Microgravity measurement is compared to conventional gravity measurement. The term microgravity measurement first appeared in literature in the 1960s. It refers to the accuracy of measurement and the gravity effect caused by the object of measurement exploration. It is measured in micro-ga levels. Microgravity measurement also includes the meaning that the size and scale of the measurement object are smaller than those of general gravity measurement. The results of microgravity measurements can give people a more in-depth and detailed understanding of the Earth's gravity field; the observed microgravity anomalies can better explain the shape, distribution, and structure of underground density anomalies. Microgravity measurement has more theoretical significance and practical value.

Microgravity measurement is a product of modern science and technology and is due to the emergence of high-precision gravimeters. Since the 1970s, the gradual introduction and application of micro-G level high-precision gravimeters LaCoste and Romberg (L&R instruments) have made gravity measurements with an accuracy of several micro-G levels a reality. In the past, due to the low precision of instruments, it was difficult or even impossible to detect exploration objects with weak gravity effects. But now microgravity measurements can detect and explain the existence or formation of potential safety-threatening hazards such as fractures, rock bursts, and cavities within the foundations of hydropower projects, transportation, civil engineering, and high-rise buildings.

The application of microgravity measurement can detect density anomalies such as near-surface caves, underground rivers, cavities, abandoned mine tunnels, giant-diameter pipelines, and smaller geological structural fractures and faults, solving many engineering and environmental problems. It also plays a role in disaster forecasting, exploration of oil and natural gas resources, detection of geothermal resources, monitoring of earthquake forecasts in seismic activity areas, and monitoring of terrain changes in reservoir areas caused by the storage and release of water from large reservoirs. important role and widely used.

Microgravity measurement is a new branch of science developed on the basis of classical gravimetry. Therefore, the basic theories and concepts of the gravity potential field are basically the same as those of classical gravity, and have their uniqueness, but their particularity is that they highlight the "micro" nature and characteristics, so in the following chapters, they are referred to as " Describe the characteristics of "micro".

1. Basic theories and concepts

The basic theories and concepts of microgravity measurement are consistent with those of classical gravity measurement, so the basic theories and concepts of classical gravity measurement are quoted here.

1. Gravity and its changes

(1) Gravity

In addition to the gravity of the earth, objects on the earth are also affected by the inertia generated by the rotation of the earth. The effect of centrifugal force. The resultant force of the earth's gravity and inertial centrifugal force on an object is the gravity G, which can be expressed as

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The direction is roughly towards the center of the earth, that is, roughly along the gravity F direction. The space where gravity exists inside and near the Earth is called the Earth's gravity field.

Assume that the earth is an oblate spheroid with uniform internal density distribution and mass M (Figure 4-4-1). According to Newton's law of universal gravitation, the gravity and mass of the earth experienced by an object with mass m m, M and the distance from the object to the center of the earth are related. The inertial centrifugal force experienced by an object is related to the mass m, the angular velocity of the Earth's rotation and the distance r from the object to the Earth's rotation axis (Figure 4-4-1).

The Earth’s gravity field is described by its gravity field strength. The strength of the gravity field is numerically equal to the gravitational acceleration g of an object when it is affected by gravity, that is,

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Figure 4-4-1 The gravity of an object

In the gravity method, the quantity studied is the acceleration due to gravity, and is simply called gravity.

In the CGS unit system, the unit of g is m/s2. This unit is usually called Gal, and the practical unit is "mGal", which is one thousandth of a Gal, or "μGal". The legal unit of gravity is the International System of Units, that is, g.u. (gravity unit), 1g.u.=10-1mGal.

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(2) Distribution of Gravity

Gravity on the earth changes with location and time.

The reasons for the change of gravity in space are: first, the earth is not a perfect sphere, but an oblate spheroid with two poles compressed, with an equatorial radius of about 6378 km and a pole radius of about 6356 km; Second, due to the ups and downs of the earth's surface, that is, gravity changes with height; third, objects at different latitudes have different rotation radii r and experience different inertial centrifugal forces, so gravity changes with latitude; fourth, the density of underground materials Uneven distribution will also cause changes in gravity.

