When the ferromagnetic mineral grows or changes chemically in the magnetic field under its curie point, it gets chemical or crystalline magnetization. In this paper, application of rock magnetism arising from chemical or crystalline remanent magnetization to economic geology was attempted. In order to achieve this purpose, the Shinyama ore body of Kamaishi iron and copper ore deposits was selected, because it is a typical contact metasomatic ore deposit and is a well exploited one. The Shinyama ore body consists of two iron ore bodies that contain magnetite and several copper ore bodies that contain pyrrhotite. Oriented samples were collected from many localities of the ore deposits systimatically. The direction and intensity of remanent magnetization, the magnetic susceptibility and other magnetic properties of ores were measured. The direction of remanent magnetization of each ore body is as follows. the iron ore…………………………Inclination 90°Down The 2nd copper ore body (closed to the iron ore body)……………{Declination 12°E INclination 48°Down The 4th copper ore body (200m distant from the iron ore body)…{Declination 333°E Inclination 53°Down From the view point of mechanism of magnetization, it is sure that the iron ore body gets the remanent magnetization which, roughly speaking, agrees with the direction of elongation of the ore body; and the 2nd copper ore body gets the remanent magnetization in the direction compounding geomagnetic field and magnetic flux of the iron ore body. By means of remanent magnetization together with other magnetic properties of ores, the direction of elongation of an ore body and the order of mineralization can be predicated.

Geothermal zones and hot mineral springs are primarily associated with magmatism along the Eurasian Continental Margin. In our study, we examine the correlation between geothermal anomalies observed in Bulgarian territory and the magnetic anomalies produced by magmatic and metamorphic bodies. Specifically, if these magnetic sources are of relatively recent origin, they could contribute to heating the surrounding geological environment, thereby influencing geothermal patterns. We use geological and hydrological information, along with temperature distribution at depth, to clarify the geological environment. Next, we apply geophysical data processing and direct inversion techniques to compute the total field modulus and Euler solutions, refining the interpretation of magnetic sources and enabling a more precise evaluation of subsurface magnetic structures and their potential impact on geothermal activity. A thorough analysis of the geomagnetic field, including over forty described anomalies, is presented for the examined territory. The correlation between geothermal and magnetic anomalies is confirmed using the Chi-square test. The null hypothesis is rejected, and the zones that significantly contributed to this result are outlined. Our study identifies seven regions with the strongest correlations in terms of temperature and magnetic anomalies: (1) A broad area in the western part of the Moesian platform near Kozloduy, characterized by increased temperatures and moderate magnetic anomalies; (2) A well-defined area east of Sofia, the capital city, which features high temperatures and intense magnetic anomalies, as well as numerous hot springs; (3) The region north of Blagoevgrad, located at the foothills of the Rila Mountains, where the hottest spring, Sapareva Banya, can be found; (4) The lower course of the Struma River, near the town of Petrich, where higher temperatures coincide with a distinct group of magnetic anomalies caused by granite bodies locally enriched in ferromagnetic iron minerals; (5) A local area near Velingrad exhibiting high temperatures alongside magmatic bodies, set in a complex tectonic environment; (6) A confined zone to the north of Dospat, where rhyolites are exposed along a significant fault line; (7) A broad zone with the highest temperatures (exceeding 100 °C at a depth of 1,000 m) that aligns with magnetic anomalies from extensive outcrops of Precambrian metamorphic rocks. We analyze these relationships in the context of identified hydrogeological zones, providing a detailed examination of the spatial distribution of geothermal and magnetic features. Furthermore, we explore the correlation between these anomalies and the depth to the Curie point, as inferred from magnetic data, to better understand the thermal and magnetic structure of the region. Graphical Abstract The present study aims to identify the spatial distribution of geothermal zones in Bulgaria that coincide with the appearance of magnetic anomalies. To delineate geothermal anomalies, we use temperatures at a depth of 1000 m below the surface, obtained from temperature logs of wells. Magnetic anomalies are calculated from the vertical component of the anomalous geomagnetic field across Bulgaria. Additionally, we support our interpretation with geological information and hydrogeological zoning of mineral waters, including the distribution of hot springs. Two types of analysis are performed: (1) geophysical data processing in terms of magnitude calculation and the direct inverse method of Euler deconvolution, which outlines the magnetic sources, and (2) statistical examination using a Chi-square test. A uniform grid with over 200,000 cells is generated, with each cell containing geothermal and magnetic data organized by classes (levels). A contingency table is compiled that records the frequency of each combination of geothermal and magnetic classes. The Chi-square test results indicate a highly significant statistical correlation between geothermal and magnetic anomalies. Consequently, we reject the null hypothesis and conclude that geothermal activity and magnetic variations are closely linked, possibly due to shared geological structures, heat-altered magnetism, or fault systems. The cells that contribute most to this correlation are identified and discussed.

