阅读说明：本技术 一种非结构化等效源的重磁数据延拓与转换方法 (Gravity-magnetic data continuation and conversion method of unstructured equivalent source ) 是由 左博新 于 2020-12-29 设计创作，主要内容包括：本发明公开了一种非结构化等效源的重磁数据延拓与转换方法,包括：通过观测得到物理场数据d-0；给定反演初始模型和反演网格边界,利用现有的四面体网格生成方法生成非结构化四面体网格,作为等效源反演网格空间；根据地磁场参数,对物理场数据d-0进行带规整化项的PDE三维反演计算,同时对反演等效源模型周围的反演网格空间,进行加密剖分动态调整,在满足迭代条件后得到异常体的最终等效源模型；根据实验需要设定不同的正演参数,利用步骤S3得到的等效源模型进行相应的三维正演计算,得到等效源产生的上延拓数据或下延拓数据、化极数据、异常总场数据、异常分量数据以及梯度张量数据。本发明实现对复杂环境中的异常数据进行高精度延拓和数据转换。(The invention discloses a gravity and magnetic data continuation and conversion method of an unstructured equivalent source, which comprises the following steps: obtaining physical field data d by observation 0 (ii) a Giving an inversion initial model and an inversion grid boundary, and generating an unstructured tetrahedral grid by using the existing tetrahedral grid generation method as an equivalent source inversion grid space; for physical field data d according to geomagnetic field parameters 0 Carrying out PDE three-dimensional inversion calculation with regularization terms, simultaneously carrying out encryption subdivision dynamic adjustment on an inversion grid space around an inversion equivalent source model, and obtaining a final equivalent source model of the abnormal body after an iteration condition is met; different forward modeling parameters are set according to experiment needs, corresponding three-dimensional forward modeling calculation is carried out by using the equivalent source model obtained in the step S3, and upper extension data or lower extension data, polarization data, abnormal total field data, abnormal component data and gradient tensor data generated by the equivalent source are obtained. The invention realizes the compoundAnd carrying out high-precision continuation and data conversion on abnormal data in the heterogeneous environment.)

1. A gravity-magnetic data continuation and conversion method of an unstructured equivalent source is characterized by comprising the following steps: the method specifically comprises the following steps:

s1, obtaining physical field data d by observation in gravity or magnetic detection₀(ii) a The physical field is a gravity field or a magnetic field;

s2, giving an inversion initial model and an inversion grid boundary, and generating an unstructured tetrahedral grid by using the existing tetrahedral grid generation method to serve as an equivalent source inversion grid space;

s3, according to the geomagnetic field parameter, for the physical field data d₀Performing PDE three-dimensional inversion calculation with regularization terms, performing encryption subdivision dynamic adjustment on an inversion grid space around an inversion equivalent source model while performing inversion iteration, and obtaining a final equivalent source model of an abnormal body after an iteration condition is met;

and S4, setting different forward modeling parameters according to experimental needs, and performing corresponding three-dimensional forward modeling calculation by using the equivalent source model obtained in the step S3 to obtain upper extension data or lower extension data, polarization data, abnormal total field data, abnormal component data and gradient tensor data generated by the equivalent source.

2. The method of claim 1, wherein the method comprises: in step S3, the inversion calculation uses a three-dimensional finite volume, a three-dimensional scalar finite element, or a three-dimensional vector finite element method.

3. The method of claim 1, wherein the method comprises: in step S3, the objective function of the inversion calculation is:

wherein the content of the first and second substances,expressing an inversion target function, and solving by adopting nonlinear optimization; d ═ F (B)₀M), m is not less than 0, which represents the sum d obtained by forward calculation₀Corresponding physical field data; f (-) represents the PDE three-dimensional forward calculation of the equivalent source model; m represents an equivalent source model; phi is a_regRepresenting a regularization function; b is₀Representing the ambient field, i.e., the earth's magnetic field, as determined by the geographic location.

