An assessment of long-wavelength magnetic anomalies over Canada

1996 ◽  
Vol 33 (1) ◽  
pp. 12-23 ◽  
Author(s):  
Mark Pilkington ◽  
Walter R. Roest
Keyword(s):  
Magnetic Field ◽  
Magnetic Anomalies ◽  
Time Span ◽  
Magnetic Data ◽  
Data Sets ◽  
Independent Data ◽  
Data Set ◽  
Long Wavelength ◽  
The Magnetic Field

The reliability of the long-wavelength portion (> 300 km) of the magnetic field over Canada, as represented by the national aeromagnetic anomaly database compiled by the Geological Survey of Canada (GSC), is assessed by comparison with two independent data sets: a high-altitude country-wide survey carried out by the former Earth Physics Branch (EPB) and data from the MAGSAT and POGO satellite missions. The different altitudes at which each data set was measured (300 m, ~4 km, and ~400 km), and their different resolution and time span of observations allow a determination of the integrity of selected wavelength bands in each data set. The (upward-continued) EPB and MAGSAT–POGO fields compare well for wavelengths of 300–2500 km. The GSC data show significant differences to the former, indicating that the levelling and merging of several hundred individual surveys has degraded the longer wavelength components of the magnetic field. Replacing the GSC wavelength components >300 km with those from the EPB field produces a magnetic data set containing more dependable information within the largest possible waveband.

Automatic boundary extraction from magnetic field data using triangular meshes

Geophysics ◽  
2016 ◽  
Vol 81 (3) ◽  
pp. J47-J60 ◽  
Author(s):  
Nathan Leon Foks ◽  
Yaoguo Li
Keyword(s):  
Magnetic Field ◽  
Synthetic Data ◽  
Magnetic Data ◽  
Lake Area ◽  
Boundary Extraction ◽  
Data Set ◽  
Magnetic Field Data ◽  
Line Segments ◽  
Extraction Algorithm ◽  
The Magnetic Field

Boundary extraction is a collective term that we use for the process of extracting the locations of faults, lineaments, and lateral boundaries between geologic units using geophysical observations, such as measurements of the magnetic field. The process typically begins with a preprocessing stage, where the data are transformed to enhance the visual clarity of pertinent features and hence improve the interpretability of the data. The majority of the existing methods are based on raster grid enhancement techniques, and the boundaries are extracted as a series of points or line segments. In contrast, we set out a methodology for boundary extraction from magnetic data, in which we represent the transformed data as a surface in 3D using a mesh of triangular facets. After initializing the mesh, we modify the node locations, such that the mesh smoothly represents the transformed data and that facet edges are aligned with features in the data that approximate the horizontal locations of subsurface boundaries. To illustrate our boundary extraction algorithm, we first apply it to a synthetic data set. We then apply it to identify boundaries in a magnetic data set from the McFaulds Lake area in Ontario, Canada. The extracted boundaries are in agreement with known boundaries and several of the regions that are completely enclosed by extracted boundaries coincide with regions of known mineralization.

scholarly journals Characterization of the umbra–penumbra boundary by the vertical component of the magnetic field

2020 ◽  
Vol 638 ◽  
pp. A25
Author(s):  
P. Lindner ◽  
R. Schlichenmaier ◽  
N. Bello González
Keyword(s):  
Magnetic Field ◽  
Near Infrared ◽  
Statistical Study ◽  
Vertical Component ◽  
Fundamental Property ◽  
Field Vector ◽  
Data Sets ◽  
Data Set ◽  
Geometric Difference ◽  
The Magnetic Field

