Enhancement of the total horizontal gradient of magnetic anomalies using the tilt angle

Geophysics ◽  
2013 ◽  
Vol 78 (3) ◽  
pp. J33-J41 ◽  
Author(s):  
Francisco J. F. Ferreira ◽  
Jeferson de Souza ◽  
Alessandra de B. e S. Bongiolo ◽  
Luís G. de Castro
Keyword(s):  
Tilt Angle ◽  
Real Data ◽  
Magnetic Anomalies ◽  
Detection Methods ◽  
Magnetic Data ◽  
Horizontal Gradient ◽  
Geometrical Properties ◽  
Different Depths ◽  
Deep Sources ◽  
Derivatives Of

Magnetic anomaly maps reflect the spatial distribution of magnetic sources, which may be located at different depths and have significantly different physical and geometrical properties, complicating the identification of the corresponding geologic structures. Filtering techniques are frequently used to balance anomalies from shallow and deep sources, and to enhance certain features of interest, such as the edges of the causative bodies. Most methods used for enhancing magnetic data are based on vertical or horizontal derivatives of the magnetic anomalies or combinations of them, and the edges or centers of the sources are identified by maxima, minima, or null values in the transformed data. Normalized derivatives methods are used to equalize signals from sources buried at different depths. We present an edge detector method for the enhancement of magnetic anomalies, which is based on the tilt angle of the total horizontal gradient. The notable features of this method are that it produces amplitude maxima over the source edges and that it equalizes signals from shallow and deep sources. The method is applied to synthetic and real data. The effectiveness of the method is evaluated by comparing it with other edge detection methods that have been previously reported in the literature and that make use of derivatives. The results show that our method is less sensitive to variations in the depth of the sources and that it indicates the position of the edges of causative bodies in a more accurate fashion, when compared with previous methods, even for anomalies due to multiple interfering sources. These results demonstrate that the proposed method is a useful tool for the qualitative interpretation of magnetic data.

scholarly journals Estimating Effectiveness of Some Methods for Detecting Edge of Potential Field Sources

2020 ◽  
Vol 36 (4) ◽  
Author(s):  
Pham Thanh Luan ◽  
Le Thi Sang ◽  
Vu Duc Minh ◽  
Ngo Thi To Nhu ◽  
Do Duc Thanh ◽  
...  
Keyword(s):  
Gravity Anomaly ◽  
Tilt Angle ◽  
Northeast China ◽  
Signal Amplitude ◽  
Analytic Signal ◽  
Detection Methods ◽  
Magnetic Data ◽  
Horizontal Gradient ◽  
North Vietnam ◽  
Gradient Amplitude

This paper presents a comparative study of effectiveness of edge detection methods such as total horizontal gradient, analytic signal amplitude, tilt angle, gradient amplitude of tilt angle, theta map, horizontal tilt angle, tilt angle of total horizontal gradient, tilt angle of analytic signal, improved theta map, and total horizontal gradient of improved tilt angle. The effectiveness of each method was estimated on synthetic magnetic data and synthetic gravity anomaly data with and without noise. The obtained results show that the tilt angle of gradient amplitude can detect all the edges more clearly and precisely. The applicability of each method is demonstrated on the aeromagnetic anomaly data from the Zhurihe region of Northeast China, and Bouguer gravity anomaly data from a region of North Vietnam. The results computed by the tilt angle of horizontal gradient were also in accord with the geologic structures of the areas.

Magnetization vector imaging for borehole magnetic data based on magnitude magnetic anomaly

Geophysics ◽  
2013 ◽  
Vol 78 (6) ◽  
pp. D429-D444 ◽  
Author(s):  
Shuang Liu ◽  
Xiangyun Hu ◽  
Tianyou Liu ◽  
Jie Feng ◽  
Wenli Gao ◽  
...  
Keyword(s):  
Conjugate Gradient Method ◽  
Conjugate Gradient ◽  
Gradient Method ◽  
Magnetization Vector ◽  
Real Data ◽  
Magnetic Anomalies ◽  
Magnetic Data ◽  
Preconditioned Conjugate Gradient ◽  
Magnetization Direction ◽  
Magnetization Intensity

