Application of dipole modeling to magnetic anomalies

Schmitz, Dave R.; Frayser, Jeffory; Cain, J. C.

doi:10.1029/gl009i004p00307

Cited by 11 publications

(3 citation statements)

References 5 publications

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“…When satisfactory solutions are found, however, the method can be very efficient in representing short-wavelength features [Mayhew, 1979[Mayhew, , 1982Mayhew et al, 1980;yon Frese et al, 1981]. Schmitz et al [1982] suggest that rather than using an array of dipoles of fixed positions, it may be sufficient to fit a small number of dipoles if not only the dipole magnitudes are determined but also their directions and positions. In this case, the anomalies are calculated relative to a spherical harmonic model of degree greater than 20 rather than the customary 13.…”

Section: Introductionmentioning

confidence: 99%

Magsat vertical field anomalies above 40°N from spherical cap harmonic analysis

Haines¹

1985

J. Geophys. Res.

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A new technique, the method of spherical cap harmonic analysis, has provided the means to construct an analytical model of the vertical field above 40°N from Magsat data. With this technique, proper consideration can be taken of altitude variation and of the source‐free constraint that the vertical field be the vertical derivative of a potential function. Maps can be produced at any altitude. Here they are given at the lower and upper limits of the Magsat satellite, 6700 and 6900 km radial distance or approximately 335 and 535 km geodetic altitude. A novel decimation procedure, applied to NASA “Investigator B” data screened for low disturbance levels, results in an approximately uniform distribution in area, about 1 point per (150 km)2. Each of the 5087 data points represents an average over 5 s of time or 37 km of satellite track. The harmonic model, of maximum index 22, includes only those basis functions that are necessary for fitting a potential which need not be differentiable in colatitude and that are therefore completely orthogonal over the spherical cap. Although there are 276 coefficients in the theoretical model, only 190 were found to be statistically significant. The resulting scatter, or standard error of estimate, was 3.6 nT.

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Section: Introductionmentioning

confidence: 99%

Magsat vertical field anomalies above 40°N from spherical cap harmonic analysis

Haines¹

1985

J. Geophys. Res.

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“…The ratio of anomaly maximum to minimum indicated a near-horizontal inclination, implying a strong remanent magnetization, and an origin for Broken Ridge much nearer the equator. A similar result was found by Schmitz et al [1982]. This is in conflict with paleomagnetic data, indicating that Broken Ridge has never been farther from the pole than now [Schmidt and Embleton, 1982].…”

Section: Models For Interpretationmentioning

confidence: 52%

A review of problems and progress in studies of satellite magnetic anomalies

1985

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The incomplete removal of time‐varying effects in the Magsat data leads to the necessity of fitting low‐order polynomial functions in order to minimize differences between passes. This process makes the estimation of the zero level in the crustal anomaly field very difficult. Refinements in main field analysis, ring current estimation, and the identification of other time‐varying field sources should provide us with a physical realization of these differences. The spectral overlap of core and crustal anomaly fields makes it particularly difficult to interpret the longer wavelengths of crustal anomalies and refined filtering techniques may need to be developed to improve their resolution. Equivalent source techniques for the inversion of the satellite data to a constant thickness, varying‐magnetization source layer solution are well developed. Magnetization and reduced‐to‐the‐pole anomaly maps are now available for many regions. Problems of instability of mathematical inversions are still present and are particularly noticeable near the magnetic equator. The equivalent source solutions contain the same lack of information regarding zero level as the data from which they are derived. Forward modeling techniques are available to model discrete source distributions and are useful in testing among various interpretive hypotheses. Forward modeling techniques are inappropriate for complex geological situations, and some hybrid techniques are envisaged, in which the inverted magnetizations are constrained by some a priori estimates of their distribution and variability in order to provide geologically meaningful solutions. There is now sufficient information on the Curie points and magnetic properties of rocks which have been derived from the deep crust and upper mantle to begin to develop magnetization‐depth models for a number of tectonic environments. While the familiar Fe‐Ti oxides are the magnetic minerals characteristic of the crust, relatively nonmagnetic chromium spinels and magnesian ilmenites are characteristic of the subcontinental mantle; thus the Moho is the magnetic bottom of the continents except where the Curie isotherm is elevated within the crust. Intensity of magnetization in the continental crust is strongly dependent on metamorphic grade and, in general, increases with basicity: mafic granulites tend to form the most magnetic laterally extensive zones of the crust. Some large‐scale mafic plutonic complexes also have very large magnetizations. Model studies and laboratory measurements are in agreement that the more magnetic rocks of the crust typically have magnetization values in the range 2–6 A/m. In the deep crust, Curie points are predominantly near that of magnetite, although in hot, anhydrous environments in active tectonic zones, Curie temperatures may be reduced below 300°C. A comprehensive laboratory program to characterize further the magnetic properties of all lithologies important in the crust is in progress, and much work remains. The goal is an understanding of how crustal evolutionary p...

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“…An isolated anomaly of the crustal field may be approximated by a single magnetic dipole or an array of dipoles (e.g., SCHMITZ et al, 1982). To show more clearly the significance of the long wavelength components of the crustal field, we examine the Fourier spectra of the field due to a shallow-seated dipole.…”

Section: Significance Of Long Wavelength Components In Crustal Fieldmentioning

confidence: 99%

Understanding models of the geomagnetic field by Fourier analysis.

Wang

1987

J. geomagn. geoelec

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Fourier spectra of X, Y, and Z components for both latitudinal and longitudinal circular profiles are deduced analytically from a n*=64 spherical harmonic model of the internal geomagnetic field. Expressions and numerical results demonstrate that the spectra behave differently for different field components and different profiles. Simplified anomaly model studies indicate that, despite being much less in amplitude with respect to the spectra of the core field, the spectra with long wavelengths of crustal field are generally more significant than those with short wavelengths of the same source. Wavelength should not be used as a criterion to discuss the depth of the field sources. The crustal field may be seriously distorted by high-pass filtering, whereas truncation at the commonly accepted level of spherical harmonic model of the core field has little effect on localized patterns of the anomalies.

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Application of dipole modeling to magnetic anomalies

Cited by 11 publications

References 5 publications

Magsat vertical field anomalies above 40°N from spherical cap harmonic analysis

Magsat vertical field anomalies above 40°N from spherical cap harmonic analysis

A review of problems and progress in studies of satellite magnetic anomalies

Understanding models of the geomagnetic field by Fourier analysis.

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