Regional North American gravity and magnetic anomaly correlations

The authors discuss the genesis of the regional gravity and magnetic anomalies in the northern part of Eastern Xinjiang, a correlation analysis was carried out between the regional gravity anomaly and the regional magnetic anomaly. Through data processing and integrated interpretation of the gravity and magnetic data in the study area, the Bouguer gravity anomaly and the magnetic anomaly by reduction to the pole were separated. Then, the regional gravity and magnetic anomalies across multiple scales were extracted; in addition, the correlation coefficients between the regional gravity anomaly and the regional magnetic anomaly at different scales were calculated. Finally, the features of the gravity and magnetic fields, their geological significance, and the origins of these regional gravity and magnetic anomalies were analyzed. The results showed that the regional negative gravity anomaly in the Turpan-Hami basin and the Santanghu basin was mainly caused by Cenozoic and Mesozoic strata; the regional positive magnetic anomaly was mainly caused by the Precambrian metamorphic basement. The study revealed that the regional positive gravity and magnetic anomalies resulted from the crust and the mantle substances in the eastern part of the Junggar basin. While the regional positive gravity anomaly was mainly caused by Pre-Mesozoic strata, and the regional negative magnetic anomaly by the sedimentary formation and intermediate acid rocks in the Bogeda-Harlike folded zone and the Jueluotage anticlinorium. The regional negative gravity and magnetic anomalies in the active zone of the northern margin of Tarim were mainly caused by the sedimentary formation and intermediate acid rocks.

Based on detailed analysis of Bouguer gravity anomaly and magnetic anomaly by reduced to the pole, this paper uses correlation analysis method to analyze regional gravity and magnetic anomalies of different filter scales, including filter scales of 50km, 90km, and 130km, and then discusses the contributing factors of local gravity and magnetic anomalies in eastern Xinjiang. The results show that regional gravity and magnetic anomalies are cogenetic anomalies in some areas, such as Hangou, and northern Lamamiao, Jingrquan, and western Qijiaojing, gravity and magnetic anomalies are allogenic anomalies in other areas. The study reveals that regional negative gravity anomalies are chiefly caused by Cenozoic – Mesozoic with larger thickness and lower density, and regional positive magnetic anomalies are mainly caused by Precambrian metamorphic basement with stronger magnetic property uplifting in Turpan – Hami and Santanghu basin. Mantle substances uplifting mainly cause regional positive gravity and magnetic anomalies in Junggar basin. Regional positive gravity anomalies are mainly caused by Pre-Mesozoic with higher density uplifting, and regional negative magnetic anomalies are chiefly caused by sedimentary formation and intermediate acidity rock with larger thickness and weaker magnetism and Precambrian metamorphic basement.

An integrated geodynamic model of the Nankai subduction zone and neighboring regions from geophysical inversion and modeling

A study of the features of gravity and magnetic fields in the vicinity of oil and gas reservoirs in West Siberia demonstrated a spatial relationship with the hydrocarbon deposits. The relevant magnetic and gravity anomalies cover approximately [Formula: see text] in northern West Siberia. Amplitude and frequency were investigated initially using double Fourier spectrum (DFS) analysis. This was followed by (1) application of transformations, filtering, and “moving windows” analysis; (2) compilation of maps of regional and local anomalies, and potential field derivatives; and (3) investigation of the distribution of parameters in areas of known deposits. Hydrocarbon deposits are located mostly at the slopes of positive regional gravity and magnetic anomalies which are interpreted as relating to deep riftogenic structures. At the same time, it is established that the location of hydrocarbon depositions coincides commonly with local gravity and magnetic minima generated by lows in basement density and magnetization. All known hydrocarbon deposits in northern West Siberia are in areas characterized by comparatively high gradients of constituent of gravity anomalies with a wavelength of about 90–100 km. These newly revealed links between reservoirs and potential field parameters may be a means to predict new discoveries in poorly explored territories and seas, primarily in Russia's Arctic shelf.

