b'Metadata standard for MTTechnical noteMetadata standards for magnetotelluric time-series dataJingming Duan, Alison Kirkby, Darren Kyi, Wenping Jiang, Marinato locate the MT site and electric-field electrode locations is a Costelloe, Adrian Hitchman required piece of metadata. Geoscience Australia recommends that the Geocentric Datum of Australia 2020 (GDA2020) be used. Geoscience Australia, Canberra, Australia This is the current coordinate reference frame recommended by Adrian.Hitchman@ga.gov.au the Intergovernmental Committee on Surveying and Mapping for use in Australia (icsm.gov.au/gda2020). However, where its use is not possible, the proposed standard has the flexibility for Introduction fieldworkers to use a coordinate reference frame compatible with the GPS receivers they use.Modern magnetotellurics (MT) offers a multiscale capability to image the electrical properties of Earths crust and upperLeonard etal. (2020) also recommend clarity about the time mantle. The data it provides and the models derived from it arereference system to be used when recording dates and times important geophysical contributions to understanding Earthsrelevant to MT surveys. The proposed metadata standard geology and resource potential. provides for either Coordinated Universal Time (UTC) and/In Australia, MT data is acquired by the resource explorationor local time to be used. When local time is used, additional industry, university-based research groups, and Federal, Stateinformation is needed, such as the time correction to UTC and and Territory geological surveys. To ensure this data can be usedthe time zone in which the measurement is made. Whereas, to its full potential, including by groups and individuals whowhen UTC dates and times are recorded such additional may not have been involved in its acquisition, it is importantinformation is not required and its omission is unlikely to that community-agreed standards be adopted for the acquiredresult in incomplete metadata. Additionally, some MT surveys data and its associated metadata. may cross state/territory borders and therefore be recorded in different time zones such that inadequately captured local-time metadata may jeopardise the correct interpretation of date Metadata for field-acquired MT data and time information. For these reasons, Geoscience Australia recommends UTC as the preferred time reference system and Several steps have been taken to solicit the Australian MTendorses its classification as compulsory, and local time as communitys views on metadata. Kirkby (2019) sought feedbackoptional, in the metadata standard.on a draft set of metadata parameters for field-acquired MT data. Rees etal. (2019) describe field-acquired data as Level 0Rees etal. (2019) describe various levels of time-series data data. Intrepid Geophysics subsequently conducted a series ofdepending on the amount of processing it has undergone. The interviews with MT stakeholders in Australia and overseas. TheLeonard etal. (2020) metadata standard is recommended for use results of this consultation are described in Leonard etal. (2020). in the acquisition of raw and Level 0 MT time-series data. This The key recommendations of the Intrepid Geophysics report aredata is not typically released publicly but is the foundation for that: a range of higher-level data products that have been processed to varying degrees and can be made publicly accessible on Australian MT practitioners adopt a standard set of metadatadelivery platforms managed by the acquiring organisation and parameters to be recorded with acquired MT field data by domain-specific platforms managed by others.The required parameters be only those necessary to adequately represent the data Metadata and data formats for higher-level MT data JSON be used in preference to XML as the metadata markup language productsGeoscience Australia nominates suitable time and spaceThere is significant overlap between the metadata that should reference systems for the collection of MT data in Australia be recorded for field-acquired Level 0 time-series data and the The Australian MT community adopts an internationallymetadata that should accompany published Level 1A and 1B accepted standard for the public release of MT time-seriestime-series data; there are also distinct differences (Figure 1). data. Level 0 metadata should be sufficient to identify, quality assure Achieving the ultimate goal of findable, accessible, interoperableand later process the field-acquired data without being too and reusable (FAIR; Wilkinson, 2016) MT data and metadataonerous for field parties to record. Level 1A and 1B metadata begins with the acquisition of high-quality field data and theshould record much of this information and add further metadata required to adequately describe and quality assureinformation about what processing the data has undergone, it. The metadata standard proposed by Leonard etal. (2020)who has been responsible for the processing and archiving of has been developed in consultation with the Australian MTthe data, assigned DOIs, papers published using the data, etc. community to reflect current and possible future Australian fieldMuch of this additional metadata is not known at the time of practices for the acquisition of MT data. This standard is the basisfield acquisition. Figure 1 gives some examples of this overlap on which metadata for higher-level MT data products (Rees etal.,and these differences using a Venn diagram representation.2019) is founded. In this paper, Geoscience Australia recommends that Australian MT practitioners adopt the metadata standard forThe similar but different metadata requirements for the different field data that Leonard etal. (2020) propose. levels of data suggest the need for different standardsone for field-acquired data and another for data that is ultimately Leonard etal. (2020) identify the need for clarity on themade publicly available. The first could feasibly be a locally or coordinate reference frame used for MT data collection. Innationally agreed standard, the second should preferably be an the proposed standard the coordinate reference frame usedinternationally accepted standard.61 PREVIEW DECEMBER 2021'