Study of a Permafrost Area in the Northern Part of Siberia Using Controlled Source Radiomagnetotellurics

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.