A prediction method for deep-sea hydrothermal areas based on shear deformation of oceanic crust

Abstract

The present invention discloses a method for predicting deep-sea hydrothermal areas based on oceanic crust shear deformation, which comprises the following steps: First, select an oceanic crust deformation analysis area and establish a geometric model of the analysis area; Secondly, calculate the elastomechanical parameters based on the oceanic crust stratification structure of the study area, establish a geomechanical model, and conduct finite element numerical simulation on the established geomechanical model to obtain the shear strain distribution on the seabed surface. Then, use the shear strain distribution to predict the favorable positions of hydrothermal areas. The method of the present invention combines the favorable distribution characteristics of deep-sea hydrothermal areas with the shear fractures on the seabed surface. Compared with other methods, this method can quickly, completely and with relatively high resolution predict the favorable positions where hydrothermal areas appear, providing effective support for searching for deep-sea polymetallic sulfide resources.

Description

Technical Field

[0001] The present invention relates to the field of metallogenic prediction in deep - sea hydrothermal areas, and particularly to a prediction method for deep - sea hydrothermal areas based on oceanic crust shear deformation. Background Art

[0002] In recent years, with the consumption of land mineral resources and the development of deep - sea survey technologies, the exploration and development of seabed mineral resources have gradually become the target of competition among countries for research and development. Submarine polymetallic sulfides, due to their relatively shallow occurrence depth and rich in a large number of heavy metal elements, are a mineral resource with potential commercial exploitation value and are expected to become the first target of deep - sea mining, carrying the future of marine resources.

[0003] The hydrothermal circulation process in the deep sea is an important factor for the accumulation of seabed sulfide resources on the deep - sea surface. An important factor in maintaining hydrothermal circulation is to provide a high - permeability channel for the migration of hydrothermal fluids, and these channels often form during the stretching and shear deformation of the earth's crust during evolution. Therefore, to quickly and effectively find deep - sea hydrothermal sulfide deposits, finding high - permeability channels in the earth's crust is a very effective method.

[0004] At present, the research on the deep - seated deformation field in deep - sea hydrothermal areas is very limited, and most of the research focuses on the inversion of seismic focal mechanism solutions. The current seismic focal mechanism solutions can only obtain the stress state at one or several points in the earth's crust, and the cost of arranging a micro - seismic detection once is huge, and the inversion results are extremely dependent on the layout azimuth of the stations and the inversion technology, which is a time - consuming and resource - consuming method. In contrast, the numerical simulation method can obtain the stress state of the entire study area and almost does not consume human and material resources, which is a fast, economical and effective way and has important value for understanding the metallogenic mechanism of sulfide resources, rock fracture and earthquake occurrence processes in deep - sea hydrothermal areas. Summary of the Invention

[0005] The purpose of the present invention is to propose a prediction method for deep - sea hydrothermal areas based on oceanic crust shear deformation in view of the deficiencies of the prior art. By using the oceanic crust stratification results obtained from ocean drilling and seabed topography data for mechanical numerical simulation, and according to the distribution characteristics of shear deformation on the seabed surface, combined with the analysis method of geomechanics, the occurrence mechanism of hydrothermal circulation in the oceanic crust of deep - sea hydrothermal areas is revealed, in order to make new explorations for understanding the dynamic geological processes in deep - sea hydrothermal areas and solve the above - mentioned deficiencies in the prior art. Under the existing technical conditions, it has the advantages of being fast, economical and effective.

[0006] To achieve the above object, the present invention adopts the following technical solutions: A prediction method for deep - sea hydrothermal areas based on oceanic crust shear deformation, comprising the following steps:

[0007] Step 1: Select the oceanic crust deformation area as the area to be analyzed; and establish a geometric model of the area to be analyzed based on the seabed topography data, where the upper surface of the model is the topography of the area to be analyzed;

[0008] Step 2: Obtain the elastomechanical parameters based on the layered structure of the oceanic crust in the area to be analyzed, and establish a geomechanical model;

[0009] Step 3: Conduct a finite element numerical simulation on the established geomechanical model; the boundary conditions of the finite element numerical simulation are set according to the specific geological structure conditions of the area to be analyzed; the meshing process of the model adopts a free meshing strategy to adapt to the undulations of the seabed topography, and the load applied inside the model is the gravity load, which is applied to each grid point, and the shear deformation distribution of the oceanic crust on the seabed surface after the finite element numerical simulation is obtained;

[0010] Step 4: Analyze the stress state of the shear deformation distribution of the oceanic crust simulated in Step 3 to predict the distribution positions of favorable hydrothermal areas.

