A deep matter thermal state quantitative estimation method based on an electrical structure model

Abstract

The application discloses a kind of deep matter thermal state quantitative estimation method based on electrical structure model, first according to the magnetotelluric sounding data collected, establish the deep three-dimensional electrical structure model of study area;Again, according to the relationship between rock conductivity and its temperature, the relationship between the total conductivity and temperature of the upper mantle in solid mixed system and solid-melt mixed system is considered respectively, the quantitative relationship model of the total conductivity of the upper mantle and temperature, rock melting percentage content is established;Finally, according to the quantitative relationship model, the conductivity data measured by actual magnetotelluric is brought in, the rock melting percentage content and temperature in the scale range of the upper mantle are calculated, and the deep thermal structure model of exploration area is established.The application can directly obtain deep temperature and rock melting percentage content by combining the electrical structure model established by rock physics and magnetotelluric sounding.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of geophysical exploration, in particular to a deep material thermal state quantitative estimation method based on an electrical structure model. BACKGROUND

[0002] The magnetotelluric method (MT) is an important geophysical method for studying the electrical structure of the earth's interior using natural electromagnetic fields as the source. Based on the principle that electromagnetic waves of different frequencies have different skin depths in conductors, the electromagnetic response sequence of the earth from high frequency to low frequency is measured on the ground, and the electrical structure of the earth from shallow to deep is obtained through related data processing and analysis. Compared with other electrical exploration methods, the magnetotelluric method can effectively detect ultra-deep depths of tens to hundreds of kilometers below the surface, can penetrate high-resistance layers, and has the advantage of being sensitive to low-resistance layers. Therefore, this method has been widely used in the study of deep material structure, mineral resource exploration, etc.

[0003] In recent years, the magnetotelluric method has been widely used in the exploration of deep geothermal resources. For example, Li Shuai et al. (Li Shuai, et al. "Deep electrical structure and thermal characteristics of the Zhangshu-Ningde magnetotelluric profile in South China." Chinese Journal of Geophysics 65.04 (2022): 1354-1375.) used the magnetotelluric method to obtain the electrical structure model of the Zhangshu-Ningde lithosphere in South China, and found that the Fu'an-Nanjing fault zone and the Zhenghe-Dapu fault zone were low resistance, and there was a high-conductivity anomaly in the southeast coastal margin zone where the two faults were located. Combined with the tectonic evolution characteristics of South China, it is speculated that it is a channel for basaltic magma upwelling; Guan Dawei (Guan Dawei. Application of audio magnetotelluric method in geothermal exploration in Sanjiangkou area of Rucheng County [J]. Energy Technology and Management, 2022, 47(03): 163-166.) used audio magnetotelluric method to obtain the deep electrical structure model of Sanjiangkou area in Rucheng County, and based on the electrical characteristics of the profile, inferred the distribution of fault structures and the spatial extension of the fractured water-filled zone in the work area. Combined with surface and hydrogeological data, six favorable exploration areas for geothermal resources in the study area were delineated.

[0004] However, the above-mentioned magnetotelluric method in the exploration of deep geothermal resources is only an indirect means, which can only indirectly obtain the deep heat source "channel" by detecting the location and occurrence of faults, and cannot directly provide deep thermal structure information such as temperature and rock melting percentage content.

[0005] In addition, in terms of deep dynamics research, the electrical structure model obtained by magnetotelluric sounding usually has a significant low-resistance area distribution in the lower crust and upper mantle. For example, Zhao Guoze et al. (Zhao Guoze, et al. Study on the relationship between crustal electrical structure and block deformation in the northeastern margin of the Qinghai-Tibet Plateau [J]. Science in China (Series D: Earth Sciences), 2004 (10): 908-918.) believed that the relatively low-resistance layer in the upper part of the lower crust in the northeastern margin of the Qinghai-Tibet Plateau was a fluid with good connectivity according to the established magnetotelluric profile of the northeastern margin of the Qinghai-Tibet Plateau. Dong Ji'en et al. (Dong Ji'en, et al. Tectonic characteristics, mineralization and dynamic mechanism in the northeastern part of the South China block: Insights from magnetotelluric sounding. Geology Review 68.03 (2022): 921-937. doi:10.16509 / j.georeview.2022.02.021.) found that there was a significant deep thermal material upwelling feature near the northeastern Jiangxi fault zone according to the two-dimensional electrical profile of the lithospheric scale in the northeastern part of the South China block, mainly a low-resistance anomaly, and there was a significant magma hydrothermal underplating phenomenon in the deep part.