4. The method of claim 2, wherein the method comprises: in step S4, the total abnormal field data, the abnormal component data, and the gradient tensor data are different according to the physical field, and specifically include:

if the physical field is a magnetic field, the total magnetic field data, the abnormal component data, and the gradient tensor data are total magnetic field abnormal data, three-component magnetic abnormal data, and magnetic gradient tensor data, respectively, as follows:

the magnetic three-component abnormal data is expressed by formula (2):

in the formula (2), the reaction mixture is,B_svector form of magnetic three-component abnormal data;three components of magnetic three-component anomaly data;three components representing the earth's magnetic field; phi denotes a gravitational potential or a magnetic potential; mu-mu₀(1+χ)，μ₀Representing the vacuum permeability, and x representing the magnetic susceptibility of the unstructured grid model;

the magnetic total field abnormal data is expressed by formula (3):

the magnetic gradient tensor data is expressed by equation (4):

if the physical field is a gravity field, the abnormal total field data, the abnormal component data and the gradient tensor data are respectively gravity abnormal data, gravity three-component abnormal data and gravity gradient tensor data, and the method comprises the following steps:

the gravity three-component abnormal data is expressed by formula (5):

wherein, g^x、g^yAnd g^zData representing three components of a gravity anomaly, respectively;

the gravity anomaly data is expressed by equation (6):

the gravity gradient tensor data is expressed by equation (7):

5. the method of claim 4, wherein the method comprises: in step S4, performing corresponding three-dimensional forward calculation by using the equivalent source model obtained in step S3, wherein for the gravity field, the forward formula is as follows:for magnetic fields, the forward equation is:where γ represents the gravitational constant and ρ represents the density of the unstructured grid model.

6. The method of claim 4, wherein the method comprises: in step S4, the space coordinate p (x) is determined from the original observation position₀,y₀,z₀) And the experiment needs to set the extended position space coordinate p of the observation surface_up(x,y,z)/p_down(x, y, z), and further obtaining physical field data after upper extension and/or lower extension as follows: b ═ CB_sWherein, B' represents the data after the upper continuation or the lower continuation, and C represents the interpolation function obtained according to the position of the observation surface after the continuation and the original observation position.

7. The method of claim 4, wherein the method comprises: when the magnetic field data polarization calculation is performed according to the experimental requirement in step S4, B 'is given by using the equivalent source model m determined in step S3'₀Performing PDE forward calculation to obtainPolarized electrode data B'_s＝F(B′₀,m)。

Technical Field

The invention relates to the technical field of geophysical surveying, in particular to a gravity magnetic data continuation and conversion method of an unstructured equivalent source.

Background

In gravity or magnetic detection, measurement is distributed on a fluctuating observation curved surface, and the main abnormal information content reflected by gravity and magnetic data is different due to different distances between a measurement position and an abnormal body. In practical application, the data often needs to be converted into required parameters and types, such as conversion of different observation heights (prolongation), abnormal components, tensor data, and the like, the main challenge of the gravity and magnetic data type conversion is to measure a curved surface of a fluctuation measurement of aeromagnetic (ground magnetic measurement) and an irregular measurement position (non-grid measurement) of the data, the data cannot be processed by the traditional data type conversion and chemical pole calculation, and the processing result is low in precision and cannot meet the application requirements by adopting a traditional data conversion method of a single-layer, double-layer, and three-layer equivalent source.

Prior document 1, "Thomas Gunther, Carsten Hucker, Klaus Spitzer, three-dimensional modification and inversion of dc resistance data in a structural-II. inversion [ J ]. Geophanic Journal International,2006,166 (2)" proposes an electrical inversion method based on unstructured grids, and does not see a remagnetic inversion based on unstructured grids; document 2, "damper, c.n.g. the EQUIVALENT SOURCE method, 1969,34(1): 39", proposes an EQUIVALENT SOURCE method, which uses a single-layer EQUIVALENT field SOURCE to simulate an actual measurement anomaly, and can be used for spatial extension (including surface extension), gradient calculation, component conversion, and the like of bit field data; document 3 "upward continuation of magnetic field on curved surface and component conversion [ J ] in geophysical reports, 1985,000(004): 419" proposes an upward continuation method of magnetic field on curved surface based on single-level and double-level theories; document 4 "Li y.reduction to the pole using equivalent sources [ J ]. SEG Technical Program Expanded Abstracts,1999,19(1): 2484" realizes a pole-making method for low-latitude magnetic field anomalies by providing a single layer of equivalent source at a limited depth. Document 5 "Zuo B, Hu X, Marcelo Leo-Santos, et al. Downware content and Transformation of Total-Field Magnetic alloys Into Magnetic Gradient software BetWeen Arbitraray Surfaces Using Multilayer Equivalent Sources [ J ]. geographic Research Letters,2020,47 (16)" proposes a method for Continuation and data conversion by Multilayer Equivalent source technology with depth normalization. The document "Farquhason C.G., C.R.W.Mosher,2009, Three-dimensional modeling of dimensional data using fields, Journal of Applied Geophysics,68, 417-. Documents "Jahandari h.and c.g.farquharson,2013, Forward modeling of gradient data using fine-volume and fine-element methods on unstructured grids, Geophysics,78(3), G69-G80" realize finite volume, finite element gravity three-dimensional Forward, and do not realize inversion. The document "Galley c.g., p.g.leisurefre _ and c.g.farqusson, 2020, geopysic inversion for 3D contact surface geometry, geopysics, 85, K27-K45" adopts an integral equation to realize the surface integral inversion of an unstructured grid, which is obviously different from the PDE method in the basic theory and equation.

