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.

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.

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.

Interpretation of aeromagnetic data using eigenvector analysis of pseudogravity gradient tensor

Geophysics ◽  
2011 ◽  
Vol 76 (3) ◽  
pp. L1-L10 ◽  
Author(s):  
Majid Beiki ◽  
Laust B. Pedersen ◽  
Hediyeh Nazi
Keyword(s):  
Magnetic Field ◽  
Least Squares ◽  
Field Data ◽  
Random Noise ◽  
Gravity Gradient ◽  
Aeromagnetic Data ◽  
Gradient Tensor ◽  
Magnetization Direction ◽  
Magnetic Field Data ◽  
Robust Least Squares

This study has shown that the same properties of the gravity gradient tensor are valid for the pseudogravity gradient tensor derived from magnetic field data, assuming that the magnetization direction is known. Eigenvectors of the pseudogravity gradient tensor are used to estimate depth to the center of mass of geologic bodies. The strike directions of 2D geological structures are estimated from the eigenvectors corresponding to the smallest eigenvalues. For a set of data points enclosed by a square window, a robust least-squares procedure is used to estimate the source point which has the smallest sum of squared distances to the lines passing through the measurement points and parallel to the eigenvectors corresponding to the maximum eigenvalues. The dimensionality of the pseudogravity field is defined from the dimensionality indicator I, derived from the tensor components. In the case of quasi-2D sources, a rectangular window is used in the robust least-squares procedure to reduce the uncertainty of estimations.Based on synthetic data sets, the method was tested on synthetic models and found to be robust to random noise in magnetic field data. The application of the method was also tested on a pseudogravity gradient tensor derived from total magnetic field data over the Särna area in west-central Sweden. Combined with Euler deconvolution, the method provides useful complementary information for interpretation of aeromagnetic data.

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 Structural features derived from a Multiscale Analysis and 2.75D Modelling of Aeromagnetic Data over the Pitoa-Figuil Area (Northern Cameroon)

2020 ◽  
Author(s):  
Voltaire Souga Kassia ◽  
Theophile Ndougsa-Mbarga ◽  
Arsène Meying ◽  
Jean Daniel Ngoh ◽  
Steve Ngoa Embeng
Keyword(s):  
Multiscale Analysis ◽  
Structural Features ◽  
Magnetic Data ◽  
Horizontal Gradient ◽  
Aeromagnetic Data ◽  
Near Surface ◽  
Subsurface Structures ◽  
Northern Cameroon ◽  
Processing Techniques ◽  
Geological Formations

Abstract. In the Pitoa-Figuil area (Northern Cameroon), an interpretation of aeromagnetic data was conducted. The aim of this investigation was first to emphasize lineaments hidden under geological formations and secondly to propose two 2.75D models of the subsurface structures. Different magnetic data processing techniques were used, notably horizontal gradient magnitude, analytic signal, and Euler deconvolution. These techniques in combination with the 2.75D modelling to the aeromagnetic anomaly reduced to the equator permit to understand the stratification of the deep and near surface structures, which are sources of the observed anomalies. We managed to put in evidence and characterize 18 faults and some intrusive bodies. According to Euler's solutions, anomaly sources go up to a depth of 5.3 km.

scholarly journals LOCATION OF MAGNETIC CONTACTS AND DEPTH ESTIMATES FOR THE MAGNETIC SOURCES FROM THE AEROMAGNETIC DATA OF MACEDONIA AND THRACE

2004 ◽  
Vol 36 (3) ◽  
pp. 1252
Author(s):  
Α. Σταμπολίδης ◽  
Γ. Τσόκας
Keyword(s):  
Magnetic Field ◽  
Analytical Signal ◽  
Crystalline Basement ◽  
Horizontal Gradient ◽  
Aeromagnetic Data ◽  
Local Wavenumber

In this work we present the processing that applied to digitised aeromagnetic data from Macedonia and Thrace. This processing was aiming the calculation of magnetic contacts and depths of burial of magnetic sources in the region of study. Three methods were applied for these calculations: the method of horizontal gradient, the method of analytical signal and the method of local wavenumber. All methods were successful in locating the deeper magnetic sources in basins and surface sources in regions with surface outcrops of the crystalline basement. Depending on the transformation of magnetic field we use as input for the methods of the horizontal gradient and analytical signal, we could calculate the minimum and maximum depths of burial for the magnetic contacts. The locations of magnetic contacts that were calculated for every method of analysis were combined in a map in order to help in the interpretation of proposed magnetic contacts

scholarly journals 3D magnetic amplitude inversion in the presence of self-demagnetization and remanent magnetization