Remanent magnetization and self-demagnetization change the magnitude and direction of the magnetization vector, which complicates the interpretation of magnetic data. To deal with this problem, we evaluated a method for inverting the distributions of 2D magnetization vector or effective susceptibility using 3C borehole magnetic data. The basis for this method is the fact that 2D magnitude magnetic anomalies are not sensitive to the magnetization direction. We calculated magnitude anomalies from the measured borehole magnetic data in a spatial domain. The vector distributions of magnetization were inverted methodically in two steps. The distributions of magnetization magnitude were initially solved based on magnitude magnetic anomalies using the preconditioned conjugate gradient method. The preconditioner determined by the distances between the cells and the borehole observation points greatly improved the quality of the magnetization magnitude imaging. With the calculated magnetization magnitude, the distributions of magnetization direction were computed by fitting the component anomalies secondly using the conjugate gradient method. The two-step approach made full use of the amplitude and phase anomalies of the borehole magnetic data. We studied the influence of remanence and demagnetization based on the recovered magnetization intensity and direction distributions. Finally, we tested our method using synthetic and real data from scenarios that involved high susceptibility and complicated remanence, and all tests returned favorable results.

Aeromagnetic Interpretations of the Crittenden County Fault Zone

2020 ◽  
Vol 92 (1) ◽  
pp. 494-507
Author(s):  
Christopher Marlow ◽  
Christine Powell ◽  
Randel Cox
Keyword(s):  
Seismic Hazard ◽  
Fault Zone ◽  
Detection Methods ◽  
Magnetic Data ◽  
Horizontal Gradient ◽  
Reverse Fault ◽  
Maximum Magnitude ◽  
Fault Systems ◽  
Basement Faults ◽  
Seismic Reflection Profiles

Abstract The Crittenden County fault zone (CCFZ) is a potentially active fault zone located within 25 km of Memphis, Tennessee, and poses a significant seismic hazard to the region. Previous research has associated the fault zone with basement faults of the eastern Reelfoot rift margin (ERRM) and described it as a northeast-striking, northwest-dipping reverse fault. However, we suggest that there is an incomplete understanding of the fault geometry of the CCFZ and the ERRM in this region due to significant gaps in seismic reflection profiles used to interpret the fault systems. To improve our understanding of the structure of both fault systems in this region, we apply two processing techniques to gridded aeromagnetic data. We use the horizontal gradient method on reduction-to-pole magnetic data to detect magnetic contacts associated with faults as this technique produces shaper gradients at magnetic contacts than other edge detection methods. For depth to basement estimations, we use the analytic signal as the method does not require knowledge of the remnant magnetization of the source body. We suggest that the CCFZ extends approximately 16 km farther to the southwest than previously mapped and may be composed of three independent faults as opposed to a continuous structure. To the northeast, we interpreted two possible faults associated with the ERRM that intersect the CCFZ, one of which has been previously mapped as the Meeman–Shelby fault. If the CCFZ and the eastern rift margin are composed of isolated fault segments, the maximum magnitude earthquake that each fault segment may generate is reduced, thereby, lowering the existing seismic hazard both fault systems pose to Memphis, Tennessee.

Practical considerations in the use of edge detectors for geologic mapping using magnetic data

Geophysics ◽  
2017 ◽  
Vol 82 (3) ◽  
pp. J1-J8 ◽  
Author(s):  
Mark Pilkington ◽  
Victoria Tschirhart
Keyword(s):  
Magnetic Field ◽  
Detection Methods ◽  
Magnetic Data ◽  
Horizontal Gradient ◽  
Recent Effort ◽  
Aeromagnetic Data ◽  
Geologic Mapping ◽  
Magnetization Direction ◽  
Model Studies ◽  
Edge Detectors

Locating the edges of magnetized sources provides a fundamental tool in the geologic interpretation of magnetic field data. Much recent effort has been expended on developing improvements to existing edge-detection methods, resulting in purported increases in accuracy and continuity along edges, reduction of noise effects, and limiting the influences of variable depth to source, magnetization direction, and source dip. These endeavors are valuable and provide interpreters with a wider range of tools to carry out geologic interpretations of aeromagnetic data. Nevertheless, survey parameters such as flight height and line spacing impose limits on the quality of edge locations that can be achieved. Using model studies, we quantify the effects that source size, depth, and interference between sources have on calculated edge locations. Based on the known behavior of established edge detectors, we found that many of the newer approaches offer limited advantages over older methods. Consequently, we studied an example of field mapping of geologic contacts in the Canadian Shield, supported by aeromagnetic data, using calculation of a standard edge detector: the horizontal gradient magnitude of the total magnetic field or TF-hgm. Calculated edge locations estimated from this method appear sufficiently accurate and continuous to provide a solid basis on which the mapping campaign was based and executed successfully.

