2D Modeling of Controlled Source Radiomagnetotelluric Data Observed on Buried Faults Close to St. Petersburg/Russia

Abstract

Controlled source radiomagnetotellurics (CSRMT) is a new and innovative method of near surface geophysics. We applied this method for the detection of a buried fault beneath a test area close to St. Petersburg. During this survey, a rectangular current was injected into the ground using an electrical horizontal dipole with a length of 700 m and O.5, 11.5, 30 and 105 kHz – as well as their subharmonics - were used as main frequencies. The time series of two horizontal field components and three magnetic field components were observed at 97 stations with a station interval of 20 m in the near, the transition, and the far field zone of the electrical dipole. Transfer functions were derived in the frequency interval of 1 kHz to 1000 kHz from the observed electric and magnetic time series using the newly developed processing software. They showed a smooth dependence of frequency indicating the reliability of the observed CSRMT data. The CSRMT data were interpreted in the far field zone by using a 2D inversion algorithm and in the near zone by using 2D forward calculations. The derived 2D conductivity models showed clearly the location of buried faults as large resistivity contrasts.