Reduction To Pole Research Articles

New subsurface structural insights of Northeast Vietnam: Advanced implications from high-resolution magnetic data

Northeast Vietnam is covered by sedimentary rocks, and its subsurface structures can be determined by geophysical studies. However, the structural patterns in Northeast Vietnam have so far been poorly studied and there is a lack of addressing the complex geological features of this area. Outlining such geological features is commonly achieved by applying various edge detection methods to magnetic fields. In this research, we applied nine enhancement techniques to magnetic data of Northeast Vietnam to identify geological features in this area. The methods are tested on synthetic data before applying to RTP (reduction to pole) magnetic data that are obtained from the multiple stage RTP technique. Four semiautomated depth estimation methods are also used to obtain the depth of magnetic structures in Northeast Vietnam. The results indicate that the normalized total gradient and power transform of the tilt angle of the total horizontal gradient can produce geological features more clearly than others. The findings show that the study area is dominated by NE-SW and ENE-WSW trending lineaments, and most of magnetic sources are around 1–2.5 km deep in the area. The NE-SW and ENE-WSW trending structures are possibly related to the left-lateral motion along the Ailaoshan–Red River shear zone. Our findings also show that Northeast Vietnam may be considered as a southwestern continuation of the South China Block. Our results provide a better picture of the structural features of Northeast Vietnam and overcome the gap in understanding the tectonic and geological framework of this important area.

Aeromagnetic data of the Kelâat M'Gouna inlier (Jbel Saghro, Eastern Anti-Atlas, Morocco): Geotectonic and mining implications

The Kelâat M'Gouna inlier (KM'G) is located in the Eastern Anti-Atlas of Morocco, north of the West African Craton (WAC). It offers outcrops of great lithological diversity and has a polycyclic tectonic history. Its bedrock bears traces of the Pan-African orogeny, and its Adoudou-Cambrian cover was affected by the Hercynian orogeny. Therefore, the KM'G inlier is a privileged geological region for a geo-structural study to evaluate its mining potential. In this study, high-resolution aeromagnetic and surface geological data were used to map the structural architecture of the inlier and determine its relationship to the spatial distribution of mineral deposits and showings. Several filters and operators such as Reduction to Pole (RTP), Upward Continuation (UC), Centre for Exploration Targeting (CET) and Tilt Derivative (TDR) were applied to the aeromagnetic data to reveal important magnetic signatures such as magnetic lineaments/domains in the study area. The TDR and UC subdivided the area into two magnetic morpho-structural domains: to the north, a domain dominated by negative anomalies (labeled NMA1, NMA2 and NMA3), which coincide with the Sidi Flah fault and the Azlag and Ibantarn granites; to the south, a domain dominated by positive anomalies (labeled PMA4 and PMA6) which are deeply rooted. CET and TDR have been applied to map structural complexity and depth estimation. Four main magnetic lineament trends were deduced (NE-SW, Sub E-W, NW-SE, and Sub N–S) with variable depths ranging from 45 to 2120 m. A thorough structural scheme was developed based on the CET and TDR outputs and surface geological data. This scheme shows that the main deposits and mineral occurrences are known from the KM’G geological map are in areas with high magnetic lineament density values. Thus, several potential mineralization zones in the study area have been delineated. These results should guide the exploration of possible metal deposits in the KM’G inlier.

Structural implications of potential field data on Southeastern North America

Abstract The fault system of Eastern Piedmont could be extensive in the East of the USA. Debates remain regarding the sutures zone, contacts and faults between terranes, especially underneath the coastal sediment. However, in this study, a new interpretation of the structures and contacts of the southeastern margin was based on regional land gravity anomaly and magnetic anomaly maps. To delineate and investigate the subsurface geological structures in the southern Appalachian belt and underneath the coastal sediment that covers the southeastern half of the study area, the gravity and magnetic maps were subjected to several filter techniques. The anomalies maps were enhanced by applying the reduction to pole (RTP), analytical signal (AS), tilt derivative (TDR), horizontal gradient (HG), direction filter and power spectrum techniques. The power spectrum filter was applied to separate the regional-residual anomaly. The results of regional anomaly maps display elongate high amplitude anomalies lie in the south that are related to deep-seated igneous mafic intrusive and basaltic lavas emplacement. The directional filter was used to eliminate the sutural trend of the Jurassic dikes that intruded the study area. The result displays the structural contacts clearly after removing the anomalies of the Jurassic. Finally, the edge detection filters (TDR, HG, AS) from the gravity and magnetic maps helped in mapping the anomaly contact of the subsurface bodies. The apparent structural signature of the interpreted contacts confirmed the presence of these structural features in all edge detection methods.