Tectonic structures of SW margin of Gondwana from gravity and magnetic anomalies of the Río de la Plata area and their correlation with the Beattie magnetic Anomaly in South Africa

A Rapid Graphical Solution for Determining the Presence of Remnant Magnetization in the Basement Rocks of Western Iraq

About 700 gravity and 1200 magnetic measurements have been made in north-east Aberdeenshire in order to find the attitudes of the basic intrusions. The surveys have covered the younger gabbros, the Bennachie granite and the intervening areas. A marine magnetic survey has covered an area about 40 × 30 km offshore from Banff. The Bouguer anomalies are positive over almost the whole area except the Bennachie granite; together with prominent magnetic anomalies these indicate that the exposed gabbros form part of a continuous basic sheet. This sheet varies in thickness from 1 or 2 km near Huntly to 7 km at Cabrach. Positive magnetic and negative gravity anomalies are found from Dufftown to the Moray Firth, suggesting that there are extensive granite intrusions in this area. Hidden extensions of the Cabrach mass and the Bennachie granite have been approximately delineated. Positive gravity anomalies extend from Cabrach over the northern part of the Morven mass. Models have been constructed which fit the anomalies over the Insch and Belhelvie masses; these suggest that there has been post-consolidation folding at Belhelvie. In five localities the gravity and magnetic anomalies are closely correlated and the direction of magnetization has been computed for each of these areas. These are compared with the directions found by Blundell & Read (1958) and the deformation of the basic intrusions is discussed. Gravity and magnetic anomalies at the east end of the Insch mass suggest that this mass also has been deformed.

Preliminary satellite scalar magnetic anomaly data (MAGSAT) reduced to vertical polarization and long‐wavelength‐pass filtered free‐air gravity anomaly data of South and Central America are compared to major tectonic features. A number of correlations are observed, but these must be generalized because of the preliminary nature of the geophysical data and the inherent petrophysical variations within tectonic features. Statistical analysis of the magnetic data reveals that South and Central America are more magnetic and magnetically more variable than adjacent marine areas. More obvious correlations exist between magnetic anomalies and tectonic elements of the continents than in the case of oceanic areas. No obvious correlations occur between the tectonic features of the Atlantic Ocean, including the Mid‐Atlantic Ridge, and magnetic anomalies. The continental shields generally are more magnetic than adjacent basins, oceans and orogenic belts. In contrast, the major aulacogens are characterized by negative magnetic anomalies. Positive free‐air gravity anomalies are related to the Andean Foldbelt, but the relationship of this feature to magnetic anomalies is much less obvious. However, along the west coast of South America, the magnetic anomalies of the Pacific Ocean are separated from those of the eastern platforms by north to northwest trending anomalies. South of the equator along the Foldbelt, gravity maxima are related to magnetic minima, a relationship analogous to the situation observed in the Rocky Mountain Cordillera. North of the equator in Columbia, gravity and magnetic maxima roughly correlate along the Foldbelt.

Based on detailed analysis of Bouguer gravity anomaly and magnetic anomaly by reduced to the pole, this paper uses correlation analysis method to analyze local gravity and magnetic anomalies of different filter scales, including filter scales of 50km, 90km, and 130km, and then discusses the contributing factors of local gravity and magnetic anomalies in eastern Tianshan. The results show that local gravity and magnetic anomalies are cogenetic anomalies in some areas, such as northeastern Santanghu, Shaqiuhe, southern Huangtuya, northern Dikan, and western Qijiaojing, and gravity and magnetic anomalies are allogenic anomalies in other areas. The study reveals that local gravity and magnetic anomalies are chiefly caused by Cenozoic – Mesozoic strata with lower density and weaker magnetism in northern Santanghu, and igneous rock with higher density and stronger magnetism in Shaqiuhe, and uplifting of older strata in northern Dikan, and Carboniferous strata with higher density and lower magnetism than its south and north areas in Qijiaojing. The study also reveals that local negative gravity and magnetic anomalies are chiefly caused by Cenozoic – Mesozoic strata with lower density and weaker magnetism, while local positive gravity and magnetic anomalies are mainly caused by Pre-Jurassic strata higher density and stronger magnetism in southern Huangtuya.

