Preliminary geological interpretation of long-wavelength magnetic anomalies over China and surrounding regions

Long-wavelength(>500 km)magnetic anomalies originating in the lithosphere were first found in satellite magnetic surveys.Compared to the striking magnetic anomalies around the world,the long-wavelength magnetic anomalies in China and surrounding regions are relatively weak.Specialized research on each of these anomalies has been quite inadequate;their geological origins remain unclear,in particular their connection to tectonic activity in the Chinese and surrounding regions.We focus on six magnetic high anomalies over the(1)Tarim Basin,(2)Sichuan Basin(3)Great Xing’an Range,(4)Barmer Basin,(5)Central Myanmar Basin,and(6)Sunda and Banda Arcs,and a striking magnetic low anomaly along the southern part of the Himalayan-Tibetan Plateau.We have analyzed their geological origins by reviewing related research and by detailed comparison with geological results.The tectonic backgrounds for these anomalies belong to two cases:either ancient basin basement,or subduction-collision zone.However,the geological origins of large-scale regional magnetic anomalies are always subject to dispute,mainly because of limited surface exposure of sources,later tectonic destruction,and superposition of multi-phase events.

An alternative 2D ICEEMDAN-based denoising method and its application in processing magnetic anomaly data

Due to environmental noise and human factors,magnetic data collected in the field often contain various noises and interferences that significantly affect the subsequent data processing and interpretation.Empirical Mode Decomposition(EMD),an adaptive multiscale analysis method for nonlinear and non-stationary signals,is widely used in geophysical and geodetic data processing.Compared with traditional EMD,Improved Complete Ensemble EMD with Adaptive Noise(ICEEMDAN)is more effective in addressing the problem of mode mixing.Based on the principles of 1D ICEEMDAN,this paper presents an alternative algorithm for 2D ICEEMDAN,extending its application to two-dimensional scenarios.The effectiveness of the proposed approach is demonstrated through synthetic signal experiments,which show that the 2D ICEEMDAN exhibits a weaker mode mixing effect compared to the traditional bidimensional EMD(BEMD)method.Furthermore,to improve the performance of the denoising method based on 2D ICEEMDAN and preserve useful signals in high-frequency components,an improved soft thresholding technique is introduced.Synthetic magnetic anomaly data testing indicates that our denoising method effectively preserves signal continuity and outperforms traditional soft thresholding methods.To validate the practical application of this improved threshold denoising method based on 2D ICEEMDAN,it is applied to ground magnetic survey data in the Yandun area of Xinjiang.The results demonstrate the effectiveness of the method in removing noise while retaining essential information from practical magnetic anomaly data.In particular,practical applications suggest that 2D ICEEMDAN can extract trend signals more accurately than the BEMD.In conclusion,as a potential tool for multi-scale decomposition,the 2D ICEEMDAN is versatile in processing and analyzing 2D geophysical and geodetic data.

Identification of the Caroline Plate boundary:constraints from magnetic anomaly

The Caroline Plate is located among the Pacific Plate,the Philippine Sea Plate,and the India Australia Plate,and plays a key role in controlling the spreading direction of the Philippine Sea Plate.The Caroline Submarine Plateau(or Caroline Ridge)and the Eauripik Rise on the south formed a remarkable T-shaped large igneous rock province,which covered the northern boundary between the Caroline Plate and the Pacific Plate.However,relationship between these tectonic units and magma evolution remains unclear.Based on magnetic data from the Earth Magnetic Anomaly Grid(2-arc-minute resolution)(V2),the normalized vertical derivative of the total horizontal derivative(NVDR-THDR)technique was used to study the boundary of the Caroline Plate.Results show that the northern boundary is a transform fault that runs 1400 km long in approximately 28 km wide along the N8°in E-W direction.The eastern boundary is an NNW-SSE trending fault zone and subduction zone with a width of tens to hundreds of kilometers;and the north of N4°is a fracture zone of dense faults.The southeastern boundary may be the Lyra Trough.The area between the southwestern part of the Caroline Plate and the Ayu Trough is occupied by a wide shear zone up to 100 km wide in nearly S-N trending in general.The Eauripik transform fault(ETF)in the center of the Caroline Plate and the fault zones in the east and west basins are mostly semi-parallel sinistral NNW-SSE–trending faults,which together with the eastern boundary Mussau Trench(MT)sinistral fault,the northern Caroline transform fault(CTF),and the southern shear zone of the western boundary,indicates the sinistral characteristics of the Caroline Plate.The Caroline hotspot erupted in the Pacific Plate near the CTF and formed the west Caroline Ridge,and then joined with the Caroline transform fault at the N8°.A large amount of magma erupted along the CTF,by which the east Caroline Ridge was formed.At the same time,a large amount of magma developed southward via the eastern branch of the ETF,forming the northern segment of the Eauripik Rise.Therefore,the magmatic activity of the T-shaped large igneous province is obviously related to the fault structure of the boundary faults between the Caroline Plate and Pacific Plate,and the active faults within the Caroline Plate.

