A gravity and magnetic detection method for the target area location of ultrabasic rock deposits

Abstract

The present invention discloses a gravity and magnetic detection method for target area positioning of ultrabasic rock deposits, belonging to the technical field of geophysics, and comprising the following steps: S1: determining the fracture distribution; S2: obtaining the regional field and residual field of gravity and magnetic anomalies; S3: performing normalization processing on the first-order vertical derivatives of the residual field of magnetic anomalies and the residual field of gravity anomalies; S4: performing cross-correlation processing on the normalization results; S5: using the Parker method to obtain the Moho depth for the regional field of gravity anomalies; S6: using the central point method to obtain the Curie depth for the regional field of aeromagnetic anomalies; S7: respectively performing three-dimensional focused inversion on the residual fields of gravity and magnetic anomalies to obtain the remaining density anomaly and magnetic susceptibility anomaly of the crust; S8: setting the prediction criteria for the target area of ultrabasic rock deposits according to the field measured data of ultrabasic rocks, and constructing relevant prediction methods. The present invention can effectively improve the prediction ability of ultrabasic rock deposits and contribute to the exploration of mineral resources.

Description

Technical Field

[0001] The present invention relates to the technical field of geophysics, and particularly to a gravity and magnetic detection method for locating target areas of ultrabasic rock deposits. Background Art

[0002] Ultrabasic rock deposits are solid mineral resources formed inside or at the edge of ultrabasic rock bodies, including important ore types such as magnetite, chromite, copper-nickel ore, lead-zinc ore, etc. Since these deposits are generally hosted in complex and rugged orogenic belts or plateaus, on the one hand, due to the high work difficulty, it is difficult to effectively carry out seismic exploration methods or magnetotelluric methods; on the other hand, the effects of seismic or magnetotelluric methods are not ideal either. These two unfavorable conditions restrict the exploration intensity and effect of ultrabasic rock deposits.

[0003] Aerial gravity and aeromagnetic surveys can quickly and efficiently obtain high-precision geophysical data over large areas due to their high work efficiency and low cost. On the other hand, because ultrabasic rocks have high density and strong magnetism, with significant differences in physical properties from the surrounding rocks, they are well displayed in gravity and magnetic data, so the advantages of gravity and magnetic data can be greatly exploited.

[0004] The information disclosed in this background art section is only intended to enhance the overall understanding of the present invention and should not be regarded as an admission or any form of suggestion that this information constitutes prior art already known to those of ordinary skill in the art. Summary of the Invention

[0005] The purpose of the present invention is to provide a gravity and magnetic detection method for locating target areas of ultrabasic rock deposits, which can effectively improve the prediction ability of ultrabasic rock deposits and contribute to the exploration of mineral resources.

[0006] To achieve the above purpose, the present invention provides a gravity and magnetic detection method for locating target areas of ultrabasic rock deposits, including the following steps:

[0007] S1: By performing first-order vertical derivative processing on aerial gravity and magnetic data, determine the fracture distribution according to the positions of the derivative extreme points;

[0008] S2: By selecting reasonable filtering parameters, perform frequency-domain filtering processing on the gravity and magnetic data to obtain the regional field and residual field of the gravity and magnetic anomalies;

[0009] S3: Normalize the first-order vertical derivatives of the Gaussian residual field of the magnetic anomaly and the Gaussian residual field of the gravity anomaly obtained in step S2;

[0010] S4: Perform cross-correlation processing on the normalization results obtained in step S3;

[0011] S5: Calculate the Moho depth for the regional field of gravity anomalies using the Parker method;

[0012] S6: Obtain the Curie depth for the regional field of aeromagnetic anomalies using the central point method;

[0013] S7: Conduct 3D focused inversion on the residual fields of gravity and magnetic anomalies respectively to obtain the remaining density anomalies and magnetic susceptibility anomalies in the crust;

[0014] S8: Set the prediction criteria for the target areas of ultramafic rock deposits based on the field measured data of ultramafic rocks and construct relevant prediction methods.