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

Imaging buried geological boundaries is one of a major objective during the interpretation of magnetic field data in Geophysics. Therefore, edge detection and edge enhancement techniques assist a crucial role on this aim. Most of the existing edge detector methods require to obtain special points such as in general the maxima of the resulting image. One of the useful tools in estimating edges from magnetic data is the tilt angle of the analytical signal amplitude due to its value slightly dependence on the direction of magnetization. In this study, the maxima of the tilt angle of analytical signal amplitudes of the magnetic data was determined by a curvature-based method. The technique is based on fitting a quadratic surface over a 3×3 windows of the grid for locating any appropriate critical point that is near the centre of the window. The algorithm is built in Matlab environment. The feasibility of the algorithm is demonstrated in two cases of synthetic data as well as on real magnetic data from Tu Chinh-Vung May area. The source code is available from the authors on request.ReferencesAkpınar Z., Gürsoy H., Tatar O., Büyüksaraç A., Koçbulut F., Piper, JDA., 2016. Geophysical analysis of fault geometry and volcanic activity in the Erzincan Basin, Central Turkey, Complex evolution of a mature pull-apart basin. Journal of Asian Earth Sciences, 116, 97-114. Beiki M., 2010. Analytic signals of gravity gradient tensor and their application to estimate source location, Geophysics, 75(6), 159-174.Blakely R. J., and Simpson R.W., 1986. Approximating edges of source bodies from magnetic or gravity anomalies, Geophysics, 51, 1494-1498.Chen An-Guo, Zhou Tao-Fa, Liu Dong-Jia, Zhang Shu, 2017. Application of an enhanced theta-based filter for potential field edge detection: a case study of the LUZONG ORE DISTRICT, Chinese Journal of Geophysics, 60(2), 203-218.Cooper G.RJ., 2014. Reducing the dependence of the analytic signal amplitude of aeromagnetic data on the source vector direction, Geophysics, 79, 55-60.Cordell L., 1979. Gravimetric Expression of Graben Faulting in Santa Fe Country and theEspanola Basin, New Mexico. In Ingersoll, R.V., Ed., Guidebook to Santa Fe Country, New Mexico Geological Society, Socorro, 59-64.Cordell L and Grauch V.J.S., 1985. Mapping Basement Magnetization Zones from Aeromagnetic Data in the San Juan Basin, New Mexico, The Utility of Regional Gravity and Magnetic Anomaly Maps, Society of Exploration Geophysicists, Tulsa, 181-197.Hsu S.K., Coppense D., Shyu C.T., 1996. High- resolution detection of geologic boundaries from potential field anomalies: An enhanced analytic signal technique, Geophysics, 61, 1947-1957.Le D.C., Application of seismic exploration methods to identify geological structural characteristics supporting for hydrocarbon potential assessment in TuChinh - Vung May basin, Ph.D. Thesis, Hanoi University of Mining and Geology.Li X., 2006. Understanding 3D analytic signal amplitude: Geophysics, 71(2), 13-16.Miller H.G. and Singh V., 1994. Potential Field Tilt a New Concept for Location of Potential Field Sources, Journal of Applied Geophysics, 32, 213-217.Nabighian M.N., 1972. The analytic signal of two-dimensional magnetic bodies with polygonal cross-section: Its properties and use of automated anomaly interpretation, Geophysics, 37, 507-517.Nguyen N.T., Bui V.N., Nguyen T.T.H., 2014. Determining the depth to the magnetic basement and fault systems in Tu Chinh - Vung May area by magnetic data interpretation, Journal of Marine Science and Technology, 14(4a), 16-25.Nguyen X.H, San T.N, Bae W., Hoang M.C, 2014. Formation mechanism and petroleum system of tertiary sedimentary basins, offshore Vietnam, Energy Sources, Part A, 36, 1634-1649.Phillips J.D., Hansen R.O. and Blakely R.J., 2007. The use of curvature in potential-field interpretation, Exploration Geophysics, 38(2), 111-119.Rao D.B., and Babu N.R., 1991. A rapid method for three-dimensional modeling of magnetic anomalies, Geophysics, 56(11), 1729-1737.Roest W.R., Verhoef J., and Pilkington M., 1992. Magnetic interpretation using the 3-D analytic signal, Geophysics, 57, 116-125.Tran N., 2017. Sediment geology of Vietnam, VNU Press.Tran T.D., Tran N., Nguyen T.H., Dinh X.T., Pham B.N., Nguyen T.T., Tran T.T.T.N., Nguyen T.H.T., 2018. The Miocenedepositional geological evolution of Phu Khanh, Nam Con Son and Tu Chinh - Vung May basins in Vietnam continental shelf, VNU Journal of Science: Earth and Environmental Sciences, 34(1), 112-135.Vo T.S., Le H.M., Luu V.H., 2005. Three-dimensional analytic signal method and its application in interpretation of aeromagnetic anomaly maps in the Tuan Giao region, Proceedings of the 4th geophysical scientific and technical conference of Vietnam, Publisher of Science and Engineering 2005.Wijns C, Perez C and Kowalczyk P, 2005, Theta map: Edge detection in magnetic data, Geophysics, 70, 39-43.

