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Role of fine-scale layering and grain alignment in the electrical anisotropy of marine sediments

机译：精细分层和晶粒排列在海洋沉积物电各向异性中的作用

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摘要

Electrical resistivities of seafloor sediments determined by controlled source electromagnetic (CSEM) surveys have been found to be significantly greater than those measured by electrical well logging, in some instances by a ratio of as much as 5:1. Because borehole logging techniques invariably measure electrical resistivity using currents circulating in horizontal planes and CSEM surveys are sensitive to the currents circulating in vertical planes, a possible cause of this discrepancy is strong electrical anisotropy of the sediments. We have examined electrical log data from vertical exploration and appraisal wells and deviated production wells in the North Sea. By correlating the same sedimentary units between wells, we are able to compare resistivities measured at different borehole inclination angles. As the inclination angle changes, the amount of vertical and horizontal resistivity contributing to the resistivity changes. Hence we can estimate the electrical anisotropy in the sediments. Results indicate that anisotropy ratios of 1:5 and greater are present within the shale-dominated units. We show that fine-scale horizontal layering can make only a relatively small contribution to this anisotropy, but that a model of horizontal alignment of highly oblate spheroidal grains can account for most or all of it.
机译：已经发现，通过受控源电磁（CSEM）调查确定的海底沉积物的电阻率明显大于通过电测井测得的电阻率，在某些情况下其电阻率高达5：1。由于钻孔测井技术始终使用在水平平面中循环的电流来测量电阻率，并且CSEM测量对在垂直平面中循环的电流敏感，因此，这种差异的可能原因是沉积物的强电各向异性。我们检查了北海垂直勘探和评估井以及偏斜生产井的电测井数据。通过关联井之间的相同沉积单位，我们能够比较在不同井眼倾角下测得的电阻率。随着倾斜角的变化，有助于电阻率的垂直和水平电阻率的量也变化。因此，我们可以估计沉积物中的电各向异性。结果表明，在以页岩为主的单元中存在1：5和更大的各向异性比率。我们表明，精细的水平分层只能对这种各向异性做出相对较小的贡献，但是高度扁球形球体的水平排列模型可以解决其中的大部分或全部问题。

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