[0006] For the explanation of the low-resistance area of the deep electrical structure, the above analysis is mainly qualitative, and lacks quantitative explanation. For example, there is a significant low-resistance layer in the lower crust and upper mantle of the Songpan-Ganzi block in the eastern margin of the Qinghai-Tibet Plateau, and most scholars believe that the low-resistance layer is caused by rock partial melting. Rock melting requires a high-temperature environment, and whether the temperature exceeds the solidus temperature of the rock is one of the conditions for rock melting. However, the magnetotelluric sounding data has not been able to directly determine the temperature of the low-resistance layer, and thus the percentage of rock melting is not obtained.

[0007] In summary, the magnetotelluric method has been used as an indirect means for speculation and analysis in the exploration of deep geothermal resources and deep dynamics interpretation, and the results are mainly qualitative and lack quantitative explanation. The current magnetotelluric sounding method cannot directly obtain the deep thermal structure (temperature, rock melting percentage content), and cannot achieve quantitative estimation of the deep thermal state. SUMMARY

[0008] In view of the above problems, the purpose of the present application is to provide a deep material thermal state quantitative estimation method based on electrical structure model, which overcomes the limitation that the current magnetotelluric method cannot directly obtain the deep thermal structure (temperature, rock melting percentage content) in the exploration of deep geothermal resources and deep dynamics interpretation. By combining the electrical structure model established by rock physics and magnetotelluric sounding, the relationship between the electrical conductivity of the upper mantle rock and the rock melting percentage content and temperature is established, so that the deep temperature and rock melting percentage content can be directly obtained. The technical scheme is as follows:

[0009] A deep material thermal state quantitative estimation method based on electrical structure model, comprising the following steps:

[0010] Step 1: Collect the magnetotelluric sounding data, and after Fourier transform, spectrum filtering and Robust impedance tensor estimation, the continuous sounding curve is obtained, the qualitative parameters of the study area are obtained by phase tensor decomposition, and the three-dimensional electrical structure model of the deep part of the study area is established;

[0011] Step 2: Based on the relationship between rock conductivity and temperature obtained by high temperature and high pressure conductivity experiment of upper mantle rock, the relationship between total conductivity and temperature of upper mantle in solid state mixed system and solid state-melted state mixed system is considered respectively, the quantitative relationship between melt percentage, temperature and upper mantle conductivity is introduced, and the quantitative relationship model between total conductivity of upper mantle and temperature, rock melt percentage is established;

[0012] Step 3: According to the established quantitative relationship model between total conductivity of upper mantle and temperature, melt percentage, the rock melt percentage and temperature in the range of upper mantle scale are calculated by introducing the actual magnetotelluric conductivity data, the deep thermal structure model of the exploration area is established, and the quantitative estimation of the melt percentage and the corresponding temperature in the range of upper mantle scale of the study area is realized.

[0013] Further, the step 2 specifically comprises:

[0014] Step 2.1: According to the actual geological conditions of the study area, the Moho surface depth and water content value C of the study area are determined w , and different values of water content are set according to the actual situation; different geological structures of different study areas correspond to different ranges of water content of upper mantle, such as 250-2500 ppm of water content in subduction area and 300-1000 ppm of water content in ocean island basalt mantle. Therefore, the water content of the study area should be determined in combination with its geological background.

