The sources for the magnetotelluric (MT) method are naturally occurring electromagnetic field variations. Electromagnetic currents are excited over a wide frequency range in the earth’s ionosphere by solar activity and global lighting discharges. While these source are available everywhere on earth and at all times, their signal strength varies considerably.

The so-called remote-reference technique is an effective way to improve magnetotelluric data quality by referencing the electromagnetic fields recorded at a local site to simultaneously recorded, undisturbed fields at a remote site. The remote reference approach has become standard for magnetotelluric field work, particularly in areas with high levels of man-made electromagnetic noise; i.e. almost everywhere in central Europe. Identifying a suitable location for a clean remote site is difficult and time-consuming. Maintaining such a site in addition to normal field operations is expensive and logistically challenging.

A permanently installed reference station simplifies MT measurements considerably. The radius for which source fields are coherent depends mostly on the frequency content of the signals and the latitude of observations. According to our own tests, signals are coherent at frequencies of up to 10 kHz over distances of 1000 km.

After a reconnaissance operation during which several locations all over Germany were tested, we could eventually identify an urban forest near the town of Wittstock in northern Germany as a suitable location for a reference station. A preliminary installation of the reference station was accomplished by mid-2010. The permanent installation in a wooden hut and operating with a range of sensors and sampling rates is available since November 2010. Meanwhile the data of the Wittstock site have been used very successfully to re-process a number of magnetotelluric stations in Germany.

• Broad-band horizontal magnetic field data for magnetotelluric remote reference processing
• Two sets of Metronix induction coils (MFS06/07) measuring data in the range of a few Hertz to KHz
• A 3-component Geomagnet fluxgate magnetometer records at a sampling rate of 5 Hz
• Data acquisition is with two SPAM Mk IV data loggers

The magnetotelluric data of the Permanent Magnetotelluric Reference Station Wittstock, Germany is freely available upon request and may be used under the Creative Commons Licence (CC-by-sa 4.0 Unported). Please send your data requests to Oliver Ritter (oritter@gfz-potsdam.de).

Recommended citation of the datasets: 
Ritter, O., Muñoz, G., Weckmann, U., Klose, R., Rulff, P., Rettig, S., Müller-Brettschneider, C., Schüler, M., Willkommen, G., Eydam., D. (2015): Permanent Magnetotelluric Reference Station in Wittstock, Germany (Datasets), GFZ Data Services.
DOI: http://doi.org/10.5880/GFZ.2.2.2015.001

Recommended citation of the data report:
Ritter, O., Muñoz, G., Weckmann, U., Klose, R., Rulff, P., Rettig, S., Müller-Brettschneider, C., Schüler, M., Willkommen, G., Eydam., D. (2015): A Permanent Magnetotelluric Reference Station in Wittstock, Germany (Report), (Scientific Technical Report - Data ; 15/09) (GIPP Experiment- and Data Archive), Potsdam : Deutsches GeoForschungsZentrum GFZ, 10 p.
DOI: http://doi.org/10.2312/GFZ.b103-15092

The EMERALD Data Format for Magnetotelluric Data is described in detail in:
Ritter, O., Klose, R., Weckmann, U. (2015): EMERALD Data Format for Magnetotelluric Data, Scientific Technical Report - Data; 15/08, Potsdam: Deutsches GeoForschungsZentrum GFZ.
DOI: http://doi.org/10.2312/GFZ.b103-15082

All data are recorded as time series using two SPAM Mk. IV systems. For ease of use, the data are re-organized as three virtual MT sites, covering a wide frequency range. Data streams can be continuous or in scheduled modes.

Site 991 comprises a 3-component Geomagnet fluxgate magnetometer (Bx, By, Bz) and electric field (Ex, Ey) recordings. Recording is continuous at a sampling rate of 5 Hz.

Site 996 comprises one (or more) continuously recorded data streams using MFS06 induction coil magnetometers (Bx, By, coil switch in LF mode, chopper on) and electric fields (Ex, Ey). Over time, a range of sampling rates were tested (50 Hz, 250 Hz, 500 Hz).

Site 997 records high frequency data which are typically sampled in scheduled mode. Over time, a range of sampling frequencies were tried (2.5 kHz, 5 kHz, 6.25 kHz, 12.5 kHz, 25 kHz). The magnetic fields are recorded with Metronix MFS07 sensors (Bx, By, coil switch in HF mode, chopper off).

Detailed information on available time-series for each site and year can be found on: https://gipp.gfz-potsdam.de/wittstock