Characterization of silty to fine‐sandy sediments with SH waves: full waveform inversion in comparison with other geophysical methods

Abstract

We apply seismic full waveform inversion to SH‐ and Love‐wave data for investigating the near‐surface lithology at an archaeological site. We evaluate the resolution of the applied full waveform inversion algorithm through ground truthing in the form of an excavation and sediment core studies. Thereby, we investigate the benefits of full waveform inversion in comparison with other established methods of near‐surface prospecting in terms of resolution capabilities and interpretation security. The study is performed in a presumed harbour area of the ancient Thracian city of Ainos. The exemplary target is the source of a linear magnetic anomaly oriented perpendicular to the coast, which was found in a previous magnetic gradiometry survey, suggesting a mole. The SH‐wave full waveform inversion recovered a subsurface SH‐wave velocity model with submeter resolution showing lateral and vertical velocity variation between 40 and 150 m/s. To tame the non‐linearity of the full waveform inversion, a sequential inversion of frequency bands has to be combined with time‐windowing in order to separate the Love wave from the reflected SH wavefield. We compare the full waveform inversion results with multichannel analysis of surface waves, standard seismic reflection imaging, electrical resistivity tomography and electromagnetic induction. It turns out that the respective depth sections are correlated to a certain degree with the full waveform inversion results. However, the structural resolution of the other geophysical methods is significantly lower than for the full waveform inversion. An exception is the reflection seismic imaging, which shows the same resolution as full waveform inversion but can only be interpreted together with the full waveform inversion–based velocity model. An archaeological excavation as well as coring data allows ground truthing and a direct understanding of the geophysical structures. The results show that the target was a sort of near‐surface trench of about 3–4 m width and 0.8 m to 1.0 m depth, filled with silty sediment, which differs from the layered surrounding in colour and composition. The ground truthing revealed that only SH‐wave full waveform inversion and seismic reflection imaging could image the trench and sediment structure with satisfying lateral and depth resolution. We emphasize that the velocity distribution from SH‐wave full waveform inversion agrees closely with the excavated subsurface structures, and that the discovered changes in seismic velocity correlate with changes in the sand content in the respective sediment facies sequences. The study demonstrated that SH‐wave full waveform inversion is capable to image structural and lithological changes in the near subsurface at scales as low as 0.5 m, thus providing the high resolution needed for archaeological and geoarchaeological prospection.

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.…”

Mid- to late-Holocene sea-level evolution of the northeastern Aegean sea

“…However, the different mechanisms, which controlled sea-level evolution in recent millennia in this portion of the Aegean, are poorly constrained. To fill this knowledge gap, the results of geoarcheological (Brückner et al, 2015) and geophysical (Schwardt et al, 2020; Seeliger et al, 2018) studies offer possibilities to produce new relative sea-level data and furnish fresh perspectives on its evolution.…”

Understanding Wetlands Stratigraphy: Geophysics and Soil Parameters for Investigating Ancient Basin Development at Lake Duvensee

“…The Full Waveform Inversion (FWI) is a recent tool which is able to improve the resolution of seismic data. At the Fossa Carolina FWI was able to improve the definition of the canal basement and to resolve small-scale velocity anomalies correlated with features in the archaeological documentation [47,48,92]. A further application of this tool could foster the identification of shallow structures and improve the understanding of the seismic signals visible in the reflection profile.…”

“…A further application of this tool could foster the identification of shallow structures and improve the understanding of the seismic signals visible in the reflection profile. The multichannel analysis of surface waves (MASW, e.g., [48,93]) and spectral analysis of surface waves (SASW) can also be used to improve resolution for mapping subsurface stratigraphy.…”

“…The application of seismics for geoarchaeological research is not widespread but has lately been intensified due to exemplary results [23,[42][43][44][45][46][47][48]. SH-wave seismics are directly sensitive to the rigidity of soil layers and are capable of resolving stratigraphic interfaces at much larger depths than GPR [42,49].…”