A STUDY OF MAGNETOTELLURIC STATIC DISTORTION IN THE CONTEXT OF INTRUSIVE VOLCANISM1

Abstract

AbstractMT data from a detailed 84‐site grid array (28 × 12 km) in the Paraná Basin (Brazil) reveal a high degree of frequency‐independent and parallel amplitude distortion. The Palaeozoic sediments across the survey area are covered by 1.2 km of flood basalts. A deep well‐log provides some control regarding the emplacement of thin diabase sills but information regarding vertical feeder dikes is non‐existent. The degree of parallel behaviour is identified using anisotropy ratios which quantify the extent and bandwidth of the distortion characteristics.A 2D modelling study is carried out using the concept of a horizontally layered (1D) basin with superimposed, small‐scale inhomogeneities. Two likely geological distortion structures are considered. The first represents at‐ or near‐surface (0 to 100 m) inhomogeneities. The second represents thin but vertically elongate dikes. The two distortion effects are found to produce different and characteristic behaviour in the H‐polarization (TM) mode model data. The modelled E‐polarization (TE) mode data are undistorted and provide control. The model developed to represent vertical dikes gives rise to a characteristic frequency dependence which is distinct from that of the undistorted response. This model also generates subparallel distortion effects which give rise to a form of variance in the response estimates as a function of location. The behaviour occurs, importantly, in both the amplitude and phase of the distorted data. A comparison of the normalized amplitude data, observed and modelled, indicates a high degree of correspondence down to a low frequency limit (< 0.1 Hz) where additional and deep contributions to the observed response become apparent. The modelling results and data characteristics indicate that the survey area is underlain by a series of closely parallel, thin intrusive dikes (a dike swarm).