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2. Methods and innovations

The Search for Sikyon’s Sunken Harbor

Active and passive seismic mapping in an urban environment
Katharina Rusch, Martin Thorwart, Silke Müth, Konstantinos Kissas et Wolfgang Rabbel
p. 197-200

Résumé

– Horizontal-to-Vertical Spectral Ratio is used for mapping the thickness of the alluvial fill.

– Recording of ambient noise with mini arrays is used to map the shear-wave velocity.

– Imaged depression filled with silty material might be connected to former harbor.

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Texte intégral

The project ‘Finding Old Sikyon’ is a cooperation between the National Museum of Denmark, the Ephorate of Antiquities of Corinth, the Danish Institute at Athens, and Kiel University and is generously supported by the Carlsberg foundation in Denmark and also acknowledges funding by the Gösta Enbom Foundation. Our thanks go to Carina Milde, Birte Driehaus, and Rebekka Mecking for their help collecting the data and also to Eastern Atlas GmbH & Co KG for the good cooperation during the Sikyon campaigns.

Introduction

1A geophysical approach that has been proven successful to distinguish silted-up harbor basins, canals, and river beds from undisturbed geological layers in the past is the mapping of the shear- wave velocity (Ostia, Wunderlich et al., 2018; Miletos et al., 2014; Rabbel et al., 2004; Selinunte et al., 2014; Köhn et al., 2019). This is due to the fact that the sediments deposited in harbor situations are typically fine-grained clay or silt and show a lower degree of compaction and mechanical stiffness, all being factors that influence the seismic propagation velocity of the material. It is, however, always a question how the shear-wave velocity is mapped.

2Typically refraction or reflection seismic soundings are conducted either onshore or offshore depending on the situation of the harbor. In the case of Old Sikyon, Northern Peloponnese, Greece, however, the harbor basin that silted up in the course of the regional tectonic uplift is expected to be located underneath the outskirts of the modern Kiato (Fig. 1). Active seismic measurements (first arrival traveltime tomography [FATT] and multi-channel analysis of surface waves [MASW]) revealed a depression filled with silty sediments (Rusch et al., 2020) but the measurements also showed the disturbing effect of the ambient noise by the urban environment.

Figure 1. a) Location of Sikyon on the Northern Peloponnese, Greece. b) Layout of all conducted measurements in the area.

Figure 1. a) Location of Sikyon on the Northern Peloponnese, Greece. b) Layout of all conducted measurements in the area.

3This urban environment and the large area to be surveyed motivated the search for a different approach. We, therefore, used arrays of very small aperture to record the ambient noise to determine the dispersion of the Rayleigh-waves and map the shear-wave velocity in regard to depth and area. Additionally, the horizontal-to-vertical spectral ratio (HVSR) is calculated to estimate the thickness of the first sedimentary layer. In this paper we want to present the results of our first test survey and show the potential of these methods for a large scale geophysical survey in an urban environment.

Method

4Four areas were surveyed with a grid of arrays that consists of 9 stations each. The receiver spacing ranges between 3 to 9 m and each station is equipped with a 4.5 Hz 3C geophones in combination with a DataCube that recorded the noise for 20 minutes (Fig. 1b).

5The HVSR are calculated using the Geopsy software and the identified resonance frequencies are picked and by establishing our own relationship between the resonance frequency and the thickness of the sediment cover using a priori information of seismic lines and augerings we are able to map the thickness over the survey area (Abu Zeid et al., 2017; Ibs-von Seht & Wohlenberg, 1999).

6For the array processing we remove the linear trend from the vertical component before using the beamforming of the array processing tool of the ObsPy package (Beyreuther et al., 2010) on a total of 16 narrow frequency bands between 1.5 to 46 Hz to determine the apparent phase velocity of the Rayleigh-wave. The resulting dispersion curves are transferred into pseudo-depthsections using the empirical relationship z=0.4•λ (ranging in between of λ/2 and λ/3 given by Foti et al., 2014) for a first overview of the subsurface before they are inverted into depthsections in regard of shear-wave velocity using the forward modeling tool disrc96 (Herrmann, 2013).