However, these techniques have the following problems: 1) the subdivision accuracy and efficiency of the structured hexahedron (cube) grid on the model space and the abnormal body are limited by conditions: on one hand, when the cube unit is adopted to divide a complex three-dimensional structure, the discretization error of the model is easily overlarge, and the fitting precision is low; on the other hand, if a large number of fine cube units are adopted to subdivide a complex three-dimensional structure, the three-dimensional forward and backward calculation amount is too large, and the requirements on hardware performance and calculation time are high; 2) the mesh subdivision of a structured hexahedron is carried out on the free space and the model space, so that the number of model units is difficult to reduce while the model subdivision accuracy of an observation point, a terrain and a field source is improved; 3) it is difficult to represent the space from the observation point to the terrain range by adopting a non-uniform sparse grid in the aviation data processing.

Disclosure of Invention

Aiming at the technical problems, the invention provides a gravity and magnetic data continuation and conversion method based on an unstructured equivalent source, which utilizes the existing tetrahedral mesh generation method to generate an unstructured mesh, and based on a forward and backward theoretical frame of a nonlinear Differential PDE (Partial Differential equation), three-dimensional inversion calculation is carried out by a three-dimensional finite volume, three-dimensional scalar finite element or three-dimensional vector finite element method to obtain an unstructured tetrahedral equivalent source model of an abnormal body, and the model is correspondingly calculated based on the three-dimensional finite volume, three-dimensional scalar finite element or three-dimensional vector finite element forward calculation of the PDE, so that the abnormal data in the complex environment can be subjected to self-adaption, high-precision continuation and data type conversion calculation.

The invention provides a gravity-magnetic data continuation and conversion method of an unstructured equivalent source, which specifically comprises the following steps:

s1, obtaining physical field data d by observation in gravity or magnetic detection₀(ii) a The physical field is a gravity field or a magnetic field;

s2, giving an inversion initial model and an inversion grid boundary, and generating an unstructured tetrahedral grid by using the existing tetrahedral grid generation method to serve as an equivalent source inversion grid space;

s3, according to the geomagnetic field parameter, for the physical field data d₀Performing PDE three-dimensional inversion calculation with regularization terms, performing encryption subdivision dynamic adjustment on an inversion grid space around an inversion equivalent source model while performing inversion iteration, and obtaining a final equivalent source model of an abnormal body after an iteration condition is met;

and S4, setting different forward modeling parameters according to experimental needs, and performing corresponding three-dimensional forward modeling calculation by using the equivalent source model obtained in the step S3 to obtain upper extension data or lower extension data, polarization data, abnormal total field data, abnormal component data and gradient tensor data generated by the equivalent source.

Further, in step S3, the inversion calculation uses a three-dimensional finite volume, a three-dimensional scalar finite element or a three-dimensional vector finite element method.

Further, in step S3, the objective function of the inversion calculation is:

wherein the content of the first and second substances,expressing an inversion target function, and solving by adopting nonlinear optimization; d ═ F (B)₀M), m is not less than 0, which represents the sum d obtained by forward calculation₀Corresponding physical field data; f (-) represents the PDE three-dimensional forward calculation of the equivalent source model; m represents an equivalent source model; phi is a_regRepresenting a regularization function; b is₀Representing the background field, i.e. the earth's magnetic field, byThe geographic location of the location is determined.