Identification of Gold Mineralization Zones Using Magnetic Data at Gunung Gupit Area, Magelang, Central Java

Indonesia is one of the biggest gold producing countries in the world. the discovery of new gold reserves makes the passion of the gold mining industry increase. The Gunung Gupit area of Magelang Regency in Central Java has discovered new gold potential, but has not been thoroughly explored. The magnetic method is applied to identify and determine the distribution of the presence of gold mineralized zones based on the magnetic properties of rocks. A total of 1001 points on 41 magnetic line were acquired using Proton Precession Magnetometer (PPM) Geometrics model G-856 in an area of 2.5 km2. Field data is obtained as Magnetic Field Anomaly. Reduction to pole (RTP) and low-pass filter process is applied to the magnetic anomaly. The results showed the Gunung Gupit area was divided into 4 zones based on magnetic anomaly responses, i.e strong, medium, weak anomaly and igneous rock zones. The 2.5 D modeling result shows the presence of northwest-southeast normal fault and strike slip fault that is perpendicular to the normal fault. These faults control the hydrothermal alteration in Gunung Gupit and its surroundings. A strong mineralization zone lies between these two faults. The presence of gold and other metal minerals lies in this strong mineralization zone, and moderate alteration. The coverage area is quite wide and distributed along the two faults.

Archaeological Investigation at the Urartian Sites of Bastam Castle Using Magnetics Method

Archaeological Investigation at the Urartian Sites of Bastam Castle Using Magnetics Method

New insights on Kimberlite emplacement around the Bundelkhand Craton using integrated satellite-based remote sensing, gravity and magnetic data

The present study attempts a conjugate analysis of satellite-based remote sensing, gravity, and magnetic data for evaluation of structures and tectonic for new insights on Kimberlite emplacement around the Bundelkhand Craton. Initially, Landsat Operational Land Imager (OLI) data has been enhanced using high-pass filtering for the delineation of lineaments. A combined lineament density map has been prepared from the processed Landsat image, Bouguer gravity, and RTP (Reduction to pole) magnetic data, which are further correlated with the reported Kimberlite/Lamproite pipes. The depth to the lithospheric Mantle of the study area has been estimated from EIGEN6C4 Bouguer gravity data using the Parker-Oldenburg algorithm. The association of the reported Kimberlite/Lamproite pipes in the study area have been analyzed with the i) composite lineament density map, ii) different mantle blocks of high and low Moho depths, iii) lithospheric mantle density, and iv) tectonic setting. Some new potential zones for Kimberlite/Lamproites emplacement have been identified based on the integrated analysis of the above mentioned parameters.

DELINEATION OF SUBSURFACE STRUCTURES OF SEMANGGOL FORMATION, NORTH PERAK USING MAGNETIC DATA

The Semanggol Formation has been studied widely and published by many, but only focus on areas of northern and southeast Kedah. Limited data have been studied and published on Perak area, especially Gunung Semanggol. The research is aimed to augment the presently limited knowledge on the hill, particularly the subsurface structure. In addition, new lithology such as a breccia formation with tens of meters width of what appears to be a breccia granitic matrix was identified in an abandoned quarry. Therefore, a study on the application of geophysical methods covering the Semanggol Formation located at Semanggol area has been carried out. The main aim of this study was to conduct a magnetic survey to study the subsurface and possibility of granitic bodies in Gunung Semanggol. The total magnetic field data were obtained by using a proton free-precision magnetometer, recording the total ground magnetic field along the transverse. There was a total of 184 magnetic stations established during the survey with a 500 m to 1 km spacing. Data reduction was processed using Oasis Montaj software to produce Total Magnetic Intensity (TMI), Reduction to Pole (RTE), Regional Anomaly and Residual Anomaly maps for qualitative interpretation. The interpreted field data gave the results of total component measurements of magnetic anomaly varying between minimum negative value around 180 nanoTesla (nT) and maximum around 510 nanoTesla (nT) as shown in TMI map. Besides, the regional magnetic map shows a gradual decrement of magnetic intensity towards east and southeast. And residual magnetic map reveals that the dominant orientation for magnetic lineaments in the NE-SW direction. However, geochemical tests have to be conducted to study the possibility of granitic bodies.

