An underground seismic-anisotropy experiment in a salt mine
R. James Brown, Zandong Sun
In order to study possible seismic anisotropy due to crystal alignment in salt, we have carried out an in-situ experiment in the Devonian Prairie Evaporite about 1 km underground in the Allan potash mine, near Saskatoon, Saskatchewan. The work has been carried out using a high-resolution (~1-kHz) seismic acquisition system and a tomographic (transmission) type of experimental design. The main purpose of the study has been to establish whether or not anisotropic seismic properties in these salts, which are believed to have undergone recrystallization and thermomechanical alteration, show any promise of being useful in the mapping of such salt units.
A pillar of potash ore (roughly 30% halite and 30% sylvite) was selected for the experiment on the basis of accessibility and rock integrity. A hammer was employed as the energy source (frequencies from about 100 to 2000 Hz) and each record was obtained by vertically stacking the records from rive repeated hammer blows. Geophones were affixed to one face of the pillar in three orthogonal orientations. Records were acquired in three separate steps, each with identical geometry but different orientations of the geophone polarization: first normal to the wall, then vertically tangential to the wall, and finally horizontally tangential.
We observe shear-wave splitting for many of the three-component record sets acquired in this experiment. This could conceivably be caused by cracking or fracturing in the rock as a result of de-stressing following mine excavation or by the alignment of salt crystals in the rock in response to the stress field prevailing at the rime of recrystallization. Indications are that the anisotropy observed here is a result of crystal alignment, rather than post-excavation effects. We are led to this conclusion because the observed symmetry axes are consistent with the known trends of fracture lineaments and principal stress axes on the western Canadian plains but inconsistent with the orientations of the excavated rock faces.