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Address : 1University of Oulu, Finland;2University of Helsinki, Finland;3-, Finland
Celebration date : 20 Saturday September 2008
Author name : Silvennoinen, Hanna1; Kozlovskaya, Elena1; Yliniemi, Jukka1; Tiira, Timo2; FIRE, Working Group3

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  • Article title : Interpretation of wide-angle reflection and refraction recordings of Vibroseis signals and 3-D gravity modelling along FIRE4 profile, northern Finland
    Article type : Tectonic & Seismotectonic
    Location : International Geological Congress,oslo 2008

    Fulltext :

    We present the results of the interpretation of wide-angle measurement of Vibroseis signals along the southern part of the FIRE4 profile located in the northern Finland and 3-D density modelling of the area around the profile. The Finnish Reflection Experiment (FIRE) was a deep CMP reflection seismic survey made by Vibroseis technique along four profiles in Finland during 2001 - 2003. During the experiment thirteen recording stations were deployed along the southern part of FIRE4 profile for the purpose of recording wide-angle signal from vibrator sources. The FIRE4 wide angle reflection and refraction profile is 235 km long and crosses Archaean granitoids, early Proterozoic Peräpohja Schist Belt and Central Lapland Granitoid Complex. The first arrivals and P-waves reflected from boundaries in the uppermost crust penetrate to a depth of 5 km and can be traced to offsets of 20 - 60 km. Using these arrivals, we obtained a P-wave velocity model of the uppermost crust with both forward raytrace modelling and inversion. There are three main layers in the model with nearly horizontal layer boundaries that can be traced through the whole model. The major geological units can be seen in the model as horizontal variations in the P-wave velocity. The velocity at the surface in the Archean granitoids, Peräpohja Schist Belt, and Central Lapland Granitoid Complex, are about 5.90 km/s, 5.70 km/s, and 5.85 km/s respectively. The most interesting feature in the velocity model is a zone of high P-wave velocity (about 6.30 km/s) at a depth of about 2 - 3 km in the middle of the Central Lapland Granitoid Complex that is marked also by a high reflectivity on FIRE4 reflection section. A large-scale maximum of the Bouguer anomaly is also observed above this area. In order to constrain the depth of this feature and explain it in terms of rock composition, we applied modelling and inversion of Bouguer anomaly and calculated a 3-D density model of the uppermost crust for the area around the profile. The modelling showed that the source of this anomaly is a body with density of about 2780 - 2800 kg/m3 located at the depth of about 2.5 - 6.0 km. These density and velocity values and high reflectivity could indicate that the Central Lapland Granitoid Complex is underlayed by a highly deformed and folded structure composed of rocks with contrasting elastic properties.

     
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