Terrestrial and Airborne Gravimetry
Topic Leader: 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.
Terrestrial gravity observations comprise the measurement of the vertical component of the gravity acceleration g on the Earth's surface either to determine spatial or time-dependent gravity variations. Besides the attraction of masses gravity depends on the earth's rotation (centrifugal acceleration). Density changes in the underground result in an increased or reduced attraction which can be detected with gravimeters. Therefore spatial gravity observations can be used to derive information about geological structures such as fault zones, salt domes, volcanic structures or to explore ore deposits.
Since gravity g is depending on the mass attraction (Newton's law), not only changes in spatial mass distributions but also temporal mass shifts produce effects. The biggest signals come from the changes in the positions between earth, moon, and sun and the resulting deformations of the earth's body (tides).
Examples for gravity changes are:
• between pole and equator due to earth’s oblateness: Δg ~ 5•10-2 m/s²
• 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
For the study of time-dependent gravity changes on ground two instrument types are deployed: absolute (g) and relative (Δg) gravimeters. For a high-precision continuous monitoring of gravity variations Superconducting Gravimeters are deployed. It is an integrating sensor measuring gravity variations associated with mass redistributions in its near and far surroundings. Therefore, the recordings include gravity effects of different sources. Because matter has the properties of gravity and inertia, the sensor inside the gravimeter (a test mass) reacts to time variations of
- Gravitational forces (Newtonian attraction) caused by redistribution and density variations of all surrounding masses and
- Inertial forces caused by accelerations, i.e., the second time derivative of the vertical position of the gravimeter site.
For separating the different gravity effects special analysis methods and additional data are necessary e.g. meteorological and hydrological data. Research topics among others are:
• Gravity changes induced by mass redistribution in the atmosphere and hydrosphere
• Validation of satellite derived gravity variations
• Validation of ocean tide models
• Validation of hydrological models
• Inner core translation (Slichter Triplet)
• Core resonance in the tidal band (Nearly Diurnal Free Wobble)
• Polar motion
• Free oscillations of the Earth
Presently about 25 superconducting gravimeters are being operated worldwide whose records provide the basis for the "Global Geodynamics Project" (GGP). After successfully running for several years at Potsdam, since 2000 GFZ is operating a dual sensor instrument at the South African Geodynamic Observatory Sutherland (SAGOS) . Meanwhile a second, single sensor instrument has been installed in Sutherland.
Satellite gravimetry can map the gravity field of the Earth in a very homogeneous way, however, with lower resolution due to the height of the orbits. Traditional terrestrial gravimetry can measure the gravity with high resolution, but the data is often inhomogeneous and the measurement is limited by the environment conditions. Airborne gravimetry measurements can be used to fill the data gaps of the traditional and satellite gravimetry. 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.
GFZ’s instrumentation for Airborne Gravimetry are a mobile gravity meter Chekan-AM made by CSRI "Elektropribor" , an Inertial Navigation System (INS) AEROcontrol-II and 4 GPS receiver JAVAD Delta G3T
Measurement activities using these instruments were up to now:
First use of the instruments for ship gravimetry on lake Müritz.
Project GEOHALO, a geoscientific Earth observation project over Italy with the High Altitude and LOng Range Research Aircraft (HALO) (originally planned in the Aegean Region, shifted to Italy because no flight permission was granted over Greece)
Measurements on Lake Constance together with Bundesamt für Kartographie und Geodäsie (BKG)