Context. The vertical component of the magnetic field was found to reach a constant value at the boundary between penumbra and umbra of stable sunspots in a recent statistical study of Hinode/SP data. This finding has profound implications as it can serve as a criterion to distinguish between fundamentally different magneto-convective modes operating in the sun. Aims. The objective of this work is to verify the existence of a constant value for the vertical component of the magnetic field (B⊥) at the boundary between umbra and penumbra from ground-based data in the near-infrared wavelengths and to determine its value for the GREGOR Infrared Spectrograph (GRIS@GREGOR) data. This is the first statistical study on the Jurčák criterion with ground-based data, and we compare it with the results from space-based data (Hinode/SP and SDO/HMI). Methods. Eleven spectropolarimetric data sets from the GRIS@GREGOR slit-spectograph containing fully-fledged stable sunspots were selected from the GRIS archive. SIR inversions including a polarimetric straylight correction are used to produce maps of the magnetic field vector using the Fe I 15648 Å and 15662 Å lines. Averages of B⊥ along the contours between penumbra and umbra are analyzed for the 11 data sets. In addition, contours at the resulting B⊥const are drawn onto maps and compared to intensity contours. The geometric difference between these contours, ΔP, is calculated for each data set. Results. Averaged over the 11 sunspots, we find a value of B⊥const = (1787 ± 100) gauss. The difference from the values previously derived from Hinode/SP and SDO/HMI data is explained by instrumental differences and by the formation characteristics of the respective lines that were used. Contours at B⊥ = B⊥const and contours calculated in intensity maps match from a visual inspection and the geometric distance ΔP was found to be on the order of 2 pixels. Furthermore, the standard deviation between different data sets of averages along umbra–penumbra contours is smaller for B⊥ than for B∥ by a factor of 2.4. Conclusions. Our results provide further support to the Jurčák criterion with the existence of an invariable value B⊥const at the umbra–penumbra boundary. This fundamental property of sunspots can act as a constraining parameter in the calibration of analysis techniques that calculate magnetic fields. It also serves as a requirement for numerical simulations to be realistic. Furthermore, it is found that the geometric difference, ΔP, between intensity contours and contours at B⊥ = B⊥const acts as an index of stability for sunspots.

scholarly journals Modelling of the ring current in Saturn's magnetosphere

2004 ◽  
Vol 22 (2) ◽  
pp. 653-659 ◽  
Author(s):  
G. Giampieri ◽  
M. K. Dougherty
Keyword(s):  
Magnetic Field ◽  
Ring Current ◽  
Current Model ◽  
Temporal Variations ◽  
Data Sets ◽  
Data Set ◽  
Equatorial Current ◽  
The Difference ◽  
Current Systems ◽  
The Magnetic Field

Abstract. The existence of a ring current inside Saturn's magnetosphere was first suggested by Smith et al. (1980) and Ness et al. (1981, 1982), in order to explain various features in the magnetic field observations from the Pioneer 11 and Voyager 1 and 2 spacecraft. Connerney et al. (1983) formalized the equatorial current model, based on previous modelling work of Jupiter's current sheet and estimated its parameters from the two Voyager data sets. Here, we investigate the model further, by reconsidering the data from the two Voyager spacecraft, as well as including the Pioneer 11 flyby data set. First, we obtain, in closed form, an analytic expression for the magnetic field produced by the ring current. We then fit the model to the external field, that is the difference between the observed field and the internal magnetic field, considering all the available data. In general, through our global fit we obtain more accurate parameters, compared to previous models. We point out differences between the model's parameters for the three flybys, and also investigate possible deviations from the axial and planar symmetries assumed in the model. We conclude that an accurate modelling of the Saturnian disk current will require taking into account both of the temporal variations related to the condition of the magnetosphere, as well as non-axisymmetric contributions due to local time effects. Key words. Magnetospheric physics (current systems; planetary magnetospheres; plasma sheet)

Mitigating remanent magnetization effects in magnetic data using the normalized source strength

Geophysics ◽  
2013 ◽  
Vol 78 (3) ◽  
pp. J25-J32 ◽  
Author(s):  
Mark Pilkington ◽  
Majid Beiki
Keyword(s):  
Magnetic Field ◽  
Remanent Magnetization ◽  
Field Data ◽  
Magnetic Data ◽  
Magnetization Distribution ◽  
Source Strength ◽  
Inversion Algorithm ◽  
Data Set ◽  
Magnetic Field Data ◽  
The Magnetic Field

We have developed an approach for the interpretation of magnetic field data that can be used when measured anomalies are affected by significant remanent magnetization components. The method deals with remanent effects by using the normalized source strength (NSS), a quantity calculated from the eigenvectors of the magnetic gradient tensor. The NSS is minimally affected by the direction of remanent magnetization present and compares well with other transformations of the magnetic field that are used for the same purpose. It therefore offers a way of inverting magnetic data containing the effects of remanent magnetizations, particularly when these are unknown and are possibly varying within a given data set. We use a standard 3D inversion algorithm to invert NSS data from an area where varying remanence directions are apparent, resulting in a more reliable image of the subsurface magnetization distribution than possible using the observed magnetic field data directly.

