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Improved controlled source audio‐frequency magnetotelluric method apparent resistivity pseudo‐sections based on the frequency and frequency–spatial gradients of electromagnetic fields
Geophysical Prospecting ( IF 2.6 ) Pub Date : 2020-11-27 , DOI: 10.1111/1365-2478.13059
Ming Zhang 1, 2, 3 , Colin G. Farquharson 3 , Changsheng Liu 1, 2
Affiliation  

Although most electromagnetic data can be inverted to actual resistivity, ways of quickly getting a real‐time interpretation of a data set are still valuable. Such methods are useful when we are testing instrumentation or assessing data quality during a survey, or when we need to get a general understanding of the geological structure during a field survey. Apparent resistivity is a good way to satisfy these desires. However, one of the disadvantages of apparent resistivities is that the traditional apparent resistivity formulations are poor at recognizing boundaries, mainly because abnormal responses get stretched into deeper parts of the image (a shadow effect). In order to improve the recognition ability of boundaries, we propose improved apparent resistivity pseudo‐sections based on the formulae for the frequency and frequency–spatial gradients of the fields in the far‐field region of frequency‐domain controlled‐source audio‐frequency magnetotelluric surveys. The new pseudo‐sections are found to be better than those produced from a traditional method when applied to a number of 3‐D examples. The performance of this apparent resistivity method is closely related to using an appropriate transmitter–receiver distance: when a proper value is used, good results can be obtained in which the horizontal locations of vertical boundaries and the positions of top and bottom boundaries can be identified clearly. Finally, the usefulness of the proposed method for practical applications is evaluated with a field‐data example, for which the results of the proposed apparent resistivity imaging method are compared with traditional apparent resistivities, as well as with the results from a 2‐D inversion of DC resistivity data from the same survey line and with what is known about the geology of the area. This comparison demonstrates the improved capabilities of the new apparent resistivities over traditional approaches, including an improved capability to accurately reveal the bottoms of targets.

中文翻译:

基于电磁场的频率和频率-空间梯度的改进的受控源音频-大地电磁方法视电阻率伪截面

尽管大多数电磁数据都可以转换为实际电阻率,但是快速获得数据集实时解释的方法仍然很有价值。当我们在调查过程中测试仪器或评估数据质量时,或者当我们需要在野外调查过程中对地质结构有一个总体了解时,此类方法非常有用。视电阻率是满足这些需求的好方法。但是,视在电阻率的缺点之一是传统的视在电阻率公式在识别边界方面很差,主要是因为异常响应扩展到了图像的较深部分(阴影效应)。为了提高边界的识别能力,我们基于频域受控源音频大地电磁测量的远场区域中场的频率和频率-空间梯度的公式,提出了改进的视电阻率伪剖面。当将这些伪截面应用于许多3D实例时,发现它们比传统方法产生的截面更好。这种表观电阻率方法的性能与使用适当的发射器-接收器距离密切相关:当使用适当的值时,可以获得良好的结果,其中可以确定垂直边界的水平位置以及顶部和底部边界的位置清楚地。最后,通过现场数据示例评估了所提出方法对实际应用的有效性,将拟议的视电阻率成像方法的结果与传统的视电阻率进行比较,并与来自同一测量线的直流电阻率数据的二维反演以及该地区的地质情况进行了比较。这种比较证明了新的视电阻率相对于传统方法的改进能力,包括改进了准确显示目标底部的能力。
更新日期:2021-01-18
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