Rock magnetism and magnetic surveys in the iron ore deposit of El Encino, Mexico

The purpose of the study. It needs to substantiate that sources of magnetic anomalies with wavelengths of the first thousand kilometers detected at the present time might have a magneto-mineralogical origin due to the existence of magnetic minerals at the mantle depths, in particular magnetite, hematite, native iron, as well as iron alloys. It should be also shown that present temporal changes of long-wave magnetic anomalies should be induced by changes of the magnetic properties of these minerals due to thermodynamic and fluid modes. According to numerous authors, the transformations of magnetic minerals occur in special tectonic zones of the upper mantle of the Earth, in particular at junction zones of lithospheric plates of different types, rifts, plumes, tectonic-thermal activation, etc. Areas of the upper mantle with temperatures below the Curie temperature of magnetite can be magnetic, such as subduction zones, cratons, and regions with the old oceanic lithosphere. Iron oxides might be a potential source of magnetic anomalies of the upper mantle besides magnetite and native iron, in particular hematite (α-Fe2O3), which is the dominant oxide in subduction zones at depths of 300 to 600 km. It was proved experimentally by foreign researchers that in cold subduction slabs, hematite remains its magnetic properties up to the mantle transition zone (approximately 410-600 km). Conclusions. A review of previous studies of native and foreign authors has made it possible to substantiate the possibility of the existence of magnetized rocks at the mantle depths, including native iron at the magneto-mineralogical level, and their possible changes due to thermodynamic factors and fluid regime. It has been experimentally proven by foreign researchers that in subduction zones of the lithospheric slabs their magnetization might be preserved for a long time at the mantle depths, as well as increase of magnetic susceptibility may observed due to the Hopkinson effect near the Curie temperature of magnetic minerals. Practical value. Information about the ability of the mantle to contain magnetic minerals and to have a residual magnetization up to the depths of the transition zone was obtained. It should be used in the interpretation of both modern magnetic anomalies and paleomagnetic data.

An important task for the White Sea region, Russia’s second largest diamond-producing province, is the search for magmatic bodies overlapped by sedimentary cover via magnetometer survey. The models, linking local and magnetic anomalies with their sources, are essential for interpretation of search results. The aim of the study is to build a 3D magnetic model of the Earth’s crust for the White Sea region using aeromagnetic data and the modeling technologies of the Integro software package. The simulation is basing on a digital map of the pole-reduced anomalous magnetic field. The sources of magnetic anomalies are believed to be located in the Earth’s crust. The researchers obtained 3D distribution of the relative magnetic susceptibility of rocks by solving the inverse problem of magnetic prospecting. To separate the magnetic sources by spatial frequencies and depth, the model magnetic field was recalculated upward, as well as the TDR derivatives, which determine the lateral boundaries of the sources of positive magnetic field anomalies, were calculated. The researchers further analyzed 2D distributions of the magnetic sources of the model for vertical and horizontal sections with depths of 10, 15 and 20 km, thus proving the relationship between the surface and deep structures of the magnetic sources of the Earth’s crust in the region.