Assume that the shape of the earth is a two-axis spheroid, and the material inside is distributed in uniform concentric layers. In this way, the formula for the gravity of each point on the ideal earth surface (which should be the geoid) can be derived - the normal gravity formula:

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Where: gφ is the normal gravity value at any point on the earth's surface, ge is the gravity value at the earth's equator, generally 9.78049m/s2, β, β1 are constants related to the shape of the earth and the earth's rotation angular velocity, generally β=0.0052884, β1=- 0.0000059, φ is the latitude of the calculation point. The normal gravity value at any point on the earth's surface can be calculated using the above formula.

There are two types of changes in gravity over time: one is the change in gravity caused by the gravity of celestial bodies such as the moon and the sun. Its performance has a certain periodicity, which is also called tidal change or gravity solid. Tide; the other is that changes in the shape of the earth and changes in the distribution of underground materials cause gravity to change over time at the same place, which is manifested as non-periodic, also known as non-tidal changes.

2. Gravity anomaly

Comparing the measured gravity value in the field with the theoretically calculated gravity value at this point, you will find that there is a certain difference between the two. The reasons for this difference mainly include the following two aspects.

First, the gravity value calculated using equation (4.4.3) is the gravity value on the geoid, and the gravity measurement is carried out on the natural surface of the earth. The natural surface of the earth and the geoid The matter in between causes changes in gravity. In order to eliminate this effect, the measured gravity values ??need to be corrected.

Second, the density distribution of matter inside the earth is uneven, resulting in a certain difference between the measured gravity value and the normal field value.

Therefore, the measured gravity value in the field includes not only the influence of natural terrain changes, but also the gravity changes caused by uneven underground density. In order to study the latter, the former must be eliminated. In gravity exploration, we call the remaining gravity value after subtracting the normal gravity value from the measured gravity value and eliminating the influence of terrain and other factors, called gravity anomaly. That is

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In the formula: Δg is the gravity anomaly; g is the gravity observation value; Δg is the terrain correction value; Δg is the middle layer correction value ; Δg height is the height correction value; gφ is the gravity value on the geoid.

3. Application conditions of microgravity exploration

Gravity exploration can be used for geological exploration, geodesy, natural earthquake and other aspects of research, as well as directly solve certain hydrological and engineering geological problems. However, these problems can be effectively solved only when the geological body being detected can cause large enough gravity anomalies and the interference factors are small, or the interference factors can be separated by certain methods. The prerequisites for gravity exploration are: 1) For gravity anomalies to occur, density inhomogeneities must first exist. In addition, if the object we are studying is very small, although there is a certain density difference between it and the surrounding rock, the gravity anomaly cannot be measured due to the small remaining mass and the limited accuracy of the measuring instrument.

2) Density difference alone does not necessarily produce gravity anomalies. There must also be density changes in the horizontal direction. For example, for a group of horizontal rock layers, although the density of each layer is different, there is no fluctuation in the horizontal direction, and it cannot cause gravity anomalies.

3) When using gravity measurements to study geological structural issues, it is required that there is a large enough density difference between the upper rock layer and the lower rock layer, and the rock layer has an obvious dip angle, or the fault has a large drop.

4) Flat terrain is also a favorable condition for gravity exploration. This can reduce a lot of work and improve the reliability of exceptions.

5) The smaller the interfering anomalies (such as anomalies caused by uneven surface density, density changes in deep rocks, etc.), the better.

The density values ??of the main rocks (ores) are now listed in Table 4-4-1. The unit is g/cm3 and is for reference only.

Table 4-4-1

2. Microgravity meter

The gravimeter is a measuring instrument that measures the relative change in gravity acceleration. Distinguished by the materials used in their elastic systems, the commonly used gravimeters mainly include metal spring gravimeters and quartz spring gravimeters. Their working principles are basically the same. They determine the relative changes in gravity by measuring the displacement of a certain static balance system when the gravity changes. They measure the difference in gravity between the measuring point and the base point, rather than the absolute value of gravity at a certain point.

As for metal spring gravimeters, the Lacoste gravimeter (L&R for short) produced by the American LaCoste & Romberg Gravimeter Company represents the world's leading level of this type of product. It can be divided into two types: L&R-D type (exploration type) and L&R-G type (geodetic type). This type of instrument has a series of advantages such as high resolution and observation accuracy, and small zero drift, and is widely used in countries around the world.

