ABSTRACT In near‐surface transient electromagnetic studies, it is desirable to measure the transient response starting from the earliest possible time. This requires the current in the transmitter loop to be switched off quickly, which necessitates working with a low transmitter current. As for deep‐target transient electromagnetic studies, the transmitter current is as high as possible. The transmitter current's turn‐off waveform and total duration affect the transient voltage response, especially at early times, which is to be accounted for when interpreting transient electromagnetic data. This article discusses the difference in switching off low and high current in a horizontal loop used as the source of the primary magnetic field in the transient electromagnetic method. Low and high currents are turned off in fundamentally different ways. When the current to be switched off is low, the loop can be represented as a symmetric combination of two transmission lines grounded at the middle of the loop perimeter. Such a representation of a loop allows calculating the current turn‐off waveform at any point of the loop. The waveform and total duration of switching off a low current does not depend on its magnitude, but is determined by the period of natural oscillations of the current in the loop and the resistance of a shunting resistor. Switching off a low current in a loop can be represented as the sum of stepped current waves travelling along the loop wire. As a consequence, the current at different points of the loop perimeter is turned off at different times. In contrast to a low current, a high current is switched off linearly in time and synchronously at all points of the loop perimeter. The wave phenomena appear only at the very beginning of the current shutdown for a time interval that is much less than the total current turn‐off duration. Presentation of the loop using a simple lumped‐circuit model predicts the waveform and duration of the high current turn‐off that coincide with the measured ones. There are two reasons why the article may be of interest to those engaged in the theory and/or practice of electromagnetic geophysical methods. First, it contributes to a general understanding of how the current in the transmitter loop is turned off. Second, the article shows how the parameters of a transmitter loop determine the current turn‐off duration and thus the minimum depth of the transient electromagnetic sounding method.

中文翻译：

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关闭瞬态电磁法中使用的变送器回路中的低电流和高电流

摘要 在近地表瞬态电磁研究中，希望从尽可能早的时间开始测量瞬态响应。这需要快速关闭发射器回路中的电流，这需要使用低发射器电流。对于深目标瞬态电磁研究，发射机电流尽可能高。发射机电流的关断波形和总持续时间会影响瞬态电压响应，尤其是在早期，这在解释瞬态电磁数据时要考虑在内。本文讨论了在瞬态电磁法中用作初级磁场源的水平回路中关闭低电流和高电流的区别。低电流和高电流的关闭方式完全不同。当要关断的电流很低时，环路可以表示为在环路周长中间接地的两条传输线的对称组合。这种环路表示允许计算环路任意点的电流关断波形。关断小电流的波形和总持续时间不取决于其大小，而是由回路中电流的自然振荡周期和分流电阻器的电阻决定。关闭回路中的低电流可以表示为沿回路导线行进的阶梯电流波的总和。因此，环路周长不同点处的电流在不同时间关闭。与低电流相比，大电流会在环路周边的所有点上及时线性关闭。波动现象仅出现在电流关断的最开始，其时间间隔远小于总电流关断持续时间。使用简单的集总电路模型表示的回路可以预测与测量值一致的高电流关断的波形和持续时间。从事电磁地球物理方法理论和/或实践的人可能会对本文感兴趣的原因有两个。首先，它有助于大致了解发射机回路中的电流是如何关闭的。其次，本文展示了发射机回路的参数如何确定电流关闭持续时间，从而确定瞬态电磁测深方法的最小深度。