[0011] Furthermore, in Step 1, the upper surface of the model is the topography of the study area, with depth undulations, and hydrostatic pressure is applied according to the undulations of the topography.

[0012] Furthermore, in Step 2, the elastomechanical parameters include Poisson's ratio, density, and Young's modulus.

[0013] Furthermore, the method for setting the boundary conditions in Step 3 is as follows: Expansion rates of 8.7 mm / y and 5.2 mm / y are applied to the southern and northern boundaries respectively, hydrostatic pressure is applied to the upper boundary of the model according to the seabed depth, and the calculation formula is represented by formula (1), and the lower boundary of the model is fixed;

[0014] P = ρgh (1)

[0015] In the above formula, P represents hydrostatic pressure, with the unit of Pa, ρ represents seawater density, with the unit of kg / m 3 , h represents the seabed depth, and g represents the gravitational acceleration in the seabed area.

[0016] Furthermore, the stress state analysis method in Step 4 is specifically as follows: Extract the shear deformation values on the seabed surface according to the simulation results, where positive values represent right-handed shear and negative values represent left-handed shear. Predict the distribution positions of favorable hydrothermal areas according to the shear deformation distribution of the oceanic crust simulated in Step 3, and select the areas where the oceanic crust shear strain is greater than the strain threshold as favorable ore-forming areas; Combine the simulation results of shear deformation with the geological phenomena in the analysis area to analyze the geodynamic process mechanism of the deformation analysis area and reveal the processes of rock earthquake occurrence and hydrothermal fluid migration in the oceanic crust.

[0017] Advantages of the present invention: The method of the present invention combines the distribution characteristics of shear deformation on the seabed with hydrothermal activities, and proposes a method for predicting deep-sea hydrothermal areas based on oceanic crust shear deformation. Compared with other methods, this method can quickly, completely, and with relatively high resolution establish the strain distribution characteristics of the study area, so as to quickly and effectively determine the location of favorable hydrothermal metallogenic areas in the study area. Brief Description of the Drawings

[0018] Figure 1 is the flowchart of the implementation process of the present invention;

[0019] Figure 2 is the seabed topographic map of the study area during the implementation process of the present invention. The black five-pointed star indicates the location of the proven hydrothermal area. The horizontal and vertical coordinates represent the distance, and the unit is km;

[0020] Figure 3 is the geometric model established during the implementation process of the present invention, and the upper surface is the seabed surface;

[0021] Figure 4 is the shear deformation field obtained during the implementation process of the present invention; Detailed Embodiment

[0022] The following combines the drawings and introduces the implementation manner of the present invention in detail through specific specific examples. Those skilled in the art can easily understand the implementation steps, effects, and advantages of the present invention according to the content of the specification. The present invention can also be applied to other types of prospecting work on the seabed. Those skilled in the art can make adjustments to various details according to different application scenarios without departing from the spirit of the present invention, such as the acquisition methods of various data, the accuracy of terrain data, etc. It should be noted that any detailed adjustments made by those skilled in the art within the scope of the claims belong to the protection scope of the present invention.

[0023] Please refer to Figure 1 the flow schematic diagram of, the present invention provides a method for predicting deep-sea hydrothermal areas based on oceanic crust shear deformation, including the following steps:

[0024] Step 1: Select the deformation analysis area, generally select the area near the deep-sea sulfide ore area;

[0025] Step 2: Establish a geometric model of the analysis area according to the selected area and seabed terrain data; The depth selection should consider the crust thickness of the study area. The upper surface of the model is the seabed terrain of the study area, and the left and right boundaries generally select rectangular boundaries, which can be changed according to specific research objectives.

