An important concern for time-lapse studies using the controlled-source electromagnetic (CSEM) method is repeatability of acquisition parameters for the base and monitoring surveys. We consider a challenging case when variations in source and receiver positions, conductivity of seawater, etc. will lead to differences in the recorded EM fields that exceed EM response due to production-induced changes in the reservoir resistivity. We show that even in that case, 4-D response can often be clearly resolved if the base and monitor EM data sets are inverted to produce 3-D resistivity distributions. More precisely, for a synthetic model of the Snøhvit gas field, changes in the inverted resistivity maps caused by such non-repeatability will be at least 10 times smaller than the time-lapse differences in the reservoir resistivity. By contrast, measurement errors and poor knowledge of background resistivity may reduce the resolution of 4-D CSEM to a much stronger degree. Analysis of field CSEM data from the Wisting oil field supports our conclusion about strongly relaxed repeatability requirements when 4-D effects are established by examining inverted resistivity volumes rather than by looking at raw EM data.

中文翻译：

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延时CSEM：调查重复性有多重要？

使用受控源电磁（CSEM）方法进行时移研究的一个重要问题是基础和监视调查的采集参数的可重复性。当源和接收器位置的变化，海水的电导率等会由于储层电阻率的生产引起的变化而导致记录的电磁场中的差异超过电磁响应时，我们认为这是一个具有挑战性的情况。我们表明，即使在这种情况下，如果将基础和监测器EM数据集反转以产生3-D电阻率分布，则通常也可以清楚地解决4-D响应。更准确地说，对于Snøhvit气田的综合模型，由这种不可重复性引起的反演电阻率图的变化至少比储层电阻率随时间的差异小10倍。相比之下，测量误差和对背景电阻率的了解不足可能会使4-D CSEM的分辨率降低到更大的程度。来自Wisting油田的现场CSEM数据的分析支持我们关于通过检查反演电阻率而不是通过查看原始EM数据建立4-D效应时强烈放宽重复性要求的结论。