Three-Dimensional Forward Modeling for SOTEM from Frequency-Domain Model Reduction

The short-offset transient electromagnetic method (SOTEM) has become a popular research topic in recent years. In this paper, we present the vector finite element method based on model reduction, which greatly improves computational efficiency. Based on the Krylov subspace projection method, the electromagnetic response of multi-frequency is quickly obtained using model reduction, and the acceleration ratio reaches 33. The electromagnetic field in the time domain is obtained using cosine transformation. A translation algorithm is used to calculate the full-time apparent resistivity. The correctness of the algorithm is verified using uniform half-space model and a complex geoelectric model. Then the amplitude of the transient electromagnetic apparent resistivity anomaly of the 1D, 2D, and 3D geoelectric models is compared. Results show that the electromagnetic response amplitude from the 1D model is much greater than the ones from the 2D and 3D models. Special attention needs to be paid to the processing and interpreting field data using the 1D forward and inversion method. Additionally, we analyze the impulse responses at the different offsets for the conductive body, and the results indicate that with the decrease in the offset, the impulse response and anomalous amplitude are greatly enhanced, and exploration depth and resolution are also improved. The porphyry Cu-Mo deposit model experiment further demonstrates that three-dimensional SOTEM forward modeling combined with full-time apparent resistivity imaging can effectively identify low-resistivity mineralized anomalies within a complex geological model at depths of approximately 200–500 m, thereby confirming the applicability of the method to realistic mineral deposit exploration.