Metadata : Northern Territory Statewide Geophysics - Gravity

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Metadata Details:

Name: AS/NZS ISO 19115 Geographic Information - Metadata, ANZLIC Metadata Profile

Version: 1.0

Date Metadata Extracted: 2020-08-03

Date Metadata Last Updated: 2019-11-19

Current URL (HTML format) : http://www.ntlis.nt.gov.au/metadata/export_data?type=html&metadata_id=FAD227C240E247A8E040CD9B214430D3

Current URL (XML format) : http://www.ntlis.nt.gov.au/metadata/export_data?type=xml&metadata_id=FAD227C240E247A8E040CD9B214430D3


Citation

ANZLIC Identifier: FAD227C240E247A8E040CD9B214430D3

Title: Northern Territory Statewide Geophysics - Gravity

Citation Date: 2007-03-01

Date Type: creation

Custodian: Department of Primary Industry and Resources


Description

Abstract:

On a background of older GA-collected gravity data, further surveys by NTGS and exploration companies have been merged, gridded and imaged on the NT-wide scale. Mostly collected on the ground, gravity data has also been collected from an aircraft.

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Dataset Status

Dataset ID:

Language: English

Character Set: Latin 1

Progress: completed

Maintenance and Update Frequency: annually

Data Currency Start Date: 1950-01-01

Data Currency End Date:

Access Constraint: Copyright Northern Territory of Australia (Northern Territory Geological Survey) 2018
With the exception of the Northern Territory of Australia logo, other government and corporate logos and where otherwise noted, all material in this publication is provided under a Creative Commons Attribution 4.0 International licence (https://creativecommons.org/licenses/by/4.0/legalcode).
You are free to re-use the work under the licence, on the condition that you attribute the Northern Territory of Australia (Northern Territory Geological Survey) and comply with the other licence terms.

Disclaimer: While all care has been taken to ensure that information contained in this publication is true and correct at the time of publication, changes in circumstances after the time of publication may impact on the accuracy of its information. The Northern Territory of Australia gives no warranty or assurance, and makes no representation as to the accuracy of any information or advice contained in this publication, or that it is suitable for your intended use. You should not rely upon information in this publication for the purpose of making any serious business or investment decisions without obtaining independent and/or professional advice in relation to your particular situation. The Northern Territory of Australia disclaims any liability or responsibility or duty of care towards any person for loss or damage caused by any use of, or reliance on the information contained in this publication.


Data Quality

Lineage: Point datasets of Bouguer gravity have been sourced from NTGS and GA (including AGSO and BMR) and supplemented with data from open file company reports. Data is processed within NTGS to produce NT-wide geophysical grids and images. Grids and images are periodically updated and re-processed with the inclusion of new reports and surveys.
The original sampling points have been preserved as sharp points in the final interpolated grid. Also the 2007 grid includes three interpolation grids of successively finer resolution which may appear as if sample points.
The coloured "gravity" image has been sun shaded from the west and assigned equally-distributed hues from the colour wheel between purple up to red.
The grey scale "gravity hipass" image is based on the Bouguer grid after it has been high-pass filtered in the Fourier domain to remove the longest wavelengths. Consequently only local changes in the Bouguer gravity are displayed.

Gravity worms (aka strings) are a derived product described in the 'Other Comments' section below.

Positional Accuracy: Data collected by road in the 60s and 70s generally has the accuracy of sun-sights, ie 50 m N-S and 200 m E-W. Data collected by helicopter in the 70s and 80s appears to have been done by dead reckoning and air photos, as we have found it to be up to a kilometre wrong. Details may be sought from GA. Ground surveys done by NTGS in the 80s and 90s were done on air photos, and after 1993, by GPS.

Attribute Accuracy: Altitude accuracy has been surveyed at benchmarks only and interpolated by barometer in between, with implied accuracy of perhaps 4 m and thus creates an uncertainty of 8 um/s2 in the Bouguer gravity. Helicopter-collected data by GA in the 60s and 70s appears to have a much more severe uncertainty, almost certainly limited by their altitude measurements. After 2000, vertical accuracy became nearly of geodetic quality, of 10 cm and better. Consequently the uncertainty of the gravity became less than 1 um/s2, limited by the instrument itself. Airborne gravity is heavily filtered, so the accuracy is wavelength dependent, worse at shorter wavelengths. Crossovers at intervals of tens of kilometres may be as good as 50 um/s2, but see documentation for the West Arnhem survey .

Logical Consistency: Not Applicable

Completeness: Dynamic dataset. Updated during the year as new data arrives. New fields may be added.


Contacts

NameOrganisationPositionRolePhoneFaxEmail
Manager Geoscience Business SystemsDepartment of Primary Industry and ResourcesManager Geoscience Business Systemscustodian(08) 8999 5211(08) 8999 6824geoscience.spatial@nt.gov.au

Data Dictionary

No data dictionary defined for this dataset

Supplementary Information

Search Words: Geophysics, Gravity, Bouger

Gravity Worms (Strings):

The second horizontal derivative of the gravity or magnetic field reaches a maximum over the edge of an extensive body. Consequently this value can be
used to trace a "worm" automatically around an extensive magnetic or dense body.

The second horizontal derivative is sensitive to nearby objects, so some of the outlines are obtained by standing off further from the sample plane.
Upward continuation is obtained by smoothing in the Fourier domain.

The gravity image has large areas of particular sparse sampling, creating an illusion of fine detail around the occasional sample. A more accurate picture
is obtained in these areas by standing further above, so colours are used to show how much stand-off is been used for each contour.

When you use worms to delineate a body, it is important to check the implied boundary with other indications you have of the body of interest.

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