Geo-Blog: New survey of the Helsinki–Tallinn railway tunnel route

Kimmo Alvi, Geologist
Ossi Ikävalko, Senior Specialist
Keijo Nenonen, Chief Expert, Science and Innovations

In this new GTK’s Geo-Blog text Kimmo Alvi, Ossi Ikävalko and Keijo Nenonen tell about the new survey of the Helsinki-Tallinn railway tunnel route.

The Geological Survey of Finland (GTK) conducted acoustic-seismic surveys of the possible routes  for an undersea railway tunnel between Helsinki and Tallinn. The report was commissioned by the Helsinki-Uusimaa Regional Council (Figure 1).

The survey provided a detailed overview of the geological structure of the sea floor basement and deposits between Finland and Estonia. The results are promising, even though some areas near the Estonian coast require additional soundings (Figure 2).

–A new finding was that the crystalline basement sea floor continues all the way to Naissaar and to the shallows near the Estonian coast, Chief Expert at GTK, Keijo Nenonen, explains (Figure 3).

This acoustic-seismic survey is the first of its kind between Helsinki and Tallinn, conducted with modern and accurate sounding equipment (Figures 4 and 5). The survey was carried out in August 2016, with additional soundings in October and November

The results of the study have been published in English. The report does not directly comment on the constructability of the basement, but provides new and useful information for further surveys.

The feasibility of a Helsinki–Tallinn tunnel is being studied

The soundings bring more information for  the FinEst Link project between Finland and Estonia, which is examining the possibilities of building a railway tunnel. The next phase will be to examine the technical and economic feasibility of the project, in addition to its cost-effectiveness and impacts. The results are expected during the present year. The viability of planning the tunnel can only be assessed after receiving these results.

The undersea railway tunnel would be 90 kilometres long and the train journey from Helsinki to Tallinn would take about 30 minutes.

The thorough research on the feasibility of the tunnel project was enabled by EU funding under the Interreg Central Baltic programme. The overall budget of the two-year FinEst Link project is 1.3 million euros. The project is led by the Helsinki-Uusimaa Regional Council in cooperation with the City of Helsinki, the Finnish Transport Agency, the City of Tallinn, the Estonian Ministry of Economic Affairs and Communications and the Harju County Government.


Alvi, K. 2017. Acoustic-seismic survey along the proposed railway tunnel route options, between Helsinki and Tallinn. Merenmittauslupa T- 037-16 22.7.2016 (AM13753). Report: Helsinki –Uusimaa Regional Council, Geological Survey of Finland, 24 p.

> Acoustic-seismic survey along the proposed railway tunnel route options, between Helsinki and Tallinn

Anttikoski, U. Vilo, A., 1999. Baltic Sea Circular Link via Rock Tunnels. World Tunnel Congress (Challenges for the 21st Century). Oslo, 29 May – 3 June. 1999. p. 473- 480 in. Proceedings of the World Tunnel Congress ’99. Ed. T Alten et al. Balkema 965 p.

Ikävalko, O., Vähäaho, I and Suuroja S. 2013.  Soil and bedrock conditions to be expected in Tallinn-Helsinki tunnel construction. STATENS VEGVESENS RAPPORTER Nr. 213, Strait Crossings 2013 16 – 19 June 2013 Bergen, Norway p. 790–799.

Koistinen, T. J. (ed.) 1996. Explanation to the map of Precambrian basement of the Gulf of Finland and surrounding area 1: 1 mill. Geological Survey of Finland. Special Paper 21. Espoo: Geological Survey of Finland. 141 p. + 1 app. map.

Nenonen, K. ja Ikävalko, O., 2012. Tunneli läpi harmaan kiven Tallinnaan with English summary; A tunnel connection to Tallinn through the hard grey bedrock. Geologi 64:75–80.

Suuroja, S., Suuroja, K., Ploom, K., Kask, A. & Soosalu, H., 2012. Tallinn – Helsinki ‐tunnel soil‐ and bedrock construction conditions. Compilation of a geological database for the possible Tallinn‐Helsinki tunnel area (in Estonian EEZ). Geological Survey of Estonia, Department of Geophysics, Marine and Environmental Geology, Department of Geological mapping. Report. Tallinn, pp. 19.

