A method for extracting residual density field of the Earth's crust based on magnetic mineral density effect separation

Abstract

This application discloses a method for extracting the residual density field of the Earth's crust based on the separation of magnetic mineral density effects, relating to the field of geophysics. The method includes: obtaining the three-dimensional density and magnetization structure of a target region; obtaining the three-dimensional temperature structure of the target region; converting magnetization into magnetite content; and stripping away the density effect of magnetite to obtain the residual density of the Earth's crust. This invention, through quantitative estimation and separation of the magnetic mineral density effect, explicitly removes the contribution of magnetic minerals to the density field from the total density structure, avoiding interference from magnetic mineral enrichment in the interpretation of crustal density anomalies. It estimates the volume fraction of magnetite using magnetic information and combines it with the temperature-corrected density structure to establish a crustal density component decomposition method based on a volume fraction mixing model, achieving physical differentiation between magnetic and non-magnetic components in the density field. It does not rely on specific gravity or magnetic inversion algorithms and has good versatility and scalability.

Description

Technical Field

[0001] This invention relates to the field of geophysics, and in particular to a method for extracting the residual density field of the Earth's crust based on the separation of magnetic mineral density effects. Background Technology

[0002] Gravity methods are an important geophysical tool for studying the density structure and composition of the Earth's crust. However, gravity inversion results essentially reflect the combined effects of multiple subsurface material components and various physical processes, resulting in a significantly non-unique origin for density anomalies. Particularly in magnetic rock masses or areas rich in magnetic minerals, magnetic minerals are not only the primary source of magnetic anomalies, but also significantly influence gravity anomalies and the density structure obtained through inversion due to their typically significantly higher density than the non-magnetic silicate matrix. However, in existing crustal density inversion and interpretation techniques, the contribution of magnetic minerals to the density field is often implicitly included in the total density anomaly. The lack of quantitative characterization and explicit separation of the density effect of magnetic minerals leads to the overlapping of different causal factors in density anomalies, making it difficult to accurately distinguish between density anomalies caused by magnetic mineral enrichment and changes in non-magnetic components.

[0003] Under current technological conditions, magnetic information is mainly used to characterize the magnetic structure of the Earth's crust, while gravity information is mainly used to invert crustal density distribution. These two methods are typically used as relatively independent information sources or combined through inversion to comprehensively constrain subsurface structures. While these methods improve model stability to some extent, their goal remains the same: to obtain the overall density or magnetic structure. They do not specifically address the fact that magnetic minerals simultaneously control magnetic and density anomalies at the physical mechanism level. In areas with significant magnetic anomalies, these methods can easily misinterpret density anomalies caused by magnetic mineral enrichment as changes in the composition of non-magnetic materials, thereby reducing the reliability and distinguishability of crustal density structure interpretation. Summary of the Invention

[0004] In view of this, and addressing the problem that the density effect of magnetic minerals is not quantitatively separated in existing technologies, this invention, starting from the physical fact that magnetic minerals simultaneously influence magnetic anomalies and density anomalies, provides a method for extracting the residual density field of the Earth's crust based on the separation of magnetic mineral density effects. This method aims to eliminate the interference of magnetic minerals on density anomalies, thereby more purely reflecting the density changes of the non-magnetic matrix. First, the three-dimensional density and magnetization structure of the Earth's crust is obtained through gravity and magnetic data inversion. Then, a three-dimensional temperature field is constructed under isothermal constraints at the bottom interface of the magnetic mineral layer to perform temperature correction on the crustal density. Subsequently, the magnetization is converted into magnetic susceptibility, and based on the empirical relationship between magnetic susceptibility and the volume fraction of magnetite, the three-dimensional spatial distribution of magnetite is quantitatively estimated. On this basis, a volume fraction mixing model is used to strictly decompose the temperature-corrected crustal density structure into magnetite components and non-magnetic matrix components, directly obtaining the non-magnetic matrix density field unaffected by magnetic minerals, i.e., the residual density structure of the Earth's crust.