[0015] Step 2.2: The relationship expression between temperature T and melt percentage V is obtained from the set water content value and upper mantle depth value, and the corresponding solidus temperature T solidus and two-pyroxene peridotite melt temperature

[0016]

[0017]

[0018]

[0019] T solidus =A1+A2P+A3P 2

[0020]

[0021] P = (3.5 / 110) x (depth-40) + 1

[0022] In the formula, V is the percentage of melting content to be solved, which is an unknown number for representing the temperature T; P is the pressure corresponding to different depths of the upper mantle; depth is the depth of the upper mantle, taking the value of the Moho surface depth of the study area; Cw is the water content percentage, which is the water content Cw of the study area; K is the distribution coefficient of water between the solid upper mantle and the melt; is the mass fraction of water in the liquid phase; A1, A2, A3 are constants that determine the solidus temperature T solidus ; B1, B2, B3 are constants that determine the temperature of the two-pyroxene peridotite melt ; β1 is the index of the melting function; is the solidus temperature drop caused by the water content in the melt; K, γ are both determinants;

[0023] Step 2.3: Using the conductivity formulas of olivine, garnet, orthorhombic pyroxene, and clinopyroxene, and the boundary conditions of the upper mantle solid-state mixed system HS, according to the solved solidus temperature T solidus , the lower limit value σ HS- of the boundary condition is obtained, that is, the value of the upper mantle solid-state conductivity σ solid ;

[0024]

[0025]

[0026]

[0027]

[0028]

[0029] In the formula, σ ol is the conductivity of olivine, σ Grt is the conductivity of garnet, σ Opx is the conductivity of orthorhombic pyroxene, and σ Cpx is the conductivity of clinopyroxene; R is the ideal gas constant; is the rth power of the water content Cw; v i is the proportion of the four rocks in the mixed system; σ i is the conductivity of the four rocks; the temperature T adopts the solidus temperature T solidus , k is the Boltzmann constant; σ 0i , σ 0h , σ 0p are all pre-factors; H i ​H h H p are activation enthalpy, wherein H p is the activation enthalpy of proton conduction at very low water concentration; a is a geometric factor; r is a constant; V1, V2 are molar volumes; σ max，min corresponds to the minimum value of electrical conductivity in the four kinds of rocks;

[0030] Step 2.4: when the upper mantle is a solid-melt mixed system, the temperature T is the temperature of the two-pyroxene peridotite melt solved in step 2.2 The partial electrical conductivity σ melt of the upper mantle melt is solved.

[0031]

[0032]

[0033]

[0034] wherein, is the activation energy of volatilization, is the weight percentage of volatiles, which is the proportion of water in the melt part of the upper mantle system in the depth range of the upper mantle; R is the ideal gas constant; a, b, c, d, e are all constants; is the pre-exponential factor.

[0035] Further, the step 3 specifically comprises:

[0036] Step 3.1: the solid-state electrical conductivity σ solid , the melt part electrical conductivity σ melt , and the measured magnetotelluric electrical conductivity data are taken as the upper limit σ HS+ of the boundary condition, and are brought into the boundary formula of the solid-melt mixed system HS, so as to calculate the melt percentage V:

[0037]

[0038] Step 3.2: according to the melt percentage V obtained and the relationship expression of the temperature T about the melt percentage V established in step 2.2, the temperature T corresponding to the melt percentage is directly calculated.

[0039] The beneficial effect of the present application is that the magnetotelluric sounding method can only qualitatively determine the state of deep matter, the shallow result analysis can be combined with other detection means, and the upper mantle scale range cannot be combined with other exploration methods, therefore, aiming at the limitation that the current magnetotelluric method cannot directly obtain the deep thermal structure (temperature, rock melting percentage content) in the aspects of deep geothermal resource exploration and deep dynamics interpretation, the electrical structure model obtained by the rock physics and the magnetotelluric sounding method is combined, the relationship between the upper mantle rock conductivity and the rock melting percentage content and temperature is established, so that the corresponding deep temperature and rock melting percentage content can be directly obtained according to the magnetotelluric sounding data, and the deep thermal structure model of the research area is established. This not only breaks through the limitation of the magnetotelluric sounding in the analysis of the upper mantle deep result, but also provides a basis for the development of geothermal resources in the research area, the research on the material migration of the Qinghai-Tibet Plateau and the like. BRIEF DESCRIPTION OF DRAWINGS

[0040] Figure 1 The method flow chart of the deep matter thermal state quantitative estimation method based on the electrical structure model of the present application. DETAILED DESCRIPTION

[0041] The present application will be further described in detail below in combination with the drawings and specific embodiments.