7Next to the HVSR and array analysis’, five active refraction seismic lines, three electrical resistivity tomography lines and three manual augerings are available in the area (Fig. 1b).

Results

8The mini arrays allow a mapping of the shear-wave velocity in regard to depth but also area. The results show that the depression filled by silty sediments identified in the previously conducted active seismic measurements (Fig. 2c-f) can be resolved in the depthsections of the array analysis as well. They show an approximately 15 m deep depression filled by sediment with shear-wave velocities typical for fine-grained silty sediments. Since these sort of sediments are typically deposited in rivers, lagoonal or marine environments a connection to the former harbor appears plausible. This course of this depression can be traced in northwestern direction (Fig. 2a).

Figure 2. a) Map of the Rayleigh-wave phase velocity in relation to the depression found in the first arrival traveltime analysis (FATT) and multi-channel analysis of surface waves (MASW). b) Map of the layer depth determined by the HVSR method. c) Dispersion of the Rayleigh-wave (array analysis) along the line B4. d) Dispersion of the actively excited Love-wave on the neighboring profile L4000. e) Pseudosection of the array line B4. f) Pseudosection of the Love-wave profile.

Figure 2. a) Map of the Rayleigh-wave phase velocity in relation to the depression found in the first arrival traveltime analysis (FATT) and multi-channel analysis of surface waves (MASW). b) Map of the layer depth determined by the HVSR method. c) Dispersion of the Rayleigh-wave (array analysis) along the line B4. d) Dispersion of the actively excited Love-wave on the neighboring profile L4000. e) Pseudosection of the array line B4. f) Pseudosection of the Love-wave profile.

9Using the HVSR method a layer interface that most likely represents the bottom the alluvial fill can be mapped. This first stiffer layer can be found in depths of 1 to 3 m of depth, which correlates nicely with the results of previously conducted seismic and ERT measurements. We can also identify the course of the depression in the area (Fig. 2b).

10With both methods together we can image of the subsurface that is in accordance with the other applied methods that shows a deeper depression filled with potentially marine/submarine silty to clayey sediments that are disrupted by a layer of stiffer material like sand or gravel in a depth of about 2.5 m. A similar stratification sequence can also be observed in the port of Lechaion, only 15 km away, where Hadler et al. (2013) identified deposits of a tsunamigenic event as a transition between marine and lagoonal environments.

Conclusion

11By recording the ambient noise of the urban environment of the modern city of Kiato with mini arrays we were able to image an about 15 m deep depression that is filled with silty sediments and might be connected to the former harbor of Old Sikyon. So far, the survey areas are very small compared to the area that could be considered as a possible harbor area, but they show the potential of the presented method to map the shear-wave velocity and thus find the former harbor by using the otherwise disturbing noise of the urban environment.

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Bibliographie

Beyreuther, M., Barsch, R., Krischer, L., Megies, T., Behr, Y., Wassermann, J., 2010. ObsPy: A Python Toolbox for Seismology. Seismological Research Letters, 81: 530-533.

Brückner, H., Herda, A., 2014. On the lion harbour and other harbours in Miletos: recent historical, archaeological, sedimentological, and geophysical research. Proceedings of the Danish Institute at Athens, 7: 49-103.

Foti, S., Lai, C.G., Rix, G.J., Strobbia, C., 2014. Surface Wave Methods for Near-Surface Site Characterization. UK: Taylor & Francis.

Hadler, H., Vött, A., Koster, B., Mathes-Schmidt, M., Mattern, T., Ntageretzis, A.K., Reicherter, K., Willershäuser, T., 2013. Multiple late-Holocene tsunami landfall in the eastern Gulf of Corinth recorded in the palaeotsunami geo-archive at Lechaion, harbour of ancient Corinth (Peloponnese, Greece). Zeitschrift für Geomorphologie, Supplementary Issues, 57: 139-180.

Herrmann, R.B., 2013. Computer Programs in Seismology: An Evolving Tool for Instruction and Research. Seismological Research Letters, 84: 1081-1088.

