Head: Dr. Christoph Förste

Gravimetry has a long tradition on the Telegraphenberg hill in Potsdam, where the former Geodetic Institute started to carry out precise gravimetric pendulum measurements more than 100 years ago.  Later on, spring gravimeters were used. Nowadays in our Section 1.2 at GFZ we apply two modern gravimetric techniques: Superconducting and Airborne Gravimetry.

According to Newton’s law the acceleration of free fall of a dropped body is proportional to its mass. That means the acceleration of free fall and the gravitational attractive force are equivalent. Therefore in Geodesy and Geophysics the acceleration of free fall is used as a measure for the gravity field.

Terrestrial gravity observations comprise the measurement of the vertical component of the  acceleration of free fall g (g = Gravity) on the Earth's surface to determine spatial as well as temporal gravity variations. In this context one has to take into account that Gravity on the Earth’s surface is a superposition of the gravitational attraction of masses (Newton’s law) and the significantly smaller but outwards directed centrifugal acceleration due to the rotation of the Earth. Furthermore, mass density inhomogeneities as well as temporal mass displacements cause spatial and temporal variations of the gravitational attraction on the Earth’s surface which can be detected with gravity meters. In this context, spatial gravity observations can be used to derive information about geological structures such as fault zones, salt domes and volcanic structures or to explore ore deposits.

The largest signals of gravity variations are caused by the periodic changes of the relative positions between the Earth, the Moon and the Sun and the thereby induced deformations of the Earth's body (for instance Ocean and Earth tides and Ocean tide induced loading effects).

Examples of gravity variations on the Earth’s surface are (expressed as acceleration of free fall):

• The gravity difference between pole and equator due to Earth’s oblateness (the poles are closer to the Earth’s centre of mass and the centrifugal acceleration due the Earth’s rotation is zero at the poles and maximum at the equator):
  Δg ~ 5•10^-2 m/s²
• Gravity differences between deep sea and highest mountains: Δg ~ 5•10^-2 m/s²
• Earth tides: up to Δg ~ 3•10^-6 m/s²
• Gravity changes by mass redistribution in the atmosphere: up to Δg ~ 2•10^-7 m/s²
• Gravity changes by long-term terrestrial mass displacements: in the order of Δg ~10^-7 m/s²
• Increase of Gravity due to 1 m groundwater rise:  Δg ~ 4•10^-8 m/s²

Temporal gravity changes can be recorded with absolute and relative gravimeters. The most precise and temporally stable relative gravimeters are superconducting gravimeters (SG). In contrast to classic spring gravimeters, superconducting gravimeters do not have a mechanical spring but a "virtual spring” design where a liquid helium cooled diamagnetic superconducting sphere is levitating in the super-stable magnetic field of a superconducting electromagnet.

As all gravity sensors a superconducting gravimeter is an integrating sensor measuring the sum of all gravity variations caused by mass variations and deformations in the Earth system from the immediate and wider surroundings of the measuring point. This means that the SG recordings include gravity effects from various sources. In addition, the sensor in a gravimeter is recording both changes in Newton's gravitational force (due to mass shifts and density changes) and inertial forces caused by accelerations due to Einstein's equivalence principle.

In order to separate the various gravity components in the time series of a gravimeter, complex analysis procedures are necessary, including other measurement data such as meteorological and hydrological recordings.

The research topics related to superconducting gravimetry include:

• Gravity changes due to mass transfers in the atmosphere and hydrosphere as well as interactions with the lithosphere due to loading effects
• Evaluation and combination of satellite data from GRACE and follow-on missions
• Evaluation of oceanic and hydrological models
• Oscillations of the Earth's inner core (Slichter triplet) and free oscillations of the Earth.

Currently, 68 sensors are permanently installed at 48 stations worldwide. They are part of the IAG service International Geodynamics and Earth Tide Service (IGETS). The GFZ operates the IGETS database and provides the data from its three superconducting gravimeters from the geodynamic observatories SAGOS (South African Geodynamic Observatory Sutherland), ZUGOG (Zugspitze Geodynamic Observatory Germany) and HELGOG (Helgoland Gravimetric Observatory Germany).

