Seismic site characterization in and around the COSC-2 drillhole

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The project COSC (Collisional Orogeny in the ScandinavianCaledonides) focuses on the mid Paleozoic Caledonide Orogen inScandinavia in order to better understand orogenic processes. TheScandinavian Caledonides provide a well preserved deeply erodedexample of a Paleozoic continent-continent collision of Himalayandimensions. The current structure consists of the allochthons and theunderlying autochthonous units, and the shallow 1-2 degreeswestdipping décollement surface that separates the two units andconsists of a thin skin of Cambrian black shales. The basement of theFennoscandian Shield, underlying the Caledonides is dominated bycrystalline rocks of Paleoproterozoic age. The structure of thebasement is highly reflective and dominated by either mafic sheets ordeformation zones of Caledonian or pre-Caledonian age. The COSCproject will examine the structure and physical conditions of theseunits, in particular the Caledonian nappes and the underlyingbasement, with two approximately 2.5 km deep boreholes, located inwestern Jämtland. In addition to that, the boreholes will provideunique information about the present temperature gradient in theCaledonides, the porosity and permeability of the rock formations, andthe petrophysical properties of the rocks at depth. The boreholeCOSC-1, drilled in 2014, investigated the emplacement of high-grademetamorphic allochthons, the Seve Nappe Complex. The secondborehole COSC-2 will investigate the character of the underlyinglower allochthonous thrust sheets, the main Caledonian décollementand the Precambrian basement. Existing regional seismic andmagnetotelluric data have imaged the geometry of the upper crust,and several pre-site seismic reflection surveys were performed inorder to define the exact drill site locations. The surrounding of theCOSC-1 drill site was characterized 2014 through an extensiveseismic survey consisting of several borehole and surface basedexperiments. The goal of this current proposal is to perform similarinvestigations around the new COSC-2. This will be achieved by acombination of seismic transmission and reflection experiments usinga borehole geophone system and complemented by two long offsetlines at the surface equipped with 3C geophones. Sources andsurface receivers will be aligned at different azimuths and centredaround the borehole location. These land-based investigations will be complemented by an airgun and OBS profile within the adjacent lake.Two supplementary experiments, a zero-offset VSP and ahighfrequency walkaway VSP will help to characterize the directvicinity of the borehole. The data processing will employ recentlydeveloped advanced imaging techniques and will focus on theanalysis of anisotropic effects. The results of our investigations will behigh-resolution images of the fine-scale structures around the boreholewhich is vital information for a reliable extrapolation of structural andpetrophysical borehole findings.