Estimating sedimentary pile thickness, structural lineaments and heat flow in parts of North Central Nigeria from aeromagnetic data

Evaluation, analysis and interpretation of aeromagnetic data acquired over parts of North Central Nigeria were carried out in order to characterise structural features and lineaments, sedimentary pile thickness, geothermal gradient and heat flow in the study area. Fourier domain digital filtering and reduction to pole (RTP) operations were done to enhance the data, while spectral analysis and source parameter imaging (SPI) were used to analyse the data. Results from the 2-D spectral analysis of the enhanced magnetic data suggest that the depth range to the top of magnetic sources Zt is 0.33–0.73 km (average depth of 0.55 km); depth range to magnetic centroid Zo is 2.51–5.83 km (average depth of 3.94 km); Curie point depth range is 4.41–10.96 km (average depth of 7.32 km); and average sedimentary rock cover thickness is 3.94 km. This compares with the results from SPI which gave depth ranges to various magnetic sources as 0.27–3.01 km. Structural lineament analysis indicates the major trend of the structural lineament is in the NE–SW, NW–SE and NNE-SSW. Average geothermal gradient of 82.87°Ckm−1 and heat flow of 207.19 mWm−2 were estimated, with a general increase in heat flow from the southern parts towards the northern parts of the study area. These relatively high geothermal gradient and heat flow may be associated with the mineralogical, thermal and tectonic history of the area, suggesting the study area has the potential of geothermal energy.

Remanent magnetization mapping: A tool for greenfields magmatic Ni-Cu-PGE exploration undercover: Part 2

Remanent magnetization dominates induced magnetization in several types of mafic-ultramafic magmatic rocks, and where it is strong, allows remote targeting of mafic lithologies based purely on their magnetization direction. We evaluate practical methods for utilising remanent magnetization as a first pass tool for Ni-Cu-PGE exploration in remote, undercover areas, using examples from the Huckitta area, Central Australia. Modeling results demonstrate that measured magnetizations can be used to accurately model target intrusions regionally. However, of the two automated methods tested for remote mapping of magnetization, neither produced results consistent with prior modeling, or with measured magnetization directions. A customised reduction to pole (RTP) grid transformation based on measured magnetization directions is a simpler, more effective method of mapping specific mafic suites regionally, and was used to guide target selection in an undercover area in the east Arunta region. Several model geometries (e.g., plunging prism, ellipsoid, etc.), constrained by the regionally dominant magnetization, were used to provide a range of end-member target bodies for evaluation. Target models’ volume and depth below surface were used to constrain estimates of economic mineral potential (i.e., volume of modeled body × conservative grade) and “minability” (distance from infrastructure + depth of overburden), as a method of vetting targets in a greenfields exploration program. This is an effective method to discard prospects with almost no chance of being economically viable, and allowed exploration to be focused on follow-up airborne electromagnetic (EM) surveys on targets in the region that were most likely to be economically significant.

Depth Estimation of Source Bodies Using 2D Magnetic Gradient Ratios

Depth estimation remains an important interpretation goal of the exploration geophysicist, especially when cover of uncertain thickness is present. There are many methods of estimating depth using magnetic data ranging from simple 1D (profile) analysis to complex 3D inversion, each with respective strengths and weaknesses. The method discussed here involves analysis of standard 2D magnetic grids.The dipolar nature of magnetic anomalies is exploited, with the location of the nearest pole in relation to the magnetic sensor the parameter estimated. For sphere-like sources the result plots the centre of the body, where-as for bodies with extensive strike in 1D (pipe) or 2D (dyke) the result plots at the pole nearest the sensor (the depth to top).Reduction to pole (RTP) filtering is performed on the standard 2D total magnetic intensity grid to make the anomalies symmetrical and centred over source bodies (assuming induced magnetism). The 1st order total horizontal gradient (THG) and vertical gradient (1VD) of the RTP grid are then generated. The ratio of 1VD/THG is then calculated and gridded with contouring used to highlight the isograd with a value of 1 (THG=1VD). The depth estimate is calculated by dividing the shortest diameter of this isograd by 2 and subtracting the ground clearance. Dip information can also be interpreted. Forward modelling of simulated data illustrates the method, proves the concept and discusses weaknesses. Several different scenarios identify the role of source topology. A real world example using open file magnetic data with known depths from drilling is then presented.

