The Yığma Tepe of Pergamon: stratigraphic construction of a monumental tumulus from seismic refraction measurements

Abstract

The monumental tumulus Yı gma Tepe is an important part of the cityscape of the ancient city of Pergamon. The tumulus construction is estimated in the Hellenistic period, the internal structure and exact purpose have been unknown so far. Its height of 32 m and diameter of 158 m make the deep interior of the tumulus practically inaccessible for excavations. Therefore, we applied a combination of geophysical measurements and archaeological sondages to explore the structure of the Yı gma Tepe. The investigations centre on P-wave refraction soundings. They were carried out to clarify the stratigraphic structure and soil composition as a prerequisite for a better understanding of construction techniques, building history and soil deposition.Interactive seismic raytracing, traveltime tomography and visco-elastic forward modelling were applied to derive a three-dimensional seismic velocity model, which was validated by excavations and soil analysis, historical records and additional geoelectric measurements. Our results reveal that the tumulus is composed of three layers, each about 10 m thick, separated by first-order seismic discontinuities which were locally verified. These layers form a stack of conical disks, the interfaces of which define two internal plateaus. The analysis of soil samples showed that the seismic velocity increase at the interfaces is likely to be caused by anthropogenic compaction applied to soils showing increased fractions of silt and clay. By combining topographic data with results from seismic and geoelectric sounding on the tumulus, the surrounding trench and the unaltered surrounding area, we show that the tumulus was built almost completely from the surrounding soils and that 17% of its original volume was displaced by destruction and erosion. Based on this mass balance the strongly destroyed and eroded original surface of the tumulus and its surrounding trench is reconstructed.

The Yığma Tepe of Pergamon: Internal construction of a monumental burial mound from shear wave reflection sounding and wavefield modelling

“…Directly in the centre of the mound at 1.5 m below the top's surface (Figure 3, Pe17/So04), a strongly hardened layer was found, matching a high-velocity zone (HVZ) detected before by P-wave refraction measurements (Mecking et al, 2020).…”

“…The measurements did not distinguish whether only the surface layer of this first construction phase is compacted or if Phase 1 is homogeneously consolidated. A thin layer with the same P-wave velocity as Phase 1 (800 m/s) was found at the top of the mound and excavated for verification in 2017 (Mecking et al, 2020;Meinecke, 2019a).…”

“…The mound and surrounding soil consist of river sediments from the rivers Bakır Çayı (ancient: Kaïkos) and Bergama Çayı (ancient: Selinus) (Philippson, 1912). They consist of layers of sand and gravel with maximum diameters of 0.5 m (Mecking et al, 2020). Refraction seismic measurements showed that two overlapping alluvial fans exist in the area of the Yı gma Tepe, distinguishable by their P-wave velocity and composition.…”

“…The mound consists of three construction phases, each about 10 m high, which could be distinguished by their P-seismic velocities (Mecking et al, 2020;Mecking et al, 2015Mecking et al, , 2016Mecking et al, , 2018. Phase 1 located at the basal surface of the mound shows significantly higher seismic velocities (ca.…”

Geological Challenges of Archaeological Prospecting: The Northern Peloponnese as a Type Location of Populated Syn-Rift Settings

“…Although magnetic surveys are still the backbone of an archaeological survey it is often only in combination with other methods possible to interpret the geophysical results correctly. These complementing methods include ground-penetrating radar (GPR) (e.g., [4,5]), electrical resistivity tomography (ERT) (e.g., [6][7][8]), electromagnetic induction (EMI) (e.g., [9]) to different sorts of seismic methods ( [10][11][12][13]). Whereas multi-ha-scale areas can be surveyed easily with magnetics and GPR in plain open area, the terrain conditions found on the Corinthian Rift are more difficult, and the rather small contrasts between geological background and the cultural remains mostly excludes the use of magnetics.…”