Further, it is characterized in that: in step S4, the total abnormal field data, the abnormal component data, and the gradient tensor data are different according to the physical field, and specifically include:

if the physical field is a magnetic field, the total magnetic field data, the abnormal component data, and the gradient tensor data are total magnetic field abnormal data, three-component magnetic abnormal data, and magnetic gradient tensor data, respectively, as follows:

the magnetic three-component abnormal data is expressed by formula (2):

in the formula (2), B_sVector form of magnetic three-component abnormal data;three components of magnetic three-component anomaly data;three components representing the earth's magnetic field; phi denotes a gravitational potential or a magnetic potential; mu-mu₀(1+χ)，μ₀Representing the vacuum permeability, and x representing the magnetic susceptibility of the unstructured grid model;

the magnetic total field abnormal data is expressed by formula (3):

the magnetic gradient tensor data is expressed by equation (4):

if the physical field is a gravity field, the abnormal total field data, the abnormal component data and the gradient tensor data are respectively gravity abnormal data, gravity three-component abnormal data and gravity gradient tensor data, and the method comprises the following steps:

the gravity three-component abnormal data is expressed by formula (5):

wherein, g^x、g^yAnd g^zData representing three components of a gravity anomaly, respectively;

the gravity anomaly data is expressed by equation (6):

the gravity gradient tensor data is expressed by equation (7):

further, in step S4, performing corresponding three-dimensional forward calculation by using the equivalent source model obtained in step S3, wherein for the gravity field, the forward formula is as follows:for magnetic fields, the forward equation is:wherein gamma represents a universal gravitation constant, and rho represents the density of the unstructured grid model;

further, in step S4, the space coordinate p (x) is determined according to the original observation position₀,y₀,z₀) And the experiment needs to set the extended position space coordinate p of the observation surface_up(x,y,z)/p_down(x, y, z), and further obtaining physical field data after upper extension and/or lower extension as follows: b ═ CB_sWherein, B' represents the data after the upper continuation or the lower continuation, and C represents the interpolation function obtained according to the position of the observation surface after the continuation and the original observation position.

When the magnetic field data polarization calculation is performed according to the experimental requirement in step S4, B 'is given by using the equivalent source model m determined in step S3'₀Calculating forward calculation of PDE to obtain polarized data B'_s＝F(B′₀,m)。

The beneficial effects provided by the invention are as follows: the technical scheme provided by the invention is based on a PDE framework to carry out a three-dimensional finite volume, three-dimensional scalar finite element or three-dimensional vector finite element method, can carry out upper and lower continuation operation and data type conversion on abnormal data generated by an abnormal body, and has good applicability to physical field distribution in a calculation space; the invention utilizes the existing tetrahedral mesh generation method to generate the unstructured tetrahedral mesh, has higher flexibility by using the unstructured tetrahedral mesh to describe the geometric shape of any model, can generate the required upper extension or lower extension data and finish the data type conversion by self-adapting, fast, high-efficiency and accurately for the abnormal body, and has higher stability and precision.

Drawings

FIG. 1 is a flow chart of a method of gravity magnetic data continuation and conversion for an unstructured equivalent source;

FIG. 2 is a schematic diagram of an inversion space provided by an embodiment of the present invention;

fig. 3 is a schematic diagram of up/down extension and polarization conversion provided by the embodiment of the invention.

Detailed Description

In order to make the objects, technical solutions and advantages of the present invention more apparent, embodiments of the present invention will be further described with reference to the accompanying drawings.

Referring to fig. 1, an embodiment of the present invention provides a heavy magnetic data continuation and conversion method based on an unstructured equivalent source, and the method for continuation and conversion of magnetic field data of an unstructured tetrahedral equivalent source includes the following steps:

s1, inputting the existing physical field data (gravity field or magnetic field) d₀；

S2, giving an inversion initial model and an inversion grid boundary, and generating an unstructured tetrahedral grid by using the existing tetrahedral grid generation method to serve as an equivalent source inversion grid space;

s3, according to the geomagnetic field parameter, for the magnetic field data d₀Carrying out PDE three-dimensional inversion calculation with regularization items, wherein the inversion calculation can use a three-dimensional finite volume, a three-dimensional scalar finite element or a three-dimensional vector finite element method, carrying out encryption subdivision dynamic adjustment on an inversion grid space around an inversion equivalent source model while carrying out inversion iteration, and obtaining a final equivalent source model of the abnormal body after an iteration condition is met;

and S4, setting different forward modeling parameters according to experimental needs, and performing corresponding three-dimensional forward modeling calculation by using the equivalent source model obtained in the step S3 to obtain one or more of upper extension and/or lower extension data, polarization data, abnormal total field data, abnormal component data and gradient tensor data generated by the equivalent source.