Microscopic and rock-magnetic studies of the ores and host rocks of the El Laco iron oxide deposits permit us to characterize the magnetic mineralogy and the processes affecting natural remanent magnetization (NRM) during emplacement and evolution of the deposits. Particular attention was devoted to identifying the magnetic mineral composition (magnetite and/or titanomagnetite, and hematite and/or titanohematite, and titanomaghemite) and grain size variations of both ores and host rock. Rock-magnetic data are used to clarify magnetic domain states and remanence acquisition processes, and to assess their significance as a source of magnetic anomalies. Microscopy under reflected light demonstrates that magnetic carriers are mainly magnetite, with significant amounts of ilmenite-hematite minerals. Magmatic titanomagnetites in the andesitic rocks show trellis textures, compatible with high-temperature oxy-exsolution processes. Supergene reactions in ore deposits under eruption conditions are indicated by goethite and hematite oxide minerals. Grain sizes range from a few microns to >100 μm. Hysteresis measurements point to pseudo-single-domain states. Thermal spectra, continuous temperature-dependent susceptibility measurements, and isothermal remanent magnetization (IRM) acquisition suggest predominance of spinels (titanomagnetite or titanomaghemite) with low-Ti contents as magnetic carriers. Although the presence of (titano)hematites is indicated by hysteresis and IRM studies, their contribution to the total remanence seems to be minor. The Fe-oxides in the ore are typically poor in Ti, whereas in the rocks they are Ti-bearing. For the modeling of the magnetic anomalies, we used data on bulk susceptibility and NRM intensity and direction in order to constrain the relative contributions of induced and remanent magnetization components, and to obtain improved control regarding depth and geometry of source bodies. The deep magnetic source corresponds to an ENE-striking tabular body, steeply inclined 65° to the north.

Basement geology and tectonic development of the greater New Zealand region: an interpretation from regional magnetic data

    Magnetic anomalies commonly contain anomalies generated by crustal rocks that have variable mineral compositions and natural remanent magnetizations. Understanding the magnetic susceptibility, remanent magnetization, magnetization direction, and distribution is important for studying the spatial location, formation, and evolution of underground rocks. However, superposition of magnetic anomalies leads to nonnegligible errors of inversion and interpretation. To overcome the interpretation problems caused by source interference, it is necessary for the target magnetic anomaly to be extracted from the observed magnetic anomaly data. Different methods have been developed to separate the magnetic anomalies of different sources using the spectral differences of regional and residual anomalies. Such methods, which include matched filtering, Wiener filtering, and wavelet analysis, have been successfully applied to solve many geological problems. However, these methods cannot extract the anomalies caused by the interference of sources at similar depths because the spectra of the target and residual magnetic anomalies are similar. Effective techniques to obtain additional magnetic information regarding the distribution of rocks at different layers and with different magnetization directions remain lacking.    Unlike existing regional-residual separation methods used for separating superimposed magnetic anomalies caused by sources with a large depth separation, this study focuses on magnetic anomalies generated by variability of the magnetic parameters and source interference with and without depth differences. We propose a new and useful method for extracting a target magnetic anomaly from an observed magnetic anomaly field. An optimization scheme is proposed for approximating the low-rank component of an observed magnetic anomaly field on the basis of low-rank theory. The magnetic dipole layout is added as a constraint based on the assumed source location. The optimal magnetizations of the magnetic dipoles are then obtained to minimize the objective function. The sum of the magnetic anomalies generated by the magnetic dipoles is calculated as the target magnetic anomaly. The synthetic and field data experiments indicate that the proposed method can accurately and robustly recover target magnetic anomalies. In the field data experiments, the magnetization information of the various isolated sources is derived via 3D fuzzy C-means inversion using the extracted magnetic anomalies. The results show that the proposed method can extract the geometric and physical information of each target magnetic source, even when the observed magnetic anomaly field is generated by various superimposed sources with target source interference at similar depths. The proposed method has the potential for dealing with the separation problems of potential field data with different scales, including the separation of the geomagnetic core field and the lithospheric magnetic field as well as the extraction of target magnetic anomalies from satellite magnetic measurements. Therefore, this approach could be of great importance for geological investigations and mineral exploration.