[0005] To achieve the above objectives, the present invention provides the following technical solution:

[0006] In a first aspect, the present invention provides a method for extracting the residual density field of the Earth's crust based on the magnetic mineral density effect separation, comprising the following steps:

[0007] Gravity anomaly data and aeromagnetic anomaly data of the target area are acquired, and a stable potential field inversion algorithm is used to perform three-dimensional density and magnetization inversion to obtain the three-dimensional density structure and three-dimensional magnetization structure of the crust.

[0008] The spectral characteristics of the magnetic anomaly were analyzed from the aeromagnetic anomaly data to determine the depth of the magnetic layer bottom interface;

[0009] By using the interface between the Earth's surface and the bottom of the magnetosphere as isothermal boundary constraints, and minimizing the gradient energy of the temperature field in three-dimensional space, a three-dimensional temperature structure inside the Earth's crust is constructed.

[0010] Based on the three-dimensional temperature structure inside the Earth's crust, the three-dimensional density structure of the Earth's crust is temperature-corrected to obtain the temperature-corrected crustal density.

[0011] Under conditions where inductive magnetization dominates, the magnetization intensity of the three-dimensional magnetization intensity structure is converted into volume magnetic susceptibility;

[0012] Based on the obtained volume magnetic susceptibility, the volume fraction of magnetite in the target area is estimated using the empirical relationship between magnetic susceptibility and magnetite volume fraction.

[0013] Based on the volume fraction mixing model, the temperature-corrected crustal density is decomposed into magnetite component and non-magnetic matrix component, and the resulting non-magnetic matrix density field, which is unaffected by magnetic minerals, is used as the residual density field of the crust.

[0014] As a further aspect of the present invention, when using a stable potential field inversion algorithm to perform three-dimensional density and magnetization inversion, the objective function of the inversion algorithm is:

[0015] ;

[0016] in, Let be the three-dimensional physical property vector to be determined, representing the three-dimensional density structure of the Earth's crust. or three-dimensional magnetization structure , It is a spatial position vector; This is the observation data vector, corresponding to gravity anomaly data. Or aeromagnetic anomaly data ; Forward modeling operators are used to model subsurface physical properties. The theoretical field value mapped onto the Earth's surface or the air, wherein the theoretical field value is the theoretical gravitational field or the theoretical magnetic field; A weighting operator for the data domain, used to adjust the data fitting weights for different observation points; Weighting operators for the model domain are used to apply prior constraints; This is a priori model or initial model; and For regularization parameters, The magnitude of the deviation of the control model from the prior model, Control the smoothness of the model space; It is a three-dimensional gradient operator.

[0017] As a further aspect of the present invention, the frequency domain Centroid method is used to analyze the spectral characteristics of the magnetic anomaly data to obtain the depth of the magnetic layer bottom interface. The calculation relationship is expressed as:

[0018] ;

[0019] In the formula, This represents the effective bottom interface depth of the magnetic mineral layer. This represents the power spectrum of the magnetic anomaly in the wavenumber domain. Radial wave number, For wave numbers in the low wavenumber domain, The wavenumber value in the high wavenumber domain; the effective bottom interface depth of the magnetic mineral layer. The average temperature represented is 550℃.

[0020] As a further aspect of the present invention, the ground surface is denoted as The bottom interface of the magnetic layer is denoted as Based on satisfying the isothermal boundary conditions, a three-dimensional temperature structure inside the Earth's crust is constructed by minimizing the gradient energy of the temperature field in three-dimensional space; the objective function of the three-dimensional temperature structure inside the Earth's crust is:

[0021] ;

