Combined surface and borehole seismic investigations at the ICDP COSC-1 and COSC-2 drillholes (Sweden)

The ICDP funded project COSC (Collisional Orogeny in the Scandinavian Caledonides) is investigating mountain building processes with the help of two ~2.5 km deep fully cored boreholes in Central Sweden. While borehole COSC-1, drilled in 2014, studied the emplacement of the high-grade metamorphic allochthons, borehole COSC-2, drilled in 2020, focuses on defining the character and age of deformation of the underlying greenschist facies thrust-sheets, the main Caledonian décollement and the Precambrian basement.

We have performed combined surface and borehole seismic investigations at both drill sites in order to characterize the Earth’s upper crust in the direct vicinity of the boreholes. Both surveys were designed as multi-azimuthal walkaway VSP surveys that have the potential to yield not only a 3D seismic image around the borehole both also to derive information about seismic anisotropy related to the drilled rock units.

During the COSC-1 survey in 2014, three surface lines were acquired centered radially around the COSC-1 drillsite. In the central part up to 2.5 km away from the borehole a hydraulic hammer was used as the seismic source, while for larger offsets up to 5 km explosives were employed. The wavefield of both source types was recorded using an array of 15 three-component receivers with a spacing of 10 m deployed at 7 different depth levels in the borehole. Simultaneously, the wavefield was recorded at the surface by 180 standalone three-component receivers along each of the three up to 10 km long lines, as well as by a 3D array of single-component receivers in the central part of the survey area around the borehole.

The COSC-2 survey in 2021 comprised two surface lines across the COSC-2 drillsite with densely spaced single- and three-component receivers and maximum source-receiver offsets of ~11 km. The location of the COSC-2 borehole right next to lake Liten made it necessary to design the survey as an amphibious seismic experiment using a 32 t Vibroseis truck and wireless geophones on land along the lake as well as an airgun and three-component OBS along the profile part across the lake. An array of 17 three-component receivers with a spacing of 10 m recorded the seismic wavefields of both sources along the entire borehole length.

In both cases, a 3D velocity model including anisotropy information was obtained from the seismic data by first-arrival traveltime tomography. In the case of COSC-1, the anisotropic velocity model was used to perform an anisotropic prestack depth migration of the surface data, while for COSC-2 this part of the data processing and imaging is still ongoing. We show a comparison of the characteristics of both data sets, compare the obtained results and present lessons learnt for the planning of similar projects in the future.