Interpretation of the reflections in the Siljan Ring area and the nature of fracture zones at depth based on the results from the Gravberg-1 borehole

Abstract

Constraints on small-scale heterogeneity in the upper crust of the Baltic Shield were obtained from stochastic modelling of a variety of seismic data from the BABEL and Siljan Ring surveys. A non-linear least-squares inversion scheme was applied to wavefield fluctuations in seismic reflections to obtain statistical parameters of the medium through which the wave energy propagated. The reflections are from dolerite sills intruded at depths of 6–12 km into the Palaeoproterozoic upper crust of the central Baltic Shield. Analysis of the BABEL data set reveals that the upper crust is a weakly scattering stochastic medium, with velocities characterised by an exponential autocorrelation function with a correlation length of 150 ± 50 m and rms velocity perturbation of 1.5 ± 0.5%. Further inversions show that most of the scattering occurs within the upper few kilometres of the crust. Analysis of the Sijan seismic data set reveals heterogeneity with length scales of 210 ± 50 m and a velocity perturbation of about 1.0%. The autocovariance and power spectrum of the sonic log, and studies of frequency-dependent Q in VSP data, from the Gravberg-1 borehole, give consistent estimates of basement heterogeneity. The upper crust in this region is characterised by a correlation distance of 35 ± 5 m and rms velocity perturbation of 4–6% in the uppermost 1–2 km of crust, associated with near-surface fracturing, and a correlation distance of 140 ± 50 m and rms velocity perturbation of 1.5 ± 0.5% in the uppermost 6–8 km, associated with compositional heterogeneity.