[0022] In the formula, A temperature vector consisting of the temperature values ​​of all nodes within the three-dimensional computational mesh. ; For the Earth's surface The selection operator is used to select regions from the temperature vector. Extract the temperature values ​​of surface nodes; Magnetic layer bottom interface The selection operator is used to select regions from the temperature vector. Extract the temperature value of the bottom interface node; For the Earth's surface The isothermal constraint vector on; Magnetic layer bottom interface The isothermal constraint vector on; For the Earth's surface Boundary constraint weight matrix, Magnetic layer bottom interface Boundary constraint weight matrix; It is a three-dimensional gradient operator; As a smoothing control factor;

[0023] Within a unified underground 3D computational grid, the isothermal boundary conditions applied to the temperature field constraint surface include:

[0024] , ;

[0025] , ;

[0026] In the formula, , is a spatial position vector; This represents the three-dimensional temperature field structure at the described location; For the Earth's surface Average annual temperature; Magnetic layer bottom interface The corresponding isothermal value is taken as 550℃.

[0027] As a further aspect of the present invention, the expression for the temperature-corrected crustal density is:

[0028] ;

[0029] In the formula, This refers to the temperature-corrected three-dimensional density of the Earth's crust. The three-dimensional density structure of the Earth's crust without temperature correction; is the coefficient of thermal expansion of rock volume, used to describe the linear response of density to temperature changes; For position The three-dimensional temperature structure inside the Earth's crust. , is a spatial position vector; The average surface temperature is used as a reference temperature.

[0030] As a further aspect of the present invention, the conversion relationship between the magnetization intensity of the three-dimensional magnetization structure and its volume magnetic susceptibility is as follows:

[0031] ;

[0032] In the formula, For position The volume magnetic susceptibility at that location is dimensionless. For position The induced magnetization intensity at that location; Permeability in vacuum ; The total geomagnetic field intensity of the study area is taken as the spatial average value of the study area or the spatial variation value assigned by the grid.

[0033] As a further aspect of the present invention, the empirical relationship between the magnetic susceptibility and the volume fraction of magnetite is as follows:

[0034] ;

[0035] In the formula, For position The volume fraction of magnetite in the target area; For position Volume magnetic susceptibility at that location; and This is an empirical coefficient. , .

[0036] As a further aspect of the present invention, the temperature-corrected equivalent density is expressed as follows:

[0037] ;

[0038] In the formula, This is the temperature-corrected crustal density; To estimate the volume fraction of magnetite; This is the characteristic density of magnetite; Let be the equivalent density of the non-magnetic matrix, i.e., the residual density of the crust; where, the residual density field of the crust is obtained by solving:

[0039] ;in, ; This represents the characteristic density of magnetite.

[0040] Compared with existing technologies, the crustal residual density field extraction method based on magnetic mineral density effect separation provided by this invention has the following advantages:

[0041] This invention presents a method for extracting the residual density field of the Earth's crust based on the separation of magnetic mineral density effects. This method systematically achieves quantitative estimation and separation of the density effects of magnetic minerals, explicitly separating the contribution of magnetic minerals to the density field from the total density structure, thus avoiding interference from magnetic mineral enrichment in the interpretation of crustal density anomalies. Furthermore, this invention estimates the volume fraction of magnetite using magnetic information and combines it with the temperature-corrected density structure to establish a crustal density component decomposition method based on a volume fraction mixing model, achieving a physical distinction between magnetic and non-magnetic components in the density field. Based on the fact that magnetic minerals are simultaneously sources of magnetic anomalies and important density sources, this method uses the magnetic bottom interface as a key constraint to construct a crustal temperature field. It then first eliminates thermal effects and then eliminates magnetic mineral effects, achieving a deep fusion of gravity, magnetic, and geothermal geophysical information at the physical mechanism level. This method does not rely on specific gravity or magnetic inversion algorithms and can be implemented independently as a post-processing step for existing three-dimensional density models, exhibiting good versatility and scalability.

