Non-invasive characterization of a burial tumulus with use of seismic P-wave velocity and attenuation tomography

Abstract

Knowledge of the engineering quality of soil material making up ancient earthen structures such as tumuli (burial mounds) is essential for planning and protecting their structural integrity during excavation and for designing preservation measures. A non-invasive method for assessing the engineering quality of a tumulus would be of benefit before archaeological exploration and during excavation, and in cases where excavation is not a choice. The relation of seismic velocity and attenuation to the engineering quality of geological materials is well established. We use seismic P-wave velocity and attenuation tomography to explore non-invasively soil differentiations and assess the bulk soil quality of a tumulus' lower embankment and foundations. The method is applied to a tumulus located in Northern Greece. Our results show that P-wave velocity patterns can image important structural and lithological differentiations by determining boundaries of soil conditions. Attenuation provides additional details on the spatial heterogeneity of soil quality within the embankment, as it appears to be sensitive to different factors related to soil quality. The combined results allowed us to outline the bedrock morphology of the tumulus foundations and identify areas of reduced soil strength in the embankment that we discuss with respect to the construction and usage of the tumulus. With use of the tomographic velocities we obtained parameter values for the soils of a tumulus body for the first time, that allowed us to assess the combined effect of grain size, porosity, saturation and consolidation on the bulk soil quality of the tumulus. We suggest that seismic tomography can be an efficient tool for spatially continuous and whole-scale non-invasive investigation of the soil quality and structural integrity of tumuli and similarly shaped structures, effectively contributing to the understanding of the structure's history and assisting excavation and protection design.