Abstract
SUMMARY Strong anisotropy of seismic velocity in the Earth's crust poses serious challenges for seismic imaging. Where in situ seismic properties are not available, the anisotropy can be determined from velocity analysis of surface and borehole seismic profiles. This is well established for dense, long-offset reflection seismic data. However, it is unknown how applicable this approach is for sparse seismic reflection data with low fold and short offsets in anisotropic metamorphic rocks. Here, we show that anisotropy parameters can be determined from a sparse 3-D data set at the COSC-1 borehole site in the Swedish Caledonides and that the results agree well with the seismic anisotropy parameters determined from seismic laboratory measurements on core samples. Applying these anisotropy parameters during 3-D seismic imaging improves the seismic image of the high-amplitude reflections especially in the vicinity of the lower part of the borehole. Strong reflections in the resulting seismic data show good correlation with the borehole-derived lithology. Our results aid the interpretation and extrapolation of the seismic stratigraphy of the Lower Seve Nappe in Jämtland and other parts in the Caledonides.
Highlights
Thrust sheets are common tectonic features in the Earth’s crust associated with mountain building processes
Poorly constrained velocity information in connection with the strong seismic anisotropy can result in large uncertainties in the seismic stratigraphy, which hampers the correct localization of reflections and their correlation with lithology, for example from core or borehole data
The Seve Nappe Complex (SNC) lie the lower nappes of the Middle Allochthons including the Sarv and Offerdal Nappes, which are composed of lower-grade metamorphosed clastic and carbonate sediments intruded by Ediacaran dike swarms from the Baltoscandian margin
Summary
Thrust sheets are common tectonic features in the Earth’s crust associated with mountain building processes. The Seve Nappe Complex (SNC) is a prominent thrust sheet (so-called nappe) in the central Scandinavian Caledonides, which is investigated by the COSC-1 drilling project with the aim to better understand deep orogenic processes in mountain belts (Gee et al 2010; Lorenz et al 2015b). The SNC lie the lower nappes of the Middle Allochthons including the Sarv and Offerdal Nappes, which are composed of lower-grade metamorphosed clastic and carbonate sediments intruded by Ediacaran dike swarms from the Baltoscandian margin. These are underlain by the metasedimentary successions of the Lower Allochthon, which are derived from the Baltoscandian platform and of even lower metamorphic grade. At the Caledonian front, a thin low-angle thrust zone with Cambrian alum shale at the base of the Lower Allochthon separates the allochthonous units from the underlying Autochthonous Precambrian basement
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