[0042] These or other aspects of the invention will become more apparent from the following description of embodiments. It should be understood that the foregoing general description and the following detailed description are exemplary and explanatory only, and are not intended to limit the invention. Attached Figure Description

[0043] To more clearly illustrate the technical solutions in the embodiments of the present invention, the accompanying drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. In the drawings:

[0044] Figure 1 This is a flowchart of a method for extracting the residual density field of the Earth's crust based on the magnetic mineral density effect separation according to the present invention.

[0045] Figure 2 This is a schematic diagram of the data flow of a crustal residual density field extraction method based on magnetic mineral density effect separation according to the present invention.

[0046] Figure 3 This is a sub-flowchart of the three-dimensional density and magnetization structure inversion in a crustal residual density field extraction method based on magnetic mineral density effect separation according to the present invention.

[0047] Figure 4 This is a sub-flowchart of the three-dimensional temperature field construction in a crustal residual density field extraction method based on magnetic mineral density effect separation according to the present invention.

[0048] Figure 5 This is a sub-flowchart of magnetite density effect separation and residual density field extraction in a crustal residual density field extraction method based on magnetic mineral density effect separation according to the present invention. Detailed Implementation

[0049] The specific embodiments of the present invention will now be described in detail with reference to the accompanying drawings, but it should be understood that the scope of protection of the present invention is not limited to the specific embodiments.

[0050] Unless otherwise expressly stated, throughout the specification and claims, the term "comprising" or its variations such as "including" or "comprises" shall be understood to include the stated elements or components without excluding other elements or other components.

[0051] See Figures 1 to 5 As shown, one embodiment of the present invention provides a method for extracting the residual density field of the Earth's crust based on the magnetic mineral density effect, comprising the following steps:

[0052] Step S10: Obtain gravity anomaly data and aeromagnetic anomaly data of the target area, and use a stable potential field inversion algorithm to perform three-dimensional density and magnetization inversion to obtain the three-dimensional density structure and three-dimensional magnetization structure of the crust.

[0053] This step involves the inversion of the three-dimensional density and magnetization structure of the Earth's crust. It is based on the acquired gravity anomaly data for the target region. and aeromagnetic anomaly data Under a unified three-dimensional computational grid, a stable potential field inversion algorithm is used to perform three-dimensional density and magnetization inversion on gravity anomalies and aeromagnetic anomalies, thereby obtaining the three-dimensional density structure of the Earth's crust. and three-dimensional magnetization structure .

[0054] In this embodiment, when using a stable potential field inversion algorithm to perform three-dimensional density and magnetization inversion, the objective function of the inversion algorithm is:

[0055] ;

[0056] in, Let be the three-dimensional physical property vector to be determined, representing the three-dimensional density structure of the Earth's crust. or three-dimensional magnetization structure , It is a spatial position vector; This is the observation data vector, corresponding to gravity anomaly data. Or aeromagnetic anomaly data ; Forward modeling operators are used to model subsurface physical properties. The theoretical field value mapped onto the Earth's surface or the air, wherein the theoretical field value is the theoretical gravitational field or the theoretical magnetic field; A weighting operator for the data domain, used to adjust the data fitting weights for different observation points; Weighting operators for the model domain are used to apply prior constraints; This is a priori model or initial model; and For regularization parameters, The magnitude of the deviation of the control model from the prior model, Control the smoothness of the model space; It is a three-dimensional gradient operator.

[0057] Step S20: Analyze the spectral characteristics of the magnetic anomaly data to determine the depth of the magnetic layer bottom interface.

[0058] This step involves determining the interface at the bottom of the magnetic layer. This is done for the aeromagnetic anomaly data. The spectral characteristics of the magnetic anomaly were analyzed using the frequency domain Centroid method to obtain the depth of the magnetic layer bottom interface. The calculation relationship is expressed as follows:

[0059] ;

[0060] In the formula, This represents the effective bottom interface depth of the magnetic mineral layer. This represents the power spectrum of the magnetic anomaly in the wavenumber domain. Radial wave number, For wave numbers in the low wavenumber domain, The wavenumber value is in the high wavenumber domain. The effective bottom interface depth of the magnetic mineral layer is also mentioned. The average temperature represented is 550℃.

