Metadata standards for Magnetotelluric Time Series Data

Magnetotellurics (MT) is an electromagnetic geophysical method that is sensitive to variations in subsurface electrical resistivity. Measurements of natural electric and magnetic fields are done in the time domain, where instruments can record for a couple of hours up to mulitple months resulting in data sets on the order of gigabytes. The principles of findability, accessibility, interoperability, and reuse of digital assets (FAIR) requires standardized metadata. Unfortunately, the MT community has never had a metadata standard for time series data. In 2019, the Working Group for Magnetotelluric Data Handling and Software (https://www.iris.edu/hq/about_iris/governance/mt_soft) was assembled by the Incorporated Research Institutions for Seismology (IRIS) to develop a metadata standard for time series data. This product describes the metadata definitions. Metadata Hierarchy: Survey - Station - Run - Channel The hierarchy and structure of the MT metadata logically follows how MT time series data are collected. The highest level is "survey" which contains metadata for data collected over a certain time interval in a given geographic region. This may include multiple principle investigators or multiple data collection episodes but should be confined to a specific project. Next, a "station" which contains metadata for a single location over a certain time interval. If the location changes during a run, then a new station should be created and subsequently a new run under the new station. If the sensors, cables, data logger, battery, etc. are replaced during a run but the station remains in the same location, then this can be recorded in the "run" metadata but does not require a new station entry. A "run" contains metadata for continuous data collected at a single sample rate. If channel parameters are changed between runs, this would require creating a new run. If the station is relocated then a new station should be created. If a run has channels that drop out, the start and end period will be the minimum time and maximum time for all channels recorded. Finally, a "channel" contains metadata for a single channel during a single run, where "electric", "magnetic", and "auxiliary" channels have some different metadata to uniquely describe the physical measurement. This data product contains definitions and hierarchy of metadata standards for magnetotelluric (MT) time series. Conventions are defined: Time and Date: All time and dates are given as an ISO formatted date-time String in the UTC time zone. The ISO Date Time format is "YYYY-MM-DDThh:mm:ss.ssss+00:00", where the UTC time zone is represented by "+00:00". UTC can also be denoted by "Z" at the end of the date-time string "YYYY-MM-DDThh:mm:ss.ssssZ". Note that "Z" can also represent Greenwich Mean Time (GMT) but is an acceptable representation of UTC time. If the data requires a different time zone, this can be accommodated but it is recommended that UTC be used whenever possible to avoid confusion of local time and local daylight savings. Seconds can be accurate to 9 decimal places. ISO dates are formatted "YYYY-MM-DD". Hours are given as a 24 hour number or military time, e.g. 4:00 PM is 16:00. Location: All latitude and longitude locations are given in decimal degrees in the well known datum specified at the "Survey" level. Angles: All angles of orientation are given in decimal degrees. Orientation of channels should be given in a geographic or a geomagnetic reference frame where the right-hand coordinates are assumed to be North = 0, East = 90, and vertical is positive downward. The coordinate reference frame is given at the station level "station.orientation.reference_frame". Two angles to describe the orientation of a sensor are given by "channel.measurement_azimuth" and "channel.measurement_tilt". In a geographic or geomagnetic reference frame, the azimuth refers to the horizontal angle relative to north positive clockwise, and the tilt refers to the vertical angle with respect to the horizontal plane. In this reference frame, a tilt angle of 90 points downward, 0 is parallel with the surface, and -90 points upwards. Archived data should remain in measurement coordinates. Any transformation of coordinates for derived products can store the transformation angles at the channel level in "channel.transformed_azimuth" and "channel.transformed_tilt", the transformed reference frame can then be recorded in "station.orientation.transformed_reference_frame". Units: Acceptable units are only those from the International System of Units (SI). Only long names in all lower case are acceptable. Units with multiple dimensions should be separated by a dash "-" if multiplictive or "per" if divided. For example velocity would be "meters per second" and resistivity would be "ohm-meter". These standards have been implemented in an open-source Python package called mt_metadata. This can be installed using PIP or conda-forge, see MT Metadata documentation for more information and mt_metadata (github.com) for the source code.