Geophysics ◽  
2019 ◽  
Vol 84 (5) ◽  
pp. J69-J82 ◽  
Author(s):  
Boxin Zuo ◽  
Xiangyun Hu ◽  
Yi Cai ◽  
Shuang Liu
Keyword(s):  
Remanent Magnetization ◽  
Large Scale ◽  
Magnetic Data ◽  
Inversion Problem ◽  
Aeromagnetic Data ◽  
Inversion Algorithm ◽  
Rapid Convergence ◽  
Magnetization Direction ◽  
Amplitude Inversion ◽  
Nonlinear Conjugate Gradient

We have developed a general 3D amplitude inversion algorithm for magnetic data in the presence of self-demagnetization and remanent magnetization. The algorithm uses a nonlinear conjugate gradient (NLCG) scheme to invert the amplitude of the magnetic anomaly vector within a partial differential equation framework. Three quantities— the amplitude of the anomalous magnetic field, the analytic signal, and the normalized source strength, defined as the amplitudes of magnetic data that are weakly dependent on the magnetization direction — are inverted to recover the 3D distribution of the subsurface magnetic susceptibility. Numerical experiments indicate that our NLCG amplitude inversion algorithm has a rapid convergence rate that provides a reasonable inversion solution in the absence of knowing the total magnetization direction. High-resolution aeromagnetic data collected from the Pea Ridge iron oxide-apatite-rare earth element deposit, southeast Missouri, USA, are used to illustrate the efficacy of our amplitude inversion algorithm. This algorithm is generally applicable for tackling the large-scale inversion problem in the presence of self-demagnetization and remanent magnetization.

The magnetization of [Fe(OD 2 ) 6 ] 2+ in ammonium ferrous Tutton salt studied by polarized neutron diffraction - a canted paramagnet

1990 ◽  
Vol 428 (1874) ◽  
pp. 113-127 ◽  
Keyword(s):  
Magnetic Field ◽  
Neutron Diffraction ◽  
Iron Atom ◽  
Magnetic Data ◽  
Ligand Field ◽  
Magnetization Direction ◽  
Magnetization Density ◽  
Magnetic Structure Factor ◽  
Polarized Neutron ◽  
The Magnetic Field

A polarized neutron diffraction experiment has been done on deuterated ammonium ferrous Tutton salt at 1.5 K, 4.6 T with orientations of the magnetic field along the crystal b and c * axes. The flipping ratios of 303 and 280 reflections respectively were used, after correction for extinction, to give 121 and 118 unique values of the magnetic structure factor F M, Z (hkl) (eff). Those values were used in refinements of models for a description of the magnetization density in the crystal. All models resulted in substantial (37° and 45°) canting of the magnetization direction in the paramagnet away from the magnetic field, to an almost constant direction with respect to the O 6 ligand framework, indicating large magnetic anisotropy at the iron atom sites. There is delocalization of magnetization density away from the iron atom into the Fe-O overlap region ( — 4.5%) and onto the OD 2 ligands (6.5%), values comparable with the delocalization of spin from the metal atom in other Tutton salts studied. An earlier ligand field model for the electronic structure of the ion based upon spectroscopic and magnetic data is shown to be inadequate, because it is incompatible with the observed anisotropy in the magnetization around the iron atoms.

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.

Magnetic gradient techniques on digitized aeromagnetic data of Ibadan area, south-western Nigeria

2013 ◽  
Vol 5 (3) ◽  
Author(s):  
Biodun Badmus ◽  
Musa Awoyemi ◽  
Olukayode Akinyemi ◽  
Ganiyu Saheed ◽  
Oluwaseun Olurin
Keyword(s):  
Magnetic Field ◽  
Signal Amplitude ◽  
Analytic Signal ◽  
Maximum Depth ◽  
Horizontal Gradient ◽  
Aeromagnetic Data ◽  
Magnetic Gradient ◽  
Gradient Magnitude ◽  
Local Wavenumber ◽  
Intensity Magnetic Field

AbstractLocations and depths to magnetic contacts were estimated from the total intensity magnetic field using the Horizontal Gradient Magnitude (HGM), Analytic Signal Amplitude (ASA) and Local Wavenumber (LWN) methods. Aeromagnetic data from the Ibadan area, in south-western Nigeria, were analyzed to estimate depths to magnetic sources as well as source locations. The minimum/maximum depth limits of the HGM and LWN are relatively close and comparable, while shallow source depths limits are greater than expected in the ASA method when compared with the HGM and LWN functions.

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