Accurate seismic inversion techniques in prospecting for small size archaeological features: the case of Akrotiri excavation area on Thera Island, Greece1

Abstract: A high resolution seismic inversion technique was used to explore the volcanic basement rock at Akrotiri archaeological site on Thera Island. The survey was carried out in the context of a big scale project, where the old monuments protective roof cover is being replaced by a new environmental and friendly construction which will be supported by 95 pillars embedded in the volcanic basemen rock. The target of the geophysical prospect was the man-made or natural cavities unveiled during the excavations of the foundation shafts, the presence of which might affect the construction response during an earthquake. The cross-shaft seismic tomography technique was adopted for this purpose. A number of synthetic examples and a calibration experiment at a shaft with a known natural cavity clearly indicated that the tomographic inversion is capable of providing highly resolved 2-D models. High S/N ratios ensured field seismic records of high quality. A set of stability tests was run to check the consistency of the method. Travel time residuals verified the validity of the final velocity depth sections while model complexity trends showed a consistency between models after a certain number of iterations. The reconstructed velocity fields were quite consistent with the expected velocities. Impressive low-velocity structures attributed to natural or man-made cavities were reported to the constructin