[0015] In an embodiment of the present invention, in step S1, perform first-order vertical derivative calculation on the gridded data of airborne gravity and airborne magnetic method. The specific calculation formula is as follows:

[0016]

[0017] Where P is the potential field value, which is the potential field value of the gravity field Δg or the magnetic field ΔT; z is the direction perpendicular to the ground downward, and VDR is the first-order vertical derivative;

[0018] Through the above formula, the distribution map of the first-order vertical derivative of Bouguer gravity and aeromagnetic data can be obtained, and then the distribution of the fracture boundary can be obtained.

[0019] In an embodiment of the present invention, in step S2, according to the characteristics of gravity and magnetic anomalies, select an appropriate filtering wavelength and perform Gaussian regional field filtering and Gaussian residual field filtering on the gravity and magnetic data. Among them, the filtering formula for the Gaussian residual field in the frequency domain is:

[0020]

[0021] Where P represents the gravity field Δg or the magnetic field ΔT, F(P) represents the Fourier transform of the gravity field or the magnetic field, k0 is the standard deviation of the Gaussian function of the gravity and magnetic fields; k is the Gaussian filtering wavelength;

[0022] Performing Fourier inverse transform on res[F(P)] in the above formula can obtain the Gaussian residual field Δg of gravity anomalies res and the Gaussian residual field ΔT of magnetic anomalies res ; Subtracting the gravity and magnetic Gaussian residual fields from the original gravity and magnetic anomalies can obtain the corresponding Gaussian regional gravity field Δg reg and the Gaussian regional magnetic field ΔT reg .

[0023] In an embodiment of the present invention, the selection criterion for the appropriate wavelength is that the high-frequency anomaly boundaries of the gravity and magnetic anomalies after Gaussian residual field filtering coincide with those of the original fields.

[0024] In an embodiment of the present invention, in step S3, it specifically includes the following steps:

[0025] First, perform normalization processing on the Gaussian residual field ΔT of magnetic anomalies, and the calculation formula is as follows: res The formula is as follows:

[0026]

[0027] Where, is the magnetic anomaly value after normalization, X is the magnetic anomaly value before normalization, P min and P max are respectively the minimum and maximum values of the magnetic anomaly before normalization;

[0028] Secondly, perform first-order vertical derivative processing on the Gaussian residual field Δg of gravity anomalies, and then perform normalization processing on the first-order vertical derivative data to obtain the normalization result of the first-order vertical derivative of the corresponding gravity anomaly residual field res The formula is as follows:

[0029] In an embodiment of the present invention, in step S4, when performing cross-correlation processing, a calculation window needs to be selected. The selection criterion for the calculation window is that it includes at least one complete maximum and minimum value of the gravity and magnetic anomalies; the calculation formula for cross-correlation processing is:

[0030]

[0031] Where, represents the covariance of the normalized data of the magnetic anomaly residual field and the first-order vertical derivative of the gravity anomaly residual field within the selected calculation window, and E represents the mathematical expectation; and respectively represent the variances of the normalized data of the magnetic anomaly residual field and the first-order vertical derivative of the gravity anomaly residual field.

[0032] In an embodiment of the present invention, in step S5, according to the regional gravity anomaly, the depth of the Moho surface is inverted. The calculation formula for inverting the Moho surface using the Parker method in the frequency domain is:

[0033]

[0034] Where, F[h(r)] and F[Δg(r)] are respectively the Fourier transforms of the Moho surface depth and the regional Bouguer gravity anomaly; |k| is the wave number; π is the pi; G is the gravitational constant; ρ is the density difference between the crust and the lithospheric mantle; z0 is the average depth of the Moho surface in the study area, also known as the reference depth; r is the three-dimensional coordinate vector; n is the order.

[0035] In an embodiment of the present invention, in step S6, by performing frequency-domain center-point method calculation on the Gaussian regional magnetic field ΔT reg the Curie surface is obtained; for a magnetic layer that extends infinitely in the horizontal direction, has a small thickness relative to the depth, and has a randomly distributed magnetization intensity, the radial average amplitude spectrum of the magnetic anomaly it generates is:

[0036]

[0037] wherein, A ΔT (|k|) and A M (|k|) are respectively the radial average amplitude energy spectra of the Gaussian regional magnetic field and the magnetization intensity of the underground magnetic body; |k| is the absolute value of the wave number in the wave number domain; Z t represents the top surface of the magnetic layer; Z b represents the bottom surface of the magnetic layer, and this interface is the Curie surface; B is a constant;

[0038] By simplifying and taking the logarithm of the above formula in the low wave number domain and the medium-high wave number domain, the depth Z t of the top surface of the magnetic layer and the depth Z0 of the center point can be obtained respectively; finally, the Curie surface depth Z b is calculated as follows:

[0039] Z b = 2·Z0 - Z t .

