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<p>Short-offset Transient Electromagnetics (SOTEM) is a recently developed time-domain, near-source controlled-source electromagnetic (CSEM) method. Although its response mechanisms have attracted substantial interest, they have also prompted skepticism regarding the feasibility of near-source depth sounding. To address these concerns, this paper reviews the historical evolution of EM exploration methods and establishes the theoretical basis of SOTEM by systematically comparing time- and frequency-domain approaches in terms of transfer-function representation, acquisition strategy, spatial-region definitions, the physical meaning of offset, and treatment of primary (source) currents. Our analysis shows that conventional frequency-domain EM sounding is inherently tied to the far-zone plane-wave approximation; in the near zone, the strong primary field can dominate the measured response and obscure the secondary field, thereby hindering effective depth sounding. In contrast, SOTEM leverages the off-time window following the shut-off of a bipolar pulse to achieve a natural temporal separation of primary and secondary fields, enabling observation of diffusion-dominated responses controlled by induced eddy currents. These characteristics make time-domain transient EM methods not only theoretically sound under near-source conditions but also practically advantageous, facilitating deep exploration in complex terrains or where transmitter–receiver offsets are constrained. Overall, SOTEM provides a new technical route and a robust theoretical framework for deep mineral exploration.</p>

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Reflections on the Near Source Time Domain Electromagnetic Field Response

  • 期刊论文

  • 发表日期：

    2026年6月17日

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  • Guo-qiang Xue,
  • Xin Wu,
  • Nan-nan Zhou,
  • Wei-ying Chen,
  • Xiu Li,
  • Jian Wang,
  • Yan-bo Wang,
  • Xiao-yin Ma,
  • Jian-xin Liu

摘要

Short-offset Transient Electromagnetics (SOTEM) is a recently developed time-domain, near-source controlled-source electromagnetic (CSEM) method. Although its response mechanisms have attracted substantial interest, they have also prompted skepticism regarding the feasibility of near-source depth sounding. To address these concerns, this paper reviews the historical evolution of EM exploration methods and establishes the theoretical basis of SOTEM by systematically comparing time- and frequency-domain approaches in terms of transfer-function representation, acquisition strategy, spatial-region definitions, the physical meaning of offset, and treatment of primary (source) currents. Our analysis shows that conventional frequency-domain EM sounding is inherently tied to the far-zone plane-wave approximation; in the near zone, the strong primary field can dominate the measured response and obscure the secondary field, thereby hindering effective depth sounding. In contrast, SOTEM leverages the off-time window following the shut-off of a bipolar pulse to achieve a natural temporal separation of primary and secondary fields, enabling observation of diffusion-dominated responses controlled by induced eddy currents. These characteristics make time-domain transient EM methods not only theoretically sound under near-source conditions but also practically advantageous, facilitating deep exploration in complex terrains or where transmitter–receiver offsets are constrained. Overall, SOTEM provides a new technical route and a robust theoretical framework for deep mineral exploration.