Geophysical Prospection of Monumental Burial Mounds

Tumuli are major components of ancient land- and cityscapes, with the more monumental examples reaching tens of metres in height. Investigating their interior is a challenging task because archaeological excavations are usually restricted to the surface layers, and conclusions by analogy drawn from other, known, mounds are problematic because no general regularities of construction have been found so far. From a geophysical perspective the investigation of monumental grave mounds consists in locating and classifying possible metre-scale archaeological objects at up to tens of metres’ depth. This causes a significant resolution and range problem for most of the sounding methods typically applied in archaeological prospecting. In the present article we analyse the capabilities of geophysical prospection methods in detecting grave chambers in the interior of monumental tumuli by determining method-specific detection limits of cavities. These thresholds represent the minimum size of a cavity for being locatable as a function of depth, instrument sensitivity and measurement conditions. Based on this review we develop a prospection concept that includes three groups of targets: (1) Grave chambers and dromoi, understood as voids with typical diameters of a few metres: These cannot be detected with magnetics, geoelectrics or ground radar if their depths are greater than 10 m as an order of magnitude. The most promising method is shear wave (S-wave) reflection seismic, which enables sounding depths of several tens of metres at metre- scale resolution. (2) Stratigraphy: Layered construction phases of tumuli and internal levels of varying soil compaction and moisture contents can be identified and classified in terms of seismic wave velocity (both compressional [P-] and S-wave velocities) and electric ground resistivity. These can be reliably derived from a combination of reflection seismic, seismic refraction tomography and electric resistivity tomography. (3) Near-surface archaeological targets down to depths of a few metres, such as remains of ritual architecture and secondary usage, can be investigated with all standard methods of archaeological prospection. Generally we recommend combining geophysical measurements with targeted test excavations to optimise excavations and groundtruth geophysical findings.

Keywords:

Archaeological prospection, geophysics, dectection limits, grave chambers, dromoi