Table 4-4-2 List of microgravity measuring instruments

The quartz spring gravimeter is represented by the CG-3 automatic gravimeter launched by the Canadian Scintrex Company in 1987. It is based on a microprocessor and has many brand-new features, such as high precision, high self-awareness and intelligence, and extremely convenient field operations. At present, Scintrex Company has launched the CG-5 automatic reading gravimeter based on the CG-3 type improvement. It has all the functions of the CG-3, but has higher resolution and accuracy, and has some special functions: a very solid sensor and high noise. Amplitude reduction, the most portable fully automatic gradient meter, fast data transmission via USB and RS-232, standard 1 μg resolution, small power efficient battery, flexible data format, large 1/4VGA graphic display, 27-key character keyboard , automatic calibration of the instrument, online terrain correction, startup to enter the instrument self-test.

The instruments produced by the above two companies are the two types of instruments that currently meet the micro-Gamma level measurement accuracy requirements.

According to the product introduction, the resolution of both instruments is (1~5)×10 -2g.u. (1~5 microgal), and the observation accuracy is 0.05~0.1g.u. (5~10 microgal). The British Geospatial Survey conducted field measurement tests on the CG-3 and L&R-G gravimeters in 1991. The results showed that the observation accuracy of both instruments was better than 0.1g.u. (10 microgal).

3. Field work methods

1. Classification of microgravity measurements

In microgravity measurements, field observation methods are the same as conventional gravimetry, generally divided into There are two types of profile measurement and area measurement: ① Profile measurement is generally perpendicular to the assumed direction of linear geological bodies (such as faults, anticlines, synclines or hidden river channels, etc.); ② Area measurement mainly detects the size, shape and distribution of underground geological bodies. .

Whether it is a profile measurement or an area measurement, the location and relative elevation of the gravity measuring point must be determined by geodetic methods in order to make various corrections. In order to ensure that the errors of various corrections are lower than the specified requirements, the errors of point measurement and relative elevation measurement should be less than 1 m and 0.3 m respectively.

2. Point layout principles for microgravity measurements

1) Arrange the detected objects or anomalies in the center of the measurement line or measurement area;

2) Measure The line or survey area should be as close to the geological body related to the detection object as possible;

3) The direction of the survey line should be as perpendicular to the direction of the detection object as possible and consistent with the known geological profile as much as possible;

4) The distance between measuring points should be less than 1/2 to 1/3 of the width of the credible anomaly, ensuring that at least four measuring points can reflect the above anomalies;

5) Measuring The line distance should not be greater than 1/2 to 1/3 of the projected length of the geological body on the ground.

A base point should be established when carrying out microgravity measurements. Multiple measurements should be made at the base point, and the results can be used to correct changes in gravity over time and the zero drift of the instrument itself. In addition, the density of the middle layer must be accurately measured in order to make corrections to the middle layer. In terms of operating technology, the placement, leveling and elevation measurement of the instrument chassis should be carried out in accordance with strict requirements and regulations.

In gravity observation for the purpose of land subsidence monitoring, a long-term gravity monitoring network should be deployed to conduct regular repeated observations and joint measurements with absolute gravity points.

3. Requirements and content of field records for microgravity measurements

Field microgravity measurements should include a microgravity measurement record book and a near-instrument object measurement record book.

Record content of microgravity measurement: ①Optical shift sensitivity; ②Reading line; ③Transportation method; ④Instrument name and number; ⑤Readings at both ends of the longitudinal water bubble; ⑥Readings at both ends of the horizontal water bubble; ⑦Gravity readings Time and reading; ⑧The distance between the ground (measuring point pile) and the bottom edge of the instrument; ⑨Air pressure, air temperature and internal temperature of the instrument; ⑩Description of external interference; ?Point description;?Description of the terrain and landforms around the measuring point.

Record of near-instrument objects: ① Plane sketch in the work area; ② Sketch and editor’s note number of each measured object; ③ If the sketch of the measured object is divided into several regular geometric bodies, each A detailed drawing must be drawn for each segmented body, and the number of the segmented body shall be consistent with that of the original drawing.

IV. Microgravity observation data sorting and data interpretation

1. Microgravity measurement data sorting

Microgravity observation data sorting, in addition to conventional In addition to the same items of gravity observation data sorting (correction), in order to ensure that the quality requirements of micro-G-level observation data are met, it is also necessary to correct the influence of near-instrument objects and the influence of buildings within a certain range. Corrections in these aspects are generally not needed in conventional gravity observation data compilation. As for some correction items that are the same as conventional observation data correction, the requirements are also different. For microgravity measurement data, it is required that the elevation of the measurement point, the density value of the middle layer and the density of the rock used in terrain correction must be accurately measured.