[0040] In an embodiment of the present invention, in step S3, the minimum gradient support focusing inversion is performed on the Gaussian residual gravity field Δg res and the Gaussian residual magnetic field ΔT res obtained in step S2 to obtain the distribution of high-density and strong magnetic bodies with clear and sharp boundaries; wherein, the objective function of the focusing inversion is as follows:

[0041]

[0042] In the formula,

[0043]

[0044] wherein, m is the model parameter to be inverted, representing the density parameter during gravity inversion and the magnetic susceptibility parameter during magnetic force inversion; is the gravity and magnetic data fitting function; s(m) is the model regularization term; α is the regularization factor; W d is the weighted matrix of the observed data; W m is the weighted matrix of the model; A is the forward operator of the gravity anomaly or magnetic anomaly of a cube under the condition of uniform density or magnetic susceptibility; W e is the minimum gradient support weighted matrix; is the Hamiltonian operator; both β and e are small values less than 1; d is the Gaussian residual gravity field Δg res or the Gaussian residual magnetic field ΔT res ;

[0045] Based on the statistical results of rock density and magnetic susceptibility measured in the field, with these statistical results as boundary condition constraints, the conjugate gradient method is used to iteratively solve the objective function of the focused inversion to obtain the density and magnetic susceptibility distributions in the shallow layer of the underground space.

[0046] In an embodiment of the present invention, in step S8, the prediction criteria for the target area of the ultrabasic rock type deposit are as follows: integrating the fracture distribution determined in step S1, the cross-correlation of the normalized results of the Gaussian residual magnetic field and the Gaussian residual gravity field determined in step S4, the Mohorovicic discontinuity depth map inverted in step S5, the Curie depth map calculated in step S6, and the density and magnetic susceptibility distributions of the underground space inverted in step S7, to establish the positioning criteria for the target area of the ultrabasic rock type deposit.

[0047] Compared with the prior art, according to a gravity and magnetic detection method for positioning the target area of an ultrabasic rock type deposit of the present invention, based on gravity and magnetic data, it integrates various geophysical information such as fracture distribution, cross-correlation of gravity and magnetic residual field normalization, Mohorovicic discontinuity depth and Curie depth, and high-precision gravity and magnetic inversion under the minimum gradient support condition, and establishes a method system and principle for positioning the target area of the ultrabasic rock type deposit. Through the method proposed by the present invention, the prediction ability of the ultrabasic rock type deposit can be effectively improved, helping with mineral resource exploration. BRIEF DESCRIPTION OF THE DRAWINGS

[0048] Figure 1 is a flowchart of a gravity and magnetic detection method for positioning the target area of an ultrabasic rock type deposit according to an embodiment of the present invention.

[0049] Figure 2 is a numerical calculation flowchart of a gravity and magnetic detection method for positioning the target area of an ultrabasic rock type deposit according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0050] The following will describe in detail the specific embodiments of the present invention with reference to the accompanying drawings, but it should be understood that the protection scope of the present invention is not limited by the specific embodiments.

[0051] Unless otherwise clearly stated, throughout the specification and claims, the term "comprising" or its variations such as "comprises" or "including" etc. will be understood to include the stated elements or components, without excluding other elements or other components.

[0052] As Figures 1 to 2As shown, a gravity and magnetic detection method for target area location of ultrabasic rock deposits according to a preferred embodiment of the present invention utilizes the advantages of high working efficiency, low cost, and the ability to quickly and efficiently obtain high-precision geophysical data over a large area in airborne gravity and aeromagnetic surveys. In addition, due to the large density and strong magnetism of ultrabasic rocks, there are significant differences in physical properties from the surrounding rocks, which are very well shown in the gravity and magnetic data. Therefore, the advantages of gravity and magnetic data can be greatly exploited.