[0061] Step S30: Using the interface between the Earth's surface and the bottom of the magnetic layer as isothermal boundary constraints, a three-dimensional temperature structure inside the Earth's crust is constructed by minimizing the gradient energy of the temperature field in three-dimensional space.

[0062] This step involves constructing a three-dimensional temperature field based on isothermal boundary constraints and gradient smoothing. The Earth's surface is denoted as... The bottom interface of the magnetic layer is denoted as Based on satisfying the isothermal boundary conditions, a three-dimensional temperature structure within the Earth's crust is constructed by minimizing the gradient energy of the temperature field in three-dimensional space. In this embodiment, the objective function for the three-dimensional temperature structure within the Earth's crust is:

[0063] ;

[0064] In the formula, A temperature vector consisting of the temperature values ​​of all nodes within the three-dimensional computational mesh. ; For the Earth's surface The selection operator is used to select regions from the temperature vector. Extract the temperature values ​​of surface nodes; Magnetic layer bottom interface The selection operator is used to select regions from the temperature vector. Extract the temperature value of the bottom interface node; For the Earth's surface The isothermal constraint vector on; Magnetic layer bottom interface The isothermal constraint vector on; For the Earth's surface Boundary constraint weight matrix, Magnetic layer bottom interface Boundary constraint weight matrix; It is a three-dimensional gradient operator; This is a smoothing control factor.

[0065] Within a unified underground 3D computational grid, the isothermal boundary conditions applied to the temperature field constraint surface include:

[0066] , ;

[0067] , ;

[0068] in, , is a spatial position vector; This represents the three-dimensional temperature field structure at the described location; For the Earth's surface Average annual temperature; Magnetic layer bottom interface The corresponding isotherm value is taken as 550℃. By solving the optimization problem corresponding to the objective function of the three-dimensional temperature structure inside the Earth's crust, a value consistent with the Earth's surface is obtained. Interface with the bottom of the magnetic layer Isothermal constraint and temperature distribution that varies smoothly in three-dimensional space .

[0069] Step S40: Based on the three-dimensional temperature structure inside the Earth's crust, perform temperature correction on the three-dimensional density structure of the Earth's crust to obtain the temperature-corrected crustal density.

[0070] This step involves temperature correction of the three-dimensional density of the Earth's crust. Based on the three-dimensional temperature structure of the Earth's crust obtained in step S30... The three-dimensional density structure obtained in step S10 Temperature correction is performed to eliminate the interference of temperature changes on density distribution.

[0071] In this embodiment, the expression for the temperature-corrected crustal density is:

[0072] ;

[0073] In the formula, This refers to the temperature-corrected three-dimensional density of the Earth's crust. The three-dimensional density structure of the Earth's crust obtained in step S10 without temperature correction; is the coefficient of thermal expansion of rock volume, used to describe the linear response of density to temperature changes; The position obtained in step S30 The three-dimensional temperature structure inside the Earth's crust. , is a spatial position vector; The average surface temperature is used as a reference temperature.

[0074] This step introduces a temperature correction relationship to separate the thermal effect in crustal density from density anomalies, avoiding misjudging density changes caused by geothermal structures as differences in material composition. This provides a physically consistent density basis for the subsequent quantitative calculation of magnetic mineral density effects and the extraction of the crustal residual density field.

[0075] Step S50: Under the condition of inductive magnetization dominance, the magnetization intensity of the three-dimensional magnetization intensity structure is converted into volume magnetic susceptibility.

[0076] This step involves converting the three-dimensional magnetization structure into a three-dimensional volumetric magnetic susceptibility structure. Based on the three-dimensional magnetization structure obtained in step S10... Under conditions where induced magnetization dominates, the magnetization intensity is converted into volume magnetic susceptibility. The conversion relationship between the magnetization intensity and volume susceptibility of the three-dimensional magnetization structure is as follows:

[0077] ;

[0078] In the formula, For position The volume magnetic susceptibility at that location is dimensionless. For position The induced magnetization intensity at that location; Permeability in vacuum ; The total geomagnetic field intensity of the study area is taken as the spatial average value of the study area or the spatial variation value assigned by the grid.

