The time-domain controlled source electromagnetic method is a geophysical prospecting tool applied to image the subsurface resistivity distribution on land and in the marine environment. In its most general set-up, a square-wave current is fed into a grounded horizontal electric dipole, and several electric and magnetic field receivers at defined offsets to the imposed current measure the electromagnetic response of the Earth. In the marine environment, the application often uses only inline electric field receivers that, for a 50% duty-cycle current waveform, include both step-on and step-off signals. Here, forward and inverse 1D modelling is used to demonstrate limited sensitivity towards shallow resistive layers in the step-off electric field when transmitter and receivers are surrounded by conductive seawater. This observation is explained by a masking effect of the direct current signal that flows through the seawater and primarily affects step-off data. During a step-off measurement, this direct current is orders of magnitude larger than the inductive response at early and intermediate times, limiting the step-off sensitivity towards shallow resistive layers in the seafloor. Step-on data measure the resistive layer at times preceding the arrival of the direct current signal leading to higher sensitivity compared to step-off data. Such dichotomous behaviour between step-on and step-off data is less obvious in onshore experiments due to the lack of a strong overlying conductive zone and corresponding masking effect from direct current flow. Supported by synthetic 1D inversion studies, we conclude that time-domain controlled source electromagnetic measurements on land should apply both step-on and step-off data in a combined inversion approach to maximize signal-to-noise ratios and utilize the sensitivity characteristics of each signal. In an isotropic marine environment, step-off electric fields have inferior sensitivity towards shallow resistive layers compared to step-on data, resulting in an increase of non-uniqueness when interpreting step-off data in a single or combined inversion.

中文翻译：

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使用接地电偶极子的时域控制源电磁方法中的步进与步进信号

时域控制源电磁法是一种地球物理勘探工具，用于对陆地和海洋环境中的地下电阻率分布进行成像。在最一般的设置中，方波电流被馈入接地的水平电偶极子，几个电场和磁场接收器与施加的电流有一定的偏移，测量地球的电磁响应。在海洋环境中，该应用通常仅使用在线电场接收器，对于 50% 占空比电流波形，包括步进和步进信号。在这里，当发射器和接收器被导电海水包围时，正向和反向 1D 建模用于证明对步进电场中浅电阻层的有限敏感性。这一观察结果可以通过流经海水并主要影响阶跃数据的直流信号的掩蔽效应来解释。在阶跃测量期间，这种直流电流比早期和中期的电感响应大几个数量级，限制了对海底浅电阻层的阶跃敏感性。与步进数据相比，步进数据在直流信号到达之前测量电阻层，从而导致更高的灵敏度。由于缺乏强大的覆盖导电区和直流电流的相应掩蔽效应，在陆上实验中，踩踏数据和踩踏数据之间的这种二分行为不太明显。由合成一维反演研究支持，我们得出的结论是，陆地上的时域受控源电磁测量应该在组合反演方法中应用步进和步进数据，以最大化信噪比并利用每个信号的灵敏度特性。在各向同性海洋环境中，与步进数据相比，步进电场对浅电阻层的敏感性较差，导致在解释单个或组合反演中的步进数据时的非唯一性增加。