[0053] As Figure 1 shown, a gravity and magnetic detection method for target area location of ultrabasic rock deposits of the present invention includes the following steps:

[0054] S1: By performing first-order vertical derivative processing on airborne gravity and magnetic data, determine the fracture distribution according to the positions of the derivative extreme points.

[0055] Specifically, perform first-order vertical derivative calculation on the gridded data of airborne gravity and aeromagnetic methods. The specific calculation formula is as follows:

[0056]

[0057] where P is the potential field value, which can be the potential field value of the gravity field Δg or the magnetic field ΔT, z is the direction perpendicular to the ground downward, and VDR is the first-order vertical derivative.

[0058] Through formula (1), the first-order vertical derivative distribution maps of Bouguer gravity and aeromagnetic data can be obtained. There are multiple local maximum points in the distribution map. Connecting the locally adjacent maximum points that are approximately on the same straight line can obtain the distribution of a single fracture. According to this method, the distributions of multiple fracture boundaries with different directions and lengths can be obtained.

[0059] S2: By selecting reasonable filtering parameters, perform frequency-domain filtering processing on the gravity and magnetic data to obtain the regional field and residual field of the gravity and magnetic anomalies.

[0060] Specifically, according to the gravity and magnetic anomaly characteristics, select appropriate filtering wavelengths, and perform Gaussian regional field filtering and Gaussian residual field filtering on the gravity and magnetic data. The filtering formula for the frequency-domain Gaussian residual field is:

[0061]

[0062] where P represents the gravity field Δg or the magnetic field ΔT, F(P) represents the Fourier transform of the gravity field or magnetic field, k0 is the standard deviation of the Gaussian function of the gravity and magnetic fields; k is the Gaussian filtering wavelength.

[0063] The selection criterion for the appropriate wavelength of Gaussian residual field filtering is that the boundaries of the gravity and magnetic anomalies after Gaussian residual field filtering are basically coincident with those of the high-frequency anomalies of the original field. By performing the inverse Fourier transform on res[F(P)] in formula (2), the Gaussian residual field Δg of the gravity anomaly can be obtained. res and the Gaussian residual field ΔT of the magnetic anomaly res .

[0064] Subtracting the gravity and magnetic Gaussian residual fields from the original gravity and magnetic anomalies, the corresponding Gaussian regional gravity field Δg reg and Gaussian regional magnetic field ΔT reg can be obtained, where the original gravity and magnetic anomalies can be obtained by direct reading.

[0065] S3: Normalize the first vertical derivatives of the Gaussian residual fields of the magnetic anomaly and the Gaussian residual field of the gravity anomaly.

[0066] First, normalize the Gaussian residual field ΔT of the magnetic anomaly res , and the calculation formula is as follows:

[0067]

[0068] where is the normalized magnetic anomaly value, X is the magnetic anomaly value before normalization, P min and P max are the minimum and maximum values of the magnetic anomaly before normalization respectively, and the minimum and maximum values of the magnetic anomaly can be obtained by direct reading. Through this formula (3), the value range of the normalized magnetic anomaly value can be limited between 0 and 1.

[0069] Secondly, perform the first vertical derivative processing on the Gaussian residual field Δg of the gravity anomaly res , and then normalize the first vertical derivative data to obtain the normalized result of the first vertical derivative of the corresponding gravity anomaly residual field

[0070] For the same ultrabasic rock geological body underground, since the attenuation rate of the gravity anomaly is smaller than that of the magnetic anomaly. If the Gaussian residual field Δg of the gravity anomaly res is directly normalized as shown in formula (3), it will lead to the weakening of the boundary correspondence relationship after gravity and magnetic normalization. Therefore, in this step S3, the Gaussian residual field Δg of the gravity anomaly res is first vertically differentiated and then normalized, so that the normalized data of the gravity and magnetic anomalies have a better homologous correspondence relationship.

[0071] S4: Perform cross-correlation processing on the normalized results obtained in step S3.

[0072] When performing cross-correlation processing, it is necessary to select an appropriate calculation window. The selection criterion for the window is that it should contain at least one complete maximum and minimum value of the gravity and magnetic anomalies. The calculation formula for cross-correlation processing is as follows:

[0073]

[0074] Where represents the covariance of the normalized data of the first-order vertical derivative of the residual magnetic anomaly field and the residual gravity anomaly field within the selected calculation window, and E represents the mathematical expectation; and respectively represent the variances of the normalized data of the first-order vertical derivatives of the residual magnetic anomaly field and the residual gravity anomaly field.

