Produced by: Popular Science China
Planning: Popular Science Office of the China Geological Survey (Geological Literature Center of the China Geological Survey)
Production: Deep Earth Depth Detection Center of the Chinese Academy of Geological Sciences Jilin University Fu Wei, Hou Hesheng, Yang Rong, Guorui
Producer: Computer Network Information Center of the Chinese Academy of Sciences
Human beings have a "fault" and always want to peek at the structure inside objects. Cut a watermelon with a big knife, and the arrangement of the green-skinned red-sacred black spots will be clear at a glance. The human body is not a watermelon, so it cannot be cut open at will, because touching the nerves in the body can often make people feel painful and embarrassed. The understanding of human body structure has been relied on the dissection of corpses since ancient times. What about living bodies? Traditional Chinese medicine uses finger pulses and Western medicine stethoscopes to listen to sounds to see the health of the heart and vascular system in the body.
Of course, modern technology can detect fine structures in the body such as blood vessels, polyps, tumors, etc. without direct contact with the human body. For example, the very popular "CT" technology is the abbreviation of "Computerized Tomography", that is, "computerized tomography". CT diagnosis uses X-ray to scan the human body to obtain a large amount of data. Through complex and large-scale computing by computers, the images formed by X-rays are drawn on a "form", allowing the naked eye to recognize the fine structures and mutations on the living section.
is the same as the human body, and the earth is also an opaque "black object" under visible light. There is a legend in China that "the mountain mirror" and always hopes to peek at the structure inside the earth. Get inspiration from the technology of performing CT on the human body, and similar CT technology can also peek at the internal structure of the earth. But the earth is different from the human body. The human body is small, and in ordinary CT instruments, three-dimensional three-dimensional scanning in all directions can be performed, while scanning the earth can only be performed on the surface, which is a plane two-dimensional scanning. Therefore, it is more difficult to do CT on the earth and requires "black technology". Based on similar principles and ideas of CT technology, the X-ray electromagnetic wave was replaced with elastic waves (such as sound waves) generated by the vibration of objects. Based on more powerful computer computing technology, "oblique seismic reflection offset profile computer imaging technology" appeared in 1975, laying the foundation for seismic tomography, and the structure of underground oblique formations can be drawn through seismic exploration methods to find oil and gas deposits.
What do we use to peek at the earth?
Geophysics, as the name suggests, is to use physical principles to study the earth. What geophysicians like most is to make analogies with medical CT to explain the nature of their work. Indeed, geophysical detection and medical detection of the human body have many similarities. First, let’s briefly review the working principle of medical CT. It emits an X-ray to the human body. The various tissues of the human body have different ability to absorb X-rays. Therefore, the same X-ray will change to varying degrees after passing through the human body. At this time, this differential change will be converted into an image that is easy for us to explain for diagnosis. Our geophysics also uses the differences in the physical attributes of the earth's internal structure to study its internal structure, that is, we use our instruments and equipment to do "CT" the earth.
Figure 1 Medical CT and geophysical
How do we do "CT" for the earth?
What are the main physical properties that geophysics can distinguish? After nearly a hundred years of development, people have developed a series of geophysical detection methods. These methods correspond to gravity exploration, magnetic exploration, seismic exploration and electrical exploration, which are commonly used geophysical detection methods, based on density differences, magnetic exploration, and electrical exploration, based on density differences, magnetic exploration, differences in velocity differences, velocity differences, and differences in conductivity within the earth. Among them, gravity, magnetic and electrical methods take into account the overall effect of underground medium, while seismic methods mainly target the mutation interface of changes in velocity and density structure, and have higher resolution.
