Seismic prospecting in archaeology: a 3D shear‐wave study of the ancient harbour of Miletus (Turkey)

Abstract

ABSTRACTA 3D multicomponent shear‐wave survey was performed in the area of the ancient city of Miletus (western Turkey). To achieve high spatial resolution of near‐surface targets, data were acquired with a geophone array of 1 m grid spacing. The survey served: to investigate archaeologically important details of the basement of one of the silted harbours of Miletus, and to evaluate the potential of close‐meshed shear‐wave seismics for shallow high‐resolution prospecting in a more general sense. Methodically, we could show that isotropic radiation patterns of compressional shear‐ and surface waves can be reconstructed with simple vector arithmetic even if hand‐driven hammer blows are applied as a horizontal seismic source. Presenting time‐slices of S‐wave propagation, we demonstrate that side‐swipe may significantly affect the propagation of refracted waves. It is shown that the wavefronts of surface waves may be far from circular in the case of typical near‐surface lateral heterogeneity. As a further phenomenon of wave propagation, we could visualize the coda of a ground‐coupled air wave probably caused by mud cracks. Regarding the archaeological background of our study, we could show that the basement of the Milesian Lions’ Harbour is shallow enough to provide appropriate ground for ancient construction work. However, by ‘true 3D’ refraction mapping, we could image small‐scale topographic details of the harbour basement, proving that no large ancient stone walls are hidden under the ground. Magnetic anomalies were found to coincide with a lateral increase in Rayleigh‐wave velocity. The rectangular shape of this velocity contrast may indicate the remnants of an artificially shaped rock platform or an embankment. Spectral analysis of the surface wavefield shows that this seismic anomaly will be found at a depth of 3–6 m.