The objective function of the PDE-based three-dimensional inversion calculation is:

wherein the content of the first and second substances,expressing an inversion target function, and solving by adopting nonlinear optimization; d ═ F (B)₀M), m is not less than 0, which represents the sum d obtained by forward calculation₀Corresponding physical field data; f (-) represents the PDE three-dimensional forward calculation of the equivalent source model; m represents an equivalent source model; phi is a_regExpressing a regularization function, including a depth regularization function, a minimum volume, a tightest support function, and the like; b is₀Representing the ambient field, i.e., the earth's magnetic field, as determined by the geographic location. At the same timeAnd in the inversion iteration process, according to the iteration result of each time, carrying out encryption subdivision dynamic adjustment on the inversion grid space around the current equivalent source model. Fig. 2 is a schematic diagram of an inversion space provided by an embodiment of the present invention.

In this embodiment, the forward computation uses a finite volume PDE method, and it should be noted that, in order to satisfy the finite volume solution condition, the inversion grid space needs to be expanded, and grid expansion is performed according to the finite volume method on the horizontal space below the set inversion maximum depth, above the undulation observation surface, and around the inversion grid space.

When data extension and conversion are performed according to experimental needs, corresponding three-dimensional forward modeling calculation is performed by using the inversion final model obtained in step S3, so as to obtain one or more of upper extension data or lower extension data (or upper extension data and lower extension data, which are not limited herein), polarization data, abnormal total field data, abnormal component data, and gradient tensor data generated by the abnormal body. The PDE form of the magnetic anomaly forward process is:

wherein mu is mu₀(1+χ)，μ₀Denotes the vacuum permeability, χ denotes the magnetic susceptibility of the unstructured mesh model, and φ denotes the gravitational or magnetic potential.

In the forward modeling process based on the unstructured tetrahedral equivalent source, each tetrahedral unit has uniform (same) magnetic susceptibility, and the magnetic potential phi at all the tetrahedral unit nodes is obtained by solving the PDE through a finite volume or finite element method and an unstructured tetrahedral second-order interpolation method. And calculating to obtain magnetic three-component abnormal data of any observation point in the tetrahedron by the phi values of 10 nodes of each tetrahedron unit and combining a finite element unstructured tetrahedron second-order interpolation function. And then, other types of magnetic forward and backward modeling data can be further calculated according to the three-component abnormal data.

For three-dimensional magnetic forward modeling, the three-component anomaly data is represented as

Wherein mu is mu₀(1+χ)，μ₀Denotes the vacuum permeability, χ denotes the magnetic susceptibility of the anomaly unstructured mesh model, B₀Representing the earth's magnetic field (background field),andrespectively data of three components of the earth magnetic field, phi represents magnetic potential,andrespectively representing magnetic three-component anomaly data.

Fig. 3 is a schematic diagram of the upper extension, the lower extension, and the polarization conversion of this example.

According to the inversion final model matrix m determined in the step S3 and the set polarization forward parameter B'₀(B′₀Is 90 DEG and the magnetic declination is 0 DEG, PDE forward calculation is carried out to obtain polarized data B'_s：

In step S4, the calculation process of the different types of data converted according to the experimental requirements is:

magnetic total field anomaly data expressed as

The magnetic gradient tensor data is represented as

Wherein the content of the first and second substances,the gradient operator is shown, and x, y and z respectively show the position quantities of the north direction, the east direction and the vertical direction.

Also, for the gravitational field, the forward equation is:gamma represents a universal gravitation constant, and rho represents the density of the unstructured grid model;

correspondingly, the gravity three-component abnormal data is expressed as the following formula:

wherein, g^x、g^yAnd g^zData representing three components of a gravity anomaly, respectively;

the gravity anomaly data is expressed as:

the gravity gradient tensor data is expressed by equation (7):

the beneficial effects provided by the invention are as follows: the technical scheme provided by the invention is based on a PDE framework to carry out forward and backward calculation of a three-dimensional finite volume, three-dimensional scalar finite element or three-dimensional vector finite element method, and has good applicability to physical field distribution in a calculation space; the invention utilizes the existing tetrahedral mesh generation method to generate the unstructured mesh, and the unstructured tetrahedral mesh is used for describing the geometric shape of any model, so that the method has higher flexibility, stability and precision.

The above description is only for the purpose of illustrating the preferred embodiments of the present invention and is not to be construed as limiting the invention, and any modifications, equivalents, improvements and the like that fall within the spirit and principle of the present invention are intended to be included therein.