b'Metadata standards for MTFeatureDeveloping metadata standards for time series magnetotelluric dataAlison Kirkby drill site targeting for a number of regional drilling projects. Geoscience Australia, Canberra, Australia Regional-scale Broadband MT surveys have been undertaken Alison.Kirkby@ga.gov.au along onshore deep reflection seismic transects, and over a number of potential mineral/energy provinces and greenfields Magnetotellurics (MT) is a passive geophysical method thatareas across the country (Figure 1). These data are being used to uses natural time variations of the Earths magnetic andmap the crustal-scale electrical conductivity/resistivity structure, electric fields to measure the electrical resistivity of the and to improve the understanding of basement architecture sub-surface. and resource potential in these regions. Recent high profile The electrical resistivity of rocks and minerals varies by manyscience publications have shown the relevance of the MT orders of magnitude, from very resistive crystalline igneoustechnique to exploration undercover.rocks, to very conductive saline-filled sedimentary rocks. AsMembers of the MT community (Figure2) have begun working measured by the MT method, the resistivity obtained is a bulktowards defining agreed metadata parameters to be collected property of a volume of earth material, and is associated withfor time series Magnetotelluric (MT) data. This is the first factors such as rock composition, porosity and permeability, asstep in defining a metadata standard, which will then enable well as rock fluid composition and temperature. development and standardisation of time series formats to be The Earths magnetic field varies continuously in both time andused amongst the Australian MT community. Following this, we space. The MT method requires the electric field and variationswill also build on this list to develop standards for processed MT of the magnetic field to be recorded at the same site over adata and derived resistivity models.period of time. The ratio of the electric field and variations in theTable 1 lists the metadata parameters that we believe should magnetic field provides a measure of the electrical resistivity.be collected with time series MT data. The parameters are Depth information is obtained by measuring the time variationslisted as compulsory or optional to facilitate compatibility over a range of frequencies. High frequencies only penetratewith legacy data. We are seeking feedback from others in the Earth to shallow depths, while low frequencies penetratethe MT community on the table, in particular, any additional deeper. Information can be obtained from a few hundredparameters that are needed and any changes to those listed.metres depth to hundreds of kilometres depth.All the state and territory geological surveys support this The MT technique has been used in Australia to image theinitiative.electrical conductivity/resistivity structure of the subsurface at different scales for over 60 years. High-frequency Audio-MTPlease send any comments or suggestions to Alison.Kirkby@data have been used to estimate cover thickness, and to assistga.gov.au.Figure 1.MT survey site. Figure 2.Members of the MT community.49 PREVIEW APRIL 2019'