Ibs-von Seht, M., Wohlenberg, J., 1999. Microtremor Measurements Used to Map Thickness of Soft Sediments. Bulletin of the Seismological Society of America, 89: 250-259.

Köhn, D., Wilken, D., De Nil, D., Wunderlich, T., Rabbel, W., Werther, L., Schmidt, J., Zielhofer, C., Linzen, S., 2019. Comparison of time-domain SH waveform inversion strategies based on sequential low and bandpass filtered data for improved resolution in near-surface prospecting. Journal of Applied Geophysics, 160: 69-83.

Rabbel, W., Stümpel, H., Woelz, S., 2004. Archeological prospecting with magnetic and shear-wave surveys at the ancient city of Miletos (western Turkey). The Leading Edge, 23: 690-693, 703.

Rabbel, W., Hoffmann-Wieck, G., Jakobsen, O., Özkap, K., Stümpel, H., Suhr, W., Szalaiova, E., Wölz, S., 2014. Seismische Vermessung der verlandeten Buchten des Medione und Gorgo Cotone. Hinweise zur Lage des Hafens der antiken Stadt Selinunt, Sizilien. Mitteilungen des Deutschen Archaologischen Instituts – Romische Abteilung, 120: 135-150.

Rusch, K., Stümpel, H., Gauß, W., Müth, S., Kissas, K., Rabbel, W., 2020. Geological Challenges of Archaeological Prospecting: The Northern Peloponnese as a Type Location of Populated Syn-Rift Settings. Remote Sensing, 12: 2450.

Wunderlich, T., Wilken, D., Erkul, E., Rabbel, W., Vött, A., Fischer, P., Hadler, H., Heinzelmann, M., 2018. The river harbour of Ostia Antica – stratigraphy, extent and harbour infrastructure from combined geophysical measurements and drillings. Quaternary International, 473: 55-65.

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Table des illustrations

Titre Figure 1. a) Location of Sikyon on the Northern Peloponnese, Greece. b) Layout of all conducted measurements in the area.
URL http://journals.openedition.org/archeosciences/docannexe/image/9574/img-1.jpg
Fichier image/jpeg, 1,3M
Titre Figure 2. a) Map of the Rayleigh-wave phase velocity in relation to the depression found in the first arrival traveltime analysis (FATT) and multi-channel analysis of surface waves (MASW). b) Map of the layer depth determined by the HVSR method. c) Dispersion of the Rayleigh-wave (array analysis) along the line B4. d) Dispersion of the actively excited Love-wave on the neighboring profile L4000. e) Pseudosection of the array line B4. f) Pseudosection of the Love-wave profile.
URL http://journals.openedition.org/archeosciences/docannexe/image/9574/img-2.jpg
Fichier image/jpeg, 986k
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Référence papier

Katharina Rusch, Martin Thorwart, Silke Müth, Konstantinos Kissas et Wolfgang Rabbel, « The Search for Sikyon’s Sunken Harbor »ArcheoSciences, 45-1 | 2021, 197-200.

Référence électronique

Katharina Rusch, Martin Thorwart, Silke Müth, Konstantinos Kissas et Wolfgang Rabbel, « The Search for Sikyon’s Sunken Harbor »ArcheoSciences [En ligne], 45-1 | 2021, mis en ligne le 16 août 2021, consulté le 22 mars 2023. URL : http://journals.openedition.org/archeosciences/9574 ; DOI : https://doi.org/10.4000/archeosciences.9574

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Auteurs

Katharina Rusch

Corresponding author, Institute for Geosciences, Kiel University, Kiel, Germany

Martin Thorwart

Institute for Geosciences, Kiel University, Kiel, Germany

Silke Müth

Ancient Cultures of Denmark and the Mediterranean, National Museum Denmark, Copenhagen, Denmark

Konstantinos Kissas

Classical Archaeology FB III, Trier University, Trier, Germany

Wolfgang Rabbel

Institute for Geosciences, Kiel University, Kiel, Germany

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