Seismicity Patterns and Permeability Reduction Associated With the Anisotropic Propagation of Discrete Compaction Bands in Diemelstadt Sandstone

We report results from triaxial deformation experiments on samples of Diemelstadt sandstone conducted under a confining pressure sufficient to induce compaction bands. Diemelstadt sandstone is a visibly anisotropic rock with an initial porosity of 23% and a mean grain diameter of 0.3mm. We have quantified the void-space anisotropy of this material by: (a) measuring radial elastic S and P waves as a function of azimuth around orthogonally cored samples, and (b) measuring the magnetic susceptibility anisotropy (AMS) of samples saturated with magnetic ferro-fluid. P-wave velocities show anisotropy of 7% and AMS measurements show that the void-space fabric approximates to an oblate spheroid, with the isotropy plane parallel to the bedding plane. Consequently, we have performed experiments on samples cored both normal and parallel to the isotropy (bedding) plane. Previous studies of compactive deformation have concentrated on the growth of a sequence of compaction bands through the volume of the sample. By contrast, we have concentrated on the nucleation and temporal evolution of single compaction bands. We used full wave-form acoustic emission (AE) locations from 10 transducers to record the propagation of individual compaction bands. This allowed us to compute an average band propagation velocity, which was lower than 0.1 mm.s-1. We also computed the seismic b-value from recorded AE events in order to examine any change in the scale of cracking during compaction band growth. We found samples deformed parallel to the bedding plane to be stronger than those deformed normal to it, with the compaction band geometry showing a more tortuous growth in the bedding-normal direction. This pattern of behaviour is also consistent with the very large reductions (3 to 4 orders of magnitude) in permeability observed during initial compaction band formation. A more gradual permeability reduction is observed during compaction band growth normal to the isotropy plane, commensurate with the increased mechanical resistance to compaction band development in this orientation.