[0079] By converting magnetization intensity into volume magnetic susceptibility, the magnetization field is transformed into a volume magnetic susceptibility field, realizing a unified characterization of rock physical parameters from magnetic anomaly inversion results, and providing quantitative input for subsequent magnetic mineral content inversion and magnetic mineral density effect calculation.

[0080] Step S60: Based on the obtained volumetric magnetic susceptibility, the volume fraction of magnetite in the target area is estimated using the empirical relationship between magnetic susceptibility and magnetite volume fraction.

[0081] This step involves a three-dimensional estimation of the magnetite volume fraction. The three-dimensional volumetric magnetic susceptibility already obtained in step S50... Based on this, the empirical correlation between magnetic susceptibility and magnetite volume fraction is used to estimate the magnetite volume fraction in the target area. .

[0082] In this embodiment, the empirical relationship between the magnetic susceptibility and the volume fraction of magnetite is as follows:

[0083] ;

[0084] In the formula, For position The volume fraction of magnetite in the target area; For position Volume magnetic susceptibility at that location; and This is an empirical coefficient. , .

[0085] This step uses an empirical correlation between magnetic susceptibility and magnetite volume fraction to convert the magnetic anomaly inversion results into a material indicator, thereby obtaining the three-dimensional spatial distribution of magnetite content and providing the necessary input for subsequent quantitative calculation of magnetite density effect.

[0086] Step S70: Based on the volume fraction mixing model, the temperature-corrected crustal density is decomposed into magnetite component and non-magnetic matrix component, and the obtained non-magnetic matrix density field, which is not affected by magnetic minerals, is used as the crustal residual density field.

[0087] This step involves calculating the magnetite density effect. The three-dimensional distribution of magnetite volume fraction has been obtained in step S60. Based on this, and combined with the temperature-corrected three-dimensional crustal density structure obtained in step S40, the crustal density is decomposed into components using a volume fraction mixing model to obtain the non-magnetic matrix density field. Assuming that the crustal rock consists of magnetite and a non-magnetic matrix, its temperature-corrected equivalent density can be expressed as follows:

[0088] ;

[0089] In the formula, The temperature-corrected crustal density obtained in step S40; The volume fraction of magnetite estimated in step S60; This is the characteristic density of magnetite; Let be the equivalent density of the non-magnetic matrix, i.e., the residual density of the crust; where, the residual density field of the crust is obtained by solving:

[0090] ;in, ; This represents the characteristic density of magnetite.

[0091] This step uses a volume fraction mixing model to rigorously decompose the temperature-corrected crustal density structure into magnetite and non-magnetic matrix components, directly obtaining the non-magnetic matrix density field unaffected by magnetic minerals. This non-magnetic matrix density field is the residual crustal density structure, used to reflect the crustal density anomaly characteristics dominated by variations in the non-magnetic component.

[0092] The present invention provides a method for extracting the residual density field of the Earth's crust based on the separation of magnetic mineral density effects. This method obtains the three-dimensional density and magnetization structure of a target region; acquires the three-dimensional temperature structure of the target region; converts magnetization into magnetite content; and removes the density effect of magnetite to obtain the residual density of the Earth's crust. Specifically, the method first uses gravity and magnetic data inversion to obtain the three-dimensional density and magnetization structure of the Earth's crust, and constructs a three-dimensional temperature field under isothermal constraints at the bottom interface of the magnetic mineral layer to perform temperature correction on the crustal density. Then, the magnetization is converted into magnetic susceptibility, and based on the empirical relationship between magnetic susceptibility and magnetite volume fraction, the three-dimensional spatial distribution of magnetite is quantitatively estimated. On this basis, a volume fraction mixing model is used to strictly decompose the temperature-corrected crustal density structure into magnetite components and non-magnetic matrix components, directly obtaining the non-magnetic matrix density field, i.e., the residual density structure of the Earth's crust, which is unaffected by magnetic minerals.