[0026] Step 3: Calculate the elastic mechanics parameters (Poisson's ratio, density, and Young's modulus) based on the oceanic crust stratification structure in the study area, and establish a geomechanics model. The oceanic crust stratification structure refers to the geological stratification. The oceanic crust stratification structure is calculated by using the publicly available data of ocean drilling. Taking this embodiment as an example, the ocean drilling data closest to the study area (Atlantis in the Southwest Indian Ocean) is selected, and the oceanic crust is divided into two layers (Layer A and Layer B). The specific result parameters are shown in the table:

[0027]

[0028] Step 4: Set the boundary conditions, apply the loading, mesh the model for the established geomechanics model, and select the calculation method for finite element numerical simulation. The setting of the boundary conditions for finite element numerical simulation needs to be combined with the specific geological structure conditions in the study area. The setting method is as follows: Apply the expansion rates of 8.7 mm / y and 5.2 mm / y to the southern and northern boundaries respectively, where y represents year; apply the hydrostatic pressure to the upper boundary according to the seabed depth, and the calculation formula is represented by formula (1), and the lower boundary is fixed.

[0029] P = ρgh (1)

[0030] In the above formula, P represents the hydrostatic pressure, with the unit of Pa, ρ represents the seawater density, with the unit of kg / m 3 , h represents the seabed depth, and g represents the gravitational acceleration in the seabed area.

[0031] As Figure 2 shown, in the embodiment of the present invention, the target is the Longqi hydrothermal area in the Southwest Indian Ocean, Figure 2 where the five-pointed stars are the confirmed hydrothermal areas, 65 km in the east-west direction, 55 km in the north-south direction, and 4 km in the depth direction. The depth of the model needs to ensure that the boundary effect of the bottom interface will not affect the deformation of the seabed surface and needs to be set according to the actual situation. A three-dimensional model is established, and the model geometry is as Figure 3 shown, and the upper surface is the seabed surface. The established two-dimensional oceanic crust velocity structure diagram is as Figure 4 shown. The expansion rates of 8.7 mm / y and 5.2 mm / y are applied to the north and south sides of the model respectively, the hydrostatic pressure corresponding to the specific seawater depth is applied to the seabed surface, and the bottom surface is set as the constrained surface. To adapt to the characteristics of the undulating seabed topography, the free mesh division strategy is adopted in the meshing process of the model, and this method can be applied to almost any seabed model. The load applied inside the model is the gravity load, which is loaded on each grid point, and its value is calculated according to the global gravitational acceleration calculation formula of the IEC standard, and its calculation formula is:

[0032]

[0033] Among them, is the latitude value, with the unit of degree. The north latitude is positive. Z is the altitude, with the unit of meter. Above the horizontal plane is positive. g represents the acceleration of gravity.

[0034] The specific operation process implemented in step (4) is as follows:

[0035] 1) Assign the elastic parameters calculated in step 3 to the geometric model established in step 2 to establish an elastic mechanics model;

[0036] 2) Set the boundary conditions. The loading of the boundary conditions needs to be based on the specific geological conditions of the study area. In this embodiment of the present invention, the Longqi hydrothermal area in the Southwest Indian Ocean is taken as an example. The upper boundary is the seabed surface, and the hydrostatic pressure generated by the seawater is loaded. The hydrostatic pressure at each point is related to the seabed depth, and the specific formula is represented by (1); the left and right sides are represented as the spreading rate boundaries. The research results of geology and geophysics show that the spreading in this area is asymmetric north-south spreading, and the south side (8.7 mm / y) is slightly larger than the north side (5.2 mm / y). In this embodiment, the simulation is centered on the mid-ocean ridge, and the spreading rates of 8.7 mm / y and 5.2 mm / y are loaded on the north and south sides respectively; the lower boundary of the model is a fixed interface.

[0037] 3) The load applied is the gravity load. The acceleration of gravity g is applied at each grid point, and its value can be calculated by formula (2).

