Abstract—Current turn-off processes in ungrounded antennas used in shallow depth transient electromagnetic (TEM) investigation have been studied theoretically and experimentally. Single-turn antennas in the form of a square loop laid on the Earth’s surface are analyzed as the systems with distributed electrical parameters. The theoretical calculations are based on the results following from two-wire transmission line (TWL) theory. Transient processes in loops with sizes ranging from 6 m × 6 m to 50 m × 50 m and TWLs with sizes 50 m × 1 m and 100 m × 1 m have been studied in the field experiments in the nanosecond and microsecond time range. It is shown that current turn-off ramps in TEM antennas develop in the same way as in the TWLs short-circuited at the output without losses and are described by time-descending step functions. The duration of steps is determined by the electromagnetic wave transit time along the antenna perimeter, and the amplitude of the steps decreases exponentially with the denominator depending on the resistance ratio of the damping input resistor and the input impedance of the antenna Z₀. Based on the voltage and current turn-off waveforms, per-unit-length inductances and capacitances are calculated for copper wire antennas with wire cross-section ​​0.35–1.0 mm². The input impedance Z₀ reaches ~1000 Ω if the antennas’ and TWLs’ wires do not contact vegetation and soil and decreases to Z₀ = 400–500 Ω for the antennas placed on the ground. The duration of the current turn-off ramp in the A × A (m × m) square antennas is at most T_off (ns) ≤ 150×A, which determines the depth of the dead zone of soundings \(H({\text{m}}) \leqslant 0.25\sqrt {\rho {\text{A}}} \) where ρ (Ω m) is the effective resistivity of the near-surface layer with thickness H. Within this depth interval, layer-by-layer interpretation of transient responses is impossible; however, a robust estimate can be obtained for longitudinal conductance S = H/ρ. The transient responses measured by the TEM system in small antennas after final current turn-off t > T_off demonstrate inconsistency between the theory and the experiment. In antennas smaller than 25 m × 25 m, the induction effects with the intensity and duration depending on the wire thickness are observed. We present physical interpretation of these effects which are related to the relaxation of the induced currents within the open wire circuit. It shows that at the time the transmitter’s turn-off ramp, the decaying magnetic field induces a volume closed current vortex in a wire body, similarly to what takes place in local conductors (conducting medium). The relaxation time of these currents depends on the conductivity and cross-section area s (mm²) and is τ_Cu ≈ (1.4–1.6)×s (µs) for copper wire. The induction effects in antenna wires can be reduced by using a special stranded wire (litz wire). In the case of the combined 12 m × 12 m to 25 m × 25 m receiver–transmitter antennas made of 127-core litz wire, the depth of the dead zone does not exceed \(H({\text{m}}) \leqslant 1.5\sqrt {\rho {\text{A}}} \).

摘要— 已从理论上和实验上研究了用于浅深度瞬态电磁 (TEM) 调查的未接地天线的电流关闭过程。放置在地球表面的方形环形式的单匝天线被分析为具有分布式电参数的系统。理论计算基于两线传输线 (TWL) 理论的结果。在纳秒和微秒时间范围内的现场实验中，已经研究了尺寸从 6 m × 6 m 到 50 m × 50 m 的环路和尺寸为 50 m × 1 m 和 100 m × 1 m 的 TWL 中的瞬态过程。结果表明，TEM 天线中的电流关断斜坡的发展方式与输出端短路的 TWL 相同，没有损耗，并由时间下降阶跃函数描述。Z ₀。根据电压和电流关断波形，计算出线截面为0.35-1.0 mm ^{2 的}铜线天线的单位长度电感和电容。输入阻抗ž ₀达到〜1000Ω如果天线和TWLs'电线不接触植被和土壤和减小到ž ₀ = 400-500Ω用于放置在地面上的天线。的电流关断斜坡在A×A的持续时间（M×M）的正方形天线是至多Ť_关闭（NS）≤150×A，其确定的深度死区的探空\（H（{\文本{m}}) \leqslant 0.25\sqrt {\rho {\text{A}}} \)其中 ρ (Ω m) 是厚度为H的近地表层的有效电阻率。在这个深度区间内，对瞬态响应的逐层解释是不可能的；然而，对于纵向电导S = H /ρ ，可以获得稳健的估计。最终电流关断后 TEM 系统在小天线中测量的瞬态响应t > T _off证明理论和实验之间的不一致。在小于 25 m × 25 m 的天线中，观察到强度和持续时间取决于导线厚度的感应效应。我们提出了这些效应的物理解释，这些效应与开路电路中感应电流的弛豫有关。它表明，在发射器关闭斜坡时，衰减的磁场会在导线体中感应出体积闭合的电流涡流，类似于局部导体（导电介质）中发生的情况。这些电流的弛​​豫时间取决于电导率和横截面积s (mm ² )，并且为 τ _Cu  ≈ (1.4–1.6)× s(µs) 用于铜线。使用特殊的绞合线（利兹线）可以降低天线导线中的感应效应。在由 127 芯绞合线制成的 12 m × 12 m 至 25 m × 25 m 接收-发射组合天线的情况下，死区深度不超过\(H({\text{m}}) \leqslant 1.5\sqrt {\rho {\text{A}}} \)。