Interpretation of magnetic data using tilt-angle derivatives

Geophysics ◽  
2008 ◽  
Vol 73 (1) ◽  
pp. L1-L10 ◽  
Author(s):  
Ahmed Salem ◽  
Simon Williams ◽  
Derek Fairhead ◽  
Richard Smith ◽  
Dhananjay Ravat
Keyword(s):  
Tilt Angle ◽  
Magnetic Data ◽  
Structural Index ◽  
Spectral Content ◽  
Free Data ◽  
Second Derivatives ◽  
Simple Linear Equation ◽  
Theoretical Simulations ◽  
Derivatives Of ◽  
Method Show

We have developed a new method for interpretation of gridded magnetic data which, based on derivatives of the tilt angle, provides a simple linear equation, similar to the 3D Euler equation. Our method estimates both the horizontal location and the depth of magnetic bodies, but without specifying prior information about the nature of the sources (structural index). Using source-position estimates, the nature of the source can then be inferred. Theoretical simulations over simple and complex magnetic sources that give rise to noise-corrupted and noise-free data, illustrate the ability of the method to provide source locations and index values characterizing the nature of the source bodies. Our method uses second derivatives of the magnetic anomaly, which are sensitive to noise (high-wavenumber spectral content) in the data. Thus, an upward continuation of the anomaly may lead to reduce the noise effect. We demonstrate the practical utility of the method using a field example from Namibia, where the results of the proposed method show broad cor-relation with previous results using interactive forward modeling.

scholarly journals COMBINING TILT DERIVATIVE FILTERS: NEW APPROACHES TO ENHANCE MAGNETIC ANOMALIES

2018 ◽  
Vol 36 (3) ◽  
pp. 1
Author(s):  
Fabrício Rodrigues Castro ◽  
Saulo Pomponet Oliveira ◽  
Jeferson de Souza ◽  
Francisco José Fonseca Ferreira
Keyword(s):  
Qualitative Methods ◽  
Magnetic Anomaly ◽  
Computational Cost ◽  
Magnetic Anomalies ◽  
Horizontal Gradient ◽  
Local Phase ◽  
Aeromagnetic Data ◽  
Addition And Subtraction ◽  
Deep Sources ◽  
Tilt Derivative

ABSTRACT. We extend the concept of two earlier enhancement techniques based on the local phase of the magnetic anomaly, namely the vertical (TDR) and horizontal (TDX) tilt angles, which are defined by the inverse tangent of ratios involving the total horizontal gradient and the vertical derivative. These filters are useful to locate both shallow and deep sources, because they equalize the signal amplitudes. The proposed approach is based on the addition and subtraction of TDR and TDX. The TDR+TDX filter produces constant values over the causative bodies, while TDR-TDX generates peaks over the center of bodies and is constant out of them. By applying the proposed techniques to synthetic and aeromagnetic data we show that they locate more clearly the centers and edges of the sources in comparison to TDR and TDX, respectively. The combined filters have essentially the same computational cost as TDR and TDX and can replace them as auxiliary interpretation tools.Keywords: Qualitative Methods, Local Phase Filters, Aeromagnetic Data.RESUMO. Estendemos o conceito de duas técnicas de realce baseadas na fase local da anomalia magnética: as inclinações do sinal analítico (TDR) e do gradiente horizontal total (TDX), definidos pelo arco tangente de razões envolvendo o gradiente horizontal total e a derivada vertical. Estes filtros são úteis para localizar tanto fontes rasas quanto profundas. O método proposto baseia-se na adição e subtração dos filtros TDR e TDX. O filtro TDR+TDX produz valores constantes sobre as fontes causadoras, enquanto que o TDR-TDX produz picos sobre o centro dos corpos e é constante onde fontes causadoras não são verificadas. Aplicando as técnicas propostas aos dados sintéticos e reais mostra-se que elas localizam mais claramente os centros e as bordas dos corpos em comparação com o TDR e o TDX, respectivamente. Os filtros combinados têm essencialmente o mesmo custo computacional dos filtros originais, TDR e TDX, e podem substituí-los como ferramentas de interpretação.Palavras-chave: Métodos Qualitativos, Filtros de Fase Local, Dados Aeromagnéticos. Federal

scholarly journals An edge-assisted smooth method for potential field data

2021 ◽  
Vol 18 (1) ◽  
pp. 113-123
Author(s):  
Shijing Zheng ◽  
Xiaohong Meng ◽  
Jun Wang
Keyword(s):  
Edge Detection ◽  
Potential Field ◽  
Field Data ◽  
Real Data ◽  
Linear Functions ◽  
Detection Methods ◽  
Edge Preserving ◽  
Potential Field Data ◽  
Tectonic Boundaries ◽  
Derivatives Of

