Analysis of electrojet-distorted magnetotelluric sounding curves

Abstract

South America presents several unique geomagnetic features, one of which is the Equatorial Electrojet (EEJ), a current system which extends itself east to west in Northeastern Brazil for almost 3500 km. Considering the fact that the influence of this phenomenon at low magnetic latitudes can be detected at great distances from its central axis, it is important to simulate its effect in magnetotelluric (MT) exploration. To accomplish this and by the use of an integral equation scheme, we have modeled the MT response of confined three-dimensional (3D) bodies (dykes in an homogeneous half-space) and deep 3D structures (horst and graben located at Marajo Basin in Northern Brazil). As the inductive source, we replace the classical plane wave source by a common line of current, besides gaussian and undulatory electrojets. The results of the modelling indicate that the studied effect is prominent in both one-dimensional (1D) and 3D media for periods ( T) greater than 10 s. It decreases with distance, but it is detectable as far as 3000 km from the center of the EEJ. It is also observed that for T greater than 10 s, the computed soundings can be strongly distorted, mainly by EEJ effects in the host medium which, in turn, cause changes in the final branch of the 3D soundings. For structures in the Marajo Basin, our results reveal that the 3D galvanic effect prevails in the interval 0.1< T<10 s, while source influence occurs mainly for T>10 s. On the other hand, automatic inversion of these 3D data shows that, in equatorial regions having complex geology, some errors arise in conventional 1D interpretation of the MT soundings. This is due to the superposition of the host medium response, the galvanic effect of 3D structures, and the EEJ influence.