scholarly journals Characteristics of the Taylor microscale in the solar wind/foreshock: magnetic field and electron velocity measurements

2013 ◽  
Vol 31 (11) ◽  
pp. 2063-2075 ◽  
Author(s):  
C. Gurgiolo ◽  
M. L. Goldstein ◽  
W. H. Matthaeus ◽  
A. Viñas ◽  
A. N. Fazakerley
Keyword(s):  
Magnetic Field ◽  
Field Data ◽  
Fluid Velocity ◽  
Electron Velocity ◽  
Data Sets ◽  
Velocity Data ◽  
Data Set ◽  
Magnetic Field Data ◽  
Spacecraft Data ◽  
The Magnetic Field

Abstract. The Taylor microscale is one of the fundamental turbulence scales. Not easily estimated in the interplanetary medium employing single spacecraft data, it has generally been studied through two point correlations. In this paper we present an alternative, albeit mathematically equivalent, method for estimating the Taylor microscale (λT). We make two independent determinations employing multi-spacecraft data sets from the Cluster mission, one using magnetic field data and a second using electron velocity data. Our results using the magnetic field data set yields a scale length of 1538 ± 550 km, slightly less than, but within the same range as, values found in previous magnetic-field-based studies. During time periods where both magnetic field and electron velocity data can be used, the two values can be compared. Relative comparisons show λT computed from the velocity is often significantly smaller than that from the magnetic field data. Due to a lack of events where both measurements are available, the absolute λT based on the electron fluid velocity is not able to be determined.

Integration of aeromagnetic data acquired at different times with varying elevations and line spacings

Geophysics ◽  
1979 ◽  
Vol 44 (4) ◽  
pp. 742-752 ◽  
Author(s):  
B. K. Bhattacharyya ◽  
R. E. Sweeney ◽  
R. H. Godson
Keyword(s):  
Ground Surface ◽  
Magnetic Data ◽  
Reference Level ◽  
Data Sets ◽  
Observation Data ◽  
Aeromagnetic Data ◽  
Data Set ◽  
Using Data ◽  
Quadratic Surfaces ◽  
The Magnetic Field

The methods required for integrating magnetic data sets from surveys flown with contrasting specifications at different times are described. The first requirement in the process of integration is to bring the data sets obtained at different elevations to the same reference level. Various methods are available to continue magnetic fields from one horizontal plane to another. The procedure for continuation of the magnetic field from a draped surface to a constant barometric level requires an equivalent source representation of the field at all points of observation. Data sets continued to the same elevation over adjacent areas are found to exhibit mismatch along the area boundaries. The mismatch between data values along profiles running across a boundary can be explained by changes in level, gradient, and curvature of the fields in the two data sets in the neighborhood of the boundary. The method discussed for making the two fields compatible depends, basically, upon the adjustment of quadratic surfaces representing the fields on both sides of a boundary. Examples are also provided to demonstrate the usefulness of these methods in the preparation of an integrated data set at a reference level using data sets flown at different elevations above the ground surface in the state of Nevada.

scholarly journals The INGV tectonomagnetic network

2008 ◽  
Vol 14 ◽  
pp. 65-68 ◽  
Author(s):  
F. Masci ◽  
P. Palangio ◽  
M. Di Persio
Keyword(s):  
Magnetic Field ◽  
Geomagnetic Field ◽  
Magnetic Field Intensity ◽  
Central Italy ◽  
Magnetic Anomalies ◽  
Data Sets ◽  
Intensity Data ◽  
Geomagnetic Observatory ◽  
Data Set ◽  
Latitude Range

Abstract. The Italian Istituto Nazionale di Geofisica e Vulcanologia (INGV) tectonomagnetic network was installed in Central Italy since the middle of 1989 to investigate possible magnetic anomalies related to earthquakes. The network is part of the INGV L'Aquila Geomagnetic Observatory and is located in an area extending approximately in latitude range [41.6°–42.8°] N and longitude range [13.0°–14.3°] E. Actually the network consists of four stations where the total magnetic field intensity data are collected using proton precession magnetometers. New stations will be added to the network starting from the end of 2007. Here we are reporting the whole data set of the network's stations for the period 2004–2006. No significant anomaly in the local geomagnetic field correlated to the seismic activity has been found. Some considerations about misleading structures present in the data sets are reported.

