Gravity and Magnetic Exploration

Hinze, William J.; Frese, Ralph R. B. von; Saad, Afif H.

doi:10.1017/cbo9780511843129

Cited by 349 publications

(171 citation statements)

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“…Aeromagnetic anomalies reflect lateral variations of total magnetization, the vector sum of induced and remanent magnetizations (Blakely, 1995;Hinze et al, 2013). Induced magnetization is proportional to magnetic susceptibility and has the same direction as the present day ambient field (inclination 68°and declination 3°in the study area).…”

Section: Aeromagnetic Datamentioning

confidence: 99%

Geophysical expression of a buried niobium and rare earth element deposit: The Elk Creek carbonatite, Nebraska, USA

Drenth

2014

Interpretation

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The lower Paleozoic Elk Creek carbonatite is a 6-8-km-diameter intrusive complex buried under 200 m of sedimentary rocks in southeastern Nebraska. It hosts the largest known niobium deposit in the U.S. and a rare earth element (REE) deposit. The carbonatite is composed of several lithologies, the relations of which are poorly understood. Niobium mineralization is most enriched within a magnetite beforsite (MB) unit, and REE oxides are most concentrated in a barite beforsite unit. The carbonatite intrudes Proterozoic country rocks. Efforts to explore the carbonatite have used geophysical data and drilling. A high-resolution airborne gravity gradient and magnetic survey was flown over the carbonatite in 2012. The carbonatite is associated with a roughly annular vertical gravity gradient high and a subdued central low and a central magnetic high surrounded by magnetic field values lower than those over the country rocks. Geophysical, borehole, and physical property data are combined for an interpretation of these signatures. The carbonatite is denser than the country rocks, explaining the gravity gradient high. Most carbonatite lithologies have weaker magnetic susceptibilities than those of the country rocks, explaining why the carbonatite does not produce a magnetic high at its margin. The primary source of the central magnetic high is interpreted to be mafic rocks that are strongly magnetized and are present in large volumes. MB is very dense (mean density 3200 kg∕m 3 ) and strongly magnetized (median 0.073 magnetic susceptibility), producing a gravity gradient high and contributing to the aeromagnetic high. Barite beforsite has physical properties similar to most of the carbonatite volume, making it a poor geophysical target. Geophysical anomalies indicate the presence of dense and strongly magnetized rocks at depths below existing boreholes, either a large volume of MB or another unknown lithology.

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Section: Aeromagnetic Datamentioning

confidence: 99%

Geophysical expression of a buried niobium and rare earth element deposit: The Elk Creek carbonatite, Nebraska, USA

Drenth

2014

Interpretation

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“…For a negligible remanent magnetization, the magnetic anomaly inversion yields the susceptibility distribution, which is equal to the total magnetization intensity divided by the geomagnetic field intensity. In this case, the Koenigsberger ratio (Q) (Clark & Emerson, 1991;Hinze et al, 2013)…”

Section: Extraction Of Remanent Magnetization and True Susceptibilitymentioning

confidence: 99%

Extracting Induced and Remanent Magnetizations From Magnetic Data Modeling

Liu

Fedi

et al. 2018

JGR Solid Earth

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To investigate the crustal magnetic structure, it is important to assess the susceptibility and remanence properties of rocks and ores. In this paper, we propose a method to extract the contributions of induced and remanent magnetization from modeling of magnetic anomalies. We first estimate the direction of the total magnetization vector by studying the reduced‐to‐pole anomaly and its correlation with different magnitude magnetic transforms. Then we invert the magnetic data to obtain the volumetric distribution of the magnetization intensity. As the third step, based on a priori information about the Koenigsberger ratio derived from petrophysical measurements, we extract the distributions in the source volume of the induced and remanent magnetization intensities, based on a generalized relationship involving the total and remanent magnetizations, and the true susceptibility. In this way, we are able to produce separate maps of the anomaly fields attributed to the physical magnetic source parameters: remanent and induced magnetization. After validating the method with synthetic data, we analyze the data relative to the Mesozoic and Cenozoic igneous rocks in Yeshan region, eastern China. The analysis of the separated magnetization components reveals that the intrusion of dioritic and basaltic rocks occurred at different geological periods, and the basaltic rocks were magnetized by a reversed geomagnetic field. The uncertainty analysis shows that a larger Koenigsberger ratio is beneficial to extract more reliable remanence and susceptibility information.

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“…This feature could perhaps be also used as an input in a prospectivity model if adequate geochemistry of hosting lithological units was available. A high-pass filter was applied to the gridded magnetic and gravity data to suppress long wavelength regional anomalies and enhance the locally derived anomalies possibly caused by mafic-ultramafic intrusions and associated mineral deposits (Hinze et al, 2013). In a GIS, filtering can be undertaken using a moving window of a fixed radius across the study area to calculate the local median value, which is subtracted from the original data values (e.g., Nykänen et al, 2008b).…”

Section: Input Datasetsmentioning

confidence: 99%