Sveconorwegian igneous complexes beneath the Norwegian–Danish Basin

Within the scope of this paper, Walsh transform was applied to the total magnetic field and gravity anomalies obtained from the ideal subsurface structures and the field studies and the possibility of this method for the calculation of the depths to the source structures was investigated. This method is based on the Walsh transformation which can be applied to the total magnetic field and gravity anomalies. In this method, initially normalized energy density (NED) spectrum, then the differential energy density (DED) spectrum would be obtained. DED spectrum is the difference between two successive sequency numbers of the NED. The maximum value of sequency numbers (Imax) on DED spectrum is determined and, the depth of the subsurface structure that caused the anomaly can be computed by using of this value in the proper equation. The monopole, line of monopoles, dipole and line of dipoles models were chosen as the ideal subsurface sources for magnetic and sphere, infinite extend horizontal cylinder and semi infinite vertical cylinder for gravity in the theoretical studies and and the anomalies of these structures can be calculated. The depths of the subsurface structures were estimated by using Walsh transform method to the field data and the obtained results were compared with the previously calculated depth with the various other methods. Furthermore, the theoretical and the field magnetic and gravity anomaly data were evaluated with using Fourier-Power Spectrum method.

Paleozoic to Mesozoic micro-block tectonics in the eastern Central Asian Orogenic Belt: Insights from magnetic and gravity anomalies

Joint inversion is an essential technique in potential field data processing. The current methodology largely relies on the geology model of anomalous bodies, especially for deep, complex structures. Inspired by the excellent nonlinear mapping capability of the image semantic segmentation model and the advantages of supervised learning, a regressive, end-to-end, encoder-decoder structural, convolutional neural network with a double-branch structure called PFInvNet(Potential Field Inversion Neural Network) is proposed for joint 3D inversion of physical properties from gravity and magnetic data. Its input is a four-channel dataset consisting of gravity and magnetic anomalies and their vertical gradients, and its output is a 3D matrix representing the spatial distribution of the remnant density and the magnetic susceptibility, which are predicted independently through the double-branch structure of the decoders and then concatenated in the final layer. For network training, a large amount of precisely labeled sample is exceedingly demanding; thus, forward modeling becomes a prerequisite approach. Two discretized forward modeling algorithms for gravity and magnetic anomalies of 3D homogeneous arbitrary-shaped bodies based on surface integrals are deduced and verified with analytic solutions of the sphere model. Furthermore, the neural network needs to learn from the anomalies generated by various forms of abnormal bodies with different physical properties. Therefore, different sizes and quantities of cuboids are randomly distributed in the model space to simulate different forms of abnormal bodies. The label represents the combined spatial distribution of remanent density and magnetic susceptibility for the cuboids, encompassing both spatial location information and physical properties information. With the help of the Marching Cubes(MC) algorithm, the surface of the cuboids can be easily extracted and divided into a triangular surface mesh. The surface mesh is then used to calculate the gravity and magnetic anomalies synchronously through the forward modeling algorithms. The anomalies are concatenated in the channel direction as a sample. A set of optimal network parameters has been determined, including the weight initialization method, the gradient calculation methods, the loss function, the training hyperparameters, the regularization method, and the normalization method. The PFInvNet is trained with 500 and 10000 pairs of samples and labels, respectively. The analysis and comparison of training results prove that PFInvNet has two crucial features: one is that the branch structure enables independent prediction of magnetic susceptibility and remanent density; the other is efficient anti-overfitting ability and efficient solution-finding ability .The prediction error of small samples is very close to that of large samples and is also not obviously enhanced by the noise-contaminated data , demonstrating the strong generalization and robustness of the network. Finally, the network is tested with magnetic and gravity anomalies of the Victoria Land Basin in the western Ross Sea through transfer learning and retraining, and definite 3D distributions of apparent remnant density and apparent magnetic susceptibility have been obtained and can be checked with geological evidences.