Controlled source radiomagnetotelluric (CSRMT) measurements with a horizontal electrical dipole were carried out for the planned construction of a railway bridge in the Northern part of Siberia (the Yamalo-Nenets region). The aim of this survey was the delineation of horizons of loams, which are mostly thixotropic, and the estimation of the depth of solid sands serving as a basement for bridge piles. The remote area of the investigation is characterized by a few number of broadcasting radio transmitters. Therefore, the possibilities of the conventional radiomagnetotelluric (RMT) method to study the geological structure are quite limited. Because of swamps, the construction area is only accessible in winter or early spring periods. Our CSRMT survey was carried out by using ungrounded (capacitive) electric lines. The measurements were conducted in the broadside configuration using a 580 m long grounded electrical line at a distance of about 640 m from the transmitter. At the first stage of the CSRMT data inversion, we used the 1D controlled source inversion code considering the geometry of the source and the effect of the transition zone. In addition, we selected frequencies associated with the far-field response of the electromagnetic field. To demonstrate the advantages of the controlled source, we compared the responses and inversion results obtained from both CSRMT and RMT data. The geoelectrical section obtained from CSRMT data is more detailed in the shallow part and allows to resolve the deep structures because of an extended frequency range and an increased depth of investigation. The conductivity model derived from the 2D inversion of the CSRMT data showed a good correlation with the borehole data. The CSRMT results were very useful for the construction of the railway bridge because of the reliable delineation of loams and sands horizons in the subsurface.

中文翻译：

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利用可控源大地电磁学研究西伯利亚北部永久冻土区

使用水平电偶极子进行受控源大地电磁 (CSRMT) 测量，用于计划在西伯利亚北部（Yamalo-Nenets 地区）建造一座铁路桥。本次调查的目的是勾画壤土的层位，这些壤土主要是触变性的，并估计用作桥桩基础的实心砂层的深度。调查的偏远地区有几个广播无线电发射机。因此，传统的大地电磁（RMT）方法研究地质结构的可能性非常有限。由于沼泽地，施工区只能在冬季或早春期间进入。我们的 CSRMT 调查是使用未接地（电容）电线进行的。测量是在宽边配置中使用 580 m 长的接地电线在距发射机约 640 m 的距离处进行的。在CSRMT数据反演的第一阶段，考虑到源的几何形状和过渡带的影响，我们使用一维受控源反演代码。此外，我们选择了与电磁场远场响应相关的频率。为了证明受控源的优势，我们比较了从 CSRMT 和 RMT 数据获得的响应和反演结果。从 CSRMT 数据获得的地电剖面在浅层部分更详细，并且由于扩展的频率范围和增加的调查深度允许解析深层结构。从 CSRMT 数据的 2D 反演得出的电导率模型显示出与钻孔数据的良好相关性。CSRMT 结果对铁路桥梁的建设非常有用，因为它可以可靠地描绘地下的壤土和砂层。