Researchers from the University of Bonn install extremely sensitive measuring instrument at Todenfeld Observatory
Researchers at the University of Bonn have installed a superconducting gravimeter at the Todenfeld observatory a few kilometers from the town of Rheinbach. The extremely sensitive yet portable instrument can measure tiny changes in the billionths range of gravity. It is one of only four comparable instruments in Germany and the only one operated by a university. Among other things, the superconducting gravimeter can be used to measure minimal changes in groundwater levels, the deformation of the earth due to the gravitational forces of the moon and sun, movements of the earth’s core, and earthquakes.
The heart of the gravimeter is a sphere that measures 2.5 centimeters in diameter and weighs only five grams. It is made of the transition metal niobium. This metal becomes a superconductor near absolute zero. In this state, the sphere can float freely in an external, nest-like magnetic field without any significant losses. It then changes its relative position only due to fluctuations in gravitational attraction or when the subsurface moves. The floating sphere reacts to tidal effects due to the attraction of the moon and the sun, to deformations of the solid earth or vibrations of the earth’s core, to changes in the water content in the subsurface or to seismic events such as earthquakes.
The instrument, called ,,iGrav-043," was procured by researchers at the University of Bonn with financial support from the German Research Foundation (DFG) and the state of North Rhine-Westphalia. "It is one of only four comparable instruments in Germany and the only one operated by a university," says Jürgen Kusche of the Institute of Geodesy and Geoinformation at the University of Bonn, a member of the University’s Transdisciplinary Research Areas (TRA) "Matter" and "Sustainable Futures." The sensitive gravimeter was transported from its previous location, a laboratory in a disused Luxembourg mine, to Rheinbach-Todenfeld.
Cooling to minus 269 degrees Celsius
The research team mounted the measuring device on a concrete base and cooled it to minus 269 degrees Celsius. For this, it had to be connected to a compressor to cool the sensor and a GNSS antenna for precise time synchronization. While the setup itself took only a few days, the subsequent setup proved to be a lengthy process. "Because the iGrav is a very sensitive instrument, numerous steps have to be taken into account to mechanically isolate the device and perform the sensitive fine-tuning and compensation for local gravity in the observatory at 400 meters," Kusche reports.
The team cooled the instrument in a multi-step process using gaseous and liquid helium. Then the researchers turned on several coils built into the gravimeter, one after the other, to create a stable magnetic field so the niobium sphere could float freely. "In similar experiments during setup at the previous site in Luxembourg, the sphere did not float right away, but seemed to stick to the ground," Kusche reports. "So we had to be extremely careful." Patience and perseverance were required. With support from the manufacturer in San Diego, California, the installation was finally completed. "This was excellent teamwork by everyone involved," says Kusche. "Now everyone is excited to see what the first measurements will show."
Sensitive to movements of earthquakes
Since then, the iGrav at the Todenfeld Observatory has been continuously monitoring variations in the acceleration of the gravity field. "We can already see very clearly the tidal effects of the moon and the sun," says Basem Elsaka from Kusch’s Astronomical, Physical and Mathematical Geodesy group with pleasure. And his colleague from the Collaborative Research Center ,,Regional Climate Change: The Role of Land Use and Water Management" (SFB 1502), Dr. Benjamin Gutknecht, adds: "The device is so sensitive that we can also observe movements of earthquakes. In this respect, the gravimeter is even a seismometer."
Under ideal conditions, the iGrav is theoretically capable of detecting mass changes with a time resolution of about one minute, corresponding to a water height change of only one millimeter. This metric is important for detecting changes in total subsurface water storage, such as those being studied by SFB 1502. Over the next few weeks, the team will continue to monitor the system, analyze drift and noise conditions, factor out the influence of tides from the moon and sun, and install a permanent Internet connection. Says Kusche, "In the future, it will be possible to use the University of Bonn’s iGrav at other locations as part of research campaigns."
