Total Magnetic Intensity Anomalies Research Articles

A combined Euler deconvolution and tilt angle method for interpretation of magnetic data in the South region

Introduction: The purpose of this paper is to determinate the position, depth, dip direction and dip angle the faults in the South region of Vietnam from the total magnetic intensity anomalies, that reduced to the magnetic pole (RTP).
 Methods: Based on the Oasis Montaj software, we proposed a new way to compute the positions and the depth to the top of the faults by combining the Tilt angle and the Euler deconvolution methods. In addition, the angle and direction of the dip of theses faults were also determined by considering maximum of the total horizontal derivative of the RTP upward continuation at the different height levels.
 Results: The results show that there are 12 faults along the longitudinal direction, latitudinal direction, Northwest — Southeast direction and Northeast — Southwest direction with the mazimum depth is about 3100 m and the dip angle changes in the range of 65-82◦.
 Results: These indicate that these methods are valuable tools for specifying the characteristics of geology, contribute to give and confirm the useful information on geological structure in the South region of Vietnam.

The aeromagnetic expression of New Zealand’s Alpine Fault: regional displacement and entrainment of igneous rock

ABSTRACTThe Alpine Fault is very well delineated by recent high-resolution aeromagnetic data sets over much of its length in New Zealand’s South Island. Aeromagnetic data acquired over parts of the West Coast, Tasman and Marlborough regions for the New Zealand Government reveal different types of total magnetic intensity anomalies associated with the fault that can be characterised as truncated, step, ridge, trough and broad anomalies. Fault-terminated Permian ultramafic rocks, Late Cretaceous basalt and some Early Cretaceous intrusions are attributed to the more prominent truncated anomalies. The pre-eminent step anomaly is associated with relatively magnetic Alpine Schist juxtaposed against variable but generally less magnetic Western Province rocks. Strongly positive ridge anomalies occurring in the Alpine Fault zone are associated with Mesozoic mafic metavolcanic and ultramafic rocks. A trough anomaly associated with the Alpine Fault occurs in the southernmost survey and may reflect some hydrothermal demagnetisation. Broad, low-amplitude anomalies underlying the hanging wall are attributed in one instance to the extension of the magnetic Hohonu Range granitoids and dikes extending southeast in the footwall and in other places as magnetic schist rocks in the hanging wall. The apparent great-circle displacement of the Alpine Fault based on offset of magnetic anomalies associated with the Dun Mountain Ultramafics Group rocks is c. 461 km, in close agreement with the more precise 459 km inferred from geological map offset.

The antisymmetric factor method for magnetic reduction to the pole at low latitudes

We analyze the characteristics of the wavenumber-domain factor for magnetic reduction to the pole (RTP) at low latitudes, and then propose a new wavenumber-domain method for RTP at low latitudes, herein called the antisymmetric factor method, based on modification of the RTP factor. The method applies the antisymmetric factor in a given scope of directions centered along the magnetic declination to suppress amplification of the RTP factor, stabilizing the RTP. Meanwhile it utilizes the routine RTP factor in other directions to preserve the effective RTP features. The test on the synthetic data demonstrates that the method is robust and effective. Finally, we use the new method, as well as a variable magnetic inclinations algorithm, to perform RTP on the real data of total magnetic intensity anomalies in the South China Sea, and obtain the reliable RTP anomalies. • A new method for magnetic reduction to the pole at low latitudes is presented. • The method is based on modification of the RTP factor in a wavenumber domain. • It has the advantages of few control parameters, fast and stable RTP. • It is applicable to RTP of large-scale magnetic data. • Tests on the synthetic and real data show that the method is robust and effective.

Mapeamento aero-geofísico de pegmatitos mineralizados na Província Pegmatítica de Alto Ligonha, no norte de Moçambique

Major parts of Mozambique were recently flown to acquire high resolution airborne magnetic and radiometric data. Since then it has been the intent of the National Geology Directorate of Mozambique (DNG) to interpret these data generating value-added maps that are easier to use by the exploration and mining industries than mere airborne geophysical grid data and maps. DNG and Maputo-based Eduardo-Mondlane University (UEM) entered a joint venture with the South African Council for Geoscience (CGS) to conduct a case study over the meso-Proterozoic Alto Ligonha pegmatite province in northern Mozambique to support the regional mining sector. Integration of the airborne geophysical data was achieved using the K-means algorithm for explorative unsupervised crisp clustering of the airborne geophysical data. Discriminant analysis was then used to obtain the degree of class membership of each sample previously classified by K-means. Prior to clustering, Th- and K-surface concentrations as well as apparent magnetic susceptibility and first vertical derivative data of the total magnetic intensity anomalies were determined to project a 100 m-spaced grid comprising 850,000 multivariate sample vectors on the study area. Compilation of the classification results provided the value-added map illustrating that the probability to meet pegmatite bodies is highest where amphibole gneiss occurs. The automated integration of airborne geophysical data using the K-means algorithm is a fast, objective and efficient method to compile value-added maps. The experience made in the Alto Ligonha pegmatite province encourages the adoption of this new methodology over other parts of the Mozambique Fold Belt, becoming integral part of geological mapping ongoing in the country.