[0093] The crustal residual density field extraction method of this invention quantitatively estimates and separates the density effect of magnetic minerals, explicitly removing the contribution of magnetic minerals to the density field from the total density structure, thus avoiding interference from the interpretation of crustal density anomalies caused by the enrichment of magnetic minerals. It estimates the volume fraction of magnetite using magnetic information and combines it with the temperature-corrected density structure to establish a crustal density component decomposition method based on a volume fraction mixing model, achieving physical differentiation between magnetic and non-magnetic components in the density field. It does not rely on specific gravity or magnetic inversion algorithms and can be implemented independently as a post-processing step for existing three-dimensional density models, exhibiting good versatility and scalability.

[0094] The foregoing description of specific exemplary embodiments of the invention is for illustrative and explanatory purposes. These descriptions are not intended to limit the invention to the precise forms disclosed, and it will be apparent that many changes and variations can be made in accordance with the foregoing teachings. The exemplary embodiments were chosen and described in order to explain the specific principles of the invention and its practical application, thereby enabling those skilled in the art to implement and utilize various different exemplary embodiments of the invention, as well as various different choices and variations. The scope of the invention is intended to be defined by the claims and their equivalents.

Claims

1. A method for extracting the residual density field of the crust based on the separation of the density effect of magnetic minerals, characterized in that, Includes the following steps: Gravity anomaly data and aeromagnetic anomaly data of the target area are acquired, and a stable potential field inversion algorithm is used to perform three-dimensional density and magnetization inversion to obtain the three-dimensional density structure and three-dimensional magnetization structure of the crust. The spectral characteristics of the magnetic anomalies were analyzed from the aeromagnetic anomaly data to determine the depth of the magnetic layer bottom interface; By using the interface between the Earth's surface and the bottom of the magnetosphere as isothermal boundary constraints, and minimizing the gradient energy of the temperature field in three-dimensional space, a three-dimensional temperature structure inside the Earth's crust is constructed. Based on the three-dimensional temperature structure inside the Earth's crust, the three-dimensional density structure of the Earth's crust is temperature-corrected to obtain the temperature-corrected crustal density. Under conditions dominated by inductive magnetization, the magnetization intensity of the three-dimensional magnetization intensity structure is converted into volume magnetic susceptibility; Based on the obtained volume magnetic susceptibility, the volume fraction of magnetite in the target area is estimated using the empirical relationship between magnetic susceptibility and magnetite volume fraction. Based on the volume fraction mixing model, the temperature-corrected crustal density is decomposed into magnetite component and non-magnetic matrix component, and the resulting non-magnetic matrix density field, which is unaffected by magnetic minerals, is used as the residual density field of the crust.

2. The method for extracting the crustal residual density field based on the density effect of magnetic minerals of claim 1, wherein, When using a stable potential field inversion algorithm to perform three-dimensional density and magnetization inversion, the objective function of the inversion algorithm is: ； where, is the three-dimensional physical property vector to be solved, representing the three-dimensional density structure of the crust or the three-dimensional magnetization structure , is the spatial position vector; is the observation data vector, corresponding to gravity anomaly data or aeromagnetic anomaly data ; is the forward operator, used to map the subsurface physical property model to the theoretical field values at the surface or in the air, which are the theoretical gravity field or the theoretical magnetic field; is the data domain weighting operator, used to adjust the data fitting weight of different observation points; is the model domain weighting operator, used to impose prior constraints; is the prior model or the initial model; and are regularization parameters, controlling the magnitude of the model deviating from the prior model, controlling the spatial smoothness of the model; is the three-dimensional gradient operator.

