Research on three-component responses characteristics of axial anisotropy tunnel-hole transient electromagnetic

Large interpretation errors may exist in exploration areas with obvious conductivity anisotropy when applying tunnel-hole transient electromagnetic method. In this paper, the control equation is built by introducing the conductivity anisotropy tensor, which allows the current density of the rectangular loop source to be added into the Maxwell equation Ampere loop theorem based on the finite difference time domain (FDTD) algorithm in order to realize the loading of any current source. The control equation is discretized by the difference instead of the differential to realize the forward calculation of the tunnel-hole transient electromagnetic 3D model. Forward modeling is carried out for a full-space model, a layered model and a 3D block model based on the accuracy comparison between the analytical and numerical solution. Then the influence and the mode of the conductivity anisotropy on the three-component transient electromagnetic responses of the tunnel-hole is analyzed. The results show that the vertical axis conductivity has little effect on the tunnel-hole transient electromagnetic three-component responses, and the horizontal axis conductivity has a great influence on it. The ∂B_x/∂t response is mainly influenced by the y-axis conductivity, while the ∂B_y/∂t response is mainly influenced by the conductivity of the x-axis. The orientation of the anisotropic medium and the direction of the main axial conductivity can be identified through the relationship between the shape and amplitude of the three-component responses. When the all-space medium where the anomalous body is located is anisotropic in conductivity, the anomalous response generated from the anomalous body will be overwhelmed by the anomalous response generated by the anisotropic conductivity of the all-space medium. Thus we lead the conclusion that the characteristics of conductivity axial anisotropy cannot be ignored in the process of tunnel-hole transient electromagnetic interpretation. This research could provide guidance for the anisotropic interpretation of tunnel-hole transient electromagnetic, reference for anisotropic inversion.