[0075] The range of the cross-correlation coefficient RC is between -1 and +1. Different values represent different correlation properties and degrees of correlation. Positive and negative values represent positive correlation and negative correlation respectively. The larger the absolute value of the cross-correlation coefficient RC, the greater the degree of correlation.

[0076] For rocks with high density and low magnetism such as limestone, RC is negative; for granite and diorite, etc., the range of RC changes greatly, it can be positive or negative, but the absolute value is not large; for ultrabasic rocks, due to their large density and magnetic susceptibility values, their RC values are positive and the absolute value is large, approaching +1.

[0077] In this step S4, the position of the ultrabasic rock can be well identified through formula (4).

[0078] S5: Calculate the Moho depth of the regional gravity anomaly using the Parker method.

[0079] The Moho represents the interface between the crust and the mantle, which is an important physical property discontinuity surface within the lithosphere. The rock densities on both sides of this discontinuity surface vary greatly. Therefore, the undulation of the Moho will cause a large gravity anomaly.

[0080] In this step S5, the depth of the Moho can be inverted based on the regional gravity anomaly. The depth change of the Moho can indicate the regional tectonic properties, and thus can assist in judging the regional ore-forming potential. The calculation formula for inverting the Moho using the Parker method in the frequency domain is as follows:

[0081]

[0082] Among them, F[h(r)] and F[Δg(r)] are the Fourier transforms of the Mohorovicic discontinuity depth and the regional Bouguer gravity anomaly respectively; |k| is the wave number; π is the pi; G is the gravitational constant; ρ is the density difference between the crust and the lithospheric mantle; z0 is the average depth of the Mohorovicic discontinuity in the study area, also known as the reference depth; r is the three-dimensional coordinate vector; n is the order.

[0083] In this step S5, through formula (5), the depth map of the Mohorovicic discontinuity can be obtained from the Gaussian regional gravity field Δg reg Generally, the geothermal heat flow value is relatively high in the places where the Mohorovicic discontinuity depth is relatively shallow, which can indicate the upwelling of the regional mantle hot material and is a favorable indicator for ultrabasic rock type deposits.

[0084] S6: Use the central point method to obtain the Curie depth of the regional field of the aeromagnetic anomaly.

[0085] The Curie surface is an important temperature interface inside the lithosphere. The temperature of this interface is generally 580 °C. The ferromagnetic minerals below this interface lose their ferromagnetism due to the temperature exceeding their Curie temperature and become paramagnetic minerals.

[0086] In this step S6, the Curie surface can be obtained by performing the central point method calculation in the frequency domain on the Gaussian regional magnetic field ΔT reg For a magnetic layer that extends infinitely in the horizontal direction, has a thickness that is a small value relative to the depth, and has a randomly distributed magnetization intensity, the radial average amplitude spectrum of the magnetic anomaly it generates is:

[0087]

[0088] Among them, A ΔT (|k|) and A M (|k|) are the radial average amplitude energy spectra of the Gaussian regional magnetic field and the magnetization intensity of the underground magnetic body respectively; |k| is the absolute value of the wave number in the wave number domain; Z t represents the top surface of the magnetic layer; Z b represents the bottom surface of the magnetic layer, and this interface is the Curie surface; B represents a constant, and this constant does not affect the Curie surface value.

[0089] By simplifying and taking the logarithm of formula (6a) in the low wave number domain and the medium and high wave number domains respectively, the top surface depth Z t of the magnetic layer and the depth Z0 of the central point can be obtained respectively.

[0090] For example, in the low wave number domain, the formula for simplifying and taking the logarithm is:

[0091]

[0092] In the medium and high wave number domains, the formula for simplifying and taking the logarithm is:

[0093] ln[A ΔT (|k|)] = lnD - |k|·Z t (6c)

[0094] In formulas (6b) and (6c) above, C and D are constants and do not affect the results within the scope.