Mapeamento aero-geofísico de pegmatitos mineralizados na Província Pegmatítica de Alto Ligonha, no norte de Moçambique

Major parts of Mozambique were recently flown to acquire high resolution airborne magnetic and radiometric data. Since then it has been the intent of the National Geology Directorate of Mozambique (DNG) to interpret these data generating value-added maps that are easier to use by the exploration and mining industries than mere airborne geophysical grid data and maps. DNG and Maputo-based Eduardo-Mondlane University (UEM) entered a joint venture with the South African Council for Geoscience (CGS) to conduct a case study over the meso-Proterozoic Alto Ligonha pegmatite province in northern Mozambique to support the regional mining sector. Integration of the airborne geophysical data was achieved using the K-means algorithm for explorative unsupervised crisp clustering of the airborne geophysical data. Discriminant analysis was then used to obtain the degree of class membership of each sample previously classified by K-means. Prior to clustering, Th- and K-surface concentrations as well as apparent magnetic susceptibility and first vertical derivative data of the total magnetic intensity anomalies were determined to project a 100 m-spaced grid comprising 850,000 multivariate sample vectors on the study area. Compilation of the classification results provided the value-added map illustrating that the probability to meet pegmatite bodies is highest where amphibole gneiss occurs. The automated integration of airborne geophysical data using the K-means algorithm is a fast, objective and efficient method to compile value-added maps. The experience made in the Alto Ligonha pegmatite province encourages the adoption of this new methodology over other parts of the Mozambique Fold Belt, becoming integral part of geological mapping ongoing in the country.

Interpretation of total intensity magnetic anomalies when anomalous field is arbitrarily large

Not infrequently, in mining geophysics, the anomalous field of the magnetized body is appreciably large and it varies from the direction of the earth's normal field within the vicinity of the magnetized body. Total magnetic intensity data collected on the ground over shallow magnetized bodies cannot be interpreted quantitatively, since all the available methods of interpretation assume that the resultant field lies in the direction of earth's normal field as the anomalous field is small. For geological bodies that can be magnetically represented by infinite line pole, point pole, infinite line dipole, and point dipole, this paper gives quantitative interpretation procedures for total magnetic intensity anomalies without the above assumption using the characteristic points from the theoretical curves (not reproduced). These characteristic curves can be used for the direct determination of depths and offsets by using information from suitably chosen field profiles.

A NEW METHOD FOR INTERPRETATION OF AEROMAGNETIC MAPS: PSEUDO‐GRAVIMETRIC ANOMALIES

The purpose of this paper is to describe a method of interpretation based on a transformation of the total magnetic intensity anomalies into simpler anomalies. The result of this transformation is the elimination of the distortion due to the obliquity of the normal magnetic field, so that the resulting anomalies will be located on the vertical of the disturbing magnetized bodies. The starting point of the theory is the well known relation between magnetic potential V and Newtonian potential U—relation which can be written: [Formula: see text] with: [Formula: see text] = magnetization, f = 66.7×10−9 u ⋅CGS, σ = assumed density of magnetized bodies. This relation may be considered as a partial differential equation. The boundary condition consists in the measured values of the total field T(P ) known at each point P at the datum plane. As T(P) is the derivative of a harmonic function V, we can determine this function everywhere above the datum plane. Solving then the partial differential equation, we find the Newtonian potential U and its vertical derivatives g, g′, g″⋯. Finally, we obtain these quantities as functionals of the measured magnetic anomaly T(P). For instance, one of the formula is [Formula: see text] where H(M, P) is the kernel of the transformation allowing the direct computation of g(M)—an anomalous field which will be called “pseudo‐gravimetric anomaly.” Of course, the pseudo‐gravimetric anomaly has all the usual properties of a gravimetric anomaly. The field of g(M) presents no distortion and the interpretation becomes as easy as that of a Bouguer anomaly map. To perform the actual routine calculations of this transformation, we start with the values of T taken on a rectangular or trigonal grid, as for the usual computation of the vertical derivatives. The use of punched‐card equipment speeds up considerably these calculations.