Correction of microgravity observation data mainly includes:

1) Correction of changes in gravity over time. Changes in gravity over time include the effects of solid tides and instrument zero drift;

< p>2) Normal gravity correction, eliminates gravity changes caused by different latitudes of the measuring points;

3) Intermediate layer correction, eliminates the difference between the measuring point and the base plane (the horizontal plane where the base point is located) The influence of materials on the measured gravity value;

4) Height correction, eliminating the influence of the elevation of the measuring point relative to the datum on the measured gravity value;

5) Terrain correction, eliminating the influence of the measuring point The influence of the surrounding terrain undulations on the gravity of the measuring point. Microgravity measurement requires accurate determination of the terrain correction value within 50 m from the measurement point;

6) Near-instrument object correction, which is a special content in the collection of microgravity measurement data. Near-instrument objects generally refer to objects with a certain mass that are close to the gravimeter, such as artificial buildings on the ground, instrument observation stations, etc. They will produce different gravitational effects on each measuring point, with values ??ranging from several microgals to tens of microgals. The correction method is: generally treat artificial buildings as regular-shaped geometries (such as cylinders, cuboids, etc.) or a combination of several types of regular geometries, measure their coordinate positions, geometric shapes, and densities through field measurements, and then calculate By figuring out the gravity values ??caused by these regular shapes, corrections for near-instrument objects can be made.

After the above data is sorted, the gravity value of each measuring point relative to the base point can be obtained, which can be drawn into a cross-section or plan view, called the Bouguer gravity anomaly map, which reflects the recent gravity of the measuring area. The density distribution of surface materials.

2. Processing and interpretation of gravity data

The data processing of gravity anomalies mainly includes: anomaly smoothing and division, potential field conversion, etc. The explanation of anomalies includes qualitative explanation and quantitative explanation. The former is mainly to judge the relationship between abnormal characteristics and geological bodies, while the latter is to calculate the burial depth, occurrence and other factors of the detection target based on the abnormal characteristics.

(1) Data processing of microgravity measurements

Curve smoothing processing is used to eliminate field gravity measurement observation errors and errors caused by various corrections to the measurement results.

a. Freehand smoothing method

Experienced technicians can directly smooth the abnormal curve according to the changing rules of the gravity abnormal curve. During freehand smoothing, it should be noted that the deviation of the gravity anomaly value of each corresponding point before and after smoothing should not exceed the mean square error of the measured anomaly, and the areas formed by the abnormal curves before and after smoothing should be made equal as much as possible, and the center of gravity should not change.

b. Multiple averaging method

Use the average gravity anomaly of two adjacent points as the outliers of the two points, and then smooth the curve with freehand until the desired smoothness is finally achieved. .

c. Smoothing formulas for profile anomalies (including linear smoothing formulas and quadratic curve smoothing formulas).

Use the formula given below to find the smooth value of a certain point. Linear smoothing formula:

The smoothing value at a certain point is the arithmetic mean of odd-numbered point anomalies centered on that point on the profile. From m=1, 2, 3... we can get 3, 5, 7... point smoothing formulas respectively.

Quadratic curve smoothing formula (including five-point and seven-point smoothing formulas).

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d. Smooth formula for plane anomalies

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(2) Regional anomalies and local Identification and classification of anomalies

Bougainvillea gravity anomalies can be divided into regional anomalies and local anomalies. Regional anomalies reflect deep and large geological bodies, which are characterized by large distribution ranges, stable changes, and obvious regularity; local anomalies reflect shallow and small geological bodies, which are characterized by small abnormal ranges, large gradients, and changes More obvious.

Local anomalies are superimposed on the regional background field, often causing severe distortion of the original features. However, local anomalies can still be identified from regional anomalies on the Buga anomaly map. If the Buga anomaly isoline bulges in the direction of increasing regional background gravity in a local area, as shown in Figure 4-4-2(a), it indicates that there is a local low gravity there, and conversely there is a local high gravity.

