Three-dimensional magnetotelluric survey to image structure and stratigraphy of a sedimentary basin in Hungary

Abstract

A 3D magnetotelluric (MT) survey was carried out on a subbasin of the Pannonian Basin, Hungary. The main objectives were to test and evaluate the accuracy of interpreting 3D MT survey using 2D inversion methods, and to retrieve the deeper structures in order to complement the seismic studies. The 81 magnetotelluric stations covering a large region ( 7 × 7 km) were first analyzed for strike and distortion. Models computed from 2D inversion of the transverse magnetic (TM) data along seven lines yield vertical cross-sections of the survey area. A visual tool based on 3D finite element meshing is used to interpolate between those sections. 3D forward modeling applied on the 3D resistivity model shows a good fit between the computed responses and the observed MT data. However, it is necessary to involve some anisotropy in the basement in order to adjust both TE and TM data. This resistivity anisotropy may result from the fracturing of the pre-Neogene basement and the circulation of fluids within faults. Comparison of the 3D resistivity distribution with other geophysical data (seismic, well logging) indicates that the MT model shows high correlation with the geological structure and stratigraphy. In particular, the Miocene sediments are delineated as a conductive zone that may result from the circulation and accumulation of saline fluids. Our results also demonstrate that 2D inversion of the data associated with 3D visualization and interpolation tools may be a good alternative for the analysis and interpretation of a 3D MT survey to direct 3D inversion.