VECTOR MAGNETIC ANOMALIES DERIVED FROM MEASUREMENTS OF A SINGLE COMPONENT OF THE FIELD

Geophysics ◽  
1973 ◽  
Vol 38 (2) ◽  
pp. 359-368 ◽  
Author(s):  
José Seixas Lourenco ◽  
H. Frank Morrison
Keyword(s):  
Magnetic Field ◽  
Fourier Transform ◽  
Inverse Fourier Transform ◽  
Double Fourier Series ◽  
Magnetic Anomalies ◽  
Single Component ◽  
Magnetic Data ◽  
Simple Relationship ◽  
Magnetic Components ◽  
The Magnetic Field

Three‐component magnetic data are derivable from measurements of one single component of the magnetic field over a plane. The technique involves computation of the double‐Fourier‐series coefficients of the measured magnetic anomaly, multiplication of the coefficients by a filter operator, and, finally, evaluation of the magnetic components by taking the inverse Fourier transform. The desired filter operator is obtained from a simple relationship between the components of a potential field. The scheme has been tested with excellent results on the fields of a vertical prismatic model.

The automatic determination of the location, depth, and dip of contacts from aeromagnetic data

Geophysics ◽  
2014 ◽  
Vol 79 (3) ◽  
pp. J35-J41 ◽  
Author(s):  
Gordon R. J. Cooper
Keyword(s):  
Magnetic Field ◽  
Tilt Angle ◽  
Synthetic Data ◽  
Magnetic Data ◽  
Aeromagnetic Data ◽  
Third Order ◽  
Contour Lines ◽  
Derivatives Of ◽  
The Magnetic Field

A simple new method (termed the contact-depth method) for the determination of the depth, location, and dip of contacts from pole reduced magnetic data was evaluated. The depth was obtained by computing the horizontal derivative of the tangent of the tilt angle of the magnetic field over the contact. Although it is based upon the tilt-depth method, it does not require the distance between contour lines to be measured, and it additionally allows the dip of the contact to be estimated from the gradient of the depth estimates. The horizontal location of the contact is that of the zero value of the tilt angle. The method uses second- and third-order derivatives of the magnetic field to obtain the parameters of the contact, so it is sensitive to noise. When tested on synthetic data and on aeromagnetic data from southern Africa, the method gave plausible results.

scholarly journals Joint analysis of the magnetic field and Total Gradient Intensity in Central Europe

2019 ◽  
Author(s):  
Maurizio Milano ◽  
Maurizio Fedi ◽  
J. Derek Fairhead
Keyword(s):  
Magnetic Field ◽  
Central Europe ◽  
Joint Analysis ◽  
East European ◽  
Long Wavelength ◽  
Low Degree ◽  
Total Gradient ◽  
Synthetic Models ◽  
The Magnetic Field ◽  
Different Altitudes

Abstract. In the European region, the magnetic field at satellite altitudes (~ 350 km) is mainly defined by a long-wavelength magnetic-low called here the Central Europe Magnetic Low (CEML), located to the southwest of the Trans European Suture Zone (TESZ). We studied this area by a joint analysis of the magnetic and total gradient (∇T) anomaly maps, for a range of different altitudes of 5 km, 100 km and 350 km. Tests on synthetic models showed the usefulness of the joint analysis at various altitudes to identify reverse dipolar anomalies and to characterize areas in which magnetization is weak. By this way we identified areas where either reversely or normally magnetized sources are locally dominant. At a European scale these anomalies are sparse, with a low degree of coalescence effect. The ∇T map indeed presents generally small values within the CEML area, indicating that the Palaeozoic Platform is weakly magnetized. At 350 km altitude, the TESZ effect is largely dominant: with intense ∇T highs above the East European Craton (EEC) and very small values above the Palaeozoic Platform, this again denoting a weakly magnetized crust. Small coalescence effects are masked by the trend of the TESZ. Although we identified sparsely located reversely magnetized sources in the Palaeozoic Platform of the CEML, the joint analysis does not support a model of a generally reversely magnetized crust. Instead, our analysis strongly favors the hypothesis that the CEML anomaly is mainly caused by a sharp contrast between the magnetic properties of EEC and Palaeozoic Platform.

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