Abstract Edge detection is one of the most commonly used methods for the interpretation of potential field data, because it can highlight the horizontal inhomogeneous of underground geological bodies (faults, tectonic boundaries, etc.). A variety of edge detection methods have been reported in the literature, most of which are based on the combined transformation results of horizontal and vertical derivatives of the observations. Consequently, these edge detection methods are sensitive to noise. Therefore, noise reduction is desirable ahead of applying edge detection methods. However, the application of conventional filters smears discontinuities in the data to a certain extent, which would inevitably induce unfavourable influence on subsequent edge detection. To solve this problem, a novel edge-preserving smooth method for potential field data is proposed, which is based on the concept of guided filter developed for image processing. The new method substitutes each data point by a combination of a series of coefficients of linear functions. It was tested on synthetic model and real data, and the results showed that it can effectively smooth potential field data while preserving major structural and stratigraphic discontinuities. The obtained data from the new filter contain more obvious features of existing faults, which brings advantageous to further geological interpretations.

Enhancement of the total horizontal gradient of magnetic anomalies using tilt derivatives: Part II — Application to real data

2011 ◽  
Author(s):  
Francisco J. F. Ferreira ◽  
Luís G. de Castro ◽  
Alessandra B. S. Bongiolo ◽  
Jeferson de Souza ◽  
Marco A. T. Romeiro
Keyword(s):  
Real Data ◽  
Magnetic Anomalies ◽  
Horizontal Gradient

scholarly journals Deciphering channel networks from aeromagnetic potential field data: the case of the North Sea Quaternary tunnel valleys

2019 ◽  
Author(s):  
S Brahimi ◽  
P Le Maire ◽  
J F Ghienne ◽  
M Munschy
Keyword(s):  
Magnetic Susceptibility ◽  
North Sea ◽  
Potential Field ◽  
Magnetic Anomalies ◽  
Channel Structure ◽  
Magnetic Data ◽  
Horizontal Gradient ◽  
Finite Width ◽  
The North ◽  
The North Sea

Summary High-resolution magnetic data and potential field methods have been used to perform a detailed analysis of networks of late Quaternary subglacially-cut tunnel valleys (central Viking Graben, Norwegian sector of the North Sea). High-frequency, ribbon-like, sinuous, paired magnetic anomalies interpreted to be the signature of tunnel valleys are identified. Such magnetic anomalies have 1 to 8 nT amplitudes and reflect a magnetic susceptibility contrast between valley infills and the host sediments. Fractional vertical derivative and horizontal gradient transforms provide the best control on the accurately delineation of tunnel valleys by plotting automatically the extrema. The 2D forward modelling is a very effective approach to determinating the geometric parameters and magnetic susceptibility of the modeled valleys. It allows to determine the finite-width flat horizontal thin geometry as the most appropriate simple geometry to simulate the magnetic anomaly linked to a channel structure. The application of Euler deconvolution using complex algebra allows to substantiate the structural index (n = 1.5) for simple palaeovalley geometries and to determine fair valley depth estimates.

Identifying remanent magnetization effects in magnetic data

Geophysics ◽  
1993 ◽  
Vol 58 (5) ◽  
pp. 653-659 ◽  
Author(s):  
Walter R. Roest ◽  
Mark Pilkington
Keyword(s):  
Magnetic Anomaly ◽  
Remanent Magnetization ◽  
Close Agreement ◽  
Magnetic Anomalies ◽  
Analytic Signal ◽  
Magnetic Data ◽  
Horizontal Gradient ◽  
Impact Structure ◽  
Magnetization Direction ◽  
Additional Constraints

Remanent magnetization can have a significant influence on the shape of magnetic anomalies in areas that are generally characterized by induced magnetization. Since modeling of magnetic anomalies is nonunique, additional constraints on the direction of magnetization are useful. A method is proposed here to study the possible contribution of remanent magnetization to a particular anomaly, by comparing two functions that are calculated directly from the observations: (1) the amplitude of the analytic signal, and (2) the horizontal gradient of pseudogravity. From the amplitude and relative position of maxima in these derived quantities, we infer the deviation of the magnetization direction from that of the ambient field. The approach is applied to the magnetic anomaly in the center of the Manicouagan impact structure (Canada). Our results, based only on the magnetic anomaly observations, are in close agreement with constraints on the direction of remanent magnetization from rock samples.

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