Investigation of Potential Factors on South Atlantic Magnetic Anomaly

The first part of this investigation analyzes the deep earthquake occurrences in Nazca subducting under South America. The depth taken is to get information about possible influences from the unknown materials and formations under the crust. The results revealed the presence of malleable material, which is unbreakable and, therefore, unable to trigger earthquakes. The structure of those elements is diamagnetic, attracting ionized particles from the Van Allen belt region in the ionosphere. The charged particles travel towards Earth’s surface, enhanced during the geomagnetic storms. The South Atlantic Magnetic Anomaly (SAMA) found that the deformation suffered by the anomaly moving from South Africa to South America is, possibly due to a bulge of unknown flexible material buried underneath the oceanic and continental crust. The continental part is strengthening in weakness because the background also has a high amount of diamagnetic material in this region, and it would not happen over the Atlantic Ocean, where part of the deformation is placed.

Interpretation of magnetic anomalies by horizontal and vertical derivatives of the analytic signal

Magnetic anomalies are often disturbed by the magnetization direction, so we can＇t directly use the original magnetic anomaly to estimate the exact location and geometry of the source. The 2D analytic signal is insensitive to magnetization direction. In this paper, we present an automatic method based on the analytic signal horizontal and vertical derivatives to interpret the magnetic anomaly. We derive a linear equation using the analytic signal properties and we obtain the 2D magnetic body location parameters without giving a priori information. Then we compute the source structural index （expressing the geometry） by the estimated location parameters. The proposed method is demonstrated on synthetic magnetic anomalies with noise. For different models, the proposed technique can both successfully estimate the location parameters and the structural index of the sources and is insensitive to noise. Lastly, we apply it to real magnetic anomalies from China and obtain the distribution of unexploited iron ore. The inversion results are consistent with the parameters of known ore bodies.

Regional magnetic anomaly fields:3D Taylor polynomial and surface spline models

We used data from 1960.0,1970.0,1980.0,1990.0,and 2000.0 to study the geomagnetic anomaly field over the Chinese mainland by using the three-dimensional Taylor polynomial（3DTP） and the surface spline（SS） models.To obtain the pure anomaly field,the main field and the induced field of the ionospheric and magnetospheric fields were removed from measured data.We also compared the SS model anomalies and the data obtained with Kriging interpolation（KI）.The geomagnetic anomaly distribution over the mainland was analyzed based on the SS and 3DTP models by transferring all points from 1960.0-1990.0 to 2000.0.The results suggest that the total intensity F anomalies estimated based on the SS and KI for each year are basically consistent in distribution and intensity.The anomalous distributions in the X-,Y-,and Z-direction and F are mainly negative.The 3DTP model anomalies suggest that the intensity in the X-direction increases from-100 nT to 0 nT with longitude,whereas the intensity in the Y-direction decreases from 400 nT to 20 nT with longitude and over the eastern mainland is almost negative.The intensity in the Z-direction and F are very similar and in most areas it is about-50 nT and higher in western Tibet.The SS model anomalies overall reflect the actual distribution of the magnetic field anomalies;however,because of the uneven distribution of measurements,it yields several big anomalies.Owing to the added altitude term,the 3DTP model offers higher precision and is consistent with KI.

Thermal structures of the Pacific lithosphere from magnetic anomaly inversion

Of the world's oceans, the Pacific has the most abundant distribution of seamount trails, oceanic plateaus and hot spots, and has the longest fracture zones. However, little is known of their thermal structures due to difficulties of heat flow measurement and interpretation, and in inferring thermal anomalies from low-resolution seismic velocities. Using recently published global magnetic models, we present the first independent constraint on Pacific geothermal state and mantle dynamics, by applying a fractal magnetization inversion model to magnetic anomaly data. Warm thermal anomalies are inferred for all known active hot spots, most seamount trails, some major fracture zones, and oceanic lithosphere between ~100 and ~140 Ma in age. While most Curie points are among the shallowest in the zone roughly bounded by the 20 Ma isochrons, abnormally deep Curie points are found along nearly all ridge crests in the Pacific, related to patchy, long-wavelength and large-amplitude magnetic anomalies that are most likely caused by prevailing magmatic or hydrothermal processes. Many large contrasts in the thermal evolution between the Pacific and North Atlantic support much stronger hydrothermal circulation occurring in Pacific lithospheres younger than ~60 Ma, which may have disguised from surface heat flow any deep thermal signatures of volcanic structures. Yet, at depths of the Curie points, our model argues for warmer Pacific lithosphere for crustal ages older than ~15 Ma, given a slightly higher spatial correlation of magnetization in the Pacific than in the North Atlantic.

