Seismic survey of the Danevirke: Determining the multiple construction phases of a medieval fortification system through full waveform inversion (FWI)

We present seismic cross-sections through a complexly stratified major medieval rampart system, the Danevirke wall in Schleswig-Holstein (Northern Germany), which were obtained by seismic full waveform inversion (FWI). This target is exemplary for near-surface archaeological targets characterized by a pronounced topography and by dm- to m-scale heterogeneity with especially high contrasts in the seismic parameters (hard rock embedded in soft sediments). The locations are especially suitable for a resolution assessment of FWI because near-by excavations could be used for ground-truthing. The profiling was conducted with horizontally polarized shear waves (SH-waves). The FWI was performed with a 2D time-domain visco-elastic code enabling the inversion of SH-body waves and dispersive Love-waves. A multi-stage inversion strategy was applied. It turns out that the resulting FWI velocity and density tomograms can resolve internal structure with a minimum dimension of 1 m down to about 5 m depth. Six different construction phases can be distinguished in terms of shear wave velocity and density. These include compacted softsoil, refilled ditches, internal walls made of boulders and bricks, and modern construction overprints. All seismically detected construction phases could be verified with photos of near-by excavated sections. A comparison with previously conducted electric resistivity tomography (ERT) shows that the results of the shear wave FWI correlates much closer with the lithology than the ERT because it is less affected by soil moisture. The seismic resolution of 1 m is maintained down to 5 m depth, whereas the ERT resolution decreases almost proportionally with depth. • Seismic full waveform inversion on multi-phase medieval rampart. • Ground truthing through archaeological excavation. • Reconstruction of distinct building phases with sub-meter resolution.