Advances in multi-source transmission techniques for Short-Offset Transient Electromagnetic Methods with grounded sources: a review

Abstract Short-offset transient electromagnetic (SOTEM) surveys with grounded sources have recently emerged as a promising method for mineral exploration. However, two fundamental scientific challenges persist in near-source observations: the significant increase in signal bandwidth and the pronounced near-field source effects. While the problem of bandwidth broadening has been widely investigated, a comprehensive understanding of the near-source stratified-wavefield source effect remains lacking. When an anomalous body lies between the transmitter and receiver, the stratified wavefield generated by the source illuminates the anomaly and casts a shadow zone on its far side, producing false anomalies in the recorded data and obscuring the true location of the target. In this review, we systematically summarize the research progress of multi-source transmission techniques for SOTEM, compare the excitation characteristics of different numbers of transmitters, examine recent advances in defining full-time and full-space multi-component apparent resistivity, and analyze multi-line-source observation strategies. These approaches help suppress the transverse electric (TE) component generated by the transmitter and enhance the contribution of the transverse magnetic (TM) component in field measurements, thereby enabling a predominantly TM-mode transient electromagnetic response. We further provide a critical evaluation of the advantages and limitations of existing multi-source techniques, and propose that the ‘shadow-free illumination’ observation mode and cascaded multi-transmitter systems will form key directions for future development, with detailed operational definitions and implementation paths provided.