Three-component responses of axially anisotropic formations using the transient electromagnetic method

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.