Linear magnetic anomalies and tectonic development of the middle Okinawa Trough

By analyzing the magnetic anomalies,the linear magnetic anomalies in the middle Okinawa Trough are identified.It means that the crust along the spreading axis is broken,and new oceanic crust is formed.Geophysical data have revealed that a model of three extensive episodes occurs in the Okinawa Trough,which can be named as“doming episode” from the Middle to Late Miocene(Phase I),the episode from the Pliocence to Early Pleistocene(Phase Ⅱ),and the recent“spreading episode”(Phase Ⅲ).The magnetic anomalies in the middle Okinawa Trough are very similar to those found in the middle Red Sea,indicating that the Okinawa Trough is developing towards the “Red Sea stage”.Similar to the Red Sea,there are a “main trough”and a “axial trough”in the Okinawa Trough.

Origin of Phosphorite Nodules of Lebedinsky Iron Deposit in Kursk Magnetic Anomaly (KMA) of the Russian Platform

Phosphorite has become increasingly important as the raw material for phosphatic fertilizer. Phosphorite nodules are widespread in the Kursk complex deposit (Russian platform). Genesis of these nodules has long been a matter of debate, and this has hampered understanding of the mechanism and controls in the formation of the nodules. In this paper, we report the petrographical, mineralogical, and geochemical data of the Lebedinsky phosphorite nodules. Petrographic study reveals complex phosphatic cement resulting in the replacement of apatite minerals around quartz grains. The main mineral composition consists of quartz, phosphate minerals (apatite, francolite mainly), feldspar, iron hydroxides and carbonate minerals. These results, when combined with available data, are used to address the origin of the phosphorite nodules. The nodules are characterized by the universal presence of biogenic and chemical signatures which is phosphorus crown around crystal grains. The structure of the nodules is massive. Their texture is depicted by basaltic cement and concretion, which consists mainly of apatite and its varieties, with general formula: Ca_10 (PO_4, CO_3) 6(F,OH,Cl). Variation of mineralogy appears dependent on geological setting. Microscopic observations of biogenic fossils in Lebedinsky phosphorite favor a chemical and biogenic origin of phosphorites. Weathering has been suggested to be capable of liberating as much as 20 %-35 % P_2O_5 from sedimentary rocks. Previous investigations demonstrate that weathering of the Proterozoic substratum was the main cause in the formation of Fe, Mo, Mn, Pb, Zn, and P in the Russian platform. We therefore suggest that both weathering and biochemical processes have been positive in the formation of the Lebedinsky phosporite nodules. However, whether continental weathering or oceanic bio-chemical processes are more relevant in the phosphorite accumulation remains undetermined.

Serpentinized Peridotite as Source of Aeromagnetic Anomalies

The source of long-wavelength aeromagnetic anomalies appears to originate from the earth's deep crust. Constrained by previous studies on geochemical, petrologic analysis, the eclogite and serpentinized peridotite samples from drill hole ZK703 at Donghai in the western Sulu ultrahigh-pressure (UHP) terrane, East China, were unambiguously exhumed from the lower crust and the upper mantle, providing significant information about the magnetic properties of rocks at a deeper part of the crust. Results show that the serpentinization process favors the neoformation of nearly stoichiometric magnetite, resulting in the enhancement of its magnetization up to 8.6 A/m, which is sufficient enough to contribute to some magnetic anomalies. In contrast, eclogite samples have only weaker magnetization (generally less than 0.05 A/m) compared to serpentinized peridotite. Nevertheless, experiments under the lower crustal conditions are necessary to further support these conclusions.

An analysis of the characteristics of crustal magnetic anomaly in China based on CHAMP satellite data

Based on the observation data of CHAMP satellite from 2006 to 2009, a 2D crustal magnetic anomaly model in China is established to study the distribution characteristics of crustal magnetic anomaly. In this paper, the 2D anomaly model is derived from the Legendre polynomial expansion of harmonic term N =6-50. The result shows that many elaborate structures reflected in magnetic anomaly map well correspond to the geologic structures in China and its adjacent area. The magnetic anomaly at low satellite height behaves complexly, which is mainly caused by the magnetic disturbance of shallow rocks.In contrast, the magnetic field isolines at high satellite height are relatively sparse and only magnetic anomalies of deep crust are reflected. This fact implies that the 2D model of crustal magnetic anomaly provides an important method of the space prolongation of geomagnetic field, and is of theoretical and practice importance in geologic structure analysis and geophysical prospecting.

