magnetic
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Total magnetic intensity (TMI) data measures variations in the intensity of the Earth's magnetic field caused by the contrasting content of rock-forming minerals in the Earth crust. Magnetic anomalies can be either positive (field stronger than normal) or negative (field weaker) depending on the susceptibility of the rock. The data are processed via standard methods to ensure the response recorded is that due only to the rocks in the ground. The results produce datasets that can be interpreted to reveal the geological structure of the sub-surface. The processed data is checked for quality by GA geophysicists to ensure that the final data released by GA are fit-for-purpose. These line dataset from the Murrindal, Vic, 1996 VIMP Survey (GSV3060) survey were acquired in 1995 by the VIC Government, and consisted of 15589 line-kilometres of data at 200m line spacing and 80m terrain clearance. To constrain long wavelengths in the data, an independent data set, the Australia-wide Airborne Geophysical Survey (AWAGS) airborne magnetic data, was used to control the base levels of the survey data. This survey data is essentially levelled to AWAGS.
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Standard curves for interpretation of the magnetic anomalies due to spheres have been derived. The anomalies in the vertical component, and the horizontal component in the direction of the traverse, are each found to be represented by a single family of curves. The horizontal component is found to be not represented by a single family, and separate curves for each field inclination and traverse azimuth are presented. Curves for the anomaly in the total intensity were not computed.
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During 2009-10 Geoscience Australia completed a petroleum prospectivity study in the offshore northern Perth Basin, 200 km northwest of Perth. In some parts of this basin acoustic basement is deep and not resolved in the reflection seismic data. Improvements to the magnetic ship-track database and magnetic anomaly grid produced during the study allowed for assessment of depth to magnetic sources, and estimation of sediment thickness, and provided new insight into basement trends. 2.5D models along several transects, and analysis using spectral methods indicate penetration of the lower sediments by high-susceptibility bodies is necessary to approximate the observed magnetic anomaly. The reflection seismic evidence for these bodies is not obvious, though in some cases they may be associated with interpreted faults. Where the modelled bodies penetrate the sediments, they are mostly below or within the Permian section, except in the west of the study area where sediments thin over oceanic crust. On the northern-most profiles a large positive magnetic anomaly (the Batavia Ridge) is modelled by massive bodies whose tops are 5-10 km below sea floor. On these and other profiles to the south other dyke-like bodies rarely penetrate to shallower than 5 km below the sea floor.
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