279 Geo-Investigation Procedures for Subsea Tunnels BJORN NILSEN Dept. of Geology, Norwegian Institute of Technology, Trondheim (Norway) In Norway appox. 13 km of rock tunnels underneath fjords has been exca- vated since 1974. One distinctive feature of fjord-crossing tunnels is that a major part of the tunnel will be covered by sea, and ordinary surface mapping techniques can be used only to a minor extent. As a result comprehensive and often also very expensive investigation techniques need to be used like acoust- ical profiling, refraction seismic profiling and core drilling. A description is presented of the special investigation procedure which is used for fjord-crossing tunnels in Norway. Based on the experience collected from several Norwegian fjord-crossing projects, the value of information obtained by the respective investigation techniques is discussed, and also some possible reasons for discrepancies between prognoses and reality. In most cases considerable costs are connected with the special investigations that are necessary for subsea tunnels. No investigation should therefore be started without a thorough evaluation of the value of the information that can be obtained. In general a high degree of flexibility of investigations should be aimed at with a stepwise investigation-procedure, and basing the detailed planning of the next stage of investigation on the results from the previous one. Crustal Structure of Northern Jutland - A Seismic and Gravity Interpretation N. BALLING, B.H. JACOBSEN, P.H. NIELSEN and G. LARSEN Geological Institute, Aarhus University, Aarhus (Denmark) As part of the European Geotraverse Project (EGT) , deep seismic sound- ings have been carried out in the area of southern Sweden, Denmark, and northernmost Germany (the EUGENO-S project). 50 explosions were fired at 20 shot points and recordings were made along 5 main profiles This paper describes the first results obtained from seismic ray tracing and gravity modell- ing along profile 3 from the central part of the Norwegian-Danish Basin to the Baltic Shield area in southwestern Sweden. A thin crust (26-28 km) is modelled in the Danish Subbasin, and increasing crustal thicknesses are found towards the shield area. Moho depths of up to 40 km are modelled in the coast area of southwestern Sweden. Generally, normal crustal and uppermost mantle veloc-

Geo-investigation procedures for subsea tunnels

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Geo-Investigation Procedures for Subsea Tunnels

BJORN NILSEN

Dept. of Geology, Norwegian Institute of Technology, Trondheim (Norway)

In Norway appox. 13 km of rock tunnels underneath fjords has been exca- vated since 1974. One distinctive feature of fjord-crossing tunnels is that a major part of the tunnel will be covered by sea, and ordinary surface mapping techniques can be used only to a minor extent. As a result comprehensive and often also very expensive investigation techniques need to be used like acoust- ical profiling, refraction seismic profiling and core drilling.

A description is presented of the special investigation procedure which is used for fjord-crossing tunnels in Norway. Based on the experience collected from several Norwegian fjord-crossing projects, the value of information obtained by the respective investigation techniques is discussed, and also some possible reasons for discrepancies between prognoses and reality.

In most cases considerable costs are connected with the special investigations that are necessary for subsea tunnels. No investigation should therefore be started without a thorough evaluation of the value of the information that can be obtained. In general a high degree of flexibility of investigations should be aimed at with a stepwise investigation-procedure, and basing the detailed planning of the next stage of investigation on the results from the previous one.

Crustal Structure of Northern Jutland - A Seismic and Gravity Interpretation

N. BALLING, B.H. JACOBSEN, P.H. NIELSEN and G. LARSEN

Geological Institute, Aarhus University, Aarhus (Denmark)

As part of the European Geotraverse Project (EGT) , deep seismic sound- ings have been carried out in the area of southern Sweden, Denmark, and northernmost Germany (the EUGENO-S project). 50 explosions were fired at 20 shot points and recordings were made along 5 main profiles This paper describes the first results obtained from seismic ray tracing and gravity modell- ing along profile 3 from the central part of the Norwegian-Danish Basin to the Baltic Shield area in southwestern Sweden. A thin crust (26-28 km) is modelled in the Danish Subbasin, and increasing crustal thicknesses are found towards the shield area. Moho depths of up to 40 km are modelled in the coast area of southwestern Sweden. Generally, normal crustal and uppermost mantle veloc-