[0095] Finally, the following simple mathematical formula can be used to calculate the Curie depth Z b , that is:

[0096] Z b = 2·Z0 - Z t (7)

[0097] In this step S6, by selecting a reasonable calculation window, the Curie depth at a single point can be obtained. By sliding the window, a series of Curie points arranged at equal intervals can be obtained. Finally, through minimum curvature interpolation, the Curie point depth map in the study area can be obtained.

[0098] S7: Perform 3D focused inversion on the residual fields of the gravity and magnetic anomalies respectively to obtain the remaining density anomaly and magnetic susceptibility anomaly in the crust.

[0099] Perform minimum gradient support focused inversion on the Gaussian residual gravity field Δg res and the Gaussian residual magnetic field ΔT res obtained in step S2 to obtain the distribution of high-density and strongly magnetic bodies with clear and sharp boundaries; among them, the objective function of the focused inversion is as follows:

[0100]

[0101] In the formula,

[0102]

[0103] Among them, the meanings of the parameters in formulas (8) to (10) are introduced as follows: m is the model parameter to be inverted, representing the density parameter during gravity inversion and the magnetic susceptibility parameter during magnetic inversion; is the gravity and magnetic data fitting function; s(m) is the model regularization term, whose role is to add prior information; α is the regularization factor, which can coordinate and balance the data fitting function and the model regularization term; W d is the weighted matrix of the observed data, which is related to the error of the observed data; W m is the weighted matrix of the model; A is the forward operator of the gravity anomaly or magnetic anomaly of the cube under the condition of uniform density or magnetic susceptibility, that is, the partial derivative; W e is the minimum gradient support weighted matrix; is the Hamiltonian operator; both β and e are small values less than 1, mainly playing a role in focusing and can prevent the denominator from being zero. d is the Gaussian residual gravity field Δg res or the Gaussian residual magnetic field ΔT res ;

[0104] Based on the statistical results of rock density and magnetic susceptibility measured in the field, with these statistical results as boundary condition constraints, the conjugate gradient method is used to iteratively solve formula (8), and the density and magnetic susceptibility distributions in the shallow layer of the underground space can be obtained.

[0105] S8: According to the field measured data of ultramafic rocks, set the prediction criteria for the target areas of ultramafic rock deposits and construct relevant prediction methods.

[0106] Specifically, comprehensively consider the fracture distribution determined in step S1, the cross-correlation of the normalized results of the Gaussian residual magnetic field and the Gaussian residual gravity field determined in step S4, the Moho depth map inverted in step S5, the Curie depth map calculated in step S6, and the density and magnetic susceptibility distributions of the underground space inverted in step S7 to establish the positioning criteria for the target areas of ultramafic rock deposits.

[0107] Generally speaking, the conditions conducive to the existence of ultramafic rock deposits are: having fractures or fracture intersections; the normalized results of the Gaussian residual magnetic field and the Gaussian residual gravity field are positively correlated and the correlation coefficient is close to +1; the Moho depth is a bulge in the depression; the Curie depth is shallow; both the density and magnetic susceptibility are very large, for example, the density value is generally greater than 2.85 g / cm 3 , and the magnetic susceptibility is generally greater than 2000×10 -5 . The more a certain position on the plane of the study area satisfies the above conditions, the greater the probability of the existence of ultramafic rock deposits. Table 1 shows the states of the above indicators such as fractures, the correlation coefficient of gravity and magnetic normalization, Moho depth, Curie depth, inverted density, and inverted magnetic susceptibility, and the corresponding scores. The higher the score, the greater the probability of the existence of this type of deposit. Through this set of systematic solutions, the level of predicting the target areas of ultramafic rock deposits can be improved.

[0108] Table 1 Prediction Scheme for Target Areas of Ultramafic Rock Deposits

[0109] Fracture None (0) Yes (1) Cross (2) Gravity and magnetic normalization correlation coefficient Negative value (0) Small positive value (1) Large positive value (2) Mohorovicic discontinuity depth Deep (0) Medium (1) Shallow (2) Curie depth Deep (0) Medium (1) Shallow (2) Inversion density Small (0) Medium (1) Large (2) Inversion magnetic susceptibility Small (0) Medium (1) Large (2)

[0110] The foregoing description of the specific exemplary embodiments of the present invention is for purposes of illustration and exemplification. These descriptions are not intended to limit the invention to the precise forms disclosed, and it is apparent that many modifications and variations are possible in light of the above teachings. The purpose of selecting and describing the exemplary embodiments is to explain the specific principles of the present invention and its practical applications, so that those skilled in the art can implement and utilize the various different exemplary embodiments of the present invention, as well as various different selections and modifications. The scope of the present invention is intended to be defined by the claims and their equivalents.

