Three-dimensional magnetotelluric anisotropy inversion method based on non-structural finite element method

Abstract

The invention discloses a three-dimensional magnetotelluric anisotropy inversion method based on a non-structural finite element method. The three-dimensional magnetotelluric anisotropy inversion method comprises the following steps: (1) obtaining and screening magnetotelluric sounding data with anisotropic characteristics for inversion; (2) constructing a conductivity tensor model by adopting a non-structural finite element method; (3) constructing a magnetotelluric regularization inversion objective function under the anisotropic medium condition; (4) performing forward modeling on the conductivity tensor model to obtain a predicted total impedance tensor corresponding to model response, and calculating a product of transpose and a vector of a sensitivity matrix along with forward modeling; (5) calculating the gradient of the magnetotelluric regularization inversion objective function; (6) calculating a model parameter update quantity by adopting an L-BFGS algorithm, and updating the model parameters according to the model parameter update quantity; (7) iteratively executing the step (4) to the step (6) until an iteration termination condition is reached, obtaining a parameter-optimized conductivity tensor model, and realizing three-dimensional magnetotelluric anisotropy inversion.

Description

technical field [0001] The invention belongs to the technical field of geophysical electromagnetic method inversion, and in particular relates to a three-dimensional magnetotelluric anisotropy inversion method based on an unstructured finite element method. Background technique [0002] The magnetotelluric method can effectively detect the geoelectric structure deep in the earth by using natural low-frequency signals, and has the advantages of low cost, convenient work, no shielding by high-resistivity layers, and a wide range of exploration depths. It has been widely used in deep geological surveys, Exploration of underground resources such as minerals. [0003] The purpose of magnetotelluric inversion is to deduce the real geoelectric structure distribution of the underground space through the electromagnetic response measured on the earth's surface, and to assist geological interpreters to obtain more accurate geological structure judgments. Regarding the magnetotelluric...

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Problems solved by technology

[0004] Whether the traditional magnetotelluric inversion technique is a discrete linear solution or a completely nonlinear solution, most of them are based on the isotropic medium in which the electrical structure of the underground space is uniformly distributed. For example, the three-dimensional magnetotelluric inversion method based on the finite difference method (Egbert and Kelbert, 2012), this is not consistent with the actual deep underground geological space, more and more geophysical exploration shows that the deep underground is anisotropic, and the use of isotropic inversion method to solve the anisotropic The magnetotelluric sounding data of the signal will get wrong results, so it is necessary to develop a magnetotelluric inversion method that can adapt to the anisotropic structure of the subsurface to develop deep detection technology

[0005] However, the existing magnetotelluric anisotropy inversion technology only stays in the inversion of the main axis anisotropic medium, and it cannot be inverted for any anisotropic medium. However, the existing magnetotelluric sounding anisotropy data show that, Anisotropy is directional, so it is necessary to develop an anisotropic 3D magnetotelluric inversion method capable of charged rotation angles to meet the actual needs of geophysical exploration work

[0006] The three-dimensional magnetotelluric anisotropy inversion problem is a complex multi-parameter inversion problem, which faces problems such as low calculation efficiency and large memory consumption, and has not been applied to actual geophysical exploration work for a long time.

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