Enhanced gravity and magnetic grids for Northern Australia

Full description

Geoscience Australia (GA) generated a series of gravity and magnetic grids and enhancements covering Northern Australia. Several derivative gravity datasets have been generated 1) for the North-West Shield Western Australia region (approximately between latitudes 7‒26⁰ S and longitudes 110‒130⁰ E), 2) for the Northern Territory (approximately between latitudes 7‒26⁰ S and longitudes 125.5‒141⁰ E) and for Queensland (approximately between latitudes 7‒30⁰ S and longitudes 135‒160⁰ E). The magnetic dataset has been generated only for the North-West Shield Western Australia region (approximately between latitudes 7‒26⁰ S and longitudes 110‒130⁰ E). The magnetic and gravity data were downloaded from the Geophysical Archive Data Delivery System (GADDS), website (http://www.geoscience.gov.au/cgi-bin/mapserv?map=/nas/web/ops/prod/apps/mapserver/gadds/wms_map/gadds.map&mode=browse). Satellite Free-air (FA) gravity v27.1 (released March 11, 2019) and Satellite Topography v19.1 (released January 14, 2019) data were sourced from Sandwell et al. (2014) and downloaded from the Scripps Institution of Oceanography (SIO), National Oceanic and Atmospheric Administration (NOAA), U.S. Navy and National Geospatial-Intelligence Agency (NGA) (SIO Satellite Geodesy, website, http://topex.ucsd.edu/WWW_html/mar_grav.html). The Satellite Bouguer gravity grid with onshore correction density of 2.67 gcm-3 and offshore correction density of 2.20 gcm-3 was derived from the Free-air gravity v27.1 and Topography data V19.1. This Bouguer gravity grid was used for filling areas of data gaps in the offshore region.

Data evaluation and processing of gravity and magnetic data available in the area of interest resulted in the production of stitched onshore-offshore Bouguer gravity grid derived from offshore satellite Bouguer gravity grid and GA’s onshore ground and airborne gravity survey data and a stitched Total Magnetic Intensity (TMI) grid derived from airborne and shipborne surveys (Tables 1 and 5). A Reduction to the Pole (RTP) grid was derived from the stitched TMI grid. The TMI, RTP, FA and terrain corrected Bouguer gravity anomalies are standard datasets for geological analysis. The free-air gravity anomaly provides the raw and basic gravity information. Images of free-air gravity are useful for first-pass interpretation and the data is used for gravity modelling. Magnetic anomalies provide information on numerous magnetic sources, including deep sources as arising from the structure and composition of magnetic basement and shallow sources such as intra-sedimentary magnetic units (e.g. volcanics, intrusions, and magnetic sedimentary layers). A standard TMI image will contain information from all these sources. Geosoft Oasis montaj software was used throughout the data processing and enhancement procedure and the montaj GridKnit module was used to generate the stitched gravity and magnetic grids.

Enhancement techniques have been applied to the final processed Bouguer gravity and RTP magnetic grids to highlight subtle features from various sources and to separate anomalies from different source depths. These enhancement techniques are described in the next section.

Enhancement processing techniques and results

A summary of image processing techniques used to achieve various outcomes is described in Table 1.

Data type Filter applied Enhancement/outcome

Gravity/Magnetic First vertical derivative (1VD) Near surface features (e.g. intrabasinal)

Gravity/Magnetic Upward continuation Noise reduction in data

Gravity/Magnetic Low pass filter, or large distance upward continuation Enhancement of deep features (e.g. basement)

Gravity/Magnetic High pass filter Enhancement of shallow features (e.g. surface anomalies)

Gravity/Magnetic Tilt filter and 1VD Enhancement of structure (e.g. in basement)

Gravity/Magnetic ZS-Edgezone and ZS-Edge filters Enhancement of edges

Gravity/Magnetic horizontal modulus / horizontal gradient Enhancement of boundaries

Magnetic RTP (reduction to the pole), Compound Anomaly, and Analytic Signal filter Accurate location of sources

Lineage

Maintenance and Update Frequency: asNeeded

Statement: Data were compiled for a series of grids and enhancements covering Northern Australia, extending roughly from -24° to -7° Latitude and 110° to 130° Longitude. The data were downloaded from the Geophysical Archive Data Delivery System (GADDS) and Sandwell and Smith (2014). The stitched Bouguer gravity grid (unit in mGal) has a cell size of 1 km (Figure 1). The stitched TMI grid has a grid cell size of 200 m (Figure 2). The TMI grid was reduced to the pole (RTP) and enhancement processing have been applied to the RTP grid (Figure 2). All colour shaded relief images of gridded magnetic and gravity data have been produced with a NE sun shade The description of the input datasets used to construct the stitched grids covering the North-West Shield Western Australia region are as follows: a) Input gravity datasets (downloaded from GADDS) for the stitched Bouguer gravity grid (Table 2): 1. Australia Onshore Complete Bouguer Anomaly July 2016; 2. Satellite Free-Air Gravity v27.1; 3. NE Canning Airborne Gravity Survey, Western Australia (WA), 2017 4. Tanami-King Leopold Airborne Gravity Survey, Western Australia (WA), 2017 The resolution of the gravity dataset varies from 500m (0.005 degrees) to 2000 m (0.01667 degrees) in grid cell size. In areas covered by datasets from different sources, only the best quality dataset has been selected to produce the stitched gravity grid. b) Input magnetic datasets (downloaded from GADDS) for the stitched Total Magnetic Intensity (TMI) anomaly grid (Table 3): 1. Sixth Edition Total Magnetic Intensity (TMI) Anomaly Grid of Australia, GA; (magmap_v6_2015.ers, 2015) 2. Geological Survey of Western Australia (GSWA) WA marine TMI grid 2005 (WA_marine_TMIG.ers, 2005) 3. Fourth Edition Total Magnetic Intensity (TMI) Anomaly Grid of Australia, Offshore, GA; (Magmap04_off.ers, 2004) Links to grids metadata are: • Geophysical Archive Data Delivery System (GADDS) website – http://www.geoscience.gov.au/cgi-bin/mapserv?map=/nas/web/ops/prod/apps/mapserver/gadds/wms_map/gadds.map&mode=browse • SIO Satellite Geodesy website ─ http://topex.ucsd.edu/WWW_html/mar_grav.html.

Notes

Purpose
Digital grids for public use