The method of dividing local anomalies from regional anomalies is called regional correction. The essence of this method is to find the regional anomaly value of each point according to the rules and characteristics of regional anomalies, subtract the regional anomaly value from the Buga anomaly to obtain the remaining anomaly value, and draw a local anomaly map based on the remaining anomaly value, Figure 4-4 -2(b). This kind of diagram is the basic diagram for qualitative and quantitative interpretation.

The common methods for dividing local anomalies and regional anomalies are: ① Graphical method, which is divided into two types: parallel straight line method and smooth curve method. The parallel straight line method is suitable for regional gravity anomalies that change linearly in the horizontal direction. In areas where the smooth curve method is applicable, the regional gravity anomaly contours cannot be represented by parallel straight lines but can only be represented by curves; ② Numerical calculation methods, which include bias method, circle method, and grid method; ③ Polynomial fitting method, trend analytical method.

Figure 4-4-2 is an example of using the anomaly curve smoothing method to divide local and regional gravity anomalies. (a) is the Bouguer gravity anomaly map on the Houston Salt Dome in the United States; (b) the dot-dash pattern Lines and solid lines are regional anomaly and local anomaly maps drawn using anomaly curve smoothing method respectively. The map clearly reflects a negative anomaly, which drilling exploration confirmed was caused by a salt dome.

(3) Potential field conversion

Potential field conversion is mainly to facilitate the processing of inverse problems. The main contents include:

Figure 4-4-2 Regional anomalies and local anomalies

1) The gravity observation value on the observation plane is converted into the second-order and third-order partial derivatives (Vxz, Vzz, Vzzz) of the gravity anomaly on the same plane and other order coefficients, that is Derivative conversion of gravity anomalies.

2) The Δg, Vxz, Vzz, Vzzz, etc. at any point other than the anomaly source are converted from the gravity observation values ??on the observation plane to the analytical continuation of the gravity anomaly.

(4) Microgravity measurement data inversion method

The inversion of microgravity measurement data is the basis for the quantitative interpretation of microgravity anomalies. Before inversion, it is necessary to carefully analyze the superimposed anomalies and try to extract the gravity anomalies related to the exploration targets, so that it is possible to make a quantitative explanation of the geological bodies that caused the anomalies.

a. Analytical method

The Δg, Vxz, Vzz and Vzzz of a geological body are functions of its occurrence elements, remaining mass and observation point coordinates.

On the contrary, if the occurrence elements or remaining mass of geological bodies are expressed as functions of gravity anomalies (or their derivatives) and observation point coordinates, then when the Δg (or Vxz, Vzz, Vzzz) produced by these geological bodies is known , the occurrence elements and remaining mass parameters of the geological body can be calculated based on this functional relationship. The calculation methods include solving the Δg abnormal curve and solving the Vxz, Vzz, and Vzzz curves.

b. Tangent method

Use the tangent of the abnormal curve characteristic point to obtain the approximate burial depth of the top (or center) of the object using a graphical method.

c. Selection method

Based on the basic characteristics of the profile anomaly curve of the measured gravity anomaly or the distribution and change of the gravity anomaly contour on the gravity anomaly plane map, combined with the geology and other conditions of the work area Geophysical data provide a model of the geological body that causes this gravity anomaly, and use the problem-solving method to calculate the theoretical anomaly of the model body, and then compare the theoretical anomaly with the measured anomaly. When the two are within the allowable error range internally, then the model of the given geological body is the solution sought.

d. Direct method

Directly utilize the gravity anomaly distribution on the profile curve or plan view, and solve certain parameters of the abnormal body through integral operation, such as the remaining mass and center of mass of the three-dimensional body. Coordinates or the cross-sectional area and centroid coordinates of a two-dimensional body, etc.

e. Inversion of density interface

Determining the fluctuation of underground density interface based on measured gravity anomalies is very important for studying geological structures. For this work to achieve good results, the following conditions must be met. First, the gravity anomaly used for inversion calculations is caused by the fluctuation of the density interface; secondly, the density distribution of the material layers above and below the interface is relatively uniform, and their density differences are known; thirdly, there are at least one or several in the work area. The interface depth at each point is known. Methods for solving the density interface include linear formula solving method, second-order approximation formula solving method, compressed mass surface method, etc.

f. Shallow stress field inversion

The calculation formula for calculating the shallow stress field of the earth's crust is derived based on the elastic mechanics equilibrium equation, and the measured gravity data on the surface is used to invert. Generate shallow stress fields to explore the mechanical mechanisms and stability trends of some geological bodies.