Evaluation and extraction of weak gravity and magnetic anomalies

In this paper, I introduce what are called weak gravity and magnetic anomalies and propose standards for estimating their reliability. I also introduce new techniques for processing this kind of weak anomaly. These techniques consist of interference elimination and weak signal extraction. Practical applications have proved their effectiveness, Weak gravity and magnetic anomalies will get more attention with the development of targeted exploration.

Structural Mapping of Kakobola and Its Surroundings by Analyzing Geomagnetic Data

This study focuses on the Kakobola region and its surroundings where cavities discovered in its basement may represent a major risk for the hydroelectric dam erected on the Lufuku River near the Kakobola city and the civil engineering works in the study area. In order to deepen the studies related to this understudied region and provide decision-makers with information that will enable them to make the necessary and appropriate decisions regarding the development of this area, a study based on the analysis of geomagnetic data was carried out using certain methods revealing more shallow than deep structures, and others highlighting the limits of both shallower and deeper structures. Total magnetic anomalies and reduced to equator (RTE) magnetic anomalies were used to map the subsurface of the Kakobola region and its surroundings. In order to detect the edges of magnetized structures, the horizontal gradient magnitude (HGM), the analytic signal (AS), the horizontal gradient of tilt angle (HGTA), the tilt angle (TA), the theta map (TM), the enhanced total horizontal derivative of the tilt angle (ETHDR), the tilt angle of the horizontal gradient (TAHG), and the tilt angle of analytic signal (TAAS) were used. The study area is characterized by two areas of low values of magnetic anomalies and two other sources of high magnetic anomalies located in the bed and the neighborhood of the two major rivers in the region. The shallow sources of magnetic anomalies are lying in the bed and the vicinity of the same rivers in the study area. The magnetic sources in the study area are connected and almost linear. Several magnetic lineaments identified in this region by different methods present several preferential directions, but the most predominant directions are NE-SW, NW-SE, W-E and NE-SW.

Results of Geomagnetic Studies on the Problem of Forecasting Strong Earthquakes in Uzbekistan

The article is devoted to the problem of forecasting strong earthquakes by the geomagnetic method. The geomagnetic method is widely used on this problem in seismically active regions of the world as one of the promising, informative and operational geophysical methods. The results of long-term geomagnetic studies on the problem of forecasting strong earthquakes in Uzbekistan are presented. Geomagnetic studies were carried out on the territories of the Tashkent, Ferghana, and Kyzylkum geodynamic polygons in the epicentral zones of strong earthquakes that occurred. Long-term, medium- and short-term precursors of earthquakes have been identified. Anomalous changes in the geomagnetic field associated with the decline in aftershock activity were also revealed. The dependence between the duration of the manifestation of long-term magnetic precursors and the magnitude of earthquakes is determined. Absolute proton magnetometers MMP-1, MV-01 (Russia), and G-856 (USA) were used to measure the geomagnetic field.

Combined analysis of gravity and magnetic anomalies using normalized source strength

The traditional combined gravity and magnetic analysis uses the linear regression of the first order vertical derivative of the gravity anomaly and the reduction to the pole(RTP) magnetic anomaly,and provides the quantitative or semi-quantitative interpretation by calculating the correlation coefficient,slope,and intercept.In the calculation process,due to the remanent magnetization,the RTP anomaly still contains the effect of oblique magnetization,as a result,the homologous gravity and magnetic anomalies may display irrelevant results in the linear regression calculation.To solve this problem,we present a new combined analysis using normalized source strength(NSS).Based on the Poisson's relation,the gravity field can be transformed into the pseudomagnetic field of the direction of geomagnetic field magnetization under the homologous condition.The NSS of the pseudomagnetic field and that of the original magnetic field are calculated,which are insensitive to the remanence,and then the linear regression analysis is carried out.The approach is tested using synthetic model under complex magnetization,the results show that it can still identify the gravity and magnetic anomalies from same source under strong remanence,and can establish the Poisson's ratio.Finally,this approach is applied in Wudalianchi in China.The results demonstrated that this approach is feasible and can provide the reference for further data processing and interpretation.