SUMMARY A test site containing 24 targets of various disarmed unexploded ordnance (UXO) and non-UXO items were placed on a beach on the island of Rømø (Denmark) in a 600 m × 100 m area. Scalar magnetic anomalies were measured at 3–5 m altitude using an uncrewed aerial vehicle (UAV), towing a bird with a three-sensor triangular configuration to achieve a dense coverage with flight lines of 2 m spacing. The triple-sensor data set is utilized in a probabilistic inversion setup to infer the magnetic moments of the 24 targets. The purpose of the study, is to try and distinguish between different types of ferromagnetic objects (UXO, non-UXO) using magnetic anomaly data. The inversion methodology uses different forward models (prolate spheroids, rectangular prisms) to infer target shape, size and orientation in an attempt to discriminate between UXO and non-UXO items. Stochastic inversions are carried out using different prior assumptions of remanent magnetization strength (10, 50 and 80 per cent) of the induced dipole moment. Among the three levels of remanent magnetization strength in the prior, only some cases of discrimination seem evident for the lowest strength of remanence. One item is correctly classified as a true-negative (i.e. non-UXO) when assuming low remanent magnetization strength (10 per cent of the induced moment). However, at low remanent strength, one false-negative classification emerges, making any discrimination unreliable when assuming such low remanent magnetization. In addition to the discrimination study, different covariance models are utilized to optimize the inversion by addressing correlated errors and noise in the triple-sensor data set. Three covariance models are tested to try and account for spatially correlated noise and potential errors among the three sensors of each overflight. In many cases, the covariance models presented show a potential increase in sampling efficiency and consistency between data and the noise model, suggesting a more robust approach to a noise model in magnetic anomaly inversions. If the noise model is poor, however, it may bias the results by addressing the anomaly signal as noise. The inversions with correlated noise models are compared with inversions using a simple uncorrelated noise model. For several cases of data anomalies, differences between the inversion estimates when using correlated and uncorrelated noise models were evident, indicating that some bias may appear when assuming uncorrelated noise. Due to the general high presence of correlated signals in magnetic survey data, correlated noise models can significantly improve the overall uncertainty estimate of the estimated dipole moment. The study demonstrates, in terms of the 24 targets considered, that discrimination between UXO and non-UXO using magnetics is difficult. However, when using scalar magnetic data of high quality and resolution, the estimated dipole moments are often well resolved and uniquely defined in magnitude and position. This could provide valuable posterior information for future inversion studies by building a library of inferred magnetic moments from targets that have been found and inspected.

In magnetic prospecting, the total field anomaly formula that represents the projection of the magnetic anomaly vector on the geomagnetic field is widely used because it simplifies the calculation of forward modelling and inversion of magnetic data. However, the projection anomaly yields errors relative to the true observed magnetic anomaly, especially for high-amplitude magnetic anomalies such as in iron orebody and unexploded ordnance prospecting. In this study, we analyse the difference between the projection anomaly and observed modulus difference anomaly with physical parameters, and propose to directly invert for the modulus difference anomaly by constructing a nonlinear matrix equation between the model corrections and data corrections. The inversion is then implemented using a preconditioned conjugate gradient algorithm. Synthetic and field magnetic data were used to test the inversion method. Comparison of the two types of total field anomalies shows that the error of the projection anomaly increased with increasing total-field magnetic anomaly. When the total-field magnetic anomaly was < 5,000 nT, the difference between the projection anomaly and modulus difference anomaly results can be ignored. For high-amplitude magnetic anomalies, the modulus difference anomaly inversion produced more accurate representations of both the shape and location of the magnetic sources.