Claims

1. A gravity and magnetic detection method for locating a target area of ​​an ultrabasic rock deposit, characterized in that: The following steps are involved: S1: By performing first-order vertical derivative processing on the aerogravimetric and magnetic data, the fault distribution is determined according to the position of the derivative extreme point; S2: By selecting reasonable filtering parameters, the gravity and magnetic data are filtered in the frequency domain to obtain the regional field and residual field of gravity and magnetic anomalies; S3: normalizing the first-order vertical derivative of the Gaussian residual field of the magnetic anomaly and the Gaussian residual field of the gravity anomaly obtained in step S2; S4: performing cross-correlation processing on the normalized result obtained in step S3; S5: Calculate the Moho depth using Parker's method for the regional field of gravity anomaly; S6: The Curie depth is obtained by using the center point method for the regional field of aeromagnetic anomalies; S7: Perform three-dimensional focusing inversion on the residual fields of gravity and magnetic anomalies to obtain the residual density anomaly and magnetic susceptibility anomaly of the crust; S8: According to the field measured data of ultrabasic rocks, set the prediction standard of the target area of ​​ultrabasic rock deposits and construct relevant prediction methods; Among them, in step S2, according to the characteristics of gravity and magnetic anomalies, a suitable filtering wavelength is selected to perform Gaussian regional field filtering and Gaussian residual field filtering on the gravity and magnetic data; wherein, the filtering formula of the frequency domain Gaussian residual field is: Where P represents the gravitational field Δg or the magnetic field ΔT, F(P) represents the Fourier transform of the gravitational field or the magnetic field, k0 is the standard deviation of the Gaussian function of the gravitational field or the magnetic field; k is the Gaussian filter wavelength; Performing inverse Fourier transform on res[F(P)] in the above formula can obtain the Gaussian residual field Δg of gravity anomaly: res and the Gaussian residual field ΔT of the magnetic anomaly res ; Subtracting the gravity and magnetic Gaussian residual field from the original gravity and magnetic anomaly can give the corresponding Gaussian regional gravity field Δg reg and Gaussian magnetic field ΔT reg .

2. The gravity and magnetic detection method for locating the target area of ​​ultrabasic rock deposits according to claim 1, characterized in that: In step S1, the first-order vertical derivatives are calculated for the gridded data of the aerogravity and aeromagnetic methods. The specific calculation formula is as follows: Where P is the potential field value, which is the potential field value of the gravity field Δg or the magnetic field ΔT; z is the downward direction perpendicular to the ground, and VDR is the first-order vertical derivative; Through the above formula, the first-order vertical derivative distribution map of Bouguer gravity and aeromagnetic data can be obtained, and then the distribution of the fault boundary can be obtained.

3. The gravity and magnetic detection method for positioning the target area of ​​ultrabasic rock deposits according to claim 1, characterized in that: The selection criterion of the suitable wavelength is that the gravity and magnetic anomaly after Gaussian residual field filtering coincides with the high-frequency anomaly boundary of the original field.

4. The gravity and magnetic detection method for positioning the target area of ​​ultrabasic rock deposits according to claim 1, characterized in that: Step S3 specifically includes the following steps: First, the Gaussian residual field ΔT of the magnetic anomaly res After normalization, the calculation formula is as follows: in, is the magnetic anomaly value after normalization, X is the magnetic anomaly value before normalization, and P min and P max are the minimum and maximum values ​​of the magnetic anomaly before normalization, respectively; Secondly, the Gaussian residual field Δg of the gravity anomaly res Perform first-order vertical derivative processing, and then normalize the first-order vertical derivative data to obtain the normalized result of the first-order vertical derivative of the corresponding gravity anomaly residual field 5. The gravity and magnetic detection method for positioning the target area of ​​ultrabasic rock deposits according to claim 4, characterized in that: In step S4, when performing cross-correlation processing, it is necessary to select a calculation window, and the selection criteria of the calculation window are that it contains at least a complete maximum and minimum value of gravity and magnetic anomalies; the calculation formula for cross-correlation processing is: in, The covariance of the normalized data representing the first-order vertical derivatives of the magnetic anomaly residual field and the gravity anomaly residual field within the selected calculation window, E represents the mathematical expectation; and They represent the variance of the normalized data of the first-order vertical derivatives of the magnetic anomaly residual field and the gravity anomaly residual field, respectively.

