The acceleration caused by gravity, denoted as g, varies subtly across the Earth’s surface because the distribution of mass beneath the surface is not perfectly uniform. Differences in underlying rock density, topography, and the presence of water or ice all influence the local gravitational pull. Quantifying these minute changes in g allows scientists to create detailed maps of the planet’s interior structure and monitor how mass shifts over time. This field of study, known as gravimetry, employs specialized instruments to measure these variations, providing data valuable across numerous scientific and commercial applications.
Principles of Gravitational Measurement
Understanding gravity measurement requires distinguishing between two primary approaches: absolute and relative gravimetry. Absolute measurement determines the precise value of gravitational acceleration at a single, fixed point, traceable to physical standards. This measurement is typically expressed in meters per second squared (m/s²), or in the gravimetry unit, the Gal (one centimeter per second squared, or cm/s²).
Relative measurement determines the difference in gravity between two or more locations, rather than the full absolute value at each site. These relative changes are often measured in microGals (\(\mu\)Gal) or milliGals (mGal). Relative instruments rely on a reference point with a known absolute gravity value for calibration. Absolute measurements establish the baseline, while relative measurements efficiently map small, local changes across a wide area.
Instruments for Surface-Level Measurement
Gravimeters are divided into absolute and relative categories. The most accurate absolute gravimeter measures the acceleration of an object in free-fall within a vacuum chamber. This method uses a dropped corner-cube reflector tracked precisely by a laser interferometer.
The interferometer monitors interference patterns created by the reflected laser light to measure the object’s position over time. By applying the equations of motion for a freely falling object, the local value of g can be calculated with high precision. These complex instruments are usually stationary, serving primarily as reference standards to calibrate field instruments.
Relative gravimeters are significantly more portable and are used for detailed local surveys. The most common design uses a highly sensitive spring system, suspending a fixed mass from a spring. When the instrument moves to a location with a different gravitational pull, the tension changes, causing a minute displacement of the suspended mass.
The change in the spring’s extension is recorded and converted into a gravity difference. While simpler and more mobile, these mechanical spring systems can experience drift over time due to temperature fluctuations or changes in the spring’s material properties. Therefore, relative gravimeters require periodic re-calibration against a known absolute gravity station to maintain accuracy.
Global Gravity Mapping from Space
Measuring the Earth’s gravity field globally requires instruments placed in orbit, overcoming the limitations of static ground measurements. Satellite missions like the Gravity Recovery and Climate Experiment (GRACE) and the Gravity field and steady-state Ocean Circulation Explorer (GOCE) have provided extensive global gravity data. These missions track subtle orbital perturbations caused by mass variations on Earth.
The GRACE mission used a pair of satellites flying in tandem, separated by about 220 kilometers. When the leading satellite passes over a region of higher gravity, it is momentarily pulled forward, changing the distance between it and the trailing satellite. A microwave ranging system precisely measures this minute change in separation distance, which can be as small as a few micrometers.
This method is uniquely suited for measuring time-variable gravity, which refers to changes in the gravity field that occur over periods of months or years. GOCE, in contrast, used a single satellite equipped with a highly sensitive gravity gradiometer to measure the static, or steady-state, gravity field. The gradiometer measures the differences in gravitational pull across the satellite’s body, allowing for the mapping of the Earth’s geoid.
Practical Applications of Gravity Data
The detailed gravity data collected from both ground and space instruments have a broad range of applications across Earth science and commercial sectors. A primary application is in geodesy, the science of measuring the Earth’s shape, orientation, and gravity field. Gravity data are used to determine the geoid, which is the precise surface of equal gravitational potential corresponding to the mean sea level extended globally beneath the continents.
Resource exploration relies on localized gravity surveys to identify subsurface structures. Variations in gravity indicate density anomalies: higher readings suggest denser materials, such as mineral ore deposits. Conversely, lower readings can indicate less dense structures, potentially pointing to underground sedimentary basins that hold oil and gas reservoirs.
Gravity monitoring tracks environmental change by measuring the movement of mass within the Earth system. GRACE data monitor the depletion of major groundwater reserves, as the removal of water mass results in a measurable decrease in local gravity. This principle also tracks mass loss from ice sheets in Greenland and Antarctica, indicating climate change impacts.
In volcanology, gravity surveys offer a method for monitoring the movement of magma beneath active volcanoes. As magma rises into a shallow reservoir before an eruption, the introduction of this dense material changes the local gravity field. Detecting these gravity changes, often measured in microGals, serves as an important early warning system for potential volcanic activity.