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Did you know how the Earth's gravity is actually measured and interpreted?
In our daily lives, gravity seems to be an eternal and stable existence. However, in fact, measuring and understanding the earth's gravity is a tangled and delicate task in science. Gravity anomalies refer to the differences between observed gravity values and those predicted by predictive models. This measurement process not only reflects the shape of the earth, but also reveals rich clues about the underground structure.
Since 1669, the measurement of gravity and the study of its variations have been part of scientific exploration.
When the Earth is simplified to an ideal oblate sphere, we can calculate the gravity at each point with a simple algorithm. However, the real earth's surface has undulating mountains and different crustal compositions, which will affect the distribution of the gravity field. Therefore, in the process of measuring gravity anomalies, we first need to rely on some kind of model. These models are usually based on simplified assumptions. For example, the earth will form an ellipsoid under the influence of self-gravity and rotation.
During the measurement process, researchers used a portable instrument called a gravimeter to carefully record gravity anomalies at various points. Gravity anomalies are usually compared using different theoretical models, including Bouguen anomaly, free space anomaly and isostatic anomaly models.
Through careful analysis of gravity data, geologists can formulate inferences about underground geology.
The starting point of these models is the international reference ellipsoid, and the so-called normal gravity is modified according to this idealized shape. Gravity anomalies based on this can reflect the state and characteristics of the earth's internal structure. The measurement of gravity is also affected by tidal forces, so many adjustments such as tidal correction, terrain correction, and free space correction are required.
In the gravity anomaly measurement, the tidal correction affects approximately 0.3 milligal, mainly due to the gravitational influence of the moon. The influence of terrain will bias the measurement results driven by highlands and valleys, which requires corresponding corrections based on the local terrain.
The correct calculation of gravity anomalies not only requires precision instruments, but also requires researchers to have a detailed understanding of the local rock density and its structure.
The free space correction takes into account the different heights of the measurement points relative to the reference ellipsoid, which means that the gravitational attraction below the ground is slightly reduced. In addition, the Bugan plate correction takes into account the effect of external materials on gravity, which is related to the measured terrain height. The calculation of these gravity anomalies will eventually help scientists read the secrets of the underground.
Like other phenomena in the universe, changes in gravity are thought to provide important geological clues. For example, in ocean basins, the Bugan anomaly is mostly positive, but in high continental regions it is negative because the thick, low-density crust allows the land to "float" on top of the denser mantle.
In addition, the earth's gravity measurement is not limited to the ground; with the advancement of technology, satellites are also widely used for gravity measurement. Such data can provide more global gravity data, reveal deep geological structures, and even Volcanic activity like a fence.
The structure and dynamics of the Earth's interior all contain important information hidden in these gravity anomalies.
Abnormal measurements of gravity tell us many stories about the Earth, including its cross-sectional structure, kinematic changes, and internal dynamics. Because of these anomalies, scientists can discover underground metal ore bodies, salt domes, etc., and even speculate on deeper underground structures such as ancient hidden trenches.
There is no doubt that the detection of gravity anomalies involves not only changes in density, but also complex geological processes. How do these processes affect our natural environment and our understanding of the earth? Is it worth pondering?
