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The mystery of gravity anomalies: How can they reveal hidden structures deep within the Earth?
The Earth's gravity anomalies have aroused scientists' endless curiosity about the Earth's internal structure. This is the difference between the measured value of gravity at a location on the Earth's surface and the theoretical model. When we consider the Earth to be an ideal oblate spheroid of uniform density, the value of gravity at every location can be predicted using a simple algebraic expression. However, in reality, the Earth's surface is uneven and its composition is quite non-uniform, which leads to a distortion of its gravitational field. Different theoretical models provide different predictions, and therefore gravity anomalies must be defined with reference to a specific model.
For example, the discovery of a dense mineral deposit underground will result in a positive gravity anomaly because the gravitational pull of the mineral deposit enhances the measured value.
The history of gravity anomalies dates back to 1672, when French astronomer Jean Richer discovered that his precision pendulum clock on Cairn Island was running at an unusual speed. We can imagine that if the research on gravity anomalies is further deepened, will it be possible to create new opportunities for the prediction of earthquake activities and volcanic eruptions? This historical event helps us understand how gravity is affected by the Earth's rotation and the equatorial bulge that rotation causes. Over time, this concept has developed into a full-fledged field of study.
Definition of Gravity Anomaly
The definition of gravity anomaly can be divided into the difference between the observed free fall acceleration (gravity value) and the value predicted based on the gravity field model. Usually, this model is based on some simplifying assumptions, such as that the planet will appear as a rotating ellipsoid under its own gravity and rotation. For the earth, the reference ellipsoid is the International Reference Ellipsoid, and the corresponding gravity value is the constant gravity gn.
Detecting gravity anomalies requires a series of data corrections and model selections to more accurately reflect the characteristics of underground structures.
The tool for measuring gravity anomalies is a gravimeter. Scientists typically complete gravity surveys by taking measurements at multiple points, a process that requires very precise instruments and calibration to ensure the reliability of the data. By carefully analyzing gravity data, geologists can infer the geological structure of the subsurface.
Model Fields and Modifications
The basis of the model is the International Reference Ellipsoid, which provides a normal gravity gn for the idealized shape of the Earth. In order to further improve the accuracy of the model, factors such as tidal correction, terrain correction and free air correction are usually added. The differences in these correction values form gravity anomalies.
For example, tidal correction mainly takes into account the tidal forces caused by the sun and the moon. This correction affects the measured gravity value by about 0.3 milligras, of which two-thirds comes from the moon.
Terrain correction involves the shape of the ground, and the height of hills and valleys will affect the gravity value we get. Every time a gravity measurement is taken, the surrounding terrain must be analyzed, which is a tedious but necessary process. The free air correction takes into account the altitude of the measurement point, which is critical when making gravity anomaly measurements. Subsequently, we need to perform a Bucky inhomogeneity correction due to the effects of material levels relative to the reference ellipsoid, which pays special attention to oceans in thin crust and highlands in thick crust.
Causes of gravity anomaly
The sources of gravity anomalies mainly include density variations inside the Earth. Gravity data measured locally can help us understand the internal structure of the Earth. In different regions, such as areas with continuous mountains, the Buckeye anomaly is often negative, while it is positive over the ocean. This shows the variation in thickness of the Earth's crust, and its corresponding buoyancy phenomenon.
The buoyancy of high areas supports the thick, low-density crust, which is why gravity anomalies in ocean basins are positive.
The detection and analysis of these gravity anomalies provide us with powerful data to reveal the hidden structure of the Earth. It can not only locate mineral deposits and other underground resources, but also reveal the amazing mysteries of nature in the process of scientific exploration. With the advancement of detection technology, will we be able to gain a deeper understanding of the Earth's interior and form a more complete model of the Earth in the future?