Gravity and magnetic data provide complementary information on sedimentary basins and basement/cover relations. Magnetic anomalies are usually dominated by magnetic sources in the basement and, to a lesser extent, by volcanics and associated intrusives in the sedimentary sequence. In contrast, gravity anomalies reflect changes within the sedimentary sequence as well as basement. Where the 'basement' residual gravity anomaly can be extracted, the complementary nature of gravity and magnetic data can be used to recognize composite basement geophysical domains based on uniform magnetization/density characteristics. Comparison of the vertically integrated, 3D analytic signal with reduction to the pole or pseudo-gravity data helps to overcome some of the ambiguity. Pre-processing of magnetic and gravity anomaly data is essential so that a direct comparison of the two anomalous fields is meaningful. Total magnetic intensity anomalies are related to the gravity gradient by Poissons Relation. Magnetic anomalies are differentiated one order further than gravity anomalies, therefore magnetic anomalies should be compared with the gravity gradient. It is important to try to center magnetic anomalies over the source either by reduction to the pole or analysis of integrated 3D analytic signal. It is also important that the two anomalous fields have similar wavelength contents, therefore, pre-filtering is usually required. Separation or layer filtering allows the effects of shallow sources to be removed, therefore the magnetic and gravity signatures of deeper layers can be recognized. There are a number of ways to make a direct comparison of reduced-to-the-pole magnetic data and gravity gradient data, including global correlation, local window correlation and coherency analysis. These techniques have been applied to the northern margin of the Canning Basin in NW Australia. This is a difficult area for potential field interpretation as the relationships between magnetic anomalies, gravity anomalies and basin structures are unclear. Gravity interpretation is complicated by a high degree of isostatic compensation as a result of crustal thinning. Pre-processing of the magnetic and gravity data was effective, resulting in a high degree of correlation of the two anomalous fields over much of the area, but also significant differences. The global correlation method of comparing the two anomalous fields gave the best results and improved recognition of subtle trends. The magnetic/gravity correlation data provided good resolution of the major fault systems bounding the Fitzroy Trough, the main depocentre. The data also show internal, east-west structures in the Fitzroy Trough, parallel to the major folds, of Jurassic age. A number of cross-cutting trends are interpreted as basement faults and some of these appear to be previously unrecognised.

Geophysical survey with magnetic method to interpret the iron ore deposits in the Eastern Nusawungu Coastal, Cilacap Regency, Central Java, Indonesia was carried out during six month, i.e. March –August 2017, covering the area in the geographical position of 109.3462° – 109.3718° E and 7.6958° – 7.7098° S. This survey has produced total magnetic field strength data at each measuring point in the research area. The magnetic field strength data which have been obtained, then be processed, corrected, and mapped so that the local magnetic anomaly contour map can be obtained. The local magnetic anomaly contour map shows the distribution of magnetic anomalous sources in the subsurface of research area. The 2D-modeling of magnetic anomalies data has been carried out along the AB trajectory extending on the local magnetic anomaly contour map from the position of A(109.3463°E and 7.7023°S) to B (109.3688°E and 7.7053°S), so that some subsurface anomalous objects is obtained. The modelling results of magnetic anomalies data show that the research area is estimated to have the potential of iron ore deposits. The subsurface rocks deposits containing iron ore are estimated to be located below the AB trajectory with a length about of 164.85 meters, a depth ranging of 1.709 – 31.909 meters, and a magnetic susceptibility value of 0.0122 cgs unit. These rocks are interpreted as sand deposits which coexists with silt and clay containing iron ore grains from the alluvium formation. Further, iron ore is also estimated to be present in the rocks deposits below the AB trajectory which have a depth of 24.405 – 49.809 meters and 3.989 – 11.111 meters, with the magnetic susceptibility values of 0.0093 and 0.0073 cgs units.

Long, structurally undeformed north–south trending structures show no magnetic anomaly at the magnetic equator, except at the north and south truncations of the structure. However, folding, faulting, differential erosion or other structural deformation can produce detectable magnetic anomalies in a generally north–south trending equatorial structure. Spatial variation in magnetic susceptibility or remanent magnetization can also produce anomalies in equatorial north–south structures. These anomaly patterns are often more complicated than patterns produced by similar structures at high latitudes, but interpretational insight can be gained through numerical modelling of common structures. Reduction‐to‐pole and analytic signal filters can aid in interpretation of equatorial anomalies, but these must be applied carefully because of instabilities deriving from filter design and noise amplification.

Inversion of magnetic data has long been hampered by the need to specify the direction of magnetization. We present a general approach that utilizes the minimal dependence on magnetization direction of amplitude and total gradient data and thereby overcome the difficulty associated with inversion when unknown remanent magnetization is present. To construct the inversion algorithm for the magnitude of magnetization, we solve a nonlinear minimization problem formulated using Tikhonov regularization. A positivity constraint is also incorporated to improve the solution. The algorithm will be illustrated with both synthetic and field data sets.