Improving the reliability of airborne and ground-based electromagnetic surveys by 3-D inversion (Its application to landslide investigations)

为了预防滑坡等斜坡自然灾害，通过区域地质和岩土工程调查确定可能造成破坏的区域是很重要的。电阻率是一种可用于研究地质条件的地球物理性质。航空电磁法是最适合于有效地收集与大面积大地电阻率相关的数据的方法。但它的一个问题是，没有一种实用的解释方法，可以产生二维或三维图像前的数据。在这项研究中，我们开发了一种三维反演方法，可以考虑地形检查的精度使用合成数据，并将其应用到一个真实的数据集收集的滑坡区口池县。我们证实了航空电磁资料反演的结果与地面电阻率测量的结果是相当的。第一种类型在发射机和接收机环路之间使用非常小的间隔，例如1 m，而第二种类型使用长得多的间隔，比如50米或100米。我们比较了这两种类型的反演的适用性。它的结论是第一种类型，它是适合定性定位异常，但不能用于图像的电阻率结构的地球，因为数据，特别是同相数据，是非常容易受到环境噪声。对于第二种类型，我们使用合成和真实的数据实例表明，反演可以成功地应用于数据恢复二维和三维电阻率结构。

In order to prevent natural disaster on slopes such as landslide, it is important to locate the area that can possibly cause damages by carrying out regional geological and geotechnical surveys. The electrical resistivity is a gephysical property that can be used to investigate geological conditions. The airborne electromagnetic method is the most suited for efficiently collecting the data associated with the electrical resistivity of the earth in a large area., but it has a problem that there is no practical interpretation method that can produce 2-D or 3-D images front the data. In this study, we developed a 3-D inversion method that can take into account topography checked the accuracy using synthetic data, and applied it to a real data set collected over a landslide area in Kouchi Prefecture. We confirmed that the results from inversion of airborne electromagnetic data are comparable to those from a ground-based electrical resistivity survey.Ground-based electromagnetic methods with horizontal loops as the transmitter and receivers can be grouped into two types ; the first type uses a very small separation between the transmitter and receiver loops, say 1 m, and the second type uses much longer separation, say 50m or 100m. We compared the applicability of these two types to inversion. It is concluded for the first type that it is suitable for locating anomalies qualitatively, but cannot be used to image resistivity structures of the earth because the data, particularly in-phase data, are very susceptible to ambient noise. For the second type, we showed using synthetic and real data examples that the inversion can be successfully applied to the data to recover 2-D and 3-D resistivity structures.