Implications of magnetic secular variation for interpretation of crustal field anomalies

We usually think of crustal magnetic anomalies as static (barring some major seismic or thermal disruption).&#160; But a significant portion of the crustal magnetic field is caused by the interaction of magnetic minerals with the Earth&#8217;s magnetic field.&#160; This induced magnetic effect is dependent on the direction and magnitude of the ambient field.&#160; So, of course, as the Earth&#8217;s magnetic field changes over time, the form and magnitude of induced magnetic anomalies will vary as well.&#160; These changes will often be negligible for interpretation when compared with measurement and other interpretational uncertainties.&#160; However, with the reduction of various sources of measurement noise and increased fidelity of interpretation, these temporal anomaly changes may need to be considered.In addition to considerations relating to interpretation uncertainty, these temporal anomaly changes, if they are measured in multiple magnetic epochs, can theoretically provide valuable information for use in source inversion. &#160;For example, since crustal magnetic anomalies arise from a combination of induced (dependent the ambient field) and remanent (not dependent on ambient field) magnetic sources, measurements of secular magnetic variation can assist in separating these two sources during inversion.We will report modeling of the expected form and magnitude of predicted induced anomaly variations, the possible implications of these variations for data compilation and interpretation, and on the availability of relevant data for measuring them. &#160;Recent research into the use of high-resolution magnetic anomaly maps for airborne magnetic navigation has also brought the issue of changing magnetic fields into focus. &#160;Initial work indicates that changes in induced anomalies could affect navigation accuracy in certain situations.

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Was the moon magnetized by impact plasmas?

Science Advances ◽

10.1126/sciadv.abb1475 ◽

2020 ◽

Vol 6 (40) ◽

pp. eabb1475

Author(s):

Rona Oran ◽

Benjamin P. Weiss ◽

Yuri Shprits ◽

Katarina Miljković ◽

Gábor Tóth

Keyword(s):

Magnetic Field ◽

Interplanetary Magnetic Field ◽

Magnetic Anomalies ◽

The Moon ◽

Crustal Field ◽

Impact Simulations ◽

Magnetizing Field

The crusts of the Moon, Mercury, and many meteorite parent bodies are magnetized. Although the magnetizing field is commonly attributed to that of an ancient core dynamo, a longstanding hypothesized alternative is amplification of the interplanetary magnetic field and induced crustal field by plasmas generated by meteoroid impacts. Here, we use magnetohydrodynamic and impact simulations and analytic relationships to demonstrate that although impact plasmas can transiently enhance the field inside the Moon, the resulting fields are at least three orders of magnitude too weak to explain lunar crustal magnetic anomalies. This leaves a core dynamo as the only plausible source of most magnetization on the Moon.

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7. The Earth’s magnetic field

Geophysics: A Very Short Introduction ◽

10.1093/actrade/9780198792956.003.0007 ◽

2018 ◽

pp. 106-126

Author(s):

William Lowrie

Keyword(s):

Magnetic Field ◽

Geomagnetic Field ◽

Plate Tectonics ◽

Magnetic Anomalies ◽

Earth’S Magnetic Field ◽

The Earth ◽

Earth's Magnetic Field ◽

Reversed Polarity ◽

Harmful Radiation ◽

The Magnetic Field

The Earth is surrounded by a magnetic field, which originates inside its molten core, and which for centuries has helped travellers to navigate safely across uncharted regions. The magnetic field protects life on the Earth by acting as a shield against harmful radiation from space, especially from the Sun. ‘The Earth’s magnetic field’ explains that the magnetic field at the Earth’s surface is dominantly that of an inclined dipole. The Sun’s deforming effect on the magnetic field outside the Earth is described, as are the magnetic fields of other planets. The magnetism of rocks forms the basis of palaeomagnetism, which explains how plate tectonics displaced the continents and produced oceanic magnetic anomalies whenever the geomagnetic field reversed polarity.

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DERIVATION OF MAGNETIC AND GRAVITATIONAL QUANTITIES BY SURFACE INTEGRATION

Geophysics ◽

10.1190/1.1437527 ◽

1949 ◽

Vol 14 (2) ◽

pp. 133-150 ◽

Cited By ~ 7

Author(s):

D. C. Skeels ◽

R. J. Watson

Keyword(s):

Magnetic Field ◽

Horizontal Plane ◽

Plane Surface ◽

Vertical Component ◽

Magnetic Anomalies ◽

Magnetic Intensity ◽

Artificial Data ◽

The Earth ◽

Surface Integration ◽

Derivatives Of

In an earlier paper by one of the authors, it was pointed out that if the vertical component of a gravitational or magnetic field is known over a horizontal plane surface of sufficient extent, all other components and derivatives of the field can theoretically be calculated by surface integration of the vertical component. In this paper, examples are given to show that within certain limits such calculations are not only theoretically possible but practically so. Examples are given of the calculations of curvatures from observed gradients and of horizontal magnetic intensity from observed vertical intensity, and the calculated values are compared with those obtained by observation. Examples are also given, based on artificial data, of the calculation of plumb‐line deflections from gravity and of the calculation of magnetic anomalies at a given elevation above the earth from data obtained at the surface. The purpose of these calculations is to demonstrate the nonindependence of the various derivatives of gravitational and magnetic potential.

