Airborne Gravimetry – what is it good for?
Airborne Gravimetry – what is it good for?
Airborne Gravimetry measures the Earth’s gravity field from the air using gravimeters onboard aircraft or helicopters. This technology is used for geological mapping, geodetic surveys, and tectonic analyses. To minimize disturbances caused by movement, the measuring instruments using extended Kalman-Filters (EKF), Inertial Navigation System and precise GNSS.
Airborne Gravimetry is used in various scientific and industrial fields.
- Geological exploration: Mapping of resource deposits such as oil, natural gas and minerals through the analysis of gravity anomalies.
- Geodetics and geoid determination: improving precise height measurement and creating accurate geodetic models for surveying the Earth.
- Tectonics and geophysics: Study of geological structures such as faults, volcanic regions or salt domes.
- Hydrology and oceanography: Study of water systems and ocean currents through changes in gravity.
- Environmental protection and climate research: Identification of changes in the Earth’s crust caused by climatic or geophysical processes.
Geological Exploration
Airborne Gravimetry is used in geological exploration to measure density differences in the subsurface and identify geological structures:
• Raw material exploration: By analyzing gravity anomalies, deposits of oil, natural gas and minerals can be located.
• Mapping tectonic structures: Airborne Gravimetry helps to identify faults, salt domes, and magmatic intrusions, which are important for geological modeling.
• Geothermal investigations: The method is used for the preliminary exploration of geothermal reservoirs by analyzing density contrasts between different rock layers.
• Hydrogeological studies: Changes in the gravity field can indicate the presence of underground water reservoirs or aquifers, which is relevant for water supply and environmental studies.
Hydrology & Oceanography
Airborne Gravimetry is used in hydrology and oceanography to detect changes in the Earth’s gravity field related to water movements and distributions. Here are some specific applications:
• Groundwater exploration: Airborne gravimetric measurements help identify underground water reservoirs by analyzing density differences in the soil.
• River and lake research: The method is used to investigate water mass shifts in large rivers and lakes, which is important for hydrological models.
• Ocean currents and tides: Changes in the gravity field provide clues to current patterns and tidal movements that are crucial for oceanography.
• Climate research: Airborne Gravimetry contributes to the study of water cycles and their influence on the global climate.
Geodetics
Airborne Gravimetry plays an important role in geodetics and geoid determination, providing precise data on the Earth’s gravity field. Here are some specific applications:
• Geoid modeling: Airborne Gravimetry helps to more accurately determine the geoid—the ideal level surface of the Earth’s gravity field—which is crucial for precise altitude measurements.
• Elevation and land surveying: The method contributes to the improvement of elevation systems by recording the deviations between the geoid and the Earth’s reference ellipsoid.
• Integration with satellite geodesy : Airborne gravimetric data are often combined with satellite measurements to refine global and regional geoid models.
• Improving geodetic reference systems: By combining airborne gravimetry with terrestrial and satellite measurements, more accurate geodetic models can be created.
Environmental Protection
Airborne Gravimetry is used in environmental protection and climate research to detect changes in the Earth’s gravity field related to climatic and ecological processes. Here are some specific applications:
• Monitoring ice masses: Airborne gravimetric measurements help to detect changes in glaciers and ice sheets influenced by climate change.
• Detection of land subsidence: The method is used to identify land changes caused by water extraction or tectonic processes.
• Analysis of carbon storage: Airborne Gravimetry can contribute to the study of biomass changes and carbon storage in forests and other ecosystems.
• Research into sea level changes : By measuring gravity anomalies, long-term changes in sea level and their causes can be analyzed.
Gravimetry uses the physical principle of gravity to measure changes in the Earth’s gravitational field. High-precision gravimeters, which operate on the spring balance principle, record density differences (density inhomogeneities) in the subsurface. The goal is to use these gravity anomalies to draw conclusions about material boundaries, geological structures, and tectonic faults.
Geomagnetic measurements detect anomalies in the Earth’s natural magnetic field caused by the magnetization of geological bodies. This magnetization can be induced, depending on the field strength and material properties, or remanent, independent of the external field, particularly in ferrimagnetic minerals such as iron oxides and sulfides. The goal of applied geomagnetics is to use these anomalies to draw conclusions about the magnetization, shape, size, and depth of geological structures.
Geophysical seismic measurements use artificially generated seismic waves to analyze the subsurface. Reflection seismicity maps geological structures using reflected waves, while refraction seismicity examines deeper layers using refracted waves. This method helps determine mineral deposits, tectonic structures, and geotechnical conditions.
