Prehistoric chamber tombs or geological pitfall? A multimethod case study from Ancient Aigeira with a focus on seismic full‐waveform inversion

Abstract

We show an extensive multimethod geophysical study of focusing on some enigmatic subsurface structures found at Ancient Aigeira (N Peloponnes, Greece) that could be interpreted either as prehistoric chamber tombs or complex weathering patterns of the local marl–conglomerate rock sequences. It turns out that the nonseismic methods do not allow to distinguish between an archaeological and a geological origin of the observed patterns with certainty. In contrast, we demonstrate how shear‐wave seismics and full‐waveform inversion (FWI) can be used in archaeological prospection for distinguishing between these alternative essentially different interpretational models that are not distinguishable through nonseismic prospection data. The example site Aigeira is strategically well located on a hill on the Northern Peloponnese overlooking the Corinthian Gulf and has been inhabited with occupational gaps since Middle Neolithic times until the 12th to early 14th century ce. Magnetics, ground‐penetrating radar (GPR) and electrical resistivity tomography (ERT) reveal a honeycomb‐shaped anomaly pattern that could have been interpreted as a system of prehistoric chamber tombs. The time‐domain SH‐FWI strategy based on a sequential inversion of low‐pass and band‐pass filtered data results in subsurface models for shear‐wave velocity and density that accurately fits the complicated seismic data set. A highly heterogeneous subsurface is revealed that is characterized by linear cracks on a decimetre scale. The seismic FWI results are compared in detail with GPR, ERT and among each other. It turns out that the FWI result is consistent with each of these other geophysical methods but provides a more comprehensive subsurface characterization that it is supported by corings in addition. With the help of the seismic survey, we can reject the interpretation hypothesis of a prehistoric cemetery with chamber tombs and confirm that the enigmatic geophysical patterns represent a geological weathering structure that could be addressed as a reincised fan delta draped by reddish palaeosols.

A novel seismic full waveform inversion approach for assessing the internal structure of a medieval sea dike

“…In terms of wave theory, the maximum achievable resolution corresponds to the quarter‐wavelength criterion well known from classical reflection seismics. However, previous studies have shown the high‐resolution capabilities of the applied FWI algorithm (e.g., Dokter et al, 2017; Köhn et al, 2019; Rusch et al, 2022). All studies show that structures with dimensions smaller than 1.0 m by 1.0 m could be reliably resolved, making the FWI a promising tool for the investigation of dikes.…”

“…Using synthetic resolution tests, Dokter et al (2017) investigated the spatial resolution for the same FWI algorithm. Rusch et al, 2022 applied the FWI as part of a multimethod geophysical study to distinguish between interpretation hypothesizes for observed structures obtained by nonseismic methods achieving a resolution on the decimetre scale and Mecking et al (2021) use FWI for cavity detection inside a high monumental antique grave mount. The potential of the FWI to resolve the internal structures of a sea dike was investigated in a study by Köhn et al (2018), where a high correlation between the SH‐wave velocity structure and the lithology known from corings was observed.…”

“…As objective function during inversion, we use the trace normalized global correlation norm (GCN; eq. (1); after Choi & Alkhalifah, 2012), which previously has been successfully applied in other near‐surface studies (e.g., Köhn et al, 2019; Rusch et al, 2022). The GCN is given by where ns and nr are the number of shots and receivers, respectively, T is the recording time of the seismic data, denotes the L2‐norm and u ij mod and u ij ob s are the modelled and observed data, respectively.…”

“…As objective function during inversion, we use the trace normalized global correlation norm (GCN; eq. ( 1); after Choi & Alkhalifah, 2012), which previously has been successfully applied in other near-surface studies (e.g., Köhn et al, 2019;Rusch et al, 2022). The GCN is given by…”

“…While potential field methods such as electric resistivity tomography (ERT) are used for the investigation of mainly modern dikes, seismic methods play a minor role as they usually do not meet the resolution requirements despite their good penetration depth (e.g., Bièvre & Norgeot, 2005; Fauchard & Mériaux, 2007; Jaksch, 2009; Niederleithinger et al, 2012). However, as has been shown by several previous studies, seismic full waveform inversion (FWI) is a promising tool in near surface geophysical investigation and is able to provide detailed velocity and density models of the subsurface (e.g., Köhn et al, 2019; Mecking et al, 2021; Rusch et al, 2022; Schwardt et al, 2020) as it increases the seismic resolution allowing to image objects smaller than the seismic wavelength (e.g., Dokter et al, 2017). Therefore, through the application of advanced seismic methods like FWI, a high‐resolution image of the interior structure of the dike and underlying layers can be anticipated.…”