A Possible Cause of Magnetic Anomalies on the Southern Martian Hemisphere

From the topology of a synthetic aurora map, we propose a mechanism for the magnetic anomalies on the southern martian hemisphere, i.e., impacts by asteroids when the dynamo is active. The quasi concentric circles of aurora suggest that there are two-to-three convectional cells for each impact. The whole synthetic aurora is induced by three major impacts of asteroids. The east-west lineation features of crust magnetizations are due to the east-west trending locations of three impacts. The alternatively changed sign of crust magnetization originates from the alternatively changed flow direction on the tops of adjacent convectional cells.

Study of Role of Magnetic Resonance Urography (MRU) and Comparison with Conventional Radiology in the Diagnosis of Complex Renal Anomalies: A Tertiary Care Centre Experience

Anatomical renal anomalies are one of the most commonly occurring renal anomalies. Most of these anomalies require series of investigations for the proper diagnosis. We tried to compare the results of conventional radiological investigations with magnetic resonance urography in the diagnosis of complex renal anomalies. Materials and methods: This was a prospective study done over a period of five years from 2006 to 2011. All the patients with suspected or diagnosed presence of renal anomalies were investigated by ultrasound (USG), Intravenous urography (IVU), micturating cystourethrogram (MCU), magnetic resonance urography (MRU), retrograde urethrography, cystogenitoscopy, renal scans as per the indication in the case. Results: Total sixty three patients were studied over the period. The most common complex renal anomaly diagnosed was duplex system and conventional radiological investigations were useful in the diagnosis of less than 50% patients. Female patients outnumbered male patients in having complex renal anomalies. MRU was diagnostic in most of the patients with such anomalies and excellent mode of investigation for functional and anatomical details. Conclusion: MRU is better than conventional radiological investigations in the diagnosis of complex renal anomalies.

Using Magnetic Method for the Identification of Anomalies Due to Kimberlite Pipes, Luando Area, Bié, Angola

One of the measurement geophysical methods to investigate kimberlite pipes is by using the magnetic method. The acquired field data in this study uses two proton-precession magnetometers for the mapping of magnetic anomalies due to kimberlites. Three different magnetic maps are obtained from the result of total magnetic field data processing on Oásis Montaj software programme. These maps include magnetic anomaly maps through statistical analyses, total magnetic field intensity map and map of the analytic signal. Based on the interpretation of these maps a structure is identified with SWW-NEE directions in which magnetic signatures that indicate the presence of kimberlite pipes are observed. As the interpretation of the magnetic anomalies is a complicated process due to their dipolar nature, the analytic signal is generated, where is possible to observe the typical shape of these anomalies.

Magnetic Anomaly Interpretation of the Northern Congo Craton Boundary: Results from Depth Estimation and 2.5D Modeling

A magnetic-based geophysical study was performed across the southern part of Cameroon to investigate the boundary between the Archean Congo craton and the Pan-African metamorphic belt. Magnetic gradient techniques including Euler deconvolution and Tilt derivative have been applied to an aeromagnetic data profile to determine the depth of sources and their lateral extension. 2.5D magnetic modeling shows that the prominent magnetic positive anomalies observed on total magnetic map of south Cameroon are produced by deep and strongly magnetic bodies under the Pan-African formations mainly an important dyke formation structure with a high susceptibility of 0.041 (SI units), at an average depth of 4148 m and with a lateral extension of about 10 km. These bodies are interpreted to have emplaced at high crustal levels in a continental collision zone and were subsequently metamorphosed at granulite grade conditions, during the Pan-African orogeny about 620 Ma ago.

A two-dimensional lithospheric magnetic anomaly field model of Egypt using the measurements from Swarm satellites

We use magnetic field data observed by the Swarm mission from 2014 to 2020 to construct,for the first time,a two-dimensional(2 D)lithospheric magnetic anomaly model of Egypt and its surrounding area.Nighttime data during quiet geomagnetic conditions has been expanded in terms of the Legendre polynomial in harmonic terms N=6-50.The damped least square method has been used to estimate the model coefficients based on the lithospheric magnetic data.Modeled data at two different altitudes(438-448 km and 503-511 km)were compared with the CHAOS model.Results exhibit that the 2 D model is superior to the CHAOS model in the capability of extracting more information about small-scale crustal anomaly field.At low altitudes(438-448 km),the strength of the anomaly field increases,but the noise of the external fields has greatly reduced at high altitudes(503-511 km).Besides,the magnetic anomaly field at low altitudes has illuminated short-scale anomalies that didn’t appear at high altitudes.Both the total and vertical magnetic anomaly vectors showed their ability to reveal tectonic structures compared with Moho depth map and the geological maps.