Seismic site characterization in and around the COSC-1 drillhole

COSC（斯堪的纳维亚加里东构造的碰撞造山运动）项目侧重于斯堪的纳维亚半岛的中古生代加里东构造，以便更好地了解过去和最近活跃山带的造山过程（Gee et al., 2010）。斯堪的纳维亚加里东构造提供了一个保存完好的古生代板块碰撞的例子，其表面地质与地球物理数据相结合，提供了加里东构造的几何控制，包括异源体和下伏的本土体，包括加里东逆冲片下寒武纪黑色页岩薄皮肤上的浅w倾斜的混合表面。矿区基底构造反射性强，明显以侵入晚古元古代花岗岩或中元古代火山岩和砂岩的基性片岩为主。COSC项目将通过位于Jämtland （http://www.sddp.se/COSC）西部Åre和Mörsil附近的两个约2.5公里深的钻孔，检查这些单元的结构和物理条件，特别是加里东推覆体（“热”allochthon）和下面的基底。除此之外，钻孔还将提供有关Caledonides当前温度梯度、岩层孔隙度和渗透率以及深层岩石物理性质的独特信息。现有的区域地震和大地电磁数据已经对上地壳的几何形状进行了成像，2010年和2011年进行了现场前地震反射调查，以更好地确定准确的钻探地点（Hedin et al., 2012）。本研究旨在通过基于钻孔的调查来补充这些地面地震测量，以更好地解决和定义COSC-1钻孔周围的小尺度结构（包括岩性边界、陡倾断层段、裂缝集等）。这将通过使用3C井眼检波器系统的地震传输和反射实验的组合来实现，并辅以地面的3C检波器，其中震源和地面接收器将在不同的方位角对齐，并以井眼位置为中心。数据处理将采用最新开发的先进成像技术，并将重点分析由排列裂缝和断层引起的各向异性效应及其与应力状态的关系。我们的调查结果将是井眼周围断层和裂缝精细结构的高分辨率图像。这些信息不仅对于在井眼中观察到的构造和岩石物理性质的可靠空间外推至关重要，而且对于彻底了解构造和地球动力学环境（包括但不限于过去和现在的应力状态）至关重要。

The project COSC (Collisional Orogeny in the Scandinavian Caledonides) focuses on the mid Paleozoic Caledonide Orogen in Scandinavia in order to better understand orogenic processes, both in the past and in recent active mountain belts (Gee et al., 2010). The Scandinavian Caledonides provide a well preserved example of Paleozoic plate collision, where the surface geology in combination with geophysical data provide control of the geometry of the Caledonian structure, both of the allochthon and the underlying autochthon, including a shallow W-dipping décollement surface on a thin skin of Cambrian black shales beneath the Caledonian thrust sheets. The structure of the basement underneath the décollement is highly reflective and apparently dominated by mafic sheets intruded into either late Paleoproterozoic granites or Mesoproterozoic volcanics and sandstones. The COSC project will examine the structure and physical conditions of these units, in particular the Caledonian nappes ("hot" allochthon) and the underlying basement, with two approximately 2.5 km deep drillholes, located near Åre and Mörsil in western Jämtland (http://www.sddp.se/COSC). In addition to that, the drillholes will provide unique information about the present temperature gradient in the Caledonides, the porosity and permeability of the rock formations, and the petrophysical properties of the rocks at depth. Existing regional seismic and magnetotelluric data have imaged the geometry of the upper crust, and pre-site seismic reflection survey were preformed in 2010 and 2011 to better define the exact drill site locations (Hedin et al., 2012). This present proposal is dedicated to complement these surface seismic measurements by drillhole-based investigations to better resolve and define the small-scale structures (including lithological boundaries, steeply dipping fault segments, fracture sets, etc.) around the drillhole COSC-1. This will be achieved by a combination of seismic transmission and reflection experiments using a 3C borehole geophone system and complemented by 3C geophones at the surface, where sources and surface receivers will be aligned at different azimuths and centred around the borehole location. The data processing will employ recently developed advanced imaging techniques and will focus on, amongst other things, the analysis of anisotropic effects caused by aligned fractures and faults and their relation to the stress regime. The results of our investigations will be high-resolution images of the fine-scale structure of faults and fractures around the borehole. This information is vital not only for a reliable spatial extrapolation of the structural and petrophysical properties observed in the borehole, but also for a thorough understanding of the tectonic and geodynamic setting, including, but not limited to, the past and present stress regime.