Publication Date:
2020-02-12
Description:
As support for the COSC drilling project (Collisional Orogeny in the Scandinavian Caledonides), an extensiveseismic survey took place during September and October 2014 in and around the newly drilled 2.5 km deepCOSC-1 borehole. The main aim of the COSC project is to better understand orogenic processes in past andrecently active mountain belts. For this, the Scandinavian Caledonides provide a well preserved case of Paleozoiccollision of the Laurentia and Baltica continental plates. Surface geology and geophysical data provide knowledgeabout the geometry of the Caledonian structure. The reflectivity geometry of the upper crust was imaged byregional seismic data and the resistivity structure by magnetotelluric methods. The crustal model was refined byseismic pre-site surveys in 2010 and 2011 to define the exact position of the first borehole, COSC-1.The completely cored COSC-1 borehole was drilled in Central Sweden through the Seve Nappe Complex, a partof the Middle Allochthon of the Scandinavian Caledonides that comprises units originating from the outer marginof Baltica. The upper 2350 m consist of alternating layers of highly strained felsic and calc-silicate gneissesand amphibolites. Below 1710 m the mylonite content increases successively and indicates a high strain zoneof at least 800 m thickness. At ca. 2350 m, the borehole leaves the Seve Nappe Complex and enters underlyingmylonitised lower grade metasedimentary units of unknown tectonostratigraphic position.The seismic survey consisted of three parts: a limited 3D-survey, a high resolution zero-offset VSP (verticalseismic profile) and a multi-azimuthal walkaway VSP (MSP) experiment with sources and receivers along threesurface profiles and receivers at seven different depth levels of the borehole. For the zero-offset VSP (ZVSP) ahydraulic hammer source was used and activated over a period of 20 s as a sequence of impacts with increasinghit frequency. The wave field was recorded with 3-component geophones and a receiver spacing of 2 m over thewhole borehole length.As first pre-processing steps, the three component VSP data were decoded and vertically stacked. Afterwards, theshots were merged to get a continuous shot gather. A horizontal rotation was performed, based on the S-wavearrivals.The rotated ZVSP-data show a high signal-to-noise ratio and good data quality. Signal frequencies up to 150 Hzwere observed. On the vertical component, clear direct P-wave arrivals are visible. Several P-wave reflectionsoccur below 1600 m logging depth. On both horizontal components, clear direct S-wave arrivals are visible afterrotation what suggests that the penetrated rock is anisotropic. In addition, several PS-converted waves can beidentified.In order to integrate the borehole data into the 3D surface seismic data, further processing concentrated only on theP-waves. First, deconvolution was applied to sharpen the signals and to suppress multiples. Then the wave field wasseparated into upgoing and downgoing components by median filtering. Finally, a corridor stack was generated us-ing the upgoing wave field in order to allow correlation with the borehole logging data and the surface seismic data.
Language:
English
Type:
info:eu-repo/semantics/conferenceObject
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