The conductivity of most formations and fractured water areas is anisotropic, while the anisotropy of formations has great impacts on transient electromagnetic observations, especially on the characteristics of horizontal components. To study the three-component responses of axially anisotropic formations using the transient electromagnetic method, this study introduced a conductivity tensor to construct the governing equation to achieve the three-dimensional three-component forward modeling of the axially anisotropic conductivity using the transient electromagnetic method based on the finite-difference time-domain (FDTD) algorithm. This study verified the accuracy of the three-component forward modeling by comparing the three-dimensional three-component forward modeling results with the one-dimensional analytical results of the isotropic and anisotropic half-space models. Meanwhile, this study established the anisotropic half-space, layered, and water-bearing models and calculated loop-source three-component responses of the transient electromagnetic method. The results are as follows. The horizontal conductivity anisotropy greatly affected the three-component responses. The x-axis anisotropy had greater effects on the ∂By/∂t component response than those on ∂Bx/∂t component response, the y-axis anisotropy had greater effects on the ∂Bx/∂t component response than those on the ∂By/∂t component response, while the z-axis anisotropy had almost no effect on the three-component responses. Moreover, the three-component responses were primarily affected by conductivity anisotropy of shallow formations. The x-axis anisotropy had greater effects on the three-component responses than the y-axis anisotropy when collection points were closer to an anomaly center in the x-direction than in the y-direction, and vice versa. The results of this study provide some valuable theoretical references for the three-component processing and interpretation of the transient electromagnetic method of anisotropic formations.
Kunz K S, Moran J H. Some effects of formation anisotropy on resistivity measurements in boreholes[J]. Geophysics, 1958, 23(4): 770-794.
doi: 10.1190/1.1438527
Zhang H, Gao Y, Shi Y T, et al. Crustal seismic anisotropy beneath the northern and western margins of the Ordos block[J]. Chinese Journal of Geophysics, 2020, 63(6): 2230-2247.
Deng S G, Liu T L, Wang L, et al. Analytical solution of multicomponent induction logging response in biaxial anisotropic medium[J]. Chinese Journal of Geophysics, 2020, 63(1): 362-373.
Liu Y H, Yin C C, Cai J, et al. Review on research of electrical anisotropy in electromagnetic prospecting[J]. Chinese Journal of Geophysics, 2018, 61(8): 3468-3487.
[5]
张韬. 中国主要聚煤期沉积环境与聚煤规律[M]. 北京: 地质出版社, 1992.
[5]
Zhang T. Depositional environment and coal-accumulating regularities of main coal-accumulating stages in China[M]. Beijing: Geological Publishing House, 1992.
Wu Y M, Lan H X, Huang W Q. Discussion on the relationship between elastic anisotropy and mineral distribution of Longmaxi Shale[J]. Chinese Journal of Geophysics, 2020, 63(5): 1856-1866.
[7]
Yin C C, Fraser D C. The effect of the electrical anisotropy on the response of helicopter-borne frequency-domain electromagnetic systems[J]. Geophysical Prospecting, 2004, 52(5): 399-416.
doi: 10.1111/j.1365-2478.2004.00424.x
Sun Y F, Zhao Y X, Wang X, et al. Synchrotron radiation facility-based quantitative evaluation of pore structure heterogeneity and anisotropy in coal[J]. Petroleum Exploration and Development, 2019, 46(6): 1128-1137.
Wang Y T, Liu Z. Study on visual exploration of fissure field of overlying strata in deep coal seam under subcritical extraction[J]. Coal Science and Technology, 2020, 48(3): 197-204.
Shen T, Yuan F, Song S J, et al. Application of P-wave anisotropy detection in detecting the conducting fracture zone[J]. Journal of China Coal Society, 2017, 42(1): 197-202.
Zhou J M, Liu W T, Li X, et al. Research on the 3D mimetic finite volume method for loop-source TEM response in biaxial anisotropic formation[J]. Chinese Journal of Geophysics, 2018, 61(1): 368-378.
Liu Y J, Hu X Y, Peng R H, et al. 3D forward modeling and analysis of the loop-source transient electromagnetic method based on the finiye-colume method for an arbitrarily anisotropic medium[J]. Chinese Journal of Geophysics, 2019, 62(5): 1954-1968.
Cheng J L, Huang S H, Wen L F, et al. Response characteristics of three-dimensional axial anisotropic media for transient electromagnetic method in underground whole-space[J]. Journal of China Coal Society, 2019, 44(1): 278-286.
[15]
赵慧楠. 瞬变电磁三分量接收系统设计与实现[D]. 长春: 吉林大学, 2015.
[15]
Zhao H N. Design and implementation of three-component transient electromagnetic receiving system[D]. Changchun: Jilin University, 2015.
Qi Z P, Zhi Q Q, Li X, et al. The definition of the full-zone apparent resistivity and the constrained inversion of the three components of fixed source TEM[J]. Geophysical & Geochemical Exploration, 2014, 38(4): 742-749.
Zhi Q Q, Wu J J, Wang X C, et al. Three-component interpretation technique of fixed source TEM and its experimental application in metallic ore district[J]. Geophysical & Geochemical Exploration, 2016, 40(4): 798-803.
Wu J J, Li X, Zhi Q Q, et al. Full field apparent resistivity definition of borehole TEM with electric source[J]. Chinese Journal of Geophysics, 2017, 60(4): 1595-1605.
Wang P. Study on floating coefficient space intersection and equivalent current loop inversion of downhole TEM[D]. Wuhan: China University of Geosciences(Wuhan) 2017.
Yang H Y, Li F P, Yue J H, et al. Optimal transient electromagnetic inversion of conical source based on smoke ring theory[J]. Journal of China University of mining & Technology, 2016, 45(6): 1230-1237.
Fan T. Method and application on 2D pseudo-seismic inversion of roadway-borehole transient electromagnetic detection in coal mine[J]. Journal of China Coal Society, 2019, 44(6): 1804-1816.
Sun H, Li X, Li S C, et al. Three-dimensional FDTD modeling of TEM excited by a loop source considering ramp time[J]. Chinese Journal of Geophysics, 2013, 56(3): 1049-1064.
Lu X S. Three dimensional parallel simulation of transient electromagnetic response of tunnel boring machine and the elimination of its response[D]. Xi'an: Chang’an University, 2014.
Li Z H, Huang Q H. Application of complex frequency shifted perfectly matched layer absorbing boundary conditions in transient electromagnetic method modeling[J]. Chinese Journal of Geophysics, 2014, 57(4): 1292-1299.
Sun H F, Cheng M, Wu Q L, et al. A multi-scale grid scheme in three-dimensional transient electromagnetic modeling using FDTD[J]. Chinese Journal of Geophysics, 2018, 61(12): 5096-5104.