Objective To address the bottlenecks of low computational efficiency and impracticality of multi-dimensional inversion in the advance geological prediction based on the transient electromagnetic method (TEM), this study developed a pseudo-two-dimensional (2D) magnetotelluric (MT) inversion technique for small-frame TEM based on time-frequency transformation.

Methods First, this study corroborated the significant similarity between the late-time apparent resistivity derived using small-frame TEM and the Cagniard apparent resistivity in the MT method under the parameters commonly used in advance geological prediction. Then, the optimal time-frequency transformation coefficient (C_TF) was introduced to achieve a rapid conversion from the late-time apparent resistivity to the Cagniard apparent resistivity. The maximum average conversion deviation was determined at 3.7%, suggesting generally small deviations. Last, a mature MT inversion method was employed to process the converted data.

Results In the pseudo-1D MT inversion results, the maximum relative resistivity and depth errors were determined at 6.14% and 5.73%, respectively, suggesting generally small inversion errors. In the pseudo-2D MT inversion results, the inversion results derived from survey lines above the water-bearing structure reflected the boundary locations and actual depth of the water-bearing structure, and the inverted resistivity approximated to the actual resistivity of the water-bearing structure. In contrast, the inversion results obtained using survey lines outside the water-bearing structure can reflect the presence of the low-resistivity water-bearing structure. However, certain deviations were observed between the specific inversion parameters and their actual values.

Conclusions This method takes less than 20 minutes to calculate on a common PC, which improves the calculation efficiency and prediction accuracy, and provides a new practical 2D inversion approach for TEM advanced geological prediction.