The GEOPHYS_SURV database describes geophysical surveys (air, land, and marine), the datasets derived from those surveys, and the methods used for delivery of those datasets. The database includes metadata for all surveys conducted or managed by Geoscience Australia and its predecessor agencies, as well as data and surveys from State and Territory geological survey agencies.

Absolute observations of the vector geomagnetic field were made 1.60 m above ground level at the marked centre of Darwin International Airport (DIA) compass swing site on taxi-way C4 on Monday 19 November 2012 (coordinated universal time). The observations were made during geomagnetic quiet conditions. The average observed values were +3.46°, -39.65° and 46285 nT in declination, inclination and total magnetic intensity respectively. When corrected to average undisturbed values of the field, using data from the Kakadu Geomagnetic Observatory to derive corrections, the adopted normal field values are +3.53°, -39.65°, 46272 nT in declination, inclination and total magnetic intensity respectively. Hence the magnetic declination at 1.60 m above ground level at the centre of the DIA compass swing site in November 2012 is +3.53°. This value has changed by -0.24° since it was previously determined in December 2007. Applying the November 2012 adopted value of magnetic declination to the true azimuth from the centre of the swing site to the aerodrome reference point gives the magnetic bearing from the centre of the swing site to the ARP of 123.766°.

Geoscience Australia (GA) and the Geological Surveys of Queensland (GSQ) and NSW (GSNSW) are undertaking a multi-year, multi-disciplinary collaborative project with the aim of characterising the largely unexplored southern Thomson Oregon region (Figure 1). As part of this, a number of precompetitive geophysical datasets have been acquired or are planned to be acquired within the southern Thomson region. These datasets will support an improved understanding of the: - thickness and nature of cover; - geology obscured by recent sedimentary cover or regolith; and, - mineral systems potential of the region. Magnetotellurics is a passive geophysical technique which records the earth's natural time-varying electrical and magnetic fields to provide a measure of the subsurface conductivity/resistivity (these two physical properties are related as resistivity is the inverse of conductivity). The frequencies of magnetic and electrical sources recorded provide different depth information - audiomagnetotellurics (AMT) which records higher frequency data and images shallower electrical structure, while broadband MT (BBMT) records lower frequencies that image relatively deeper electrical structure. Although the depth of investigation of the technique varies according to the local resistivity structure, in general AMT images the shallowest upper crust only (14 km depth) and BBMT images the crust (~60 km depth). For the southern Thomson region, BBMT was acquired along 2 long and one shorter transect to provide pictures of the entire crust of the region, while AMT was acquired along shorter and higher-resolution lines to provide knowledge of the cover of the region. Within this document we detail the acquisition, processing and analysis of the MT data, and present preliminary resistivity models. These data, their analyses and models are released as underpinning datasets to support future geophysical analyses and geological interpretation. Accordingly, this report focuses primarily on the core geophysical aspects from acquisition through to preliminary modelling.

Total magnetic intensity (TMI) data measures variations in the intensity of the Earth 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. This magnetic grid has a cell size of 0.0005 degrees (approximately 50m). The data used to produce this grid was acquired in 2000 by the SA Government, and consisted of UNKNOWN line-kilometres of data at 300.0m line spacing and 80.0m terrain clearance.

Total magnetic intensity (TMI) data measures variations in the intensity of the Earth 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. This magnetic grid has a cell size of 0.0005 degrees (approximately 50m). The data used to produce this grid was acquired in 2000 by the SA Government, and consisted of 8650.0 line-kilometres of data at 300.0m line spacing and 80.0m terrain clearance.

Total magnetic intensity (TMI) data measures variations in the intensity of the Earth 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. This magnetic grid has a cell size of 0.0005 degrees (approximately 50m). The data used to produce this grid was acquired in 2000 by the WA Government, and consisted of UNKNOWN line-kilometres of data at 100.0m line spacing and 60.0m terrain clearance.

Total magnetic intensity (TMI) data measures variations in the intensity of the Earth 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. This magnetic grid has a cell size of 0.002 degrees (approximately 210m). The data used to produce this grid was acquired in UNKNOWN by the UNKNOWN Government, and consisted of UNKNOWN line-kilometres of data at 200.0m line spacing and UNKNOWNm terrain clearance.

Total magnetic intensity (TMI) data measures variations in the intensity of the Earth 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. This magnetic grid has a cell size of 0.001 degrees (approximately 110m). The data used to produce this grid was acquired in 2000 by the WA Government, and consisted of 10950.0 line-kilometres of data at 400.0m line spacing and 80.0m terrain clearance.

Total magnetic intensity (TMI) data measures variations in the intensity of the Earth 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. This magnetic grid has a cell size of 0.002 degrees (approximately 200m). The data used to produce this grid was acquired in UNKNOWN by the WA Government, and consisted of UNKNOWN line-kilometres of data at 300.0m line spacing and 60.0m terrain clearance.

Total magnetic intensity (TMI) data measures variations in the intensity of the Earth 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. This magnetic grid has a cell size of 0.002 degrees (approximately 210m). The data used to produce this grid was acquired in UNKNOWN by the WA Government, and consisted of UNKNOWN line-kilometres of data at 1000.0m line spacing and UNKNOWNm terrain clearance.