So how do these physical attribute differences correspond to the deep structure of the earth? First we need to transform the geological problems of deep structure into geophysical models, in which geological phenomena will be characterized by the above physical properties. The link that connects the above geological and geophysical issues is rock physicality.Our Earth consists of a variety of rocks and fluids contained in rocks. Each rock, including each rock, has its corresponding physical attribute parameters under specific temperature and pressure conditions, which is the basis for geophysical methods to distinguish different media. Of course, these physical properties do not correspond one by one. The same physical properties may represent different rocks and different rocks in different states. This is the root of the multi-solvability of geophysical detection. Therefore, it is necessary to combine multiple geophysical exploration methods for judgment. Most of the time, it is also necessary to combine geology and geochemistry and data for judgment. We can see all the difficult and complicated diseases of
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First of all, deep geophysics research understands the earth's circle structure. The current popular model of the earth is to divide it into three basic circles: the crust, the mantle and the core. These three basic circles can be further subdivided, such as the earth's crust, which can be divided into the upper crust (longitudinal wave velocity, Vp=5.9~6.3km/s), the middle crust (Vp=6.4~6.7km/s) and the lower crust (Vp=6.8~7.6km/s), and they have obvious physical differences. Generally speaking, with the changes in the depth inside the earth, its physical attribute parameters show a shift change, but there is also a clear attribute mutation interface (Figure 2). For example, the Moho surface, geochemical research believes that it is a phase transition interface of rocks, which is mainly transformed into a mutation interface of velocity and density, with its velocity silenced to more than 8km/s and a density silenced to more than 3g/cm3. At the same time, there are also obvious low initial velocity layer (Vp6 km/s) in the shell and high conductor layers in the shell, and usually have corresponding relationships. It is usually possible to observe obvious geophysical anomalies at these physical attribute mutation interfaces (Fig. 3), where electrical and seismic methods are more sensitive in longitudinal direction (deep direction), and their anomalies are obvious, while heavy magnetic methods do not easily observe physical changes in the depth direction. By combining these observational anomalies with the physical properties of the rock, we can explain the changing characteristics of the circles inside the earth.
Figure 2 Physical properties of different depths of the earth
Figure 3 Schematic diagram of geophysical anomalies in the circle structure
At the same time, deep geophysics focuses on the lateral inhomogeneity of the earth. Research shows that our earth is not a complete circle structure, and there will be differences in the material components and states of the same circle in the horizontal direction. Just as our surface has both mountain plains, rivers, lakes and seas, this obvious difference reflects the evolution process of the earth. The movement of plates we often discuss is a kind of lateral inhomogeneity. Different plates have different evolutionary histories and material components. When they are collaged together in tectonic motion, there are obvious physical differences on both sides of their sutures. The most obvious one is the boundary between the mainland and the oceanic plates, which are usually inserted under the mainland plate in a wedge-shaped shape. The oceanic crust is usually a silicon-magnesium crust, and its velocity and density are higher than that of the continent's silicon-aluminum crust. There are obvious physical differences between the two. Geophysical methods will obtain obvious observation abnormalities during surveying across both sides of different geophysical bodies (Figure 4).
Figure 4 Schematic diagram of geophysical anomalies with different blocks
In addition, deep geophysics will also discuss deep local anomalies. Local anomalies are also the product of internal evolution of the earth. Take a deep magma capsule as an example. The magma capsule is mainly some molten substance, usually from deeper layers, whose density and velocity are usually higher than that of surrounding rocks, and may be magnetic, and its electrical conductivity is generally poor, so it is manifested as high-speed, high-density, high resistivity, and high magnetic anomalies. In geophysical detection, each method obtains a profile or curve diagram of such attributes for a certain type of attribute (Figure 5).
Figure 5 Geophysical response diagram of local anomalies
The above process talks about the positive geophysical problem, that is, what kind of deep earth structure corresponds to what kind of geophysical anomalies. Our geophysics study of the deep structure of the earth is an inverse problem, that is, inferring deep structures through observation anomalies. This requires us to comprehensively consider various parameters and conduct a comprehensive analysis of observed geophysical anomalies combined with geological and geochemical data.At the same time, it is also necessary to improve the utilization rate of geophysical observation data so that observation anomalies can be converted into intuitive and accurate.
At present, my country has CT for the earth and makes the earth a "transparent earth". It not only has seismic tomography, but also electromagnetic wave tomography, which is not only used in the field of energy mineral exploration such as oil and natural gas, but also in the field of metal minerals, non-metallic minerals, and water vapor minerals exploration, making great contributions to the development of my country's mining industry.