More information:

FinEst Link Project Manager Kari Ruohonen, Helsinki-Uusimaa Regional Council, tel. +358 40 5045 170, kari.a.ruohonen@gmail.com

Ossi Ikävalko, Senior Advisor, Geological Survey of Finland, tel. +358 29 503 2121, ossi.ikavalko@gtk.fi Kimmo Alvi, Geologist, Geological Survey of Finland, tel. +358 29 503 2115, kimmo.alvi@gtk.fi



Figure 1. Map of the marine geological surveys and acoustic-seismic surveys in the Tallinn coastal sea area. Soundings were made along several lines to examine the geological structures of the seafloor and the depth of hard crystalline bedrock basement. The survey was supported by soundings and deep drilling conducted by the Estonian Geological Survey on the coastal islands and peninsula, where the depth of crystalline bedrock was verified. The map was drawn by the Geotechnical Division of the City of Helsinki.


Figure 2. Profiles of geological units and the sea floor relief between Helsinki and Tallinn drawn according to the marine geological survey of GTK. The Precambrian hard crystalline rock is marked with red. A southward-dipping trend is clearly visible in the acoustic cross sections and the bedrock surface dips gradually down to the depth of approximately 120 m under the Cambrian –Ordovician sedimentary rocks on the Estonian coast. The topmost profile B2 goes from Helsinki city centre to Tallinn harbor, and the lower profile D via Navissaar Island to the Ulemiste traffic centre in Tallinn. The soft sedimentary rock units are marked with brown and blue in the profile. The loose Quaternary sedimentary cover is marked with green and it is tens of metres thick on the Estonian coast and just a narrow cover on the bedrock in the Finnish section of the profile. Crystalline bedrock also frequently crops out on the seafloor in the Finnish section. The profile was compiled by the Geotechnical Division of the City of Helsinki based on the acoustic-seismic survey of GTK and available geological mapping data.


Figure 3. The 3D surface of seafloor and the Quaternary sediments of the Estonian coast according to the compiled 3D model. There is an abrupt change in the thickness of the sedimentary cover at the Tallinna, Uusmadal and Naissaare shoals, which marks the border between crystalline rocks of the Fennoscandian Shield and the younger sedimentary cover of the East European Platform. The steep coastal bank is a result of former fluvial and littoral erosion. During the cold interstadials of the ice age, the water depth in the Baltic was over 100 m lower than at present. The 3D model was compiled by the Estonian Geological Survey, EGS (Suuroja et al. 2012).


Figure 4. The GTK research vessel Geomari is an aluminum-body catamaran and measures 20 m in length, 7.6 m in breadth and has a displacement of 0.9 m. The ship’s deadweight is 75 t. The ship has seismic signal equipment (ELMA 250–1300 Hz), a Klein 3000 Sidescan Sonar (100/500 kHz), a research sonar probe (MD 28 kHz) and a multibeam sonar (Atlas Fansweep 20 200 kHz), as well as bottom sediment sampling equipment.
Figure 5. Schematic illustration of R/V Geomari’s acoustic seismic survey equipment.


Kimmo Alvi

Text: Kimmo Alvi

Kimmo Alvi is a marine geologist specializing in acoustic survey methods, including sub-bottom profiling, multibeam- and side-scan surveys. He has a wide experience on ship-based research work in marine environment. He is also one of the acting captains of GTK’s research vessel Geomari.

Ossi Ikävalko

Text: Ossi Ikävalko

Ossi Ikävalko, M.Sc. is working as Senior Specialist in the Engineering Geology and Land Use unit in GTK. He has a wide experience on use of geology in underground construction, land use planning and engineering geology. He has been geologist in Geological Survey of Finland since 2006 and led urban geological research projects and participated in several large underground construction projects in Finland and now a member of an advisory group for FinestLink Tallinn Tunnel feasibility study.

Keijo Nenonen

Text: Keijo Nenonen

Keijo Nenonen PhD. (Geol.), Prof.; Working as Chief Expert in Science and innovations group of the Geological Survey of Finland GTK). Keijo Nenonen has expertise in Engineering- and Quaternary geology. He has operated during his 40 years career in GTK in various tasks from soil stratiraphical research, - exploration to engineering- and environmental geological studies including strategic planning of the survey. Now he is a member of an advisory group for FinestLink to Tallinn Tunnel feasibility study. He has published approximately 160 writings, articles, papers, abstracts and books from his field of expertise.