Subsurface structural features of the basement complex and mineralization zone investigation in the Barramiya area, Eastern Desert of Egypt, using magnetic and gravity data analysis

In the Barramiya area, the majority of gold deposits are generally related with the quartz veins that associated with shear zones cutting the crystalline basement rocks. The quartz vein system is controlled by shear zone and general faults. The present study is to delineate the general faults, shear zones, geological limits, and basement rock relief, using airborne magnetic and gravity data analysis at the Barramiya gold mine and surrounding area, Eastern Desert of Egypt. To achieve our goal, we have applied on magnetic and gravity data the following techniques: reduction to pole (RTP), analytical signal, tilt derivative, total horizontal derivative, 3D Euler deconvolution, downward continuation, and source parameter imagining power spectrum techniques. The analytical signal used to map the types of rock boundaries. Tilt derivative and total horizontal derivative filters helped to delineate fractures and the contact zones of the formations that host the main Barramiya shear zone. 3D Euler deconvolution techniques helped to delineate the fault trends which represented at the following direction: NNE–SSW and NNW–SSE. The average depths of both regional and residual causes have been estimated by applying downward continuation, source parameter imagining, and power spectrum techniques. According to the results of the present study, the depth of the basement rocks is relatively high (~ 80-m depth) in the western part of the study area and the basement rocks cropped out in the surface at the rest of the area. Our results are coinciding with the previous geological studies.

Geomagnetic Survey to Explore High-Temperature Geothermal System in Blawan-Ijen, East Java, Indonesia

Ijen geothermal area is high-temperature geothermal system located in Bondowoso regency, East Java. It is categorized as caldera-hosted geothermal system which is covered by quaternary andesitic volcanic rocks with steep topography at the surrounding. Several surface thermal manifestations are found, such as altered rocks near Mt. Kukusan and a group of Blawan hotsprings in the northern part of the caldera. Geomagnetic survey was conducted at 72 stations which is distributed inside the caldera to delineate the existence of hydrothermal activity. Magnetic anomaly was obtained by reducing total magnetic measured on the field by IGRF and diurnal variation. Reduction to pole (RTP) method was applied with geomagnetic inclination of about -32°. In general, the result shows that high magnetic anomaly is distributed at the boundary of study area, while low magnetic anomaly is observed in the centre. The low anomaly indicates demagnetized rock that probably caused by hydrothermal activity. It has a good correlation with surface alteration observed close to Mt. Kukusan as well as high temperature reservoir drilled in the centre of caldera. Accordingly, the low magnetic anomaly also presents the possibility of geothermal reservoir in Ijen geothermal area.

Geodynamics and underlying bedrock of the magnetically active crust layer of the Lut block, Eastern Iran

The Curie point depth map of Eastern Iran was constituted from spectral analysis of the aeromagnetic data. The reduction to pole (RTP) was applied to the magnetic anomaly data. The Curie point depth values from 165 overlapping blocks, 100 × 100 km in size, have been estimated. The Curie point depth method provides a relationship between the 2-D FFT power spectrum of the magnetic anomalies and the depth of magnetic sources by transforming the spatial data into the frequency domain. The centroid and top depth of the magnetic sources (respectively Z0 and Zt) is calculated from radially averaged log power spectrum for each block. Finally, the Curie point depth of Eastern Iran is obtained by Zb = 2Z0–Zt. The highest value of 24 km is located in eastern and western boundaries of the Lut block, and the lowest value of 12 km is located at north of study area. The shallow depths in the Curie-point depth map are well correlated with the young volcanic areas and geothermal potential fields. Geothermal gradient ranging from 24 to 45°C/km. The deduced thermal structure in eastern Iran has a relationship with orogenic collapse associated with delamination of thickened lithospheric root between the Lut and Afghan continental blocks.