Satellite-based gravity field determination can map the gravity field of the Earth in a very homogeneous way, however, with limited spatial resolution due to the altitude of the orbits. On the other hand, traditional terrestrial gravimetry on ground can measure the gravity with high resolution, but its data are often inhomogeneous and such measurements can be limited by difficult environmental conditions like high mountains, glaciers, mush or jungle. Furthermore, traditional terrestrial gravimeters cannot be used on sea. Air- and shipborne gravimetry measurements can be used to fill data gaps of the traditional gravimetry on ground and the satellite techniques. Thanks to the development of GNSS, air- and shipborne gravimetry nowadays can operate routinely not only for research, but also for resource investigation etc. Regional gravity field models can be developed on the basis of this technique.

Specially designed, transportable spring gravimeters have been developed for use on aircraft and ships and are installed on gyro-stabilized platforms. These include GNSS receivers, whose measurements allow the necessary reduction of the non-gravitational, kinematic accelerations of the measurement platform. To measure the gravity field on ships or aircraft, special inertial navigation systems are increasingly being used, whose accelerometers have the necessary accuracy and drift stability. These devices, also known as strapdown gravimeters, are tightly fixed on the moving vehicle and work without a gyro-stabilized platform. Furthermore, there are now promising technological developments ongoing, that allow new quantum gravimeters to be used as mobile gravimeters on airplanes and ships.

For more than ten years, the GFZ has owned a mobile gravimeter of the Chekan-AM type, which was manufactured by the Russion company CSRI "Elektropribor"and received a new sensor unit in 2017. In the field of flight gravimetry, this device was used together with several JAVAD Delta G3T GNSS receivers as part of the GEOHALO mission. Later, intensive use in ship gravimetry took place over several years in the FAMOS project .

The GFZ has recently acquired a new iCORUS strapdown gravimeter from the manufacturer iMAR Navigation & Control. The iCORUS is currently used together with the Chekan gravimeter in ship gravimetry on the North Sea. The aim of this activity is determination of the geoid on the North Sea.

Boy, J.-P., Barriot, J.-P., Förste, C., Voigt, C., Wziontek, H. (2020 online): Achievements of the First 4 Years of the International Geodynamics and Earth Tide Service (IGETS) 2015–2019. - In: (International Association of Geodesy Symposia ), Berlin, Heidelberg : Springer-Verlag. https://doi.org/10.1007/1345_2020_94

Förste, C., Ince, E. S., Johann, F., Schwabe, J., Liebsch, G. (2020): Marine Gravimetry Activities on the Baltic Sea in the Framework of the EU Project FAMOS. - ZfV: Zeitschrift für Geodäsie, Geoinformation und Landmanagement, 145, 5, 287-294. https://doi.org/10.12902/zfv-0317-2020

Ince, E. S., Förste, C., Barthelmes, F., Pflug, H., Li, M., Kaminskis, J., Neumayer, K., Michalak, G. (2020): Gravity Measurements along Commercial Ferry Lines in the Baltic Sea and Their Use for Geodetic Purposes. - Marine Geodesy, 43, 6, 573-602. https://doi.org/10.1080/01490419.2020.1771486

Ince, E. S., Barthelmes, F., Reißland, S., Elger, K., Förste, C., Flechtner, F., Schuh, H. (2019): ICGEM – 15 years of successful collection and distribution of global gravitational models, associated services and future plans. - Earth System Science Data, 11, 647-674. https://doi.org/10.5194/essd-11-647-2019

Voigt, C., Schulz, K., Koch, F., Wetzel, K.-F., Timmen, L., Rehm, T., Pflug, H., Stolarczuk, N., Förste, C., Flechtner, F. (2021): Technical note: Introduction of a superconducting gravimeter as novel hydrological sensor for the Alpine research catchment Zugspitze. - Hydrology and Earth System Sciences, 25, 9, 5047-5064. https://doi.org/10.5194/hess-25-5047-2021