3. The method for extracting the residual density field of the Earth's crust based on the magnetic mineral density effect as described in claim 2, characterized in that, For the aeromagnetic anomaly data, the frequency domain Centroid method is used to analyze the spectral characteristics of the magnetic anomaly, and the depth of the magnetic layer bottom interface is obtained The calculation relationship is represented as: ； wherein is the effective bottom interface depth of the magnetic mineral layer, is the power spectrum of the magnetic anomaly in the wave number domain, is the radial wave number, is the wave number value of the low wave number domain, is the wave number value of the high wave number domain, the effective bottom interface depth of the magnetic mineral layer The average temperature represented by T is 550°C.

4. The method for extracting the residual density field of the Earth's crust based on the magnetic mineral density effect as described in claim 3, characterized in that, The ground surface is denoted as The magnetic layer bottom interface is denoted as On the basis of satisfying an isothermal boundary condition, a three-dimensional temperature structure inside the crust is constructed by minimizing gradient energy of a temperature field in three-dimensional space; and an objective function of the three-dimensional temperature structure inside the crust is: ； In the formula, A temperature vector consisting of the temperature values ​​of all nodes within the three-dimensional computational mesh. ; For the Earth's surface The selection operator is used to select regions from the temperature vector. Extract the temperature values ​​of surface nodes; Magnetic layer bottom interface The selection operator is used to select regions from the temperature vector. Extract the temperature value of the bottom interface node; For the Earth's surface The isothermal constraint vector on; Magnetic layer bottom interface The isothermal constraint vector on; For the Earth's surface Boundary constraint weight matrix, Magnetic layer bottom interface Boundary constraint weight matrix; It is a three-dimensional gradient operator; As a smoothing control factor; Within a unified underground 3D computational grid, the isothermal boundary conditions applied to the temperature field constraint surface include: , ； , ； In the formula, It is a spatial position vector; This represents the three-dimensional temperature field structure at the described location; For the Earth's surface Average annual temperature; Magnetic layer bottom interface The corresponding isothermal value is taken as 550℃.

5. The method for extracting the residual density field of the Earth's crust based on the magnetic mineral density effect as described in claim 4, characterized in that, The expression for temperature-corrected crustal density is: ； In the formula, This refers to the temperature-corrected three-dimensional density of the Earth's crust. The three-dimensional density structure of the Earth's crust without temperature correction; is the coefficient of thermal expansion of rock volume, used to describe the linear response of density to temperature changes; For position The three-dimensional temperature structure inside the Earth's crust. , is a spatial position vector; The average surface temperature is used as a reference temperature.

6. The method for extracting the residual density field of the Earth's crust based on the magnetic mineral density effect as described in claim 5, characterized in that, The conversion relationship between the magnetization intensity of the three-dimensional magnetization structure and its volume magnetic susceptibility is as follows: ； In the formula, For position The volume magnetic susceptibility at that location is dimensionless. For position The induced magnetization intensity at that location; Permeability in vacuum ; The total geomagnetic field intensity of the study area is taken as the spatial average value of the study area or the spatial variation value assigned by the grid.

7. The method for extracting the residual density field of the Earth's crust based on the magnetic mineral density effect as described in claim 6, characterized in that, The empirical relationship between magnetic susceptibility and magnetite volume fraction is as follows: ； In the formula, For position The volume fraction of magnetite in the target area; For position Volume magnetic susceptibility at that location; and This is an empirical coefficient. , .

8. The method for extracting the residual density field of the Earth's crust based on the magnetic mineral density effect as described in claim 7, characterized in that, The temperature-corrected equivalent density is expressed as: ； In the formula, This is the temperature-corrected crustal density; To estimate the volume fraction of magnetite; This is the characteristic density of magnetite; Let be the equivalent density of the non-magnetic matrix, i.e., the residual density of the crust; where, the residual density field of the crust is obtained by solving: ;in, ; This represents the characteristic density of magnetite.

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