Terrestrial and Airborne Gravimetry

Head: Dr. Christoph Förste

Terrestrial gravimetry has a long tradition on the Telegraphenberg hill in Potsdam, where the former Geodetic Institute started to carry out pendulum and gravimeter measurements more than 100 years ago. Today 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 uswed 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/s
  • Increase of Gravity due to 1 meter groundwater rise:  Δg ~ 4•10 -8 m/s

Superconducting Gravimetry

Temporal gravity variations can be recorded by using absolute and relative gravimeters. The most accurate and stabile relative gravimeters are Superconducting Gravimeters (SG). These instruments belong to the family of relative spring-mass gravimeters. In contrast to classical spring-mass gravimeters the SG has no spring. It’s replaced by a “virtual spring design” where a liquium helium cooled diamagnetic superconducting sphere is levitating in the super-stable magnetic field of a superconducting electromagnet.

As all gravity sensors a Superconducting Gravity meter is an integrating sensor measuring the sum of all gravity variations in its near and far environments. Therefore, the SG recordings include gravity effects of different sources. Due to the equivalence of gravitational and inertial mass (i.e. Einstein’s equivalence principle) the sensor inside a gravimeter (a test mass) is sensitive to temporal variations of Gravitational forces (Newtonian attraction) caused by redistribution and density variations of all surrounding masses as well as Inertial forces caused by accelerations, i.e. the second time derivative of the vertical position of the gravimeter site.

For the separation of the different gravity constituents sophisticated modelling and analysing techniques together with additional data (e.g. meteorological and hydrological data) are necessary. Research topics in this context are among others:

  • Gravity changes induced by mass redistribution in the atmosphere and hydrosphere
  • Comparison and Interpretation of satellite derived gravity variations
  • Evaluation of ocean tide models and Hydrological models
  • Inner core translation (Slichter Triplet)
  • Core resonance in the tidal band (Nearly Diurnal Free Wobble)
  • Polar motion and Free oscillations of the Earth

Presently about 30 superconducting gravimeters are worldwide operated within the IAG service International Geodynamics and Earth Tide Service (IGETS). GFZ hosts the IGETS data base and contributes with the data from two superconducting gravimeters at the South African Geodynamic Observatory Sutherland (SAGOS)

Airborne Gravimetry

Satellite-based gravity field determination can map the gravity field of the Earth in a very homogeneous way, however, with limited resolution due to the height of the orbits. Traditional terrestrial gravimetry on ground can measure the gravity with high resolution, but the data is often inhomogeneous and the measurement is limited by the environment conditions like high-altitude mountain, djungle or open ocean. Airborne gravimetry measurements can be used to fill the data gaps of the traditional gravimetry and satellite techniques. Thanks to the development of the GNSS system, aerogravimetry nowadays can operate routinely not only for research, but also for resource investigation etc. Regional gravity field models can be developed by using aerogravimetry data.

Current Gravimeters used for Airborne Gravimetry are especially designed transportable spring-mass gravimeters which are mounted on gyrostablized platforms. Furthermore, recordings of GNSS receivers and Inertial Navigation Systems are used for the reduction of the disturbing non-gravitational accelerations of the measurement platform.

Basic principle of the Airborne Gravimetry: The gravimeter inside of the airplane measures the superposition of all vertical accelerations (yellow arrow). The non-gravitational kinematic accelerations (green arrows) are estimated from the GNSS-recorded trajectory. The difference between the measured and the kinematic accelerations is the gravitational acceleration at the trajectory. Since the used gravimeter is a relative one, the task of aerogravimetry is the determination of gravity variations between certain positions along the flight trajectory. The main challenge of this technique results from the matter of fact that these gravity variations are only millionths of the full acceleration of free fall.

GFZ’s instrumentation for Airborne Gravimetry are a mobile gravitmeter Chekan-AM made by CSRI "Elektropribor" , an Inertial Navigation System (INS) AEROcontrol-II and 4 GPS receiver JAVAD Delta G3T.

Literature

Abe, M., Kroner, C., Förste, C., Petrovic, S., Barthelmes, F., Weise, A., Güntner, A., Creutzfeldt, B., Jahr, T., Jentzsch, G., Wilmes, H., Wziontek, H. (2012): A comparison of GRACE-derived temporal gravity variations with observations of six European superconducting gravimeters. Geophysical Journal International, 191, 2, p. 545-556, doi.org/10.1111/j.1365-246X.2012.05641.x

Weise, A., Kroner, C., Abe, M., Creutzfeldt, B., Förste, C., Güntner, A., Ihde, J., Jahr, T., Jentzsch, G., Wilmes, H., Wziontek, H., Petrovic, S. (2012): Tackling mass redistribution phenomena by time-dependent GRACE- and terrestrial gravity observations. Journal of Geodynamics, 59-60, p. 82-91, doi.org/10.1016/j.jog.2011.11.003

Neumeyer, J. (2010): Superconducting Gravimetry. In: G. Xu (eds.), Sciences in Geodesy - I, Springer-Verlag Berlin Heidelberg, doi:10.1007/978-3-642-11741-1_10

Contact

Profile photo of  Dr. Christoph Förste

Dr. Christoph Förste
Global Geomonitoring and Gravity Field

Telegrafenberg
Building A 17, room 00.15
14473 Potsdam
tel. +49 331 288-1737