Three-dimensional magnetotelluric modelling of the Serra da Cangalha impact crater, northeastern Brazil

Abstract

Three-dimensional (3-D) forward modeling results of a magnetotelluric (MT) survey carried out around the Serra da Cangalha impact crater located in northeastern region of Parnaiba basin Brazil are presented. The 13 km impact structure has been investigated because of its explicit 3-D shape and scientific interest. We recorded MT data in the period range between 0.001s and 1095s at 25 sites. The sites are aligned across the crater structure along three radial profiles (NE ? SW, NW ?SE and ENE ? WSW). The MT results at some sites are strongly affected by 3-D effects. A 3-D finite-difference forward modeling algorithm was applied in order to explain the observed data. The central part of the model covers an area of approximately 169 km2. The model of the study area, constructed using a 3-D mesh of 22 ´ 19 ´ 25 (29325 cells) was based on the a priori information of the geology of the region, and the results of the two-dimensional (2-D) inversion results. Eleven frequencies were used for the 3-D modelling from 0.01 to 1000 Hz. Induction vectors that show the conductivity distribution around the crater region were calculated for the 25 MT sites. The real induction arrows at short periods point towards the center of the crater and indicate the existence of a 3-D circular structure. The resistivity structure obtained suggests perturbations of the upper crust due to the meteorite impact; the base of the uplifted structure extending to 3.0 km depth was well delineated. The observed 3-D effects are explained as a shallow resistive ring that represents a local 3-D body embedded in a regional layered earth. The results confirm the importance of 3-D forward modeling in impact crater studies. The study provided insight into the post-impact electrical signature of the upper crust.