[0042] On the basis of the existing theoretical research, the measured conductivity data of the magnetotelluric method is taken as the mixed conductivity of the upper mantle solid state-melting state, the relationship between the corresponding temperature and melting percentage content and the upper mantle body conductivity is obtained according to the established model, the corresponding temperature and melting percentage content are obtained, the flow chart is as shown in Figure 1 , and the specific process is as follows:

[0043] 1. Establishing the deep electrical structure model of the detection area by using the magnetotelluric sounding method:

[0044] Taking the magnetotelluric sounding method as the main technical means, the deep three-dimensional electrical structure model of the research area is established, and the basis for establishing the deep thermal structure model is provided;

[0045] 2. Deriving the relationship between rock conductivity and deep thermal structure:

[0046] Establishing the upper mantle thermal structure model, first, the relationship model of the upper mantle rock and mineral conductivity and temperature needs to be selected. Based on the rock conductivity and its temperature relationship obtained by the high temperature and high pressure conductivity experiment of the upper mantle rock and mineral, the total conductivity and temperature relationship of the upper mantle in the solid state mixed system (no local melting occurs) and the solid state-melting state mixed system (local melting occurs) is considered respectively, the quantitative relationship of the melting percentage content, temperature and the upper mantle conductivity is introduced, and thus the reliable quantitative relationship model of the total conductivity of the upper mantle and the temperature and rock melting percentage content is established.

[0047] 3, the relationship between rock conductivity and deep thermal structure and the deep three-dimensional electrical structure model established by magnetotelluric is combined to obtain the deep thermal structure model of the exploration area:

[0048] According to the established quantitative relationship model of the upper mantle total conductivity, temperature and percentage of melting, the percentage of melting and temperature in the upper mantle scale range are calculated by bringing the measured conductivity data of magnetotelluric into the model, and the deep thermal structure model of the exploration area is established, so that the quantitative estimation of the percentage of melting and the corresponding temperature in the upper mantle scale range of the research area is realized.

[0049] The detailed steps are as follows:

[0050] (1) First, according to the actual geological conditions of the research area, the Moho depth and water content Cw of the research area are determined: the Moho depth of different geological structures is different, which needs to be determined by the geological background of the research area; the water content in the deep part of the research area is one of the most important parameters. In order to ensure that the obtained percentage of rock melting and temperature data have high reliability, therefore, the present application can set different values of water content according to the actual situation, and multiple result analysis is used to study the deep thermal structure state of the research area.

[0051] (2) According to the following formula, the relationship expression of T and V is obtained from the set water content value and the upper mantle depth value, and the corresponding solidus temperature T is solved solidus And the temperature of two peridotite melt

[0052]

[0053]

[0054]

[0055] T solidus =A1+A2P+A3P 2

[0056]

[0057] P=(3.5 / 110)×(depth-40)+1

[0058] In the formula, V is the percentage of melting to be solved, which is used as an unknown number to represent the temperature T here; P is the pressure corresponding to different depths of the upper mantle; depth is the depth of the upper mantle, which is taken as the Moho depth of the research area here; Cw is the percentage of water content by weight. Here, it is the water content Cw of the research area; K is the distribution coefficient of water between solid upper mantle and melt; wherein, φ is the mass fraction of water in the liquid phase; A1, A2, A3 are constants determining the solidus temperature T solidus of the peridotite; B1, B2, B3 are constants determining the temperature of the peridotite melt; β1 is the index of the melting function; ΔT is the decrease of the solidus temperature caused by the water content in the melt; K, γ are both determinants. The values of the parameters are as follows:

[0059] Table 1 Values of the melt percentage-temperature formula

[0060]

[0061] (3) Using the four rock conductivity formulas and the HS boundary conditions corresponding to the upper mantle solid-state mixed system, the solved solidus temperature T solidus is brought in to obtain σ HS- , which is the value of the conductivity σ solid of the upper mantle solid-state system.

