ALBERT

Description: The sources for the magnetotelluric 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. This report describes the installation, the type of sensors and the recorded data sets, which are freely available on request. For updated material, please refer to our webpages (http://www.gfz-potsdam.de/en/section/geophysical-deep-sounding/servicesinfrastructure/geophysical-instrument-pool-potsdam-gipp/archive/mt-reference-site/)