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Abstract

The area around Trondheim Bay in central Norway is affected by landslides, both onshore and within the fjord, with several events documented to have occurred in the last century. As urban development, including land reclamation, is taking place in the harbor, assessing in situ soil conditions is paramount for infrastructure and operational safety. To obtain better insight into the harbor setting in terms of subsurface structures and potential coastal geohazards, a high-resolution multichannel SH-wave seismic-reflection land survey was carried out during summer 2008, which complements a dense network of high-resolution, single-channel marine seismic profiles over the deltaic sediments in the fjord. The SH-wave seismic reflection was chosen because the resulting interval shear-wave velocity provides a nearly direct proxy for in situ soil stiffness, a key geotechnical parameter. In total, 4.2 km of 2.5D SH-wave profiles was acquired along roads and parking places. Highly resolved images of the sediments were obtained, overlying the bedrock at a depth of about 150 m. The high quality of the data is ascribed to the quieter ambient noise conditions of the nighttime data collection and an efficient suppression of Love waves arising from the presence of a high-velocity layer at the surface. Five main stratigraphic units were identified based on reflection patterns and amplitudes. Distinct SH-wave reflection events enabled detailed S-wave velocity determination down to the bedrock. Subsequently, interval velocities were remapped into soil stiffness. Low S-wave velocities of about 100 m/s occurring in the upper 50 m of the fjord-deltaic sediment succession suggest low sediment stiffness (50 to 100 MPa) directly below the stiffer man-made fill that is 10 to 15 m thick. The results indicate that SH-wave seismic reflection is well suited for urban ground investigation.

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