6. The gravity and magnetic detection method for positioning the target area of ​​ultrabasic rock deposits according to claim 5, characterized in that: In step S5, the depth of the Moho surface is inverted according to the regional gravity anomaly. The calculation formula for inverting the Moho surface using the Parker method in the frequency domain is: where F[h(r)] and F[Δg(r)] are the Fourier transforms of the Moho depth and the regional Bouguer gravity anomaly, respectively; |k| is the wave number; π is pi; G is the gravitational constant; ρ is the density difference between the crust and the lithospheric mantle; z0 is the average depth of the Moho in the study area, also called the reference depth; r is the three-dimensional coordinate vector; and n is the order.

7. The gravity and magnetic detection method for positioning the target area of ​​ultrabasic rock deposits according to claim 1, characterized in that: In step S6, the Gaussian magnetic field ΔT reg The frequency domain center point method is used to calculate and obtain the Curie point. For a magnetic layer that is infinitely extended in the horizontal direction, has a small thickness relative to the depth, and has a randomly distributed magnetization intensity, the radial average amplitude spectrum of the magnetic anomaly is: Among them, A ΔT (|k|) and A M (|k|) are the radial average amplitude energy spectra of the Gaussian regional magnetic field and the magnetization intensity of the underground magnetic body; |k| is the absolute value of the wave number in the wave number domain; Z t Represents the top surface of the magnetic layer; Z b represents the bottom surface of the magnetic layer, and this interface is the Curie surface; B is a constant; By simplifying and logarithmically processing the above formula in the low wave number domain and the medium and high wave number domain, the top surface depth Z of the magnetic layer can be obtained respectively. t , and the depth Z0 of the center point; finally, the depth Z of the Curie surface is calculated b : WITH b =2 Z0-Z t 。 8. The gravity and magnetic detection method for locating the target area of ​​ultrabasic rock deposits according to claim 1, characterized in that: In step S3, the Gaussian residual gravity field Δg obtained in step S2 is res and Gaussian residual magnetic field ΔT res Minimum gradient support focusing inversion is performed to obtain the distribution of high-density ferromagnetic bodies with clear and sharp boundaries. The objective function of focusing inversion is as follows: In the formula, Where m is the model parameter to be inverted, which represents the density parameter in gravity inversion and the magnetic susceptibility parameter in magnetic inversion; is the gravity and magnetic data fitting function; s(m) is the model regularization term; α is the regularization factor; W d is the weighting matrix of the observed data; W m is the weight matrix of the model; A is the forward operator of the gravity anomaly or magnetic anomaly of the cube under uniform density or magnetic susceptibility conditions; W e is the minimum gradient support weighted matrix; is the Hamiltonian operator; β and e are both small values ​​less than 1; d is the Gaussian residual gravity field Δg res Or Gaussian residual magnetic field ΔT res ; Based on the statistical results of rock density and magnetic susceptibility actually measured in the field, and with these statistical results as boundary conditions, the conjugate gradient method is used to iteratively solve the objective function of focused inversion to obtain the density and magnetic susceptibility distribution of the shallow underground space.

9. The gravity and magnetic detection method for positioning the target area of ​​ultrabasic rock deposits according to claim 1, characterized in that: In step S8, the prediction standard for the target area of ​​the ultrabasic rock deposit is: based on the fracture distribution determined in step S1, the cross-correlation of the normalized results of the Gaussian residual magnetic field and the Gaussian residual gravity field determined in step S4, the Moho surface depth map obtained by inversion in step S5, the Curie surface depth map calculated in step S6, and the density and magnetic susceptibility distribution of the underground space obtained by inversion in step S7, to establish the positioning standard for the target area of ​​the ultrabasic rock deposit.

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