ERT imaging of the interior of the huge tumulus of Kastas in Amphipolis (northern Greece)

Abstract

AbstractThe geophysical investigations aiming to image the interior of the huge tumulus of Kastas in Amphipolis in northern Greece commenced on November of 2014 and continued sporadically until the first months of 2017. The electrical resistivity tomography (ERT) method was exclusively employed in both conventional and innovative schemes. Due to the immense size of the tumulus, an irregular grid was established on the tumulus whereas regular grids of ERTs were established on a small terrace created by the excavations and at the south extremity. Also, typical two‐dimensional (2D) ERTs carried out to help clarifying the geological and tectonic setting. Kastas tumulus was created by covering a hill with large amounts of transported earth material. In this respect it constitutes a huge human construction. Further, the excavations revealed a unique building containing ancient art masterpieces. The irregular grid on top constitutes the first effort of such an approach to investigate the interior of a tumulus since all known efforts have been implemented employing regular grids. Consequently, a three‐dimensional (3D) smoothness constrained inversion algorithm was modified. At first, the irregularly placed electrode locations had to fit in a finite element forward solver having particular specifications. Further, the hexahedral elements of the created mesh were manipulated to cope with neighbouring electrode positions in order to make a less dense grid. Finally, a so called ‘nudging’ process was followed to merge closely positioned electrodes belonging to different, branching, 2D data acquisition lines. The geophysical imaging helped clarify the geological and tectonic structure of Kastas. Further, it yielded several findings some of them being interpreted as natural features and some others as probable manmade. The latter comprise the targets of future excavations.