MEASUREMENTS OF P-WAVE ANISOTROPY USING ACTIVE SOURCE SEISMIC DATA IN THE HOMESTAKE MINE

We conducted a series of controlled source seismic experiments in the Homestake Mine in South Dakota. As part of these experiments, we conducted a set of nine-component seismic experiments along a drift at the 4850 ft. level in the mine using a hammer source. We also performed a pair of walkaway experiments with 1 m shotpoint intervals with vertically and horizontally directed hammer blows. Tau-P analysis yielded P and S wave velocity estimates of X +- dX and Y +- dY respectively. We shifted all data with a linear moveout using the average P-wave particle velocity to align the P-wave arrivals. We measured P-wave particle motions with two techniques: (1) a conventional principal component approach using a singular value decomposition of the three-component data in the specified time window, and (2) a novel grid search method that uses a transformation matrix to rotate these vectors over a grid of azimuthal and polar angles with the particle motion defined by a user-specified metric for the maximum amplitude. The objective of the measurements is to measure differences between the direction of propagation, which we can be determined using the experiment geometry, and P-wave particle motion. Theory predicts these two directions will differ in an anisotropic medium. The rocks at Homestake are schists and phyllites that are known to be strongly anisotropic. Results show both methods largely agree and that P-wave particle motions at each receiver are consistent between sources. Most show considerable divergence from pure longitudinal particle motion that would be expected in an isotropic medium. However, measured particle motion directions are highly variant between receivers, indicating small scale phenomena such as mine shaft resonance and near field scattering by adjacent drifts may be complicating these results.