SUMMARY Time-lapse resistivity tomography bring valuable information on the physical changes occurring inside a geological reservoir. In this study, resistivity monitoring from controlled source electromagnetics (CSEM) data is investigated through synthetic and real data. We present three different schemes currently used to perform time-lapse inversions and compare these three methods: parallel, sequential and double difference. We demonstrate on synthetic tests that double difference scheme is the best way to perform time-lapse inversion when the survey parameters are fixed between the different time-lapse acquisitions. We show that double difference inversion allows to remove the imprint of correlated noise distortions, static shifts and most of the non-linearity of the inversion process including numerical noise and acquisition footprint. It also appears that this approach is robust against the baseline resistivity model quality, and even a rough starting resistivity model built from borehole logs or basic geological knowledge can be sufficient to map the time-lapse changes at their right positions. We perform these comparisons with real land time-lapse CSEM data acquired one year apart over the Reykjanes geothermal field.

中文翻译：

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延时电阻率成像：CSEM 数据 3-D 双差反演及其在 Reykjanes 地热场中的应用

总结 延时电阻率断层扫描带来了关于地质储层内部发生的物理变化的有价值的信息。在这项研究中，通过合成数据和真实数据研究了受控源电磁 (CSEM) 数据的电阻率监测。我们提出了目前用于执行延时反演的三种不同方案，并比较了这三种方法：并行、顺序和双差。我们在综合测试中证明，当不同延时采集之间的测量参数固定时，双差分方案是执行延时反演的最佳方法。我们表明，双差分反演可以消除相关噪声失真、静态偏移和反演过程的大部分非线性（包括数值噪声和采集足迹）的印记。看起来这种方法对基线电阻率模型质量是稳健的，即使是根据钻孔测井或基本地质知识构建的粗略起始电阻率模型也足以在其正确位置绘制延时变化。我们将这些比较与在 Reykjanes 地热场上相隔一年获得的真实陆地延时 CSEM 数据进行比较。