Performance of airborne gravity gradiometers

Dransfield, Mark; Christensen, Asbjørn Nørlund

doi:10.1190/tle32080908.1

Cited by 77 publications

(35 citation statements)

References 14 publications

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“…Because AGG data are the derivatives of the gravity field, AGG anomalies better reflect shallower density contrasts than do gravity anomalies, as they emphasize shorter wavelength components of the field that are typically related to shallow sources. The AGG data are processed to remove types of noise inherent to gradiometer systems and measurements, as well as noise common to all types of airborne geophysical data (Dransfield and Christensen, 2013).…”

Section: Airborne Gravity Gradient and Gravity 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: Airborne Gravity Gradient and Gravity 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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“…Because AGG data are the derivatives of the gravity field, AGG anomalies reflect shallower density contrasts than do gravity anomalies. The AGG data were processed to remove types of noise inherent to gradiometer systems and measurements, as well as noise common to all types of airborne geophysical data (see Dransfield and Christensen, 2013, and references therein). Free air vertical gradient data are shown in Fig.…”

Section: Airborne Gravity Gradient Datamentioning

confidence: 99%

Airborne gravity gradient expression of a buried niobium and rare earth element deposit: Elk Creek carbonatite, Nebraska

Drenth

2014

SEG Technical Program Expanded Abstracts 2014

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The Elk Creek carbonatite, a buried intrusion in southeast Nebraska, hosts the largest known niobium deposit in the U.S. and a rare earth element deposit. Airborne gravity gradient (AGG) data are interpreted to study the signature of the complex, using such tools as terrain corrections, matched filtering, lineament mapping, and forward modeling. The carbonatite produces a vertical gravity gradient high, explained by high densities compared to country rocks. An unusually dense carbonatite unit, magnetite beforsite, hosts niobium ore and sources a smaller central gravity gradient high. AGG data indicate the presence of dense rocks below existing boreholes, thought to be a large volume of magnetite beforsite.

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“…Finally, to compare the calculated data with the measured data, the measured data can be filtered with the same Gaussian low-pass filter. In practice, the measured GGT data are already low-pass filtered along the flight lines so that the smallest wavelength is approximately equal to the distance between the flight lines (Dransfield and Christensen, 2013) corresponding to a random noise between 3 and 6 E, (Eötvös) depending on the system used. Thus, a reasonable goal is that the calculated terrain corrections be accurate within those limits.…”

Section: Introductionmentioning

confidence: 99%

Gravity gradient and magnetic terrain effects for airborne applications — A practical fast Fourier transform technique

2015

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We have implemented a practical fast Fourier transform technique for fast and approximate calculation of terrain effects for airborne measurement of the gravity gradient tensor and the total magnetic field. The calculations proceed in two steps. Starting from a digital terrain model (DTM), we first calculate the fields on a plane surface lying above the highest point of the terrain in the selected area. This calculation can be made arbitrarily accurate by including a sufficiently large number of terms in Parker's well-known Fourier transform technique. The second step involves a downward continuation of the fields to a draped surface describing the positions of the airborne measurements. The inherent instability of downward continuation through the level of the highest terrain is compensated for by low-pass filtering the calculated fields on the plane surface prior to downward continuation. We use a Gaussian filter with cutoff wavenumbers well below the Nyquist wavenumber corresponding to a wavelength equal to the distance between flight lines. Tests on synthetic data as well as on real data from a DTM from northern Sweden demonstrated that the method works well and provides a low-pass-filtered version of the true terrain effect.

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Performance of airborne gravity gradiometers

Cited by 77 publications

References 14 publications

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

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

Airborne gravity gradient expression of a buried niobium and rare earth element deposit: Elk Creek carbonatite, Nebraska

Gravity gradient and magnetic terrain effects for airborne applications — A practical fast Fourier transform technique

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