Peter Hedin

3D interpretation by integrating seismic and potential field data in the vicinity of the proposed COSC-1 drill site, central Swedish Caledonides

Abstract The scientific drilling project COSC (Collisional Orogeny in the Scandinavian Caledonides), designed to study key questions concerning orogenic processes, aims to drill two fully cored boreholes to depths of c. 2.5 km each at carefully selected locations in west-central Sweden. The first of these, COSC-1, is scheduled for start late spring 2014 and will target the Seve Nappe Complex, characterized by inverted metamorphism and with parts that have evidently been subjected to hot ductile extrusion. In this study available seismic sections have been combined with surface geology to produce a 3D interpretation of the tectonic structures in the vicinity of the COSC-1 borehole. Constrained 3D inverse gravity modelling over the same area supports the interpretation, and the high-density Seve Nappe Complex stands out clearly in the model. Interpretation and models show that the maximum depth extent of the Seve Nappe Complex is less than 2.5 km, consistent with reflection seismic data. The gravity modelling also requires underlying units to comprise low-density material, consistent with the Lower Allochthon, but the modelling is unable to discern the décollement separating the allochthons from the crystalline Precambrian basement.

2D magnetotelluric inversion using reflection seismic data as constraints and application in the COSC project

We present a novel 2D magnetotelluric (MT) inversion scheme, in which the local weights of the regularizing smoothness constraints are based on the envelope attribute of a reflection seismic image. The weights resemble those of a previously published seismic modification of the minimum gradient support method. We measure the directional gradients of the seismic envelope to modify the horizontal and vertical smoothness constraints separately. Successful application of the inversion to MT field data of the Collisional Orogeny in the Scandinavian Caledonides (COSC) project using the envelope attribute of the COSC reflection seismic profile (CSP) helped to reduce the uncertainty of the interpretation of the main decollement by demonstrating that the associated alum shales may be much thinner than suggested by a previous inversion model. Thus, the new model supports the proposed location of a future borehole COSC-2 which is hoped to penetrate the main decollement and the underlying Precambrian basement.

3D Seismic Processing of Crooked Line 2D Data in the Vicinity of the COSC 2.5 km Deep Scientific borehole

Two crooked line 2D seismic profiles were acquired in the vicinity of the location of the first scientific borehole of the Collisional Orogeny in the Scandinavian Caledonides (COSC) project. The planned depth of the borehole is 2.5 km and drilling will start in mid-Spring 2014. The data were previously processed in 2D and the borehole was located based on the results of this processing. In order to obtain a better image of the subsurface and provide better predictions of the expected lithology at depth the data have been reprocessed as a sparse 3D data set. The 3D processing suggests that a larger mafic lense is expected to be penetrated at about 1000 m depth and the base of a high grade metamorphic unit at about 2.2-2.3 km depth. The upper 500 m is not well imaged due to the acquisition geometry.

COSC Geophysical and Geological Site Investigations

Drilling of the first borehole, about 2.5 km deep, for the continental scientific deep drilling project COSC (Collisional Orogeny in the Scandinavian Caledonides) is scheduled to begin in the summer of 2013. Here we present the project, a 3D interpretation of seismic data, combined with surface geology and potential field data, used for locating the most suitable drill site and planning of the drilling. An evaluation of the seismic interpretations by constrained 3D inverse modeling of potential field data shows a good fit to observed data, further supporting the choice of the drill site.