It is difficult to detect the complicated 3D structures by only using a single grounded source, as it could only generate single direction current vector and will have distortion sometimes. Therefore, the authors use the electric dipole sources transient electromagnetic tensor method, which is suitable for detecting 3D structure. In combination with numerical models, the authors present the data with the tensor apparent resistivity It is found that in uniform half-space and layered earth, the coordinate invariant P₂ has a good representation for the change of underground media. In 3D model, the coordinate invariant P₂ and ellipse are formed by the coordinate invariant ρ_min and ρ_max, which are calculated for 3D resistivity models. The authors hold that profiles of LOTEM measurements across a 3D cube structure can be used to accurately reflect the changes of the 3D structure and sketch the anomaly boundary. All of these results provide theoretical guidance for further TEM data interpretation.