Characterization of subsurface crack systems by combining triaxial doublet analysis and focal mechanism analysis

The paper proposes a procedure to characterize the dynamic behaviour of subsurface crack systems on the basis of triaxial doublet analysis, which estimates precise relative source location of microseismic doublets in the downhole 3-component measurement. A doublet is a pair or a group of microseismicity with very similar waveforms despite different origination times, and is thought to express stress release on the same or similar structural plane in a crack system. The concept and procedure of triaxial doublet analysis is explained using the cross-spectrum and spectral matrix, and it is shown that this analysis provides information on the structural plane related to a subsurface crack system. The focal mechanism of microseismicity is analysed by combining a moment tensor analysis and a grid test. An understanding of both the structural plane and the moment tensor of the events allows an estimation of whether the structural plane of a crack system behaves as a single crack or multiple cracks. -from Authors