VLF (Very Low Frequency ) is an electromagnetic technique that uses transmitter waves in the range of 15 to 25 kHz , originally used for submarine communications. These waves induce eddy currents in conductive structures in the subsurface, creating a secondary electromagnetic field . The resulting total field is measured to analyze geophysical properties.
Magnetotellurics uses natural alternating electromagnetic fields for deep probing. These fields are created by magnetic variations and telluric current systems. The waves induce electrical currents in conductive structures, which in turn generate electromagnetic fields. By measuring the total field, geological structures can be analyzed from a few meters down to the Earth’s crust.
A mineral deposit can be identified in airborne gravimetry by analyzing gravity anomalies.
Airborne gravimetry is often combined with other geophysical techniques to obtain a more comprehensive picture of the subsurface
These occur when the density of the subsurface changes—for example, due to core deposits, salt domes, or sedimentary basins.
Gravity studies are based on the fundamental principle that variations in density below the surface cause corresponding fluctuations in the local gravitational field.
High-density materials, such as rocks and minerals, exert a stronger gravitational pull than low-density materials such as water or air.
By measuring these gravitational fluctuations, scientists can infer the distribution of different rock types and geological structures beneath the Earth’s surface.
• Magnetometry: Measures magnetic anomalies to identify geological structures and mineral deposits.
• Radiometry: Detects natural gamma radiation to determine rock types and their composition.
• Geoelectrics: Analyzes the electrical conductivity of the subsurface to detect aquifers or ore deposits.
• Seismics: Uses seismic waves to investigate deep structures and tectonic movements.
• Satellite gravimetry: Complements airborne gravimetry with large-scale gravity field measurements using satellite missions such as GRACE or GOCE
These combinations enable detailed geophysical analyses for mineral exploration, geodetic surveys and tectonic studies
Gravimetric anomalies are local variations in gravitational acceleration from its theoretical normal value.
The free-air anomaly describes the difference between the observed and theoretical gravitational acceleration at a given location, based on the Earth’s shape and rotation. It is the simplest form of gravity anomaly and is calculated without considering topography or mass fluctuations. It is caused by density variations and terrain differences in the subsurface.
A Bouguer anomaly is a gravity anomaly that removes the influence of topography to analyze exclusively the gravitational effect of subsurface masses. The Bouguer correction removes the gravitational effect of the terrain from the measured gravity value. This method helps identify density fluctuations and map geological structures such as basins, faults, and ore bodies.
Isostatic anomalies describe fluctuations in crustal thickness and the Earth’s isostatic equilibrium, which influences the balance between uplift and subsidence of the Earth’s crust. They arise from differences in density and thickness in the lithosphere —thicker crusts produce positive anomalies, while thinner crusts produce negative anomalies. These anomalies are essential for tectonic studies and the understanding of geological processes and past tectonic events.
How can I evaluate and interpret the gravimetry data?
Typical corrections to the data sets:
Gravitational corrections improve the accuracy of gravity field measurements through various adjustments:
- Terrain corrections remove the influence of topography using digital terrain models.
- Bouguer corrections eliminate the gravitational effects of near-surface masses to analyze subsurface density variations.
- Latitude corrections consider the centrifugal force of the Earth’s rotation, which varies depending on latitude.
- Eötvös corrections compensate for gravitational influences caused by the Earth’s rotation and shape.
These adjustments are crucial to precisely isolate gravity anomalies and correctly interpret geological structures.
Interpretation of gravity data involves the analysis of gravity anomalies to determine geological structures and density changes beneath the Earth’s surface.
- Detecting anomalies : Positive values indicate dense rocks, negative values indicate less dense areas.
- Correlation with geology : Compares anomalies with known structures such as faults or sedimentary basins.
- Regional vs. local effects: Distinguishes large-scale deep structures from near-surface anomalies.
- Bouguer & Isostatic Anomalies: Correction of topography effects for more accurate density analysis.
- Gradient analysis & depth estimation: Identification of boundaries between rock layers and estimation of the depth of density contrasts.
- Integration with other data: combination with seismic, magnetic or geoelectrical surveys.
- Exploration of raw materials: identification of hydrocarbon deposits, ore deposits and salt domes.
- Modeling & Inversion: Use of numerical techniques to reconstruct geological structures.
These methods are essential for geophysical research, mineral exploration, and tectonic analysis.