Coal fire mapping of East Basuria Colliery, Jharia coalfield using vertical derivative technique of magnetic data

The present study deals with the coal fire mapping of East Basuria Colliery, Jharia coalfield, India, using the magnetic method. It is based on the fact that rise in temperature would result significant changes in magnetic susceptibility and thermo-remanent magnetization (TRM) of the overlying rocks. Magnetism increases slowly with the rise of temperature until the Curie temperature. Generally, rock/ overburden loses magnetization and becomes paramagnetic due to heating to Curie temperature, which results with significant reduction in magnetic susceptibility. However, magnetism increases significantly after cooling below the Curie temperature. Several data processing methods such as diurnal correction, reduction to pole (RTP), first and second vertical derivatives have been used for analysis of magnetic data and their interpretation. It is observed that the total magnetic field intensity anomaly of the area varies approximately from 44850 to 47460 nT and the residual magnetic anomaly varies approximately from −1323 to 1253 nT. The range of the magnetic anomaly after RTP is approximately 1050–1450 nT. About 20 low magnetic anomaly zones have been identified associated with active coal fire regions and 11 high magnetic anomaly zones have been identified associated with non-coal fire regions using vertical derivative techniques.

A strategy for magnetic data interpretation in South China Sea

SUMMARY Directly interpreting total-field magnetic anomaly data in the South China Sea (SCS) can be difficult because of the complex patterns associated with low-latitude anomaly projection and the presence of remanent magnetization. Additional difficulty arises from the fact that the ambient field direction, thus, the total-field anomaly projection direction, varies over a wide range in the area. To alleviate these difficulties, we present a strategy by using magnetic amplitude data analyses and inversion. Equivalent source processing is used to calculate the amplitude data in the space domain since the wavenumber-domain method is no longer applicable due to low and highly variable inclination. The amplitude data serve the role of reductionto-pole (RTP) transformation for structural interpretation. We then carry out the amplitude inversion to generate a 3D subsurface distribution of effective susceptibility. The inversion results show that this approach is feasible and effective in SCS.

Curie point depth from spectral analysis of magnetic data in Taiwan

This study constructs an integrated magnetic anomaly map of Taiwan and its vicinity that combines land magnetic data and marine magnetic data. The reduction to pole (RTP) correction method was applied to the integrated magnetic anomaly data. The corrected magnetic anomaly results for a region roughly 880km in width and 660km in length fit well with geological tectonic signatures and are used to estimate the Curie point isotherm. The adopted Curie point depth method provides a relationship between the 2-D FFT power spectrum of the magnetic anomalies and the depth of magnetic sources by transforming the spatial data into the frequency domain. The basal depth of the magnetic sources is calculated from the magnetic data. Finally, the Curie point depth of Taiwan is obtained. The highest value of 17km is located in northern Taiwan, and the lowest value of 6km is located at southern Taiwan and also has the highest thermal gradient of 88°C/km. The Curie point depth is consistent with heat-flow measurements with correlation coefficient 0.62.

An improved search for magnetization direction

Magnetic field interpretation is often conducted on an incorrect assumption that remanent magnetization is insignificant and that the resultant magnetization direction is in the local geomagnetic field direction. For compact anomalies various methods exist to test this hypothesis and return estimates of magnetization direction utilising trial reduction to pole (RTP) transforms. We have developed an analysis to return the magnetization direction which generates the most symmetric RTE anomaly and have shown that this approximately also matches input magnetizations of synthetic compact anomalies. Estimation of magnetization direction from elongate anomalies is more problematic and intrinsically less reliable, but nevertheless we found that we were able to recover approximate magnetization direction from these anomalies using cross-correlation of an analytic signal function computed from vertically integrated gradients (which we term the ?total vertically integrated gradient? or TVIG) with RTP and RTE grids computed for trial magnetization directions. The various methods are readily and automatically obtained from scanning TMI grids. The resulting magnetization direction estimates are empirical rather than analytic and are approximate. They are best used as initial estimates prior to application of more rigorous, manual methods.