[0062] The HS boundary conditions corresponding to the upper mantle solid-state mixed system are as follows:

[0063]

[0064]

[0065]

[0066]

[0067]

[0068] In the formula, σ ol is the conductivity of olivine, σ Grt is the conductivity of garnet, σ Opx is the conductivity of orthorhombic pyroxene, and σ Cpx is the conductivity of clinopyroxene; wherein the respective water content Cw is the value after the total water content is proportionally distributed. Under the solid phase condition, the distribution ratio of water in the four minerals is 1 olivine: 16 orthorhombic pyroxene: 30 clinopyroxene: 1 garnet, and R is the ideal gas constant; wherein, the conductivity of garnet in the solid-state mixed system is related to the upper mantle pressure p, and the value of the pressure p is related to the depth of the upper mantle, i.e. P = (3.5 / 110) × (depth-40) + 1. When calculating the lower limit value of the boundary, σ max，min corresponds to the minimum value of the conductivity of the four rocks, and olivine conductivity is selected here; wherein, T is the solidus temperature T solidus , and k is the Boltzmann constant; the values of the parameters are as follows:

[0069] ​​Table 2. Parameter values ​​for the formulas used to calculate the electrical conductivity-temperature of four minerals in the upper mantle.

[0070]

[0071] When calculating the total conductivity, v i These represent the proportions of the four rocks in the mixture: olivine 55%, orthopyroxene 28%, clinopyroxene 11%, and garnet 10%. σ i Let represent the electrical conductivity of the four types of rocks, and find the boundary condition limit σ. HS- When the conductivity σ of olivine is... ol As σ min Substitute the values ​​into the calculation. The calculated value is the critical volumetric conductivity of the upper mantle when melting has not occurred.

[0072] (4) When the upper mantle is a solid-molten mixed system, in (2) Substituting T into the following formula, we can calculate the partial conductivity σ of the upper mantle melt. melt The calculated conductivity of the melt is also the critical value.

[0073]

[0074]

[0075]

[0076] in, The activation energy for volatilization, This represents the weight percentage of volatiles, primarily referring to water within the upper mantle depth range, and is taken as the proportion of water in the molten portion of the upper mantle system. In a solid-molten mixed system, the water distribution becomes melt: olivine: orthopyroxene: clinopyroxene: garnet = 1000: 2: 3: 20: 1. R is the ideal gas constant; the values ​​of other parameters are shown in the table below:

[0077] Table 3. Parameters for the formula of electrical conductivity-temperature of melt

[0078]

[0079] (5) Given the known solid conductivity σ solid σ of the molten part melt And measured magnetotelluric conductivity data as σ HS+ Substituting into the HS boundary formula for the mixed system, the melt percentage V can be calculated:

[0080]

[0081] (6) According to the melting percentage V and the relationship expression of temperature T about the melting percentage V, the temperature T corresponding to the melting percentage can be directly calculated. Thus, the data of the calculated temperature T and the melting percentage V can be used to establish the complete melting percentage model of the upper mantle and the corresponding temperature model.

Claims

1. A method for quantitatively estimating the thermal state of deep materials based on an electrical structure model, characterized in that, Includes the following steps: Step 1: Collect magnetotelluric sounding data. After Fourier transform, spectrum filtering and Robust impedance tensor estimation of the original time series data, obtain continuous sounding curves. Use phase tensor decomposition to obtain qualitative parameters of the study area and establish a three-dimensional electrical structure model of the deep part of the study area. Step 2: Based on the relationship between rock conductivity and temperature obtained from high-temperature and high-pressure conductivity experiments on upper mantle rocks, the relationship between total conductivity and temperature in both solid-state mixed systems and solid-molten mixed systems of the upper mantle is considered. A quantitative relationship between melt percentage, temperature and upper mantle conductivity is introduced to establish a quantitative relationship model between total upper mantle conductivity and temperature and rock melt percentage. Step 3: Based on the established quantitative relationship model between the total electrical conductivity of the upper mantle and temperature and the percentage of melting, input the electrical conductivity data obtained from actual magnetotelluric measurements to calculate the percentage of rock melting and temperature within the upper mantle scale. Establish a deep thermal structure model of the exploration area to achieve a quantitative estimate of the percentage of melting and its corresponding temperature within the upper mantle scale of the study area.