3. Qualitative explanation of gravity anomaly

(1) Interpretation steps of gravity anomaly

On the Buga gravity anomaly map, first according to the exploration mission, Starting from abnormal characteristics such as abnormal scale, shape, gradient, and peak height, we can determine which are useful anomalies related to the exploration task and which are interference anomalies unrelated to the exploration task. Then, the area correction method is used to eliminate interference and highlight and draw useful anomalies. During the interpretation process, the geology and other geophysical prospecting data of the work area should also be closely combined, and comprehensive comparative analysis should be conducted to find out the geological factors or main density interfaces that cause gravity anomalies. Finally, quantitative or semi-quantitative calculations can be made for meaningful anomalies.

(2) The relationship between gravity anomaly characteristics and geological bodies

The shape of gravity anomalies is closely related to the shape of geological bodies. When the gravity anomaly extends farther along a certain direction, as shown in the figure 4-4-3(a), this kind of anomaly is often caused by secondary geological bodies. The direction of the abnormal long axis is the direction of the geological body. If the extension length of the anomaly is similar in all directions, as shown in Figure 4-4-3(b), it mostly corresponds to short-axis or equiaxed geological bodies, such as domes, rock masses, etc. If the isolines are dense (large gradient) and arranged in parallel, the outliers will rise or fall regularly in a certain direction, which is mostly a reflection of the fault zone. Figure 4-4-3(c), the wing with reduced gravity value It is the footwall of the fault. For flat inferred layers, gravity generally shows no abnormalities. For broken zones, they are often reflected as abnormal zones with low gravity. If the abnormal values ??increase and decrease regularly in a certain direction, but the abnormal gradient changes slowly, as shown in Figure 4-4-3(d), it is mostly a monoclinic structure.

In addition, the amplitude and magnitude of gravity anomalies are also related to the characteristics of the geological body. High gravity is generally reflected by anticlines, uplifts, dikes (bodies), etc. Negative anomalies are generally caused by synclines, fracture zones, depressions, caves, collapses, etc. However, it should be pointed out that under different geological conditions, anticlines may correspond to low gravity and synclines correspond to high gravity, so the interpretation must be based on the local geological conditions. Positive and negative gravity anomalies are arranged parallel to each other, often reflected in fold belts, anticlinal structures, etc.

The abnormal amplitude is related to the volume, burial depth of the geological body and the density difference between the geological body and the surrounding rock. When the density is different and the burial depth is the same, the one with larger volume will have a larger abnormal range and higher peak value; otherwise, the abnormal range will be smaller and the peak value will be lower. If the volume and density differences are equal, a high abnormal value indicates a shallow burial depth; a low abnormal value indicates a large burial depth.

4. Quantitative explanation of microgravity anomalies

Conventional quantitative explanation of gravity anomalies can still be used to quantitatively explain microgravity anomalies. The purpose is to calculate the volume and volume of geological bodies based on the characteristics of gravity anomalies. Burial depth and occurrence factors, etc. In order to establish the relationship between the characteristics of gravity anomalies and the occurrence of geological bodies and other factors, we approximately regard different geological bodies as simple geometric bodies. For example, mine nests, salt domes, and caves can be regarded as spheres; underground rivers, underground pipelines, etc. It can be seen as a horizontal cylinder; veins, fracture zones, etc. can be seen as plate-shaped bodies, vertical faults, contact zones can be seen as steps, etc.

So in order to quantitatively interpret geological bodies, it is necessary to understand the gravity anomaly characteristics of several common simple geometric shapes. Due to limited space, readers are advised to refer to relevant textbooks.

Figure 4-4-3 The relationship between gravity anomalies and geological body morphology

As a sophisticated shallow detection technology, microgravity measurement has been used in civil engineering and environmental surveys application. It can detect geological structures such as near-surface caves, caves, abandoned mine tunnels, and smaller fractures and faults; in reservoirs and mine tunnels, microgravity can be used to dynamically monitor rock bursts that may be induced by stress changes in the rock formations; it can also be used Monitoring of ground subsidence. However, due to the limitations of the method itself, such as being strongly affected by various interference factors, high measurement costs, complex field operations, and abnormally weak gravity of the target detected in environmental applications, the application of this method is limited to a certain extent.