Multidimensional interpretation of radiomagnetotellurics and controlled-source radiomagnetotellurics data: A validation study

Radiomagnetotellurics (RMT) is an electromagnetic method that uses signals from radio transmitters broadcasting in the 10 kHz to 1 MHz frequency range. Due to its limited frequency range, RMT is commonly used as a shallow-depth investigation tool. However, in remote areas, there is a lack of radio transmitters and only signals from very low frequency (VLF) antennas (10-30 kHz frequency range) can be measured. This can give rise to low signal-to-noise ratio. To overcome this disadvantage of RMT, a controlled-source RMT (CSRMT) can be applied to measure signals of the low-frequency (LF) and mid-frequency ranges (30-1000 kHz). Moreover, the wider frequency range of the CSRMT method (down to 1 kHz) leads to a deeper sounding depth. We present the first RMT and CSRMT validation studies using two perpendicularly located horizontal electric dipoles to realize a 3D inversion of CSRMT data. The survey area in Alexandrova village in Kaluga region, Russia, a previously investigated area, was selected for a validation study. We acquired the data along 8 profiles with 175 stations. Transmitter lines for the CSRMT case were about 900 m long, and the minimum and maximum distances of the stations from transmitters were 450-1000 m, respectively. We applied 2D and 3D inversions over the far-field data and compared with the previous results. The available geophysical information as well as the borehole data indicate a high agreement between the obtained models and the geological structure. We can confirm that the CSRMT method is a reliable approach for near-surface explorations and that, the existing advanced and tested inversion tools for magnetotellurics, can be used to invert the RMT and far-field zone CSRMT data leading to comparable results.