层状各向异性介质大地电磁正反演研究

发布时间：2018-04-08 19:18

本文选题：层状各向异性介质　切入点：阻抗张量　出处：《长安大学》2017年硕士论文

【摘要】：目前常规的大地电磁测深资料的正反演是基于各向同性介质理论进行的,但是随着人们认识不断地加深,地下各向异性介质逐渐引起人们的关注,越来越多的资料显示,地壳和上地幔介质是各向异性的。地下介质的物理性质如弹性、导电性、磁性、密度等都是各向异性的,然而大地电磁阻抗响应函数对不同方向电导率很敏感,通过地表观测大地电磁响应,再进行基于大地电磁各向异性理论反演,就能够得到张量电导率。本文首先回顾了大地电磁各向异性正反演研究现状,推导总结了大地电磁各向异性正演阻抗响应解析表达式,不同于一维各向同性理论,层状各向异性介质阻抗是张量,即Z_(xx)、Z_(xy)、Z_(yx)、Z_(yy),且电导率张量一般具有对称性,所以Z_(xx)=Z_(yy),Z_(xy)≠Z_(yx)。通过对两层,三层,四层模型进行正演模拟,同时为了研究参数对正演结果的影响,对两层模型逐一改变电阻率ρ_1、ρ_2、ρ_3,旋转角度α_L、α_D、α_S得到:(1)辅助阻抗(Z_(xx)、Z_(yy)与主阻抗(Z_(xy)、Z_(yx)的数量级相差较大,因此辅助阻抗对地下介质反应程度比较低;(2)对阻抗进行参数灵敏度分析,阻抗对于某一层的各向异性角度,厚度最为灵敏,其次是电导率;(3) ρ_1、 ρ_2方向的电阻率对于正演结果影响同时存在的,而ρ_3基本不影响;(4)旋转角度α_L,α_D, α_S同时影响正演的结果,因此ρ_(xy),ρ_(yx)对于不同方向电阻率的反应是由电阻率和旋转角度共同决定的。在各向异性正演理论的基础上,通过阻尼最小二乘法进行全参数的反演,即对模型的三个方向电阻率,三个旋转角度,以及厚度进行反演,同时舍弃辅助视电阻率,拟合两个主视电阻率,选取合适的初始模型,得到了理想的反演结果。由于全参数反演模型的输入参数比较多,反演结果受初始模型影响比较大。若初始模型选取不合适,则得不到合理的结果。因此在反演中,加入了可行方向法,一定程度上抑制了反演的多解性。通过对二层,三层,四层理论模型反演计算,反演结果与实际参数基本吻合,为了验证程序的抗噪声能力,将三层模型加入5%的噪声反演,反演结果基本能与实际参数吻合。
[Abstract]:At present, the forward inversion of conventional magnetotelluric sounding data is based on the theory of isotropic medium, but with the deepening of people's understanding, the underground anisotropic media gradually attracts people's attention, and more and more data show that,The crust and upper mantle media are anisotropic.The physical properties of underground media such as elasticity, conductivity, magnetism and density are anisotropic. However, magnetotelluric impedance response function is sensitive to conductivity in different directions.Based on magnetotelluric anisotropy theory inversion, Zhang Liang conductivity can be obtained.In this paper, the present situation of magnetotelluric anisotropy forward and inverse modeling is reviewed, and the analytical expression of magnetotelluric anisotropic forward impedance response is deduced and summarized. Different from one-dimensional isotropic theory, the impedance of layered anisotropic medium is Zhang Liang.Therefore, the secondary impedance has a relatively low degree of response to the underground medium.) the impedance is the most sensitive to the anisotropic angle of a certain layer, and the thickness of the impedance is the most sensitive.Secondly, the resistivity of 蟻 _ 1, 蟻 _ s _ 2 direction has a simultaneous effect on the forward result, while 蟻 _ s _ 3 has no effect on the rotation angle 伪 _ L, 伪 _ S _ D, 伪 _ S simultaneously affects the result of forward modeling, while 蟻 _ s _ 3 has no effect on the rotation angle of 伪 _ L, 伪 _ S _ D, 伪 -S simultaneously.Therefore, the response of 蟻 _ S _ XY _ x to resistivity in different directions is determined by both resistivity and rotation angle.On the basis of anisotropic forward modeling theory, the damping least square method is used to inverse all parameters, that is, inversion of three directions resistivity, three rotation angles and thickness of the model, at the same time giving up the auxiliary apparent resistivity.Fitting the two principal apparent resistivity and selecting the appropriate initial model, the ideal inversion results are obtained.Due to the large number of input parameters of the full parameter inversion model, the inversion results are greatly affected by the initial model.If the initial model is not suitable, there is no reasonable result.Therefore, the feasible direction method is added to the inversion, which to some extent inhibits the multi-solvability of the inversion.In order to verify the anti-noise ability of the program, the inversion results of the two-layer, three-layer and four-layer theoretical models are basically consistent with the actual parameters. In order to verify the anti-noise ability of the program, 5% noise inversion is added to the three-layer model, and the inversion results are basically consistent with the actual parameters.
【学位授予单位】：长安大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：P631.325

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