Crustal Structure of Turkey from Aeromagnetic, Gravity and Deep Seismic Reflection Data

In this paper, aeromagnetic and gravity anomalies obtained from the General Directorate of Mineral Research and Exploration were subjected to upward continuation to 3 km from the ground surface to suppress shallow effects and to expose only regional, deep sources. Then, a reduction to pole (RTP) map of aeromagnetic anomalies was produced from the 3 km upward continued data. A sinuous boundary to the south of Turkey is observed in the RTP map that may indicate the suture zone between the Anatolides and African/Arabian Plates in the closure time of the Tethys Ocean. The sinuous boundary can be correlated with the recent palaeo-tectonic maps. The southern part of the sinuous boundary is quite different and less magnetic in comparison with the northern block. In addition, maxspots maps of the aeromagnetic and gravity anomalies were produced to find out and enhance the boundaries of tectonic units. Crustal thickness, recently calculated and mapped for the western Turkey, is also extended to the whole of Turkey, and the crustal thicknesses are correlated with the previous seismological findings and deep seismic sections. The average crustal thickness calculations using the gravity data are about 28 km along the coastal regions and increase up to 42 km through the Iranian border in the east of Turkey. Density and susceptibility values used as parameters for construction of two-dimensional (2D) gravity and magnetic models were compiled in a table from different localities of Turkey. 2D models indicate that all of the anomalous masses are located in the upper crust, and this could be well correlated with the earthquakes which occurred at shallow depths.

Conversions of Magnetic Anomaly and 3-D Inversion at Low Magnetic Latitudes

At low magnetic latitudes, the magnetization direction is almost horizontal, amplification effect of pole reduction factor causes band-shape fake anomaly along the direction of the magnetic declination. Experts have been solving the problem pole reduction at low magnetic latitudes (RTP-L). However, no greatly effective method has been found. The magnetic anomaly with a reduction procedure at low magnetic latitudes misleads the judgment of experts if the method is not well used. For making a reliable magnetic anomaly interpretation result at low magnetic latitudes, it is proposed that a pole reduction procedure can be approximately simulated by the total field multiplied by -1 in this paper. In addition, a comprehensive magnetic anomaly conversions and inversion method is assembled and restricted at low magnetic latitudes including non-linear filter, analytic signal and 3-D inversion to avoid the distortion of magnetic anomaly during reduction to pole (RTP). This method takes into account the magnetic anomaly characteristics at low magnetic latitudes. The application of this method can prevent the uncertain factor of RTP-L, give the boundary of magnetite, and provide shapes and size of magnetite effectively without involving the directions of magnetization. It made good effect in Pampa de Pongo Fe deposit in Peru which is located near the magnetic equator (inclination=5.5°). Therefore, a simple method to simulate RTP-L, combined with non-linear filter, analytic signal, 3-D inversion, can enhance the reliability and accuracy of magnetic anomalies interpretation at low magnetic latitudes.

Investigation of igneous rocks in Huanghua depression, North China, from magnetic derivative methods

The igneous rock study plays an important role in understanding the tectonic evolution of sedimentary basins. The distribution of igneous rocks is one of the basic aspects in the igneous rock study. Based on the magnetic susceptibility contrast of igneous and other rocks, magnetic methods are usually used in the igneous rock investigation. In this paper, we delineated the distribution of igneous rocks in Huanghua depression based on the reduction to pole (RTP) magnetic anomaly and its derivative processing results. The main methods used to enhance the anomaly character of igneous rocks include total horizontal derivative, analytic signal modules and the tilt angle. Based on the RTP anomaly and the derivative results, three types of igneous rock distribution areas are predicted in Huanghua depression. The large scale igneous rocks are mainly distributed in the north part of Huanghua depression. The string bead-shaped igneous rocks are mainly located in the north and southwest parts. Some craters are located in the middle-south part. The distribution of igneous rocks is controlled by the deep buried fault along the coastal boundary and the associated tertiary faults. Our study verified the validity of the magnetic derivative methods for the planar distribution study of igneous rocks. The study results of Huanghua depression are the basis of the seismic interpretation of igneous rocks and the tectonic study in this area.