Reprocessing of geophysical data and comparison with results from tunneling construction of the ring-road west of Bergen, Norway
G. Tassis, J.S. Rønning, T. Kirkeby and M. Wåle
Event name: SAGEEP 2017 - 30th Annual Symposium on the Application of Geophysics to Engineering and Environmental Problems
Session: Infrastructure II
Publication date: 19 March 2017
Info: Extended abstract, PDF ( 444.16Kb )
Refraction seismic data preliminarily collected along the 3.8 km long Knappe-tunnel in Bergen, were tomographically inverted using the RayfractTM software to map and characterize fractures zones in bedrock. Previously collected ERT data were simultaneously reprocessed with a newer version of Res2DInv and all results were juxtaposed with tunneling observations. Tomographic inversion of refraction seismic data is affected by the starting model employed. In this work, we used both automatically generated smooth 1D and a semi-automatic 2D models as input. The analysis shows that 2D starting models give more distinctive velocity profiles. Reprocessed ERT data reveal similar structures to the initial processing, but with better resolution. Artificial effects are suppressed and several zones present greater depth extent. Additionally, new fracture zones were inferred based on the newly processed data. However, the most pronounced zones could be caused by sulphides and/or graphite. Comparison between various interpretation results of refraction seismic shows that traditional interpretation and tomographic inversion detect similar structures regardless of starting model, the latter being preferable. Semi-automatic interpretation of the same dataset with Hagedoorn's plus-minus method using Rayfract doesn't yield adequately satisfactory results due to limited shot points. Tunneling results show big variations in bedrock quality between the two legs of the tunnel. This is due to the fact that the actual zones intersect the tunnels in sharp angles and the subjectivity in interpreting the results for each tunnel. Interpretation of traditional seismic, tomographic inversion and ERT presents the same zone detection rate. On the other hand, semi-automatic interpretation using Hagedoorn's plus-minus method yields poorer results. Occasionally, all methods indicate fracture zones where good rock quality is found. Generally, it can be concluded that refraction seismic and ERT complement each other both in terms of weak zone detection and characterization and should be employed as standard techniques in such studies.