[0038] 4) Mesh the model. Triangular meshes, hexagonal meshes, etc. suitable for the boundary terrain undulation changes can be selected. The accuracy of the mesh division only needs to meet the analysis of the target task. The "fine" accuracy is selected in this embodiment. The meshing method of the model adopts free mesh division and is solved by the finite element method. The equation used in the numerical simulation is the stress equilibrium equation:

[0039]

[0040] In the above formula, σ: normal stress, with the unit of Pa; τ: shear force, with the unit of Pa; X, Y, Z: external forces in three directions, with the unit of N, Z is gravity, with the unit of N, and X and Y are set to 0;

[0041] Step 6: Perform numerical simulation according to formulas (3), (4), and (5) to obtain the shear deformation distribution map of the seabed surface;

[0042] Step 7: The specific stress state analysis method in Step 4 is as follows: Extract the shear deformation value of the seabed surface according to the simulation results. A positive value represents right-handed shear, and a negative value represents left-handed shear. Predict the distribution position of favorable hydrothermal areas based on the shear deformation distribution of the oceanic crust simulated in Step 3. Select the area where the shear strain of the oceanic crust is greater than 0.0001 as the favorable ore-forming area. The simulation results of shear deformation can be combined with the geological phenomena in the analysis area to analyze the geodynamic process mechanism of the deformation analysis area, reveal the occurrence of rock earthquakes and the fluid migration process in the oceanic crust. The specific method is as follows:

[0043] (1) Simulate the distribution characteristics of the shear deformation field according to the established model and boundary conditions;

[0044] (2) Extract the distribution map of shear strain τ;

[0045] (3) The high-value area of shear strain (i.e., the absolute value of τ > 0.0001) corresponds to the position where rock shear failure occurs and the oceanic crust has a relatively high permeability, which is conducive to the migration of hydrothermal fluids;

[0046] The above embodiments are used to explain the present invention, rather than limit the present invention. Any modification and change made to the present invention within the spirit and scope of the claims of the present invention fall within the protection scope of the present invention.

Claims

1. A method for predicting deep - sea hydrothermal areas based on oceanic crust shear deformation, characterized in that, It includes the following steps: Step 1: Select the oceanic crust deformation area as the area to be analyzed; and establish a geometric model of the area to be analyzed based on the seabed topography data, where the upper surface of the model is the topography of the area to be analyzed; Step 2: Obtain the elastomechanical parameters based on the layered structure of the oceanic crust in the area to be analyzed and establish a geomechanical model; Step 3: Conduct a finite element numerical simulation on the established geomechanical model; the boundary conditions of the finite element numerical simulation are set according to the specific geological structure conditions of the area to be analyzed; the meshing process of the model adopts a free mesh division strategy to adapt to the undulation of the seabed topography, and the load applied inside the model is the gravity load, which is applied to each grid point to obtain the distribution of oceanic crust shear deformation on the seabed surface after the finite element numerical simulation; The setting method of the boundary conditions is as follows: expansion rates of 8.7 mm / y and 5.2 mm / y are applied to the southern and northern boundaries respectively, where y represents year, the hydrostatic pressure is applied to the upper boundary of the model according to the seabed depth, and the calculation formula is represented by formula (1), and the lower boundary of the model is fixed; P = ρgh (1) In the above formula, P represents the hydrostatic pressure in Pa, ρ represents the seawater density in kg / m 3 , h represents the seabed depth, and g represents the gravitational acceleration in the seabed area; Step 4: Conduct a stress state analysis on the oceanic crust shear deformation distribution simulated in Step 3 to predict the distribution location of favorable hydrothermal areas; The stress state analysis method is specifically as follows: extract the shear deformation value on the seabed surface according to the simulation results, where a positive value represents right-handed shear and a negative value represents left-handed shear. Conduct a stress state analysis based on the oceanic crust shear deformation distribution simulated in Step 3 to predict the distribution location of favorable hydrothermal areas, and select the area where the oceanic crust shear strain is greater than the strain threshold as the favorable ore-forming area; combine the simulation results of the shear deformation with the geological phenomena in the area to be analyzed to analyze the geodynamic process mechanism in the area to be analyzed and reveal the occurrence of rock earthquakes and the migration process of hydrothermal fluids in the oceanic crust.

2. The method for predicting deep - sea hydrothermal areas based on oceanic crust shear deformation according to claim 1, characterized in that: In Step 1, the upper surface of the model is the topography of the area to be analyzed, which has depth undulations, and the hydrostatic pressure is applied according to the undulations of the topography.

3. The method for predicting deep - sea hydrothermal areas based on oceanic crust shear deformation according to claim 1, characterized in that: In Step 2, the elastomechanical parameters include Poisson's ratio, density, and Young's modulus.

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