2D‐3D resistivity and microgravity measurements for the detection of an ancient tunnel in the Lavrion area, Greece

Abstract

ABSTRACTAt the archaeological site of Bertseko, Lavrion, Greece, the entrance of an ancient tunnel was found during archaeological excavations. Approximately 15 m from the entrance the tunnel remains inaccessible – blocked with filled material. The main aim of the geophysical survey was to verify if the tunnel is unblocked after that point and to delineate its direction. Another objective was to understand the purpose of its construction. As a first detection approach, eight 2D resistivity profiles were carried out with a pole‐pole array in a non rectangular grid, in order to image a wider area, a greater depth and to avoid several surface obstacles. Then the most promising area was outlined and selected for analysis with 3D resistivity tomography using parallel survey lines with a pole‐dipole array. Moreover, the microgravity method was used, for better resolution and verification of resistivity results. This study has shown that the influence of 3D effects in resistivity arrays, the choice of the 3D or 2D (pseudo‐3D) inversion approach, the position and the direction of the target in respect to the 2D profiles or 3D survey grid, are crucial factors that significantly affect the accuracy of the resistivity method in tunnel detection. Moreover, the design of the microgravity measurements, based on results of other geophysical methods, permits a flexible survey with a moderate acquisition time. If the geological environment is not complex, inversion of microgravity data is feasible and can be very useful, as it offers depth information about the target and can be directly comparative with resistivity models. The reconnaissance 2D resistivity survey was an important step for the optimized application of 3D resistivity and microgravity methods. The integrated results provided answers to archaeologist’s questions, delineating the direction of the tunnel with minimum ambiguity and showing that the ancient tunnel was not constructed to connect the two ancient tanks but is part of an ancient underground mine.