The Yığma Tepe of Pergamon: Internal construction of a monumental burial mound from shear wave reflection sounding and wavefield modelling
2021
The Yigma Tepe of Pergamon is one of the largest known burial mounds in the
Eastern Mediterranean. Its internal structure, especially the existence and location of
burials, remains unknown so far because its height of 32 m and the diameter of
158 m have restricted extensive excavations. To explore the interior of the Yigma
Tepe, we applied shear wave reflection profiling and traveltime tomography, locally
complemented by electrical resistivity tomography. Shear wave reflection imaging
enabled us to identify the layering and localize structures down to the basal surface
of the mound with a metre-scale resolution. A grid of crossing SH-wave profiles with
4-m line spacing covered the central mound area. The Y i gma Tepe can be subdivided
into three stratigraphic layers representing different construction phases, each about
10 m thick. The deepest is a consolidated layer with a locally compacted surface.
SH-wave reflection imaging verified the collapsed remains of a tunnel system and a
shaft at about 30-m depth that had been dug into the mound at the level of the basal
surface at the beginning of the 20th century. The tunnel collapse created a damage
zone of decreased S-wave velocity reaching up to the overlying middle layer, in
which four seismic objects of interest (SOIs) were detected. The largest of these SOIs
is a north – south-oriented structure, about 15 m long and 4 m wide. SH-viscoelastic
forward modelling showed that it likely consists of a collapsed walled cavity with a
zone of loosened soil above. The third layer covers the underlying two and is
enriched with large stone packages as verified by local excavations. We show the
capabilities of shear-wave reflection seismic to image small-scale structures under
complicated topographic conditions and how characteristics of cavities and volumes
of decompacted soil can be evaluated by wavefield modelling.
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