2D and 3D resistivity tomography of the Suímo garnet-bearing dyke, Lisbon Volcanic Complex, Portugal: A case study

Abstract

Upper Cretaceous volcanic activity in the West Iberian Margin produced a singular garnet-bearing basaltic dyke occurring on the northern slope of the Suímo hill near Lisbon. 2D and 3D resistivity tomography have been carried out to elucidate the properties of the dyke regarding the horizontal and vertical subsurface extension after historical mining. 2D large-scale resistivity profiles (GA-1 and GA-2) were measured. Ten sub-parallel 2D resistivity sections were measured in small scale to perform a 3D resistivity model. All measurements were carried out using a pole-dipole array. The basaltic dyke, nowadays not cropping out, is detected as a low resistivity layer, extending to a depth of at least 40 m. The width of the dyke ranges from 150 to about 40 m to the south. A high resistivity limestone unit bounds the dyke from east and west sides. Some karstification features can be easily observed in the limestone unit, forming very high resistivity zones. The obtained results allow the prediction that for sampling of in situ basaltic rock the extraction of an ≈20 m thick layer of loose material would be necessary. The central garnet-rich zone of the dyke would be only reached at depths higher than ≈45 m. © 2013 Sinopec Geophysical Research Institute.