Detection of near-surface heterogeneities at archaeological sites using seismic diffractions

The detection of shallow buried ancient structures or objects of cultural heritage is a primary challenge for seismic surveys at archaeological sites. The knowledge of the distribution of shallow objects can assist archaeologists’ study of the past without making excavations. Excavations lead to surface exposure of the buried objects and potential damages and preservation issues. The seismic response arising from localized archaeological targets is encoded in diffractions, which can be used to locate the objects. However, the energy of a diffracted wave is usually weak and masked behind the strong presence of other coherent signals or coherent noise in the data (e.g., surface waves, specular reflections). This makes it difficult to detect and interpret reliably. In the past decades, researchers have attempted to detect various near-surface features using diffracted waves. Landa and Keydar (1998) developed a method for identifying local targets in the shallow subsurface using diffracted waves. They constructed a so-called diffraction-point-section (D-section) by concentrating diffracted waves from diffractor points. The anomalies in this D-section can be interpreted as potential scattering objects. Shtivelman and Keydar (2005) proposed a multipath summation approach to image near-surface objects by stacking diffracted energy along all possible diffraction curves defined by all veloc-ity values within a specific range. Subsequently, Shtivelman et al. (2009) improved the resolution of this multipath summation approach by introducing image-dependent weights. The above-mentioned methods have been tested earlier on field data dominated by surface waves; no identification of diffracted waves could be found. To improve the reliability of diffraction imaging, in this paper we first apply a method that consists of seismic interferometry (SI) and adaptive subtrac-tion for the suppression of high-amplitude surface-wave noise (Konstantaki et al., 2015; Liu et al., 2018). We then present an approach based on an extension of the spatial summation of weak diffractions as proposed by Shtivelman and Keydar (2005). We utilize instantaneous-phase coherency (Schimmel and Paulssen, 1997) to enhance the optimal summation of weak but coherent diffractions. In the following, we first describe the practical steps for the implementation of each of the above methods. We then demonstrate the feasibility of our approach in locating scatterers on numerically modelled data with a low signal-to-noise ratio (S/N). Finally, we test our method on field seismic data acquired at an archaeological site.