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Intermediate‐scale magnetic anomalies of the Earth

Geophysics ◽

10.1190/1.1441902 ◽

1985 ◽

Vol 50 (12) ◽

pp. 2817-2830 ◽

Cited By ~ 29

Author(s):

J. Arkani‐Hamed ◽

D. W. Strangway

Keyword(s):

Magnetic Field ◽

External Magnetic Field ◽

Magnetic Anomaly ◽

Correlation Coefficients ◽

Sedimentary Basins ◽

Magnetic Anomalies ◽

The Earth ◽

Asymmetric Component ◽

Anomaly Map ◽

Well Correlation

Four separate magnetic anomaly maps of the earth are derived from magnetometer satellite data acquired at dawn and at dusk using two different altitude ranges. The magnetic anomalies on the two dawn maps (or dusk maps) are well correlated for spherical harmonics of degree less than 51, suggesting that the time varying external magnetic field and leveling noise have negligible effects on these harmonics. Dawn and dusk maps have an appreciable asymmetric component for harmonics of degree n ⩽ 5 and n = 15 and 17, arising from the quasi‐stable external magnetic field. Dawn‐dusk covariant harmonics of degree [Formula: see text] with signal‐to‐noise ratios greater than 1.5 correlate well. Correlation coefficients are higher than 0.75, implying that these harmonics can be repeatably derived. A global scalar magnetic anomaly map is derived based on these harmonics. The map is then converted to a magnetic susceptibility anomaly map by an inversion technique. The susceptibility anomalies delineate the ocean‐continent differences as well as the boundaries of tectonic provinces, modern uplifts, crustal rifts, and sedimentary basins.

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Magnetic data processing for hydrocarbon exploration in the Pannonian Basin, Yugoslavia

Geophysics ◽

10.1190/1.1486769 ◽

2001 ◽

Vol 66 (6) ◽

pp. 1669-1679 ◽

Cited By ~ 3

Author(s):

Milenko Burazer ◽

Milinko Grbović ◽

Vicko Žitko

Keyword(s):

Magnetic Field ◽

Oil And Gas ◽

Pannonian Basin ◽

Magnetic Anomalies ◽

Hydrocarbon Exploration ◽

Oil Fields ◽

Window Function ◽

Magnetic Data ◽

Band Pass Filter ◽

Magnetic Minerals

Because magnetic minerals may directly indicate the presence of oil and gas deposits, magnetic methods are applied to hydrocarbon exploration in oil‐bearing sedimentary basins. The basic problem in applying these methods is the isolation of weak magnetic anomalies sourced by low concentrations of the magnetic minerals present. These weak anomalies are often masked by much stronger magnetic anomalies caused by underlying magnetic rocks and/or by rocks in the basin sediments. Weak local anomalies can efficiently be isolated by applying selective 1‐D digital frequency filters. The method of filtering has been checked by data obtained using simple models of magnetic sources and using a model representative of the local geology in our study area in the southern Pannonian basin, Yugoslavia, The magnetic field frequency content was analyzed by applying the power spectral density estimation, using the maximum entropy method. The digital filters were designed using the window function method. The best results were obtained by the Kaiser window function for the chosen range of the band‐pass filter. In our study area, me isolated local magnetic anomalies have amplitudes of ±10 nT and trend in an east‐west direction parallel to the predominant structural grain. These anomalies correlate very well with the known oil and gas fields. As an example, filter processing of magnetic anomaly data, combined with the 3‐D seismic data gained in the filtered magnetic field, correlate well with one of the oil fields. The next phase of the project will concentrate on the anomalies occurring outside the established gas and oil fields.

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International geomagnetic reference field—Its evolution and the difference in total field intensity between new and old models for 1965–1980

Geophysics ◽

10.1190/1.1441450 ◽

1983 ◽

Vol 48 (12) ◽

pp. 1691-1696 ◽

Cited By ~ 17

Author(s):

Norman W. Peddie

Keyword(s):

Magnetic Field ◽

Field Intensity ◽

Upper Atmosphere ◽

Reference Field ◽

Total Field ◽

Main Field ◽

International Geomagnetic Reference Field ◽

The Earth ◽

Crustal Field ◽

The Difference

The total magnetic field near the surface of the Earth is a sum of several constituent fields. Part of the total field consists of fields that are transient or rapidly varying. These fields are caused, either directly or indirectly, by electric currents in the upper atmosphere and beyond. The part of the total field that is more permanent arises from sources that are located inside the Earth. Evidence suggests that this part has two principal constituents: the main field and the crustal field.

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