2. The method for quantitative estimation of the thermal state of deep materials based on an electrical structure model according to claim 1, characterized in that, Step 2 specifically includes: Step 2.1: Based on the actual geological conditions of the study area, determine the depth of the Moho surface and the water content C. w And set different values ​​for moisture content according to the actual situation; Step 2.2: Based on the set water content and upper mantle depth values, derive the relationship between temperature T and the percentage of molten metal V, and solve for the corresponding solidus temperature T. solidus and the temperature of the two-pyroxene peridotite melt T solidus =A1+A2P+A3P 2 P = (3.5 / 110) × (depth - 40) + 1 In the formula, V is the percentage of melting that needs to be solved, which is used as an unknown to characterize the temperature T; P is the pressure corresponding to different depths of the upper mantle; depth is the depth of the upper mantle, which is the depth of the Moho surface in the study area. is the water content by weight percentage, and is the water content C of the study area. w ; The partition coefficient of water between the solid upper mantle and the melt; A1 represents the mass fraction of water in the liquid phase; A2 and A3 are the solidus temperatures T. solidus The constants B1, B2, and B3 determine the melting temperature of the two-pyroxene peridotite. The constant; β1 is the exponent of the melting function; The solidus temperature drop is caused by the water content in the melt; K and γ are both... The determining factors; Step 2.3: Using the rock conductivity formula and the HS boundary conditions corresponding to the upper mantle solid-state mixed system, based on the solved solidus temperature T... solidus Find the limit value σ under the boundary conditions. HS- That is, as the solid conductivity σ of the upper mantle solid The value; In the formula, σ ol Let σ be the conductivity of olivine. Grt The electrical conductivity of garnet is σ. Opx σ is the conductivity of orthorhombic pyroxene. Cpx R is the conductivity of monoclinic pyroxene; R is the ideal gas constant. Moisture content C w power of r; v i The percentages of the four rocks in the mixture; σi: electrical conductivity of the four rocks; temperature T: solidus temperature T0. solidus , k is the Boltzmann constant; σ 0i σ 0h σ 0p All are pre-exponential factors; H i H h H p All are activation enthalpies, of which H p The activation enthalpy of proton conduction at very low water concentrations; α is a geometric factor; r is a constant; V1 and V2 are molar volumes; σ is used when calculating the lower boundary limit. max，min The minimum electrical conductivity value corresponding to the four types of rocks; Step 2.4: When the upper mantle is a solid-molten mixed system, the temperature T is the temperature of the olivine melt obtained in Step 2.

2. Calculate the partial conductivity σ of the upper mantle melt. melt : In the formula, The activation energy for volatilization, The value represents the weight percentage of volatiles, and within the upper mantle depth range, it is taken as the proportion of molten water in the upper mantle system; R is the ideal gas constant; a, b, c, d, and e are all constants. It is a pre-exponential factor.

3. The method for quantitative estimation of the thermal state of deep materials based on an electrical structure model according to claim 2, characterized in that, Step 3 specifically includes: Step 3.1: The solid conductivity σ solid σ of the molten part melt And measured magnetotelluric conductivity data are used as the upper limit value of the boundary condition σ. HS+ Substituting the HS boundary formula for a solid-molten mixture, the melt percentage V can be calculated: Step 3.2: Based on the obtained melt percentage V and the relationship